Land Development Handbook
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About the Author
Dewberry, headquartered in Fairfax, Virginia, is a fully integrated engineering and architecture firm operating in more
than 50 locations throughout the United States. It consistently ranks among the top 45 design firms by Engineering
News-Record, top 20 engineering/architecture firms by Building Design + Construction, and the top 5 engineering firms by
the Washington Business Journal. Working in multiple federal,
state and local, and commercial markets, Dewberry’s services
include site/civil engineering and surveying; transportation,
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transit, and ports and intermodal design; water, wastewater,
and water resources engineering; architectural and interior
design; environmental, coastal engineering and resilience
services; emergency management and mitigation; full-service
geospatial mapping and analysis; and alternative project
delivery inclusive of design-build, public-private partnerships,
and turnkey construction. The firm enjoys a reputation for
quality, deep subject-matter expertise, community engagement,
and putting the client first.
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Land Development Handbook
A Practical Guide to Planning, Engineering, and Surveying
Editor-in-Chief: Sidney O. Dewberry, PE, LS
Principal Editor: Cody A. Pennetti, PE
Editor: Christopher J. Guyan
Fourth Edition
New York Chicago San Francisco Athens London
Madrid Mexico City Milan New Delhi
Singapore Sydney Toronto
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Copyright © 2019, 2008, 2002, 1996 by Dewberry. All rights reserved. Except as permitted under the United States Copyright Act of
1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher.
ISBN: 978-1-26-044076-8
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To the hardworking Dewberry employees
who dedicate their talent, energy, and passion to building amazing places.
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Contents
Foreword ix
4.3. Conceptual Design 355
Preface xi
4.4. Schematic Design 364
Contributors xv
Chapter 5: Final Design
5.1. Components of a Site Plan and the
Approval Process 380
PART I: OVERVIEW
5.2. Existing Conditions and Field Survey
Chapter 1: Overview of Land Development
1.1. Land Development Design Process
5.3. Transportation Design 417
6
5.4. Grading 437
5.5. Stormwater
PART II: PRE-DESIGN
462
5.6. Utility Design
486
5.7. Erosion and Sediment Control 586
Chapter 2: Due Diligence
2.1. Development Program, Site Selection,
and Defining Property 20
2.2. Comprehensive Planning
2.3. Zoning
PART IV: POST-DESIGN
29
Chapter 6: Permits and Construction
41
2.4. Subdivision Ordinance, Review Process,
Building Codes, and Development Costs
6.1. Permits and Bonds 638
6.2. Construction Documents and Construction
Phase Services 650
67
2.5. Environmental, Geotechnical, and Historical
Considerations 79
PART V: APPENDIX
Chapter 3: Site Analysis and Engineering
Fundamentals
Chapter 7: Additional Resources
3.1. Feasibility Study, Site Inspection and Plan Sheet
Comprehension 126
7.1. Technical Appendix 668
3.2. Base Map and Site Diagram
7.3. Soils
171
709
7.4. Floodplain Studies 745
207
3.5. Stormwater Fundamentals
3.6. Utility Fundamentals
7.2. Types of Stormwater Management Facilities 686
144
3.3. Transportation Fundamentals
3.4. Grading Fundamentals
395
7.5. Stream Restoration
225
751
7.6. Engineering Feasibility Study
255
7.7. Detailed Case Studies
761
779
PART III: DESIGN
7.8. Special Considerations for Public Sector
Development 854
Chapter 4: Conceptual and Schematic Design
Contributors and Reviewers of Prior Editions 862
4.1. Preliminary Plan Submission and Site Studies
308
Index 863
4.2. Product Types and Development Principles 321
vii
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Foreword
Today, we are constantly reminded of the importance of infrastructure engineering—aging structures, population growth,
diminishing resources, and a changing climate are all challenges faced by the engineering community.
These challenges are best overcome through collaboration
among industry leaders and by publishing technical content
that improves the knowledge-base of our industry. The Land
Development Handbook provides an enormous depth of technical content, which is authored by a multitude of professionals
working within an architecture, engineering, and consulting
firm that has a proven history of commitment to the industry.
Led by Mr. Sidney O. Dewberry, this edition continues to keep
pace with social and technological changes. This edition builds
from the success of prior editions and has been revised with
•• A new format to support use by professionals and
students—the sequential process of how information
is organized is beneficial to both novice engineers
and industry experts.
•• A broadened approach to land development services
that include a range of public and private project
types, including retail, office, residential, recreational, institutional, mixed-use, and others.
•• Updated graphics that communicate complex technical concepts, developed specially for this book.
•• Valuable context from a geographic expansion of
content and case studies based on a variety of project
topics including feasibility studies, resiliency, transportation, stormwater management, and others.
•• A refinement to every page to ensure all content
is current and relevant to current policies and
practices.
•• New content for the environmental and sustainability
topics, which has been integrated in every applicable
topic of land development.
As a leader in the civil engineering academic community, and an active member of the American Society of Civil
Engineers, American Institute of Steel Construction, American Iron and Steel Institute and Structural Stability Research
Council, I sincerely appreciate when an industry leader sets an
example for supporting the advancement of civil engineering.
The continued publications and philanthropy of Dewberry
are examples of significant contributions to the land development community. This community includes the educators,
developers, engineers, architects, and every individual that
benefits from the road, water, power, bridge, recreation, and
building systems that are imagined and designed by the hardworking professionals.
I know that our industry will continue to face new
challenges and I am excited to see that a resource like the
Land Development Handbook is available and continuously
improved. Every year there are advancements in environmental research, management techniques, design technology, transportation policies, and a multitude of other
systems that invent ways we can improve our built environment. I congratulate Mr. Dewberry and his entire team in
the achievement of this new publication—we appreciate the
passion and dedication of everyone that supports the effort
to grow our industry knowledge.
W. Samuel Easterling, PhD, PE, F.SEI, F.ASCE
Montague-Betts Professor of Structural Steel Design and
Department Head
The Charles E. Via, Jr. Department of Civil &
Environmental Engineering
Virginia Tech
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Preface
When this business was launched in 1956 the Land Development Planning, Engineering, and Surveying professions
were largely viewed as a backwater branch of Civil Engineering and not respected as a legitimate engineering field. Other
consultants looked down their noses at anyone engaged in
this practice and felt it was not “real engineering.” Since land
development consulting was how I made a living, I strongly
resented the notion. I felt then, and still feel now, that this is
a very noble profession. It requires expertise in all branches
of civil engineering including surveying, roadway design,
grading, drainage, water systems, wastewater systems, dry
utilities, and environmental science; as well as knowledge
of the related fields including urban planning, landscaping,
archaeology, and architecture. We also have a responsibility
to understand the economics, schedule, and vision of a project. But more important than the experience gained as a land
development consultant, is the product of our diligent labor:
we work to improve our communities.
For this reason, I have devoted my career to elevating this
profession to the level it deserves.
In the early days, infrastructure was often considered an
inconvenience that reduced the overall budget of the project. Few regulations required adequate drainage, utilities,
and other infrastructure to provide good, reliable access to
housing, employment, healthcare, public transportation,
and retail developments. We often clashed with our clients
over these issues as we advocated for sustainable and resilient infrastructure that exceeded the minimum standards.
Gradually, the localities mandated better infrastructure and
improved environmental performance through enhanced
standards and regulations. These requirements are still progressing and evolving today as evidenced by the tremendous strides taken in the Green Building and environmental
movements. Today, most developers and localities acknowledge the importance of infrastructure investments.
The profession of Land Development Consulting is now
recognized and respected among the engineering disciplines.
Every major A/E consulting firm has a land development
practice. It is taught in many colleges and universities and in
some cases, as its own specialty track within the civil engineering program. Young people are aware of and attracted to
the profession. They enter this field inspired, bringing with
them new ideas, the most recent technology, and a youthful
perspective on the world that challenges us “old-timers” to
keep pace with the speed of learning, rise above convention
and truly innovate for the benefit of our clients and our communities. I feel that our firm, in its way, has greatly contributed and remains attuned to this dynamic industry with the
Land Development Handbook series.
The first Land Development Handbook began as a dream
of mine many years ago. In the mid-eighties I committed to
developing a literary resource that could be shared with the
Pictured left to right: Sid Dewberry, Jim Nealon, Dick Davis when the firm was known as Dewberry, Nealon & Davis. Leaning on Nealon’s era 1935 airplane.
xi
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xii P r e f a c e
civil engineering industry. Rather naively, I thought a book
was something you sat down to do and finished within a few
weeks’ time. How surprised I was to learn that it would take
years. The first edition of this text, in fact, took 7 years from
start to finish. When looking for interest from publishers,
I was pleased that many were anxious to publish and distribute the book. We then entered into what has become one of
the most treasured and unique business relationships I have
formed over the years with a premier technical publisher,
McGraw-Hill Publishing. After the first edition, McGrawHill told us they would want us to update the handbook
every few years (if the book was successful). By their measures this book is a best seller for the industry and continues
to serve as a great resource. I’m proud to see the book in the
offices of our clients, in the hands of students, and on the
shelves of other design firms.
This business and our communities have changed dramatically since the first edition was published. To evolve
with these changes, we regularly update the book to capture
changes in policies and identify ever-changing design processes. With this fourth edition we have also expanded the
scope of the book to better represent the expanded scope of
land development. New technologies, tightened economics, and more complex projects require a broader range of
knowledge. This fourth edition of the Land Development
Handbook has evolved into three books to focus on (1) business, (2) design, and (3) construction. Together, these texts
are the Land Development Handbook series:
1. Development of the Built Environment is all about
business and economics of public and private projects.
This book is meant to improve understanding and
communication between consultants and developers/
owners to ensure greater success of projects.
2. Land Development Handbook continues to focus on
both the process and the technical design of civil
engineering. We also emphasize the importance of
public, private, and community relationships and
involvement. We can’t design in a silo—everything is
connected to the community and the environment.
3. Construction Practices for Land Development
describes the construction and operations of a project. Our industry is quickly trending toward design/
build processes. Design is influenced by construction and operation considerations, and the design
team should be actively involved in the construction.
I want to personally thank everyone who contributed to
the fourth edition. Having been through this process several
times before, I know the success of this exciting Dewberry
endeavor is due to the dedication of each team member. A
new edition of a book is no small task, and the development of two new books is a monumental effort. This latest
edition truly represents a corporate-wide effort, as nearly
00_Land_FM_pi-xvi.indd 12
all our 52 offices have contributed in both large and small
ways. This diverse corporate presence has yielded valuable
insight and fresh perspective from across the country.
Our lead contributors, Cody Pennetti, Chris Guyan,
Kat Grimsley, and Claire White have demonstrated tremendous dedication and passion toward a shared vision of creating a great resource for the industry. This team is unique
with backgrounds in both professional consulting and academia that adds depth to the content of the texts.
•• Cody Pennetti served as an editor and contributor
for this edition of the Land Development Handbook,
and capably handled the complex task of managing
the production of the three book Land Development
Handbook series. Cody began his career with
Dewberry and has always been passionate about
the industry and continuous teaching and learning.
Cody is now pursuing his PhD at the University of
Virginia with a goal of serving in academia while
staying involved in land development consulting.
Cody lent a unique perspective and new ideas in this
undertaking and helped produce a great resource for
both the professional and academic industry.
•• Christopher Guyan was instrumental in the development of this edition of the Land Development
Handbook and operated as both an editor and contributor. Chris joined Dewberry after earning his
undergraduate degree in civil engineering from
Penn State. He is currently working on his MS in
Urban and Regional Planning at Virginia Tech. He
has shown tremendous promise with his design work
and his efforts on the Land Development Handbook.
Chris has a strong focus on the planning and design
associated with land development. This focus has
helped shape some of the underlying themes in
this fourth edition to emphasize the importance of
planned development and the community.
•• Dr. C. Kat Grimsley was the primary contributor,
writer, and editor for Development of the Built
Environment. This new text benefits from Kat’s
extensive knowledge of commercial real estate.
She is the director of the Masters of the Real Estate
Development program at George Mason University,
has served at the U.S. Department of State managing an international development portfolio, and
has private sector experience working on financial
modeling and commercial transactions. Kat completed her doctorate at the University of Cambridge
in the United Kingdom with a focus on tenure security and international property rights and has since
been appointed as a NAIOP Distinguished Fellow
and member of the UN Economic Commission for
Europe’s Real Estate Market Advisory Group under
the Committee for Housing and Land Management.
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Preface xiii
•• Claire White was the primary contributor for the
Construction Practices for Land Development. Claire
earned her bachelor’s and master’s degrees from
Virginia Tech and has experience working in the
consulting field, starting as an intern with Dewberry.
She has recently transitioned into a teaching role at
Virginia Tech where she focuses on land development and real estate courses. From her consulting
and academia roles, Claire has witnessed the critical
role of engineers and the development team during
the construction phase of a project. She has structured the text to help engineers improve design and
further contribute to project success by anticipating
construction processes.
Thank you all for your commitment to this endeavor!
I also recognize that the efforts of these individuals are supported by family and friends that work behind the scenes.
I want to sincerely thank those that are always there to support us while we work on these demanding projects. I have
continued to hold the role of editor-in-chief for the fourth
edition of the Land Development Handbook and the two
new books in the series and have been proud to work with
this team.
I also want to extend a special thank you to Dottie Spindle, my administrative assistant, who took care of the little
things, the big things, and everything in-between so that I
could focus on the things that truly matter to me, like these
books and this company. Keeping me on schedule and on
task is a challenge, but it is one she embraced with a smile
for 33 years up until her recent retirement. Her replacement,
Janice Spillan, a career executive assistant, has picked up the
pieces seamlessly and continues to move the ball forward.
Many thanks to her as well.
Peer reviewers are a critical component of our text.
Those who think writing is difficult should try peer reviewing (or editing); balancing criticism with encouragement
is a tall order. Our peer reviewers rose to this task under
tight timeframes and across great distances. Their expertise
was invaluable and that they were willing to lend it to this
endeavor speaks highly of their commitment to Dewberry,
to their practice, and to mentoring others. Dozens of other
engineers and planners within Dewberry have reviewed the
final book and reviewed any changes that we made to sections of the original editions that they had contributed. Every
region within the United States has certain procedures to go
through to obtain permits to do land development construction. Developers and public- or institutional-sponsored projects generally retain the services of local professionals who
are familiar with local regulatory requirements, but the basic
principles of design are universal.
I would like to extend my sincere gratitude to all our
clients for your continued support. Many of you have willingly offered components of your projects for inclusion in
the text and we are happy to have your cooperation in this
00_Land_FM_pi-xvi.indd 13
unique project. In this edition, we’ve introduced case studies throughout the book—these real-world engineering
cases demonstrate the technical complexities of our profession while showcasing the exciting projects that we’ve been
involved with. These projects would, of course, not be possible without the shared vision of our clients.
Two other individuals that have contributed greatly to
this edition are Matt Pennetti and Dave Huh. Matt is an artist who developed hundreds of new illustrations and had the
complex task of creating graphics of technically complex
topics. Dave is a talented photographer at Dewberry who
has an amazing eye for showcasing our projects in the best
way possible. Our profession relies heavily on communication through graphics (plans, drawings, details), and these
two professionals have contributed amazing works of art that
complement the technical content of the books.
Craig Thomas, Dewberry’s General Counsel, helped us to
initiate this project with McGraw-Hill, has overseen all the
contractual arrangements since we first published the Land
Development Handbook many years ago, and has been a valuable legal resource throughout. Thank you, once again, for
your support in this endeavor.
Finally, I want to express my deep regard for our partner in
this effort, our publisher Lauren Poplawski of McGraw-Hill.
She took over a large project and has been instrumental in
the development of the new edition of the Land Development Handbook and the expansion of the Land Development
Handbook series. Thank you for believing in us, for helping
us elevate land development consulting as a profession and
making one of my dreams—this book—come true, again!
In 1956, if you had told me that our six-person land development consulting company would grow into becoming one
of the top 50 A&E companies in the United States, I would
have thought you were nuts! I learned the hard way that real
estate development was subject to the many ups and downs
of the economy. For that reason, we sought early on to diversify our company into other facets of the A&E business. This
diversification effort has been hugely successful for us, but
land development continues to be one of the primary underpinnings of our practice. We love it and every new project
continues to get the enthusiasm and professional care that
we gave when we were first trying to get established. I would
urge the thousands of small land development consultants
throughout the United States to always consider the connection between the projects across geographies, design firms,
the environment, and communities. It’s necessary to diversify our skillset to adapt to industry changes and economics
while also considering the big picture of our responsibilities as design professionals. And ultimately, deep down, we
get supreme joy out of helping plan and build safe, healthy,
financially feasible, sustainable, and beautiful places for people to learn, work, worship, shop, play, and live.
When I was born, 91 years ago, the horse and buggy had
almost been completely replaced by the horseless carriage.
Telephone, radios, and electricity were getting to be the norm.
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xiv P r e f a c e
Television was just coming on the scene. The information
technology era was being developed. Population in the
United States had grown from 120 to over 328 million today.
Air travel was beginning to be a serious competitor of rail,
road, and water. The United States had been completely
rebuilt. It is now a different place—totally new in just one
lifetime. My first great grandchild, who is 18 months old, will
see even more rapid changes with driverless cars and intelligent machines taking over our day-to-day activities, along
with changes in healthcare. All these things will surely be a
00_Land_FM_pi-xvi.indd 14
challenge to the engineers and scientists to provide solutions.
Just in the next few years, many of these changes will have a
huge impact on society, and how the engineers and scientists
provide solutions for them will be amazing. Oh how I wish I
were a young engineer facing these opportunities to provide
solutions to the many challenges!
Sidney O. Dewberry, PE, LS
Chairman Emeritus, Dewberry
Editor-in-chief
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Contributors
Primary Editors and Contributors of the 4th Edition
Additional Contributors
Sidney O. Dewberry, PE, LS
Editor-in-Chief, contributor
C. Kat Grimsley, PhD
Cody A. Pennetti, PE
Principal editor, contributor
Christopher J. Guyan
Editor, contributor
Claire M. White, PE
Matt A. Pennetti
Illustrator
Dave Huh
Photographer
Aileen Heberer, Molly Johnson, Kimberly McVicker
Dewberry corporate information and cover art content
Craig N. Thomas, General Counsel Dewberry
Legal advice and guidance
Reviewers of the 4th Edition
Daniel T. Anderton, RLA
Site Selection, Comprehensive Planning, Zoning,
Subdivision Ordinance, Review Process, Form Based Code
Illena Ivanciu, PhD
Environmental and Historic Preservation, Post-construction
Services
Tim Belcher, PE
Subdivision Ordinance, Site Selection, Components
of a Site Plan, Approval Process, Bonds, Bond Estimates,
Permits
Gary Kirkbride
Overview, Due Diligence, Comprehensive Planning,
Zoning, Subdivision Ordinance, Review Process,
Environmental and Historic Preservation, Conceptual
Design, Schematic Design, Development Types
Andrea Burk
Historic Preservation
Brian K. Bradner, PE
Grading, Stormwater Fundamentals
Chris Cirrotti
Feasibility Study, Road Design, Site Selection, Development
Types, Conceptual Design, Schematic Design, Components
of a Site Plan
Dennis Couture, RLA
Site Selection, Development Types, Conceptual Design
Jesus H. Echevarria, LS
Base Maps, Existing Conditions, Field Survey
Bill Fissel, PE
Complete Review
Larry Smith, PE
Environmental Planning
Brian Sayre
Natural resources
Zach Davis
Cultural resources
Steve Eget, PE
Regulatory compliance
Robert “Skip” Notte, PE
Utility Design
The contributors and reviewers of prior editions, who built
the foundation of this textbook, are listed on page 862.
xv
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Part I
Overview
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Chapter 1
Overview of Land Development
The conversion of land from one use to another is the generally accepted definition of land development. This is a very
broad term, but as used in this book, this definition applies
to land conversion and infrastructure improvements completed by public, quasi-public, and private developers for a
variety of project types. Schools, parks, government facilities,
public roads, transit systems, airports, utility networks, hospitals, office complexes, residential communities, industrial
facilities, and retail centers are just some of the types of projects within the realm of land development. In all cases, land
development should be performed responsibly and sustainably to improve communities for people to live, work, worship, shop, and play.
Today, the process for finding solutions and developing
scenarios for land use that serve the greater good is a systematic one. The team working on land development projects
focus on the preparation of required documents to secure
the land entitlement approval from official having jurisdiction. These officials issue permits for the development of a
project. The process also requires the preparation of construction documents, typically consisting of construction
drawings and technical specifications, to fully detail the construction requirements of the proposed project. The engineering principles outlined in this book are applicable to any
project, regardless of its nature.
This book is, in its entirety, an overview of the land design
process as it applies to civil engineering, planning, and surveying. The term “site engineer” is used in this text to distinguish between other civil engineers that exclusively focus on
structural, geotechnical, traffic, or other disciplines that fall
within the practice of civil engineering.
PRE-DESIGN
The site engineer, planner, and surveyor are an integral
part of the development team. They are usually among the
first to arrive on the site and the last to leave after completion. They help guide and direct the process from start to
finish. This text presents the material in a sequential order
according to the typical land development design process;
however, it is rare to see a “typical” project or process.
By the nature of land that the project is built from, every
project is unique. The terms “generally,” “often,” “usually,”
“typical,” and other conditional notes are used in the text
to acknowledge the variation in project requirements.
Every professional should be aware of the applicable project requirements and project goals. For the projects, cases,
and material not specifically identified in this text, the
fundamentals and principles mentioned herein provide a
resource for design professionals.
Figure 1.1A provides a graphic representation of the content associated with each chapter of the Land Development
Handbook. The book begins with a focus on pre-design site
investigation and analysis efforts, transitions into planning
and engineering design phases, and eventually describes the
permitting and construction phase (post-design). Each chapter is divided into subchapters (e.g., Chapter 2.1, Chapter 2.2,
etc.) with detail on different topics. Chapter 7—Additional
Information has supplemental information relevant to specific topics of land development. Additionally, case studies
are included throughout the book to provide context to the
material presented. The format of how information is presented in this book is intended to follow the typical process,
but some of the earliest project efforts require expertise in all
aspects of design and consulting.
DESIGN
POST-DESIGN
Preliminary
Detailed
CHAPTER 2
CHAPTER 3
CHAPTER 4
CHAPTER 5
CHAPTER 6
Due Dilligence
Site Analysis
Conceptual &
Schematic Design
Final Design
Permits &
Construction
F i g u r e 1 . 1 A The land development design process.
3
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4 C h a p t e r 1
■
O verview
of
L and D evelopment
SPECIAL CONSIDERATION FOR PUBLIC AND SEMI-PUBLIC SECTOR DEVELOPMENT PROJECTS
Public sector developers are government entities at either the federal, state, or local level.
The public sector is an extremely active developer, responsible for both infrastructure projects, such as highways, and
building projects, such as post offices, courthouses, military hospitals, and local schools or state universities. Semi-public
and non-profit entities are also active developers whose projects can include hospitals, university facilities, and other
buildings that serve the public. The fundamental development tasks are essentially the same for both public and
private sector developers: they must find and evaluate a potential site, obtain funding, and coordinate the design and
construction of each project. The technical engineering principles cited in this Handbook are applicable to any project,
whether it be a private development or a public, semi-public, or non-profit project.
Despite high-level similarities, however, the priorities and processes of public sector developers are extremely different
from their private sector counterparts. Public sector development projects at federal, state, and local levels are all subject
to unique challenges created by funding limitations, changes in political will, and formalized bidding processes. Similar
limitations exist for semi-public and non-profit developers. Unlike private sector activities, public development is not
driven by the desire to earn profit but rather by the need to provide services to citizens. Public projects typically begin
as a function of evaluating public needs and current capacities; funding must then be appropriated and permission
obtained for the public sector developer to begin a formal procurement processes.
The government’s role in serving its citizens is directed by elected officials at the federal, state, and local levels. Political
will refers to the degree to which these officials, individually and collectively, support a project as a matter of priority
and are willing to commit resources to it. This often means allocating funds, but may also involve advocating for needed
approvals, supporting related policy changes, and championing the project despite citizen opposition or party dissent.
The willingness of elected officials to support projects depends in part on the nature of the project itself but can also be
heavily dependent on a politician’s individual beliefs or priorities, lobbying influences, current citizen responses, reelection considerations, and other competing priorities. Citizen opposition to a particular project, especially during an election cycle, can have disastrous results if/when political candidates engage in election-oriented behavior and withdraw
support in order to satisfy voters and boost reelection chances. Election results that lead to a change of office for key
political supporters of a project can easily result in a lengthy delay or even cancelation of the project.
F i g u r e 1 A Example of a public project—Tolleson Police and Municipal Court; Tolleson, AZ.
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Even when projects receive the necessary approval and support, public sector developers are accountable to tax payers
and political leaders for their results and, as such, are required to comply with a multitude of laws, regulations, and policies that guide everything from procurement to construction practices. The federal, state, and local regulatory structure
is complex and prescriptive, including components that are sometimes overlapping or inter-dependent. In instances
where a state project benefits from federal funding, such as for a National Highway System road project, the state must
comply with both relevant state and federal regulations. This serves to add another layer of complexity to any project and
often binds states to federal procedures. These controls are intended to ensure responsible stewardship of tax revenue,
fair competition, and transparency in government operations; however, somewhat ironically, this cumbersome structure
often makes the public sector process less efficient and more costly than that of the private sector.
Public sector developers do not use traditional loans to finance their projects. Instead, they rely on sources such as
appropriations from tax revenue or the issuance of bonds. Regardless of the specific form, obtaining public sector funding for development projects involves a lengthy approval process that often requires the public sector developer to submit project budget estimates a year or more in advance of undertaking a specific project. While it is understandable that
the public sector must engage in long-term budgetary planning exercises, accurate estimates for the cost of materials
and labor for future development are difficult to predict so many years in advance. Further, approval for development
projects and must compete for funds with other spending priorities. For these and other reasons, federal, state, and local
public sector developers can all find themselves facing budget shortfalls when the time comes to actually begin a development project. As a result, partnerships with the public sector are being used with increasing frequency as a strategy
to offset funding shortfalls. Public-private partnerships, also called PPP or P3s, are projects in which a public sector
government agency works in partnership with a private sector developer in order to complete a public-sector project.
Note that in a P3, the private sector does not necessarily pay for the project, but does finance it. P3s reallocate risk and
responsibilities between the public sector and private sector, often leading to cost and time efficiencies, as well as technical innovations not previously accessible to the public sector.
The challenges and unique considerations of public sector developments will be discussed in greater detail in
Chapter 7.8.
F i g u r e 1 B Example of a public highway project—Intercounty connector Route 29 over Briggs Chaney Road, aerial, Montgomery & Prince
George’s County, MD.
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Chapter 1.1
Land Development
Design Process
1.1.1. Introduction
Land development design and consulting constitute the systematic process of collecting data, studying and understanding the data, extrapolating the data, and creating plans for
reshaping the land to yield a project that is politically, economically, and environmentally acceptable. Land development ties together a wide range of interests, pressures, user
groups, and economic goals; thus it is a design field that is
heavily influenced by the surrounding context—political,
economic, environmental, and cultural—within which the
land development will take place. This contextual influence
has driven the development of laws that provide a common
framework for land planning and design, and has directed the
focus of land development efforts throughout U.S. history.
Land development consulting merges the science of city
building with the art of placemaking through a collaborative,
multidisciplinary approach to project delivery. The challenge
for land development professionals is to understand the
factors contributing to the demand for growth and expansion. Part of that understanding includes knowing where
the industry has been and how it evolved into the practices
and procedures of today. The other part is understanding
the nature of the land development industry and how to
maintain the standards of quality, flexibility, and value that
we have attained. Design professionals must meet the challenges of today while not losing sight of yesterday’s lessons
and today’s high standards.
1.1.2. Public and Private Project Types
The most important distinction of project types to make is
between developers operating in the private sector versus the
public sector. While private sector developers are individuals or corporations, public sector developers are government
entities at either the federal, state, or local level. Land development is often attributed only to private sector work, but
public sector development represents a major component of
the development industry. Examples of public sector development projects include large-scale infrastructure projects such
as highways, post offices, courthouses, government-owned
agency office buildings, and local schools or state universities.
The project type, as public or private, can influence the
land development design process. This reiterates the point
that each project is different but still generally follows the
same path. The land development process can be fluid, but
projects still have the same requirements that must be met.
Understanding the process and the requirements will lead to
success for any project.
The fundamental development tasks are essentially the
same for both public and private sector developers; they must
find and evaluate a potential site, obtain funding, and coordinate the design and construction of each project. Public and
private developers both must interview and hire technical
experts who form their development teams. Public and private
sector developers must also be knowledgeable enough about
all aspects of the development process to play an active role
in coordinating their teams’ activities. Despite these high-level
similarities, however, the priorities and processes of public and
private sector developers are extremely different.
Public Sector. Development within the public sector is
a relatively prescribed function of evaluating public sector
needs and current capacities, appropriating necessary funding, and gaining permission to begin a formal procurement
process. The mechanisms guiding public sector development
are generally inflexible, but the public sector can be just as
innovative and passionate about projects as the private sector. Perhaps unsurprisingly, as a developer the public sector
6
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1.1
is risk averse and rule oriented. Public sector developers are
accountable to tax payers and political leaders for their results
and, as such, are required to comply with a multitude of laws,
regulations, and policies that guide everything from procurement to construction practices. The federal, state, and local
regulatory structure is complex and prescriptive, including
components that are sometimes overlapping or interdependent. These controls are intended to ensure responsible stewardship of tax revenue, fair competition, and transparency in
government operations; however, somewhat ironically, this
cumbersome structure often makes the public sector process
less efficient and more costly than that of the private sector.
Private Sector. Private sector developers are (usually)
incentivized by profit-seeking activities, which can make
them appear to be unsupervised opportunists when compared to their public sector counterparts. Unlike many development team members, developers are not licensed and are
constrained only by their own partnership relationships or
sometimes, for larger development firms, investment committee approvals. Private developers tend to be optimistic
and can have a fairly high tolerance for risk, calculated or
otherwise, as compared to other professions.
Private sector developers typically generate revenue by
(1) charging fees for their work, (2) creating value for investors and selling properties after they are built or entitled,
(3) receiving rental payments from tenants in properties they
build and continue to own, and (4) by appreciation in the
value of their ownership position in properties they build and
continue to own. In virtually all cases, profit cannot be realized until after a development project is completed, meaning
that during the development process itself, developers must
take the risk that unexpected factors will reduce or eliminate
their future earnings. To safeguard future returns, developers will constantly seek to control project costs and timing.
Anything that increases either of these two critical factors
will weaken the project’s overall profitability and, in serious
circumstances, can negatively affect the developer’s ability
to repay investors and lenders. The sooner a project is completed, the sooner the developer can begin to earn revenue
and generate profit.
Some organizations may not fall under the public sector but may operate differently from other for-profit private
developers. A charitable foundation, low-income housing
developer, animal shelter, hospital, or other similar nonprofit
organization will have different goals from a for-profit developer but will be subject to many of the same permit processing and development requirements.
1.1.3. Development Team Structure
The development team, as defined with this text, includes
all members involved in the project from pre-design efforts
through construction. The team is generally categorized by
legal, business, design, and construction teams that (should)
work together to meet project requirements and develop a
successful project. The reporting structure of each project
may change based on the project’s contractual hierarchy of
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Typical Members of Development Team
Client/Developer
Legal Team
Business Team
Land Use
Attorney
Lenders
Design Team
Broker
Construction
Team
General
Contractor
Property
Manager
Sub Contractors
Marketing
Professional
Equity Partner
Building Team
Site Team
Architect
Site Engineer
Structural
Engineer
Landscape
Architect
MEP Engineer
Surveyor
Environmental
Engineer
Figure 1.1B
Geotechnical
Engineer
Traffic
Engineer
Land Planner
Typical members of the development team.
the development team, but the organization chart is generally defined by Figure 1.1B.
As shown in the organizational chart, the client/developer
represents the position of authority of establishing requirements and making project decisions. The client/developer
looks to the rest of the development team for consulting
and design services. The legal team includes the land use
attorney—a critical team member that can navigate land use
and zoning laws. The business team is primarily involved
in the financial elements of the project and includes the
broker, lenders, marketing group, property manager, and
equity partner. The design team is separated into a building team that focuses on the vertical development and the
site team that focuses on the site infrastructure (horizontal
development). The construction team is generally engaged
toward the end of the design phases and includes a general contractor with subcontractors that perform different
trades. This text focuses on the design team with an emphasis on the role of the site engineer and how they interface
with other team members.
There are several other professionals that can be included
in the development team, and in some cases, not all the team
members identified in Figure 1.1B will be needed for every
project. The type of project will influence the role of each
team member. For example, in a traditional neighborhood
project, the site engineer and land planner will be the primary members of the design team. For a roadway or utility
project, the building team would not be involved at all. If a
project focuses primarily on the redevelopment of an existing building in a city, the site engineer will hold a tertiary
position to the building team.
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For a project to be successful, the development team
requires the support of the community and local jurisdiction. While these entities are not official team members,
they will often influence the design and progression of a
project.
A detailed description of each team member’s position is
provided herein.
Client /Developer. The client/developer is referenced as the
developer in this text but can represent a land owner, owner’s
representative, public entity, or other authority that establishes the project requirements. Several examples of different
developers are listed:
•• A developer interested in subdividing a large tract of
land for a new neighborhood.
•• Homeowners interested in seeing the development
potential of their property.
•• An owner’s representative for a hospital that has
been hired to manage the development of a future
expansion.
•• A retailer considering multiple sites for a new store.
•• A landowner looking to build an addition to an existing building.
•• A government agency looking to construct additional
public buildings.
•• A department of transportation (DOT) interested in
building a new public road.
•• A utility provider extending, upgrading, or repairing
service.
•• A university redeveloping the campus for new housing or classrooms.
The distinction between the client and developer is
important—while they may often represent the same entity,
in some cases the contractual hierarchy will be structured
such that a site engineer’s client is an architect, and the architect’s client is the developer. Additionally, the client may be
the land owner, but the program manager for the development is a separate company (often referred to as the owner’s
representative). The reporting structure and relationship
should be defined in the contract language between the client
and members of the development team.
The role, experience, and responsibility of each developer
will vary. In some cases, the developer will look to the design
professionals for guidance through the process. In other
cases, the developer may have enough experience such that
they are only seeking design services from the team. Many
developers are not inherently familiar with the design process, and all team members should work together to develop
the project schedule and review project requirements. Unrealistic project schedules and late changes to design requirements will burden the design team, and often result in costly
01_Land_CH01_p001-016.indd 8
errors that arise during the construction phase. The expectations should be clarified early in the project, so all team
members understand their roles and responsibilities.
Legal Team. The land use attorney for a project is critical
throughout the entire design process and through construction. A land use attorney should be familiar with the legal
and political aspects of real estate development. The land use
attorney will assist the team on legal aspects of development,
such as deeds associated with easements, interpretations of
local codes, document recordation, and authoring proffers or
development conditions. Additionally, the attorney may serve
as an advocate for the developer during negotiations with the
public agencies and may lead the presentations to the community during public meetings. The design team, especially
the site engineer and the surveyor, will coordinate closely with
the attorney and developer throughout the project phases.
During early phases of the project, the attorney will advise
the developer on allowable uses of the land based on zoning
and other regulatory requirements. If a developer is unable to
use the land for the desired purpose, the attorney will work with
the development team to process the necessary entitlement
applications. The attorney will have a major role in projects that
require rezoning, variances in zoning, or special permits. The
involvement of the land use attorney will extend through the
design process and into construction for coordination with
the title company and lender for construction draws.
Business Team. The business team is a reference to those
team members that are primarily responsible for the marketing and financial aspects of a project. This team interacts
with the developer, and there is likely minimal coordination between the business team and the other professionals. Lenders and equity partners often provide the financial
means for project development. A marketing team may
advise the developer of potential tenants, appropriate home
prices, growth potential, demographics, and other market
conditions of the area. The brokers will often coordinate with
leasing, specifically for a commercial site, as they work with a
developer to find appropriate tenants for retail space.
The business team will likely be involved throughout the
duration of the project, focusing on marketing research in
early phases and then financing prior to design and through
construction.
Design Team. As a part of the development team, the design
team includes the building team and site team for the project.
The design team members are required to be licensed professionals including architects, engineers, and surveyors to sign
and seal plans that are submitted for permit. There is often a
significant amount of coordination between all design team
members, and in some cases, there may be an overlap of
responsibilities. For example, the architect may provide initial site layout designs to depict the proposed building location, which is then validated or refined by the site engineer.
As noted in the preceding section, the contractual hierarchy
may be established such that some design professionals act
as the client for other design professionals, or all may be contracted directly with the developer.
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1.1
Building Team. As a part of the design team, the building
team focuses on the vertical design. Team members include
the architect, structural engineer, and MEP (mechanical,
electrical, plumbing) engineers for the project. This team will
work on the building plans that are used to secure a building
permit. The building plans and permit often rely on the site
design and permit, and all information should be carefully
coordinated. The building team will have a different design
process and schedule than the site team, but ultimately both
design sets and permits are necessary prior to starting work
on a project with a building.
The architect is often involved in early phases of the project and may even provide an initial site layout for the project. In later phases of design, the architect and site team will
often refine the layout based on site engineering, landscape
architecture, and geotechnical requirements. Throughout the
design phases of a project, the building team and site team will
exchange design information on a regular basis and will balance design requirements across the various disciplines. The
building team and site team will coordinate on building size,
location, building access points, accessibility requirements,
utility connections, grading along the building face, and other
interface considerations. As noted in the preceding sections of
this chapter, a building team will not be involved in projects
that focus exclusively on infrastructure (roads, utilities, etc.).
Site Team. As a part of the design team, the site team
focuses on the infrastructure design and surveying associated with the project. Some of the team members (surveyor,
traffic engineer, geotechnical engineer, environmental engineer) are involved in various phases of design to collect and
report site data. Other team members (site engineer, landscape architect, and land planner) will have a significant role
in the planning, designing, and permitting throughout the
entirety of the project.
The surveyor is usually involved in the early phases of
design for boundary and topographic survey work and is
involved again during later phases of design to prepare easement, subdivision, consolidation, and other relevant plats.
The traffic engineer is involved in early site analysis work and
will compile traffic studies as needed. The traffic engineer
may also be involved in supporting transportation design for
a project. The geotechnical engineer will perform soil investigations and prepare reports and recommendations based
on soil conditions. Additional documentation and reporting may be necessary from the geotechnical engineer if warranted by the proposed buildings and grading conditions.
During construction, the geotechnical engineer will provide
soil investigation and testing services. The environmental
engineer will be involved during early phases of site analysis
in determining environmental conditions and requirements
of the site and in processing environmental permits and preparing mitigation plans as required.
The site engineer, land planner, and landscape architect
serve as the primary designers for the infrastructure, site
layout, and landscape design. A land planner is a role that
may be served by a certified planner or informally held by
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another member of the design team (usually, the architect
or site engineer) depending on the site characteristics. These
design team members coordinate closely to avoid conflicts
in site features while delivering a functional and attractive
site design. This text focuses primarily on the role of the site
design team with a specific focus on the site engineer.
Construction Team. As a part of the development team, the
construction team includes a contractor and subcontractors.
The general contractor is responsible for coordination between
the design team, developer, and subcontractors to perform the
construction work of a project. A contractor will have a different role, depending on whether a project follows the traditional
design-bid-build process or follows a design-build process.
With design-bid-build, the developer works with the other
members of the development team to secure a permit and
request bids from contractors to perform the work. In designbuild, the design team and the contractor work together to
prepare design documents. Additional information on project
delivery methods is described in this chapter.
The contractor is typically involved in the later phases of
the project, near the time permits are secured for construction (if the project follows a design-bid-build project delivery method). During construction, the contractor will work
with the design team and developer and may coordinate
shop drawings, request for information (RFI) documents,
project addendums, or other post–design-related tasks.
1.1.4. Public Involvement
In today’s land development practice, a working knowledge
of the public process is essential for success. The community and public agencies should be considered as part of the
development team. Even though the public agency involvement may seem mostly regulatory, in many jurisdictions,
public entities have the power to shape projects, deny applications, and grant approvals. It is necessary to understand
the applicable regulations and submit compliant plans that
are legible and complete.
Involvement by public agencies is pervasive and must
be understood thoroughly by the land designers, as there
are many agencies involved, often with conflicting goals.
Approvals must be obtained from all involved agencies
before the project can proceed. Compliance with applicable
rules and regulations is often required by ordinances or local,
state, or federal law. A brief description of representative
agencies and selected rules is included throughout this book.
The nature of public agency involvement varies greatly
from jurisdiction to jurisdiction and agency to agency. Federal regulations, however, are reasonably consistent. Each
state has its own set of rules that are dissimilar to other states.
Even within a state each county, town, and city can be different. Projects within the same ZIP (zone improvement plan)
code may have different requirements. Sometimes regional
authorities have jurisdiction for such services as sewer or
water. It is imperative that the development team thoroughly
understand the rules of all the agencies having jurisdiction
over a project.
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Citizens have become much more involved in the process
of approvals for a project. It is important to respect the opinion and goals of the community. Some design changes may
seem insignificant to the development team but could provide a benefit to the community and garner support. There
may also be opposition to a project only because of a lack
of understanding. The communication, design, and negotiation process are critical elements to a successful project, even
if it is not always a formal process or requirement.
1.1.5. Project Communication
As described in the preceding section of the development
team members, a land development project involves a multitude of people with many different expertise and roles. For
the project to be successful, good communication is paramount. The entire development team should work closely
to define the requirements for the land development design
process and each design phase of the project. There is no universal method for preparing design documents. The schematic design for one project may look different on another
project, and some projects may not even require a schematic
design phase. Therefore, communication within the teams
and between the teams is important.
A good communication process requires members
of the team to be in constant contact with each other,
approval agencies, and with citizens. A project undergoes
many changes between its inception and completion. These
changes occur very rapidly and for many reasons. It is
imperative to communicate changes and updates to the right
people at the right time. To do this effectively, one must ultimately know what has transpired and understand how and
when to communicate the information. Project information
will be presented differently when provided to a technical
expert compared to a nontechnical stakeholder. The ability to
effectively communicate through writing, presentations, and
graphics is an important credential for the land development
professional. Further, documentation of project-related correspondence is critical from a business standpoint. Consultants should maintain a prudent plan for tracking, recording,
and retrieving all forms of project correspondence.
It is important to present ideas clearly and precisely. Good
public speaking skills before small and large groups is necessary in many professions, but it is particularly important
in land development where presentations to public approval
agencies often make or break a project. This includes skills
in adapting material for a technical audience, nontechnical
group, or a mixture of the two. Accuracy should be unquestionable and authentic enthusiasm is a key ingredient. The
ability to communicate effectively, regardless of the media, is
an essential quality for a consultant.
1.1.6. Responsibilities of the Site Engineer
The term “site engineer” is used in this text to reference the
civil engineer responsible for the site infrastructure scope
of work for a land development project. The term is used
to distinguish the role of the site engineer from other civil
01_Land_CH01_p001-016.indd 10
engineering disciplines (structural, environmental, geotechnical, traffic, etc.). The roles and responsibilities of the site
engineer, an important member of any land development
team, are the focus of this book.
A registered professional engineer has the authority to
sign and seal design documents that are used for permit
issuance. As outlined in the National Society of Professional
Engineer (NSPE) Code of Ethics, a fundamental responsibility of a professional designer is to “hold paramount the
safety, health, and welfare of the public.” The public relies
on the honesty and integrity of professionals to perform
actions in a manner that adhere to the professional ethics. The technical complexity of site engineering requires
the professional to continuously apply solutions based on
engineering judgment. This responsibility extends beyond
technical analysis.
The site engineer operates as both a technical professional
and a consultant. The technical responsibility of an engineer
includes the application of natural sciences and adherence to
local land development regulations. The consultant role does
not have a set of prescribed requirements and relies heavily
on the judgment of an engineer as they provide design solutions to the development team. This responsibility involves
decision making, which relies on an understanding of project requirements while considering economic and environmental context to provide the best solutions. Most projects
have a multitude of technically correct solutions, but the best
design must be decided by the engineer and the development
team.
The work performed by an engineer is often arduous.
Each design for a project will be unique, which means the
design has never been tested—the complexity of this task
is compounded by aggressive schedules with minimal contingencies for rework. A new design must be developed for
each project based on careful analysis of available information while considering the potential for unforeseen project
conditions that may require design changes. The engineer
is responsible for following a systematic process of design
and development with attention to quality while also meeting the aggressive schedule requirements. The balance
of schedule, budget, and quality requires expert project
management.
The amount of reference material focusing on land
development and specifically project design is extraordinary. Similar is the case with the volume of resource materials that more singularly focus on specific development and
building prototypes such as residential, commercial, industrial, office, recreation, mixed use, planned communities,
waterfront, golf course developments, etc. Basic to the success of project design is the need for the designer to have
an appreciation for the concepts and standards identified in
that body of information. Historically, land development is
steeped in technical solutions. These projects satisfy a multiplicity of functional and regulatory requirements inherent
to site engineering and ultimately program constructability;
however, they do not necessarily address the environmental,
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1.1
social, sensory, or visual dimensions, which are fundamental components of the built environment. Design solutions
need to be based in a sensitivity to basic sociocultural, physical, economic, and political concerns, while reflecting the
importance of economic and marketing constraints. In private sector land development activity, a design must be capable of being constructed and it must provide a financial (or
social) incentive to warrant its undertaking. No one profession possesses a monopoly on the diverse body of knowledge
and resources required to achieve quality land development
design. Land development is a process that is dependent on
diverse disciplines and an extraordinary commitment to
promote all aspects of the project with equal fervor.
Design Elements. Every development program comprises
elements that define, shape, and establish the essence of that
use. The constituent parts include both the physical-dimensional-building blocks that house principal activities, as well
as the ancillary or support elements, which are necessary to
sustain the principal use. The former are the major spacetaking elements that characterize a land use and its related
building components. For example, the dwelling unit is the
principal building module in residential development. The
accessory uses include such considerations as connection
to vehicular or pedestrian circulation systems, and utility
requirements necessary to maintain a certain quality of life,
such as water, sewer, power, communications, etc. Collectively, they constitute an operational “whole.” Project design
must address all of these elements. There may be several
ways to orchestrate a design that satisfies some of the basic
requirements associated with a given land use or product
type. However, the successful response seeks to reduce any
conflict with program objectives and optimizes the relationship between all component parts. This approach applies to
large-scale and mixed-use projects as well. The manner in
which a site design addresses these requirements should be a
result of a conscious decision and not insensitivity to or the
ignoring of any component or relationship.
Project design requires an ability to understand the
relative needs and physical attributes of the program components. More homogeneous uses at lesser densities or
intensities are generally more easily dealt with than mixeduse programs at higher densities. As an example, there is significantly more flexibility in siting a single-family residence
on a large lot than there is for an urban mixed-use project.
The challenge of site design rests in both knowledge of the
requirements associated with a given land use or building
type and an ability to make valid judgments and establish
priorities as to which requirements should take precedence
in formulating the design response.
Scope of Work. The scope of work for a site engineer is
contractually described through agreements between the client/developer and the engineering firm. The scope of work
will generally include the preparation of design documents
that are used to obtain project-specific permits. Inherent
to the design services, the engineer is required to coordinate with the development team, public agencies, and the
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community. An engineer may be responsible for answering
to another design professional (such as the architect) and
may be responsible for managing subconsultants that provide professional services associated with the land development design.
Design coordination should be carefully managed—each
design professional is working through an iterative and
incremental design process in the same way the site engineer
is. Project design elements are often negotiated between the
building, landscape, infrastructure, environmental, geotechnical, and other site requirements. The design team should
work together to determine the best solution for the site
based on project requirements and site constraints. Because
of the importance of infrastructure, the site engineer will
often establish or validate building locations and site features. The typical scope of work for a site engineer involves
all aspects of the land development design process.
1.1.7. The Land Development Design Process
The land development design process is iterative and incremental. Many projects will investigate several design alternatives before working toward a preferred option, which will
be refined through several more iterations. By the nature
of the land a project is built from, each project is unique
and requires a tailored design process that ensures project
requirements are met. There are many different terms and
processes used for project design phases—each developer,
jurisdiction, or contract may use different terminology and
processes. It is important for the development team to identify the design process and the terminology prior to establishing requirements and starting the project. In general, the
project process will begin with analysis and feasibility, transition into design phases, and eventually move into construction and commissioning.
The stages of design encompass pre-design efforts, design
efforts, and post-design efforts. These stages are not always
linear, and some may be omitted on certain projects. The
complexity of design and variability of the development conditions will influence the process.
Figure 1.1C depicts the typical land development design
process. The exact process and terminology of each step will
depend on the client, project, and jurisdiction. It is important to remember that although it is typically sequential and
will be described as a sequential process throughout this
book, it may not always be. At any point during a project
it is possible that early phases of work will be revisited. For
example, changes to financial conditions may occur during
final design efforts, which requires a change in scope. The
development program would need to be revisited, and the
process may return to site selection. This graphic is provided
as a reference to how content is presented within this book.
Pre-Design. The pre-design stage is often identified as a
feasibility study period. During this stage, the focus of the
development team is to identify the project requirements
and to study a project site for development opportunities
and constraints. At this stage, the effort is exploratory with
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Development
Program
Pre-Design
Site Selection
Due Diligence
Site Analysis
Feasibility Study
Site Diagram
Concept Plans
Schematic Design
Preliminary Design
Conceptual
Design
Final Design
Final Site Plan
Detailed Design
Preliminary
Plan
Construction
Documents
Procurement
Post-Design
Permits
Construction
Close Out
Figure 1.1C
The land development design process.
minimal investment (compared to later design stages) as the
development team determines if the project is feasible. This
phase requires a deep knowledge of all aspects in land development design, an understanding of the political conditions,
and experience with regulatory requirements. The feasibility
of the project focuses on the economic conditions and the
physical conditions.
This initial step in the process requires an understanding
of the proposed development program and an overview of
the site characteristics and surrounding area. The objective
of the pre-design stage is to become familiar with existing
site conditions and the developer’s intended application on
the site. The physical characteristics, including site configuration, topography, soils, hydrology, utility availability, and
01_Land_CH01_p001-016.indd 12
adjacent land uses, are evaluated in the context of the client’s
proposed development plan. The physical characteristics, as
well as land use conditions, will influence the project economics and feasibility.
Pre-design efforts begin with evaluating the development
program and then performing the site selection, due diligence, and site analysis.
Development Program. The development program is
the developer’s goal and requirement for the project. The
developer should provide information on the proposed use
of the site and any supplemental design requirements for
the site. Requirements should be communicated across all
members of the development team and openly discussed to
verify the requirements are understood. The design team
may be involved in drafting the development program for
the developer because they are most familiar with design
requirements and local jurisdictional requirements. The
development program may be influenced by information
produced in later phases of design, but major requirement
changes in later design phases can negatively affect the project schedule and budget.
The development program is introduced in Chapter 2.1.
Site Selection. Site selection is the process of identifying
suitable sites for a development project. The developer may
have one site in mind or several they are deciding between.
The development team assists the developer with the site
selection process by performing due diligence studies for
each site under consideration. This may include a legal due
diligence, financial due diligence, and a site-based due diligence. The site engineer is focused on this site-based due
diligence to evaluate the project’s regulatory and physical
characteristics.
Site selection is described in Chapter 2.1.
Due Diligence. A technical site-based due diligence is performed by the site engineer to assist the developer as they
perform their site selection. This phase initiates the land
development design process for the site engineer and begins
with a general review of regulations governing each site
being evaluated. This includes understanding the proposed
development program; determining development potential
by reviewing the local ordinances, master plans, codes, and
regulations; and performing a preliminary desktop review
of each site. Constraints and opportunities should be identified for each site. At the end of this phase, an initial go/
no-go decision can be made on each potential site based on
the information collected and assessed by the development
team.
Due diligence is described throughout Chapter 2.
Site Analysis. The site analysis is a detailed analysis of the
existing physical conditions of each site as part of the due
diligence. This phase analyzes the existing site conditions
with particular emphasis on identification of environmental,
cultural, and infrastructure resources. This helps to further
identify development opportunities and site constraints. Site
constraints may require modifications to the development
plan, or else may require a new site.
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1.1
The site analysis results in a complete site inventory,
identification of usable site area, and forms the foundation
of further design efforts through provision of adequate base
mapping. These initial mapping efforts and the information
identified during the site analysis help to produce the feasibility study and create the site diagram for the project.
The feasibility study is a product of the site analysis
and represents a formal document to record information
obtained during the site analysis phase of work. This should
also identify any gaps in information or assumptions that
have been made. The feasibility study requires information
from both the site analysis and due diligence. It is important
to note that there are different types of feasibility studies, but
the site engineer focuses on an engineering feasibility study,
not an economic analysis or other type of feasibility study.
The site diagram is the first graphical representation of the
site layout that depicts the proposed site conditions. This is
illustrated on the base map during the site analysis as development opportunities and site constraints are identified. The
diagram is prepared in early project phases (pre-design) and
is meant for graphic context to evaluate geographic relationships of the project elements. The site diagram requires
information from both the site analysis and due diligence.
This site analysis often overlaps with the initial due diligence efforts. These efforts require a thorough understanding of all elements of land development and will reinforce
the go/no-go decision as the developer continues their site
selection process. These pre-design efforts will inform
the site selection and allow a developer to make the best
decision.
Site analysis is described in Chapter 3, along with engineering fundamentals that contribute to the site analysis
efforts. The feasibility study is described in Chapter 3.1. The
preparation of base maps and site diagrams are described in
Chapter 3.2.
Preliminary Design. The design tasks during the preliminary design phase provide simple design elements to communicate the intent of the project. The work associated with
conceptual designs may contribute to internal discussion or
could be used for informal conversations with public agencies and the community. The schematic design phase represents a level of detail beyond conceptual design that can be
used for formal submissions associated with rezoning applications or special permits.
Preliminary design efforts include the conceptual design
and schematic design.
Conceptual Design. Conceptual designs begin with the site
diagram prepared during pre-design efforts. The objective of
the conceptual design is to establish a preliminary framework depicting the distribution, organization, and arrangement of the development program. The conceptual design
should honor the development constraints yet take advantage of opportunities identified during the due diligence and
site analysis. The resultant concept plan may include several
alternatives that illustrate different arrangements of principal
land uses and infrastructure requirements. Each alternative
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L and Development Design Process 13
should satisfy the development program and previously
identified site requirements, and then can be presented to
the developer for review or used in informal conversations
with other stakeholders.
Conceptual design and the concept plan are described in
Chapter 4.3.
Schematic Design. Schematic designs begin with the preferred concept plan alternative, as decided by the developer
and other stakeholders. The schematic design is a refinement
of the conceptual design that provides more precise scale
and site detail of program components and supporting site
improvements. The detail included in the schematic design is
based, in part, on information obtained during the due diligence and site analysis stages and provides further assurance
that the development program can be achieved. Included
in the schematic design is a site layout, which details and
depicts the dimensions of the arrangement of program
components. The site layout should confirm that the development plan is consistent with the goals and objectives
established by the client and conforms to all regulatory
requirements.
An important element of schematic design is preliminary engineering. The purpose of the preliminary engineering analysis is to verify and document the technical
aspects of the schematic design. The result of this study
is usually in the form of a graphic such as a preliminary
plan. Checklists and/or reports are often prepared as well.
These documents represent a “final check” of the development program prior to proceeding with more detailed
final engineering.
While the content provided in schematic design is rudimentary, the design should be carefully developed to ensure
that major components are not overlooked. The schematic
design is often used by the development team to estimate
project cost and verify feasibility. At the end of the schematic
design phase, the team should have a defined layout for the
site, but modifications can be expected throughout the rest
of the design process. At the end of this phase, a complete
schematic design will be produced known as the preliminary
plan. This completed preliminary plan usually represents
about 30% of the effort required to produce the final design
drawings for the project.
Schematic design and the preliminary plan are described
in Chapter 4.4.
Detailed Design. Detailed designs build from the preliminary design efforts to focus on the engineering details necessary for permitting and construction. The scope of work
includes refinements to the site layout, performing calculations, compiling details, and writing technical narratives and
project specifications. During the detailed design phases,
major changes to the site layout or project requirements may
result in reverting back to earlier design phases. Before proceeding to the detailed design phase, it is important to validate earlier design work and verify concurrence on project
progression.
Detailed design efforts include the final design.
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Final Design. The work to produce the final design is
predominantly carried out by the site engineer, as the preliminary plans are enhanced with a level of detail sufficient
enough to construct the project. Ultimately, the final design
reflects the detail necessary for project approval by local
governing agencies for permit issuance. There may be a
series of preliminary reviews at different stages while working toward plan approval. During this time there may still
be several iterations in the design before a final layout is
developed, but major layout features should be established
from the schematic design phase. Project requirements may
be refined during these iterations. Additionally, the building
design team will (very) likely refine the building architecture, which may require adjustments in the infrastructure
design as well.
The final site plan is the product of the final design efforts.
The final site plan is submitted for formal regulatory review
and permit processing and eventually used for construction. Jurisdictional approval is necessary for permit issuance,
which is required before construction can begin. Project
specifications that are developed during this phase are usually included with the final site plan. The approval of the final
site plan is an indication that the design is complete; however, additional information is often required during postdesign efforts, prior to construction.
Final design is described in Chapter 5. More information
about the review process is described in Chapter 2.4 and the
submission process in Chapter 5.1.
Post-Design. The post-design stage includes permits,
construction documents, procurement, construction, and
closeout.
Permits. Permitting is the project phase that requires
application for and procurement of all necessary site and
building permits. This is required before any construction
can begin. A final site plan is required for permit issuance, but there are often a number of permits required
for different scope items of the project. Development
permits include demolition, land disturbance, transportation, and other permits associated with land development
work. Permits may also be required based on the scope
of site environmental conditions and building construction. Project bonds and other legal agreements are often
required before a permit is issued.
Permitting and bonds are described in Chapter 6.1.
Construction Documents. Construction documents
include the final site plan and other design documents—
many of which are produced by other members of the
design team. These documents also include technical content necessary to communicate the project design and construction requirements. Additional information required
for bidding the project, such as bid invitations, general
conditions, agreements, and bid forms, is included with
the construction documents. A contract to perform the
work is part of the construction documents, and once the
agreement has been signed, the construction documents
represent the contract documents. The contract documents
01_Land_CH01_p001-016.indd 14
establish the scope and performance requirements for the
construction team.
Construction documents are described in Chapter 6.2.
Construction. After the necessary permits are acquired and
a construction contract agreement has been signed, the project
moves into the construction phase. The permits allow for construction, the contract documents represent the project design
and construction requirements, and the final site plan is followed during construction. At this phase, the design has been
completed and any revisions to the design during construction
are processed in the form of addendum and revisions. During
construction, the general contractor will coordinate with the
design team to ensure compliance with design documents and
may request additional information as needed.
Closeout. After construction is complete, the project enters
the utilization, operations, and maintenance phase—this is
commonly referred to as commissioning. Many localities
require “as-built” or record drawings of the site infrastructure to document the actual built conditions (which could
differ from the original design documents). Design documents and record drawings are often used for operations and
maintenance of a facility and are useful when planning for
future projects.
Postconstruction services are described in Chapter 6.2.
1.1.8. Project Delivery Types
Several project delivery methods for land development
projects are available to developers. Traditionally, private
land development projects have used the design-bid-build
method, but the available methods provide options to the
development team.
Design-bid-build: Traditional delivery has design professionals under contract with the developer—the
project is bid, and the developer subsequently enters
into a contract with a builder/contractor for the project
construction.
Design-build: Developer contracts with a team that
includes the design professionals and contractor for the
project. Contractor is typically the lead for the team.
Value-based award: Developer hires design professionals
to prepare appropriate project construction documents.
Bids are solicited from contractors (sometimes from list
of contractors prequalified) with requirement for price
proposal and technical proposal. Award of contract
based on weighted evaluation of technical qualifications
and price.
Design-build-finance: Developer contracts with a team
that includes the design professionals, contractor,
and financial institution. The team is under contract
to deliver the project, including long-term project
financing.
There are other hybrid construction delivery methods,
but in general, all result in the integration of the design
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1.1
and construction. These alternative delivery methods have
given rise to the need for and development of new standard
contract documents for the design professionals and the
contractors.
1.1.9. This Handbook
Undeniably the land development design process, however
systematic it is, varies considerably throughout the United
States due to the diversity of state and local regulations controlling land use and land subdivision. Yet within the process
there are many elements common throughout the jurisdictions. Similarities do exist. Even if it were intentionally written for a particular microregion, no book could present the
specific design process due to the dynamics of the regulations. This book is a presentation of a typical design process
and multitude of engineering fundamentals, but in no way
should it be construed as the only design process.
This book helps professionals in anticipating the multiple
scenarios and requirements they will likely encounter as they
progress through the various stages of project development
01_Land_CH01_p001-016.indd 15
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L and Development Design Process 15
or as they seek to broaden their professional understanding
of complexities of land development. For public and private
developers, this book will be an invaluable tool to understand the services they will be acquiring from various design
specialists and will prepare them for the regulation labyrinth
ahead. For those entering the land design profession, whether
in the public sector or as a consultant, this handbook will aid
in developing the skills needed to be a successful, contributing member of the development team. For the practitioner, it
will prove a treasured reference tool.
The Land Development Handbook is intended to be a
practical guide to the land development industry, detailing
the intricacies of each discipline while providing a comprehensive view of the process including the interrelationships
between various disciplines. This book will answer technical questions and provide “next step guidance” through the
entire land development process. Systematic implementation
of this design process by creative, passionate, and dedicated
professionals is the basis for successful land development
projects.
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Part II
Pre-Design
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Chapter 2
Due Diligence
Chapter 2.1 focuses on the due diligence of the project, the
beginning of the land development design process. This
includes an introduction to the various regulations that must
be followed for a project. Additionally, an overview of the
development program, site selection, as well as an introduction to defining property is described.
Chapter 2.2 introduces the comprehensive plan. The
comprehensive plan is the long-range planning document
for a jurisdiction. This is a guide for future growth in the
community and may provide recommendations for a site.
Chapter 2.3 covers the zoning ordinances. Zoning is a legal
ordinance that governs each site. This includes information
on uses, dimensional standards, and other related procedures.
The local zoning ordinance must be followed for all projects.
Chapter 2.4 introduces the subdivision ordinance,
review process, building codes, and development costs.
PRE-DESIGN
These various regulations are important to understand
at the beginning of a project. The subdivision ordinance
is similar to the zoning ordinance and must be followed,
but instead defines the physical characteristics of a site and
jurisdictional procedures. This includes the review process
which will determine how the plan development will occur.
An introduction to building codes is necessary to ensure
future designs are in conformance. Finally, development
costs must be understood to be properly accounted for
throughout the project.
Chapter 2.5 is separated into parts A, B, and C to focus
on environmental, geotechnical, and historical regulations.
There are many federal, state, and local regulations that will
affect a project. These are important considerations that will
govern many aspects of the design process later.
DESIGN
POST-DESIGN
Preliminary
Detailed
CHAPTER 2
CHAPTER 3
CHAPTER 4
CHAPTER 5
CHAPTER 6
Due Dilligence
Site Analysis
Conceptual &
Schematic Design
Final Design
Permits &
Construction
F i g u r e 2 . 1 A The land development design process.
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Chapter 2.1
Development Program,
Site Selection, and
Defining Property
2.1.1. Introduction
Due diligence is the engineer’s technical site-based evaluation of potential sites, which occurs during the early phases
of the project. This assessment is regulatory focused and
assists the developer with the site selection process to apply
the development program to potential sites. This will be the
first look at a property to begin to understand development
potential by identifying constraints and opportunities. Then
in the next chapter (Chapter 3) site analysis will analyze the
physical characteristics of each specific site. Both reviews are
necessary for the developer’s site selection. While arranged
sequentially in this book, it is common for the due diligence
and site analysis to overlap and may be performed or completed at the same time.
2.1.2. Development Program
Understanding the development program is the first step of
the land development design process. The development program is the scope of work, the developer’s vision and goals
for the project. This program is achieved through design
efforts and ultimately construction. In order to ensure the
success of the project, the development program must be
understood by all members of the development team as
early as possible. The program affects the design process
through the due diligence as suitable sites are investigated,
site analysis as constraints and opportunities are identified,
and later as the design efforts begin.
The development program is initially a determination on the part of the developer regarding what type of
development is expected for a given parcel of land. For the
most part, such preliminary determinations are based on a
cursory review of zoning, planning, and market considerations as applied to a specific property. Deal breakers such
as unfavorable zoning, a complicated review process, environmental constraints, site access issues, site grading conflicts, or lack of utility services on a property could derail
a project. These constraints may be identified during the
developer’s preliminary determination, or could be determined after the design team is consulted. That is why it is
important to identify constraints during the due diligence
and site analysis stages of the project. Constraints will have
to be overcome, a new site will be required or the development program modified.
The initial development program, based on the developer’s preliminary determination, is usually first presented
to the business team, before the design team, for discussion
and refinement. The challenge of project design is formulating a response that simultaneously balances the “highest
and best land use” with the character of the site and its environs; developer and consumer expectations; economic and
marketing factors; and public and private approval requirements. Land uses and their associated building types need to
be consistent with current construction practices, as well as
consumer and user requirements. Market conditions, development costs, and numerous alternatives in development
20
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2.1
technologies afford the opportunity to develop distinctly
different designs for any given property.
In some cases, the site planner may be an initial participant
in orchestrating the research and background information
that leads to describing the development program. The role
of site planner could be held by the site engineer or other
members of the design team, such as the architect. In larger
and more complex projects, there may be a dedicated planner
on the design team. As noted, in many cases the development program is initially being spearheaded by the developer alone or in concert with their business team. The site
planner is, however, in a position to assist the developer in
refining the use associated with the development program
based on site characteristics and public planning, land use,
and regulatory controls.
The development program should be predicated upon
a thorough understanding and appreciation of the success
associated with previous land development designs. Such
awareness strengthens the posture of the site planner. The
intent is not to mimic that which has proven successful, but
rather to gain an understanding as to the reasons for success
and expand on those attributes. The fundamental requirement of land design rests in a working knowledge of both
the physical and functional characteristics and constraints
associated with specific building products.
While certain base considerations, such as site access
(vehicular, pedestrian, bike, and/or transit), represents a
common requirement for all land use types, the appropriate
design response varies substantially. This variability can be
seen when comparing low-density single-family residences
to urban mixed-use development. The developer should
consider all these factors when determining the development program. Again, this determination may occur before
the site engineer is consulted.
To secure a firm understanding of the development program, the site planner should elicit from the developer as
much information as possible and help to refine their initial
development program. Developers generally have strong,
preconceived ideas regarding the character of the project
they wish developed. At a minimum they can relate their
expectations relative to existing projects they believe most
resemble their current proposal. Even the most unseasoned
developer has certain base development objectives that have
prompted the initiation of a specific development effort. The
site planner should discuss these expectations with the developer, including an inventory of objective criteria such as unit
or building type, building dimensions, architectural finish,
parking ratios, amenity elements, sustainable or green building design goals, and more subjective statements regarding
the desired character the finished product. Although developers will often indicate the density or intensity of the development they desire, more information is needed to initiate a
site design.
After the initial development program has been refined
by the business team and site planner, the design team is
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Development Program, Site Selection, and Defining Property 21
consulted to verify the program. This is where this book
begins. The site engineer, as a member of the site team within
the design team, will be responsible for due diligence and site
analysis. The information prepared by the site engineer will
be used to provide the developer with the information necessary to select a site.
It is important that development program objectives be
discussed in the context of existing local planning and regulatory controls. Land use type and use intensity should be
reviewed with a clear understanding of existing jurisdictional
comprehensive plans and zoning. Based on this discussion,
the development program or alternative programs should
reflect a realistic proposed land use. Ultimately, the success
of the development program will be measured to the extent
it optimizes developer objectives against those of public sector expectations and standards. It is a benefit to no one to
foster questionable expectations associated with unreasonable program formulation. An assurance is necessary that all
regulatory requirements can be satisfied in conjunction with
refinements to the proposed development program. These
regulations will be discussed during the due diligence within
Chapter 2.
After the due diligence and with a clear understanding of
the proposed development program and options, the identification of site constraints and opportunities is required for
each site the developer is considering. During site selection a
positive finding is necessary to ensure that the physical and
functional characteristics envisioned by the developer can be
realized. This physical site analysis is introduced throughout
Chapter 3.
These steps are a precursor to design. It is, in reality, an
information-gathering stage, but information gathering
with specific purpose and direction. Information is expensive to compile and time consuming in its review. Therefore,
it is important to undertake these site assessment efforts
with a clear focus on the judicious expenditure of time and
resources. At this juncture in the development process, all
available existing information, including base maps, aerial
photographs, engineering information and controls, and
planning reports, should be assembled.
Most developers have completed at least a rudimentary
assessment of the possible return on their investment for a
project prior to the initiation of discussion with a design
team. The design team should attempt to verify the anticipated yield, green building certification level (if a priority) as well as preliminary budgetary allowances for such
items as utility construction, amenity development, and
landscape, to understand better the anticipated character
and quality of development. This will be completed during
the conceptual and schematic designs, as is introduced in
Chapter 4.
While it is important to define the development program
early in the project, actual site design efforts will not be
required until after the site has been selected and analyzed.
Preliminary design is further discussed in Chapter 4.
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D ue D iligence
2.1.3. Site Selection Process
The site, or a particular piece of real estate on which a development program will be implemented, affords a special set
of resources and opportunities for project design. Each site
is unique and requires an understanding of and appreciation for the specific characteristics to elicit a tailored design
response. Consideration should be inclusive of both surface
and subsurface characteristics, as well as the dynamics associated with the natural and cultural context that prescribe its
unique character. Attributes of a site normally considered
relevant to land development activity include those that bear
on the land’s ability to absorb specific development program
elements. These include both on-site and off-site considerations and include a range of issues from site configuration
to adjacent land uses.
Development Types. Typical sites encountered are greenfields, redevelopment, and continuous development. In some
cases, brownfield or grayfield sites are encountered.
Greenfield sites reference an undeveloped site or could
reference a minimally developed site (such as a farm).
These sites are attractive because there is often less
complication associated with existing buildings or utility systems, but environmental and historic features are
more likely to be discovered. Additionally, a significant
change in the development intensity may prove challenging when evaluating connections to existing utility
systems and transportation systems.
Redevelopment sites reference a project the removes an
existing development to provide an opportunity for the
new development. The original development may have
paved the way for new development opportunities, but
it could be difficult to verify all existing conditions of
the site. Without a record of the original design plans or
assurance that the site has been adequately maintained,
it is possible that unforeseen conditions may arise. An
existing development is a sign that utility systems are
available, but the capacity would need to be verified to
meet proposed demand.
Continuous development represents a case where
there is ongoing development, whether greenfield or
redevelopment. This condition is common in campus
settings, such as higher education or hospitals. These
projects are often maintained in a single site, which
may provide a benefit to site analysis. These sites will
likely have continuous operation, which should be
considered when planning and phasing new development so that utility and operation disruptions are
minimized.
Brownfield or grayfield development references a
development on a site that has a documented (or is
highly probable to have) hazardous environmental conditions. These sites require evaluation and consideration
for necessary site mitigation, but may be attractive
because of size, location and financial incentives
02_Land_CH02_p017-124.indd 22
(grants, loans, tax benefits, etc.) that encourage the
redevelopment.
Site Selection. For most projects, the developer has come
to the design team with a site in mind. They could own this
property outright or be in contract to purchase the site. In
other cases, the developer has several sites that they are considering. In each case, the site selection process is required
to ensure the development potential for each site under
consideration. This is necessary to achieve the development
program previously defined. And if there are multiple sites
under consideration, the site selection process will determine the best site for the developer.
If the developer is in contract to purchase a site, they may
have located a property that appears to satisfy the development program. To act quickly but still minimize risk, many
developers will initiate an “option” on the property, thus
making the final purchase contingent on satisfactory resolution of pertinent land design issues. These issues may
include the ability to achieve a successful rezoning or verify a
proposed use can be accommodated.
During the site selection process, or the pre-design of a
project, the developer employs the land development consulting team for the project. The team members assist the
developer with the site selection. The site engineer will
support a more detailed due diligence (Chapter 2) and site
analysis (Chapter 3).
During this site selection, the developer performs a complete analysis for the property likely including a market
survey, financing options, sales potential, pro forma financial
models of the proposed project and, not the least, a detailed
engineering feasibility study. The feasibility study is a report
of the due diligence and site analysis efforts which are introduced in Chapter 3.1.
If under contract, the developer may be able to proceed
with negotiations to purchase the property (exercise the
option) within a relatively short period of time. If a more
detailed study is required, there is still usually a limited
number of days (typically 15 to 45), although it could take
60 days or more. During this time the purchaser can release
his option to purchase the property for any number of
reasons. If the feasibility study or other analysis indicate the
potential for problems, renegotiations may be necessary.
Often during a feasibility study period, items are discovered that will cause the developer to drop the contract or
renegotiate the terms based on the new information.
Market Analysis. With the development program determined and a site in mind, all land development projects
should begin with extensive planning. Frequently the first
step in the planning process, preceding the technical planning by the engineer, is a market analysis by the developer.
This analysis can determine the type of development having potential for success and the design suited for the site
location.
This market analysis is dependent on the type of project
and will vary based on each developer. For example, a private
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2.1
residential home builder looking to build a new townhouse
development will have different needs then a public university looking to build new dorm buildings on campus. Both
will need to find a location for their project, but the university may already own land but would need to consider
larger program elements when choosing the right size and
location. The private home builder, on the other hand, may
need to locate a piece of property to purchase and would
consider product type (townhome, single family, duplex,
etc.) based on market conditions. More information about
the market analysis can be found in a “Development for the
Built Environment”, which is part of the Land Development
Handbook Series.
Due Diligence and Site Analysis. In most cases, a site
engineer begins working on a project after the developer
has identified a site of interest, or the developer may have
a few site options. For public projects, the site is likely predefined based on land owned by the public entity. With a
site selected, the developer often has a concept for a development program. The developer should work closely with the
site engineer to verify feasibility of the development program
based on an investigation of the site’s physical characteristics
and the applicable jurisdictional regulations.
Chapter 2 begins with the initial due diligence of the property performed by the site engineer. Due diligence is the first
look at a site to understand the regulations that govern the
site. This can quickly determine if the development program
is feasible on a given site. The next part of the due diligence
is the site analysis, described in Chapter 3, which is a more
detailed analysis of the existing physical conditions of each
site. The site engineer will identify constraints and opportunities that assist the developer as they perform their site
selection. Although presented sequentially in this book, the
due diligence and site analysis stages may overlap. Depending on the project, the requirements of the developer, the
experience of the site engineer, and other factors will determine the land development design process. It is important to
remember that each project is unique.
It is important to follow the plans, ordinances, and
regulations presented in Chapter 2 in all projects that are
encountered to ensure that the site is in conformance with
all requirements. Any challenges or constraints identified
will have to be overcome to ensure that the project will be
approved and successful. If not, a new site or development
program may need to be selected. The due diligence is the
starting point that gives the site engineer a framework of what
is allowed on the site and how to proceed with the project.
When satisfied with the feasibility of a project and after
understanding the process to proceed, the project will be
able to move into a more detailed site analysis and ultimately
able to produce conceptual and schematic designs, are discussed in following chapters of the book.
2.1.4. Defining Property
Before continuing into the pre-design efforts, it is important to understand key definitions related to property itself.
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Development Program, Site Selection, and Defining Property 23
These terms are important to understand when working on
any land development project.
Real property: Real property is a piece of the earth’s surface extending downward to the center of the earth and
upward into space, including all things permanently
attached to it by nature or by people, as well as the interests, benefits, and rights inherent in real estate ownership.
Fee simple estate: The maximum possible estate or right
of ownership of real property, continuing forever.
Title: The title is the right to or ownership of land. The
physical title also is evidence of ownership of land.
Plats: A plat is map indicating the location and boundaries of individual properties. This could be for an entire
community or for a specific subdivision.
Subdivision: Subdivision is a tract of land divided by the
owner, known as the subdivider, into blocks, building
lots, and streets according to a recorded subdivision plat,
which must comply with state regulations and the local
subdivision ordinance.
Parcel: A parcel is a specific tract of real estate defined by
a legal description and used for taxing purposes, among
others. Also termed a surveyor’s parcel and a tax parcel.
Conveyance: A conveyance is a written instrument that
evidences a transaction in which any interest in land is
created, alienated, mortgaged, assigned or “otherwise
affected in law or in equity.” A grantee is a person who
receives a conveyance of real property from the grantor.
The grantor then is the person transferring title to or an
interest in real property to a grantee.
Covenant: A covenant is a written agreement between
two or more parties in which a party or parties pledges
to perform or not perform specified acts with regard to
property; usually found in such real estate documents as
deeds, mortgages, leases, and land contracts.
Deeds. A deed is a written instrument used to transfer an
interest in land or real property. A deed is a form of contract
and as such a legally valid deed must meet certain requirements. A deed provides a clear definition of the description,
the transfer of land, the rights of an easement, etc. A deed
must have at least two parties—a grantor and a grantee. The
grantor must own the interest and rights being conveyed.
The interest and rights being conveyed must be described
with reasonable specificity. The laws of the jurisdiction
in which the land or real property is located must permit
the transfer of the interest or rights described. The interest or rights being described must be current at the time of
conveyance.
The writing within a deed will be in legal wording
that clearly express the intent of the parties. These words
must identify both grantor and grantee. The written words
must convey the intent of the parties. The deed must include
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a descriptive clause and statement of the consideration
involved. This is the premises. The deed must also contain
a description, called the habendum clause, which defines the
limits of the estate. This clause must agree with the premises.
Words used to close the deed are the tenendum.
Deeds must state exceptions and reservations when such
exist. The exceptions exclude parts of the estate described
from the conveyance. These exceptions must not be greater
than the whole estate. The reservations retain rights to the
grantor of an estate. A deed restriction can also be written as
clause to limit the future uses of a property.
The benefit of consideration to the grantor must exist
as stated in the instrument of transfer. Consideration must
pass between parties and that consideration must be adequate compensation. Laws require that parties to the deed
have the opportunity to read and examine the document
before its execution. Finally, the deed transfer consists of
the signing of the instrument by the parties to the agreement. This step takes place before a notary or other person
authorized to witness signatures. There is a requirement of
delivery, or the actual placement of the instrument into the
hands of the grantee. Strict interpretation of this requirement is rare.
Deeds used in the transfer of ownership in the United States
are warranty deeds, quitclaim deeds, and deeds of bargain
and sale. The warranty deed contains a covenant of title. This
covenant is a guarantee by the grantor that the deed conveys
a marketable title. Quitclaim deeds convey only the present
interest of the grantor and do not warrant or guarantee good
title. Deeds of bargain and sale convey definitive estates in
land, but do not imply a warranty.
Legal Descriptions. The description of land is provided
through a combination of graphics, such as a plat, and written words, referred to as the legal description.
The part of the deed devoted to the physical location of
the real property is known as the legal description. A general
definition of a legal description can be stated as those words
and maps or plats that uniquely delineate the tract from any
other. The description must be written in such a manner that
it will stand any test under law and litigation.
Such descriptions can be formed in various ways. Words
describing lines composed of bearings and distances and
calling for monuments are “metes and bounds” descriptions.
Some descriptions reference other documents that refer to
tracts that are already recorded in the public records. Other
types of descriptions are used for situations such as strip
descriptions (or baseline) for right-of-ways for roads, power
lines, and other utilities. An example of a strip description
would be for “That portion of____ included within a strip of
land, 10.00 feet wide, lying 5.00 feet on each side of the following described line:”
The thread common to all of them is that each describes
a specific parcel of land that cannot be applied to another
parcel. The legal description should be written such that is as
02_Land_CH02_p017-124.indd 24
brief as possible while maintaining clarity, completeness, and
accuracy. Incomplete, inaccurate, and unclear descriptions
will result in boundary disputes, which may turn into title
disputes later. A description of a specific parcel of real estate
complete enough for an independent surveyor to locate and
identify it.
Metes and Bounds. For legal descriptions that comprise
metes and bounds.
•• Metes are measured values that follow a line that can
be defined geometrically by a bearing (direction) and
distance.
○○
N 45° 05′ 29″ E (bearing) 134.25′ (distance)
•• Bounds are a general reference to another feature
such as a watercourse, building, or county boundary.
○○
Fifteen feet on each side of the utility pole line
Metes provide an accurate description of the land by providing information that can be measured relative to the starting
and ending point of each line. Bounds are not as accurate but
may be used when the accuracy is not required.
The bearings and distances associated with metes are relative to each other, but do not provide a reference to a physical location. To establish a location there must be a known
and identified point, referenced as the point of beginning
(POB). The POB provides a starting and ending point for the
description of the land. When starting from the POB and following the metes around the property the final point should
match back to the POB.
Figure 2.1B provides a simple example of a plat showing
a POB, description, owner information, and other relevant
data typically included with a plat.
Parts of a Description. The description is composed of
three parts: the caption, the body, and the qualifying clause
(such as exceptions and reservations). In some cases, augmenting clauses are added (for easements). Each part of the
description serves a definite purpose.
The caption establishes the general placement of the
subject property and limits title in the remainder of the
description to that general area outlined in the caption. A
typical caption in a description could read as follows “that
certain tract or parcel of land, situated at or near City, District, County, State and being more particularly described
as follows.”
The body of the description generally follows the caption.
A typical body in a colonial state deed description might
read as follows:
Beginning at a set stone at a corner of rail fence on the old
baggage road, and running with the said road to the Noah D.
Johnson line, and with said line to the John C. Williams line,
thence with the Williams line back to a set stone corner on
said John C. Williams line, thence a straight line to the beginning, containing three (3) acres, more or less.
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2.1
Figure 2.1B
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Development Program, Site Selection, and Defining Property 25
Example of a plat.
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Describing parcels within land subdivisions employs the use
of reference to a subdivision map of record. Such a description follows:
Being situated at or near City, County, State, and being more
particularly bounded and described as follows, to-wit:
Lot No. 98 of the Forest Hills Subdivision, as the same is
shown and designated on a map or plat thereof which is of
record in the office of the Registrar of Deeds of (County),
(State) in Map Book 8, at page 67.
The third part of the description, the qualifying clause, does
not follow any particular arrangement. These clauses may
except some part of the conveyance or reserve some part
generally for the Grantor. Qualifying clauses often follow the
body of the description. Such a clause might read
There is excepted and reserved from this conveyance a
10 foot lane, which lies on the western boundary of the
property herein described, which serves as access and
egress to Donald Thompson and this reservation is for the
benefit of the owners of the property presently owned by
Donald Thompson and to the successors in title to Donald
Thompson.
Any form of description must meet the requirements of the
law for a conveyance to be complete. Again, deeds must be in
writing and must include the grantor (vendor) and grantee
(vendee). The description must clearly identify the interest
conveyed. There must be an expressed intent to convey the
property identified by the description.
Chain of Title. The succession of conveyances, from some
accepted starting point, whereby the present holder of real
property derives his or her title.
Title Report. Identifying and addressing issues affecting title to real property is a critical element in the transfer
of ownership during the land development process. A title
examination will track the chain of title to real property over
a specified period of time and may uncover conveyance of a
portion of the property either in fee simple or in rights of use,
such as easements.
The title report is intended to reveal any information that
may influence the ability of the purchaser or owner to use
the property. It will include a description of the property, any
conveyances of ownership or use, liens or judgements against
the property, and any special exceptions that may affect how
the property can be used.
The title report is an important element in the feasibility study conducted to determine whether a property
is suitable for the user’s intended purpose. A survey of
the property will support the title report by identifying
whether its boundaries match the description noted, and
by addressing whether or not special exceptions listed
affect the property.
The Title Search. If conducting a title search, the subject
deed should be searched back in time until the property
lines of the tract involved originated. From the surveyor’s
02_Land_CH02_p017-124.indd 26
viewpoint, the search is to determine intent of the parties at
the time of the original survey or, if there was no original
survey, the intent of the parties based on the original writings. The search should establish a line of unbroken ownership in the subject property.
Having concluded the search back in time, the reverse
process—called a forward search is then conducted. The
forward search begins with the name of oldest grantee from
the search described above. In the subsequent conveyance,
this name becomes the grantor. Using the grantor index
and starting with this name, title can be traced forward in
time to the present. This provides the surveyor with the
chain of grantors. By reviewing each conveyance, it can be
determined if any owner in the chain of title has encumbered or impaired title to the land with, for example, easements, other sales, mortgages, or liens. This search will also
reveal the creation of servient estates (e.g., providing access
to the real estate across another property), reservations, and
exceptions.
This chain is composed of links. A link is a connection
or transfer of property between consecutive owners of the
property. These links are not always in the form of a deed
but can be a will, the records of an intestate estate (an estate
left by a deceased without a will), or a court order. In the
absence of a will, the actions of the court must be documented. In order to place the various documents in their
proper chronological order and context, a brief summary
of each may be prepared.
Easements. Easements are a right granted by the owner of
a parcel of land. These rights, granted to another party, are
for use of the land for a specified purpose.
A simple general definition of an easement is a nonpossessory interest held by one person in land of another. The
person holding that interest is accorded partial use of the
burdened estate for a specific purpose. An easement restricts
but does not abridge the rights of the burdened estate fee
owner to the use and enjoyment of his land.
Easements are critical to land development and this section includes more details concerning them. Various types of
easements exist for various purposes.
Many land and title features may be located within easements. In addition, easements needed to install or maintain the infrastructure of the land development must be
established. These include easements for streets, wells,
drainfields, stormwater runoff, storm and sanitary sewers,
gas lines, and power lines. Because of their importance,
the nature and location of easements that already exist
and those that are to be created must be carefully considered. Often, proposed easements will connect to existing
easements.
Types. Easements fall into three broad classifications: surface, subsurface, and aerial. Each can be classified as either
affirmative or negative. Affirmative and negative easements
are both common in occurrence. Affirmative easements
are those that allow activity on the estate burdened by the
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2.1
easement. Some activities mentioned in easements restrict
burdened estates. When such is the case negative easements
result.
Examples of affirmative easements include alleys, the
approach to airports, private roads through subdivisions
and railway rights. Other easements include pipelines, utility poles and electrical transmission lines. Party wall agreements and public utility dedications are also easements.
When appropriate, use easements to cover the use of springs
and wells. Important too, are surface rights to serve oil and
gas leases. Easements provide a way to grant rights to flood
land or drain land. Other uses for easements are to allow for
encroachments and to provide for excavation along boundaries. They also exist to carry out temporary building construction beyond the limits of the project. Some easements
listed above are nonaccess easements but most include rights
of entry for reasonable maintenance.
Negative easements, on the other hand, are those that
prevent specific activities by the servient estate owner. These
usually prevent certain types of improvements to protect the
easement owner’s rights. Such rights might include scenic
views and access to sunlight.
Creation. The manner and reason for creating an easement determine its type and category. There are many common examples of easements and nine prevalent means exist
to create them. The nine methods follow: grant, dedication,
condemnation (eminent domain), statutory layout, prescription, necessity, implication, express reservation, and estoppel.
Grant: Creation of easements by express agreement generally arises from a deed of grant; however, occasionally
one is created by verbal agreement. This is true regardless of the duration of the interest conveyed.
Dedication: Dedication of an easement consists of the
appropriation of land to the public use. The rightful
owner must make the dedications and an acceptance
must follow for the right to become public.
Condemnation: Easement by condemnation is the process whereby property of a private owner is taken for use
by the public. When this occurs without consent of the
private owner, that owner receives an award of compensation for his or her loss.
Statutory layout: Acquisitions of easements by statutory layout proceedings provide for ingress and egress
to public highways over intervening land. These easements provide for this access when no other reasonable
way is available for cultivation, timbering, mining,
manufacturing plants or public or private cemeteries.
State statutes vary as well as the methods for establishing these ways.
Prescription: Easements acquired by long and continual
use by an individual are easements of prescription. The
required period of use is usually the same as that for
02_Land_CH02_p017-124.indd 27
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Development Program, Site Selection, and Defining Property 27
accomplishing adverse possession. This period is a prescriptive period and varies among states.
Necessity: An easement of necessity arises when parties grant land but fail to provide access to a public
roadway system except over the land remaining with
the grantor. In such instances, an easement by necessity
or by implication provides for access over the seller’s
remaining land.
Implication: Implied easements evolve through implication, prior use, necessity, or prior map or plat dedication.
The crucial element for an implied easement is that of
prior uses. Often nondocumentary, these easements
must be recognizable through a reasonable inspection
of the property. A property survey should therefore
reveal easements of the implied type. The requirement
of appearance and visibility of such easements extends
beyond professional scrutiny and includes the grantee of
property. They constitute necessary and reasonable use
of the property subjected to these easements. Such easements can affect land development.
An example of an implied easement follows. Party “A”
owns two lots. Lot one contains the home of party “A.”
A storm drainage pipe runs from this home across the
second lot owned by party “A.” There is a catch basin
located on the second lot. Party “A” sells the second lot
to party “B.” The visible catch basin is sufficient evidence
and notice to party “B” that an easement for drainage,
over the lot mentioned, exists.
Express reservation: Creation of easements by reservation
and exception allows an owner who conveys a possessory interest to a party or parties to except or exclude a
corporeal interest from the terms of his grant.
Estoppel: Estoppel forms the basis of creation of some
easements. These easements can restrict the grantor
in the use of her land. In the case of Battle Creek v.
Goguac Resort Ass’n the Goguac Resort was a riparian
owner on a lake. The resort sold land for an easement
to Battle Creek knowing that the city wanted water
from the lake for municipal purposes. The resort company was later estopped from use of the lake because
the resort use contaminated the lake making the water
unfit for the city.
Identification. As part of the due diligence, it is important
to identify existing easements on a site.
The foregoing list is provided to apprise those persons
involved in land development of the many types of easements
that may exist. A visual inspection of a site, or even research
of prior plans, is not enough to determine the existence of
easements. The surveyor or engineer must uncover existing
easements before the commencement of design work on the
development project. Knowledge of the various means of
creating them should assist in this task.
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2.1.5. Eminent Domain
Eminent domain is a right of the state that affects the rights
of the private property owner. Eminent domain is the power
held by governments and certain quasipublic entities to
take private property for public use. Appropriation of private land for road construction, drainage channels, and laying water lines are common. This provides another way of
obtaining access across private land. Normally, this power
is reserved to governments. Many states, however, have
adopted legislation extending this power to private individuals and other legal entities. In these instances, the power
allows private owners to gain access to inaccessible (or
landlocked) land.
Eminent domain differs from easement by necessity. The
former requires compensation from the party claiming the
power. Just compensation for land interests condemned is
required by the United States Constitution and by some State
Constitutions. Easement by necessity requires no additional
compensation other than the amount paid in the conveyance of the property involved. Land developers might use
this power to gain rights-of-way in some situations but the
method is one of last resort because of the cost involved.
A site for a public project will likely have similar easement
and dedication conditions as a private site would. Although
a public easement on a public site may seems redundant, different public entities may have different interests in the land
rights. For example, a public storm drain easement may exist
on a public school site to allow maintenance and inspection
of the site’s storm system. Additionally, if the public entity
ever transfers the property, the easements will be conveyed
with the property.
Dedication. A dedication is a donation of land for the
public good. Governments provide for the long-term maintenance of this land. Only the fee owner or his authorized
agent can make these dedications. A dedication of land
would occur in a situation where a site requires a new highway turn lane or public sidewalk along the frontage. The turn
lane and sidewalk are required to be within the public right
02_Land_CH02_p017-124.indd 28
of way so the land from the project site would be dedicated
for that purpose.
Common law and statutory law provide for dedications.
Common Law. Common law dedication confers only an easement. This dedication is not a transfer of rights. Such dedications do prevent the burdened landowner from exercising his
rights in a manner inconsistent with the rights of the public.
This form of dedication is not a grant of land because there is no
grantee. Common law dedications transfer the land by estoppel
in pais. Estoppel in pais is an estoppel by conduct of the parties
compared to estoppel by deed, which rest on public records.
Statutory Law. Statutory dedications occur when there is
a grant of rights. These differ from common law dedications
because they pass legal title covering the area so dedicated to
a governmental body or agency. Statutory dedication must
comply with statutory law. Laws concerning dedication vary
from state to state. To constitute a dedication either expressed
or implied there must be an intention, on the owner’s part, to
grant the property to some public use. A dedication is a voluntary action on the part of the fee owner. The party alleging a
dedication must prove the intentions of the other party to do so
(Ray Hamilton Skelton, 1930).
Plats show actual intention to reserve any portion of lands
for the public good. If such a reservation is not on the plat, then
an equally certain method is essential for establishing the intention to reserve for dedication. Public notice of the intention to
reserve is equal to that given by a plat. Intentions to dedicate
land without supporting evidence or acts signify nothing.
Before dedications become binding on either party, there
must be certain proof of the acceptance of the dedication.
The acceptance can be actual, expressed, or implied. Until
therew is an acceptance by the public, the public has no
rights and neither has the public assumed any responsibility.
REFERENCES
Michael Davidson, Taming the Beast. Planning, 2002.
Ray Hamilton Skelton, Boundaries and Adjacent Properties, BobbsMerrill Company, Indianapolis, 1930, p. 435.
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Chapter 2.2
Comprehensive Planning
2.2.1. Introduction
How does a jurisdiction show where new roads are planned?
How do residents know where new parks are planned? How
do developers know where future infrastructure is planned?
The answers to these questions can usually be found in a
jurisdiction’s comprehensive plan. The comprehensive plan
is a guideline for the future land development conditions of
a community. The plans and policies contained within help
to shape growth and implement specific community goals.
A comprehensive plan will make recommendations for
the jurisdiction and include maps that depict future infrastructure improvements, changes in land use, as well as specific descriptions of planned changes. The comprehensive
plan may show a new highway, or identify that an industrial
area could become the next town center.
An understanding of the comprehensive plan is crucial
to begin any project—the information contained within will
shape all aspects of the land development design process.
This chapter describes how the content contained within a
comprehensive plan varies, and how all comprehensive plans
are different based on the locality the project is in. The plan
is often complex and continuously changing based on the
economic, environmental, and social conditions. The information within this chapter, though, will provide an understanding of comprehensive plans and how they can impact a
development project.
2.2.2. Historical Context
Jurisdictional planning processes are as old as the governing bodies themselves, but comprehensive plans and zoning processes are of 20th century vintage. One of the main
objectives of the planning process is to increase economic
efficiency by coordinating the size and location of physical facilities with projected future needs. Planning should
provide an overall design for urban expansion that will be
aesthetically pleasing and retain the natural integrity of the
land. The planning process also serves to allocate land to
varied uses necessary for stable and healthy growth. Properly executed planning will enhance the relationship of various land uses.
Cities as old as Athens and Rome saw careful planning initiatives that shaped their growth and have continued to have
a lasting impact on their development. In the United States,
many early cities were also built from detailed urban plans
including the national capital, Washington DC, in 1791. In
the 19th century, cities matured throughout the country
and became large industrial powerhouses. During this time,
several projects began to highlight the importance of welldesigned and carefully planned cities. In New York City, the
movement of creating parks and public spaces within the city
led to the creation of Central Park in the 1850s. The Chicago
World’s Fair of 1893 created an idealistic city with all the
newest technology to showcase and explore how cities of the
future could be built. These progressive ideas led to beautification efforts of cities, to update and improve public spaces
and attempt to support population growth, across the country in the early 20th century.
The first comprehensive zoning was adopted by New
York City in 1916 to control development through land use
regulations, to protect the public’s health and well-being
from new skyscrapers and potential overcrowding. The city
was divided into districts and limits were set on building
heights and setbacks, amongst other regulations. The U.S.
Department of Commerce published a Standard State Zoning Enabling Act in 1924, which was based on New York
City’s zoning regulations. The Zoning Enabling Act set out
the process for jurisdictions throughout the country to adopt
and administer their own comprehensive plans and zoning
regulations. It grants the power of zoning “for the purpose
of promoting health, safety, morals, or the general welfare
of the community.” It also requires that zoning regulations
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“shall be made in accordance with a comprehensive plan.”
Most communities quickly thereafter adopted comprehensive plans and zoning regulations based on the text of the
Zoning Enabling Act, thus making zoning commonplace
throughout the United States.
It is important to understand that zoning regulations
are the implementation tools for much of the comprehensive plan. The two work together by proposing development in the comprehensive plan and then ultimately
achieving them through zoning. The comprehensive plan
serves as the backbone for zoning regulations. The comprehensive plan is discussed in this chapter and zoning in
Chapter 2.3.
2.2.3. Comprehensive Plan
The comprehensive plan, also known as the master plan or
general plan, is a long-range planning document that serves
as a formal statement of the community’s goals and objectives. The plan establishes policies and procedures relating
to the community’s future growth, including new development of land and maturation of existing neighborhoods.
It represents the collective input of public and private sector attitudes, needs, and desires. The recommendations
included in the comprehensive plan are based on extensive
analysis of economic, social, demographic, environmental
as well as other forces evident in the community. Through
these recommendations, the comprehensive plan provides
an adopted vision of the community for some distant point
in time, typically between 5 and 20 years. The comprehensive
plan provides valuable guidance for those making important
economic decisions, including local officials, land developers, existing and prospective residents, employees, and business operators.
The comprehensive plan usually includes multiple plans
that provide the strategies and recommendations to implement and achieve the community vision and goals. These
more detailed plans are called elements and are standalone
plans within the overall comprehensive plan that cover
everything from housing to the environment. One of the
most important elements is the land use plan that sets future
land use patterns within the jurisdiction. The transportation plan is another important element that defines future
transportation infrastructure needs in the jurisdiction.
More information about these elements is discussed later in
this chapter.
Comprehensive plans, and all the elements within, are
broad in scope. They usually cover a wide geographic area
and offer strategies and recommendations at a macro level.
Usually, they do not identify individual properties, but
instead denote large areas that fall into a specific category.
For example, these categories could be a large swath of land
projected for a high-density development, or a long linear
roadway that is recommended to be upgraded with additional lanes. Of importance to note when reviewing the comprehensive plan for a project are the impacts on the specific
02_Land_CH02_p017-124.indd 30
project site (e.g., road widening along the project site frontage). The recommendations for project density or intensity
should be understood as well as other conditions required
to achieve the full potential of a given piece of property. The
plan may also include other requirements to reduce potential
impacts on the neighborhood and community that should be
followed.
In most states the comprehensive plan is a guide that
is advisory in nature and not a legally binding regulation,
unlike the zoning ordinance which indeed is law. Instead,
the comprehensive plan is executed with the zoning process,
referenced in the zoning ordinance, and enforced with zoning regulations. Zoning is based off the recommendations of
the land use plan and other elements within the comprehensive plan. Therefore, through zoning, the goals and objectives of the comprehensive plan are ultimately achieved.
The comprehensive plan also has an influence on rezoning
applications because the proposed rezone will be evaluated
with the planned use of the area identified in the comprehensive plan. More information about zoning and rezoning
is discussed in Chapter 2.3.
Purpose. Land development requires significant public,
private, and personal investment. Because of this investment, there is an increased importance on the comprehensive plan as a guide for development. To government
officials, the comprehensive plan serves several purposes.
It defines a general pattern of projected land use for the
community. It recommends policies that encourage desired
development or discourage inappropriate uses or intensity
of development. The plan establishes and reinforces community standards for appearance, design, delivery of public
services, and protection of the environment. The plan also
serves as an important guide for allocating resources used
for the provision and distribution of public facilities and services. The overall objective of the comprehensive plan is to
establish and achieve goals for a locality by considering the
needs of the community.
To the community’s residents, the comprehensive plan
provides a blueprint for the quality of life they can anticipate.
It creates identity for the neighborhoods in which they live
and defines the services they expect to receive. The decision
to move to a community and purchase a home is perhaps the
most important of personal investments made—the comprehensive plan is viewed to predict the soundness of that
investment. It identifies the location of new neighborhoods,
office and retail centers, new roads, and schools that may
affect that investment. The plan provides a way of anticipating intrusion or impacts that could reduce property value
with relative certainty.
For the business sector, the comprehensive plan is an
essential source of information on potential new markets.
The future locations of new centers of employment or residential communities are particularly important to businesses
operating support, supply, and service establishments. The
plan provides information that can be used to determine the
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potential customer base available to the business community. Large employers in need of properties with room for
expansion also rely on the comprehensive plan for guidance.
Existing and future labor forces and sales markets can be
determined from the comprehensive plan. At the same time,
businesses and employers look to the plan for an expression of long-term commitment to business and economic
development.
Much in the way that residents look to the plan as an indicator of long-term investment value, the land developer also
views the comprehensive plan as a protection of property
value. More importantly, however, the developer uses the
plan to identify new opportunities. They can use the comprehensive plan to determine the suitability of purchasing
specific land for new projects based on the planned land use
and their development program.
Many developers specialize in a specific land use and
product; therefore, the plan is an important tool in identifying the area, and sometimes the specific parcel(s), that
are best suited for the development program being pursued. The availability of public facilities and services is an
important part of identifying the potential of a property.
The plan may also aid the developer, by projecting when
public and private infrastructure and services will be available to the site. Similarly, the compatibility with and impact
of adjacent uses is an important factor in selecting property
for development. Just as a homeowner is concerned with
the neighborhood, so too is the developer concerned about
the character of the community. For example, an upscale
office developer may not believe that an adjoining industrial or warehouse facility is a suitable neighbor. The plan
helps define the market area of a site. The developer uses
this information to determine whether there are (or will be)
sufficient employees, residences, or customers to make the
project a successful venture.
It is important to understand that the comprehensive plan
does not guarantee an outcome or timeframe. The plan simply serves as a recommendation to achieve the community’s
vision. This can take years (or decades) to come to fruition,
and is subject to change. There is risk associated with an over
reliance on the comprehensive plan. Consider a developer
that builds an apartment building next to a future town center, which is identified in the comprehensive plan. If the town
center takes five years to be planned, designed, approved,
permitted, and constructed; then the developer’s apartments
may not be successful for those five years. Now consider,
if the town center proves to be too costly and the project
is abandoned, that could be detrimental to the developer’s
apartments.
Adoption. Most municipalities employ a professional planning staff, which coordinates the overall planning effort,
while smaller communities often retain outside consultants
for this purpose. The governing body relies heavily on the
recommendations of its planning staff, although decisions
about the plan and policies will ultimately be its own. The
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Comprehensive Planning 31
governing body ultimately must approve and adopt the comprehensive plan.
Since the plan is intended as a reflection of the community attitudes and desires, the process usually involves
several opportunities for citizen participation and input. In
many jurisdictions, outreach to citizen groups, such as home
owners associations (HOAs) or business associations, has
become increasingly common and important in the creation
of a plan. This outreach aims for plan development that will
be more representative of the wishes of the community and
less contentious at the mandated public hearings. Charrettes
sponsored by local authorities, city council, or community
association meetings are all vehicles for public participation.
The information gathered and attitudes voiced are an invaluable resource for preparing future proposals. The planning
staff will use this input as they prepare recommendations
for the governing body. Public opinion will sometimes have
a greater effect on a comprehensive plan adoption then any
of the other demographics or statistics that the planning
staff compiles.
A timetable for the plan development or revision and
a framework for the analysis will be prepared. Sometimes
working with an ad hoc task force, created for the sole purpose of dealing with a development issue, comprising the
community’s civic, business, and political leaders, the staff
undertakes extensive data collection. The planning staff
will compile the demographic and economic inventory
statistics needed for the analysis, as well as review existing
development patterns and activity. The process that some
municipalities follow to adopt or revise a comprehensive
plan is often lengthy and controversial. Depending on the
size of the community, the entire process can take several
months to several years. To spread the demanding workload, many municipalities update their comprehensive
plans, or portions of it, on a cyclical timetable. Some states’
legislation mandates the time within which the jurisdiction
must review and update the comprehensive plan, such as
every 5 years.
Circumstances unforeseen in prior plans, such as expansion of an airport, new rail system, a decline of a neighborhood, and/or influx of new industry, may justify an update.
Similarly, assembly of smaller parcels into a large development tract may also suggest an opportunity for a project of
grander scale than anticipated in the plan. The planning
staff and other involved groups consider these proposals
in preparing the comprehensive plan. More information
about updates to the comprehensive plan is discussed later
in this chapter.
Comprehensive Plan Contents. Comprehensive plans
generally begin with an introduction that includes a historical context for the area, plan development and adoption
process, local standards, and an overall community vision.
The vision statement summarizes the objectives of the community and identifies what is to be achieved in the future, at
a 5- to 20-year outlook. This statement is the basis for the
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whole comprehensive plan. The vision statement is usually
crafted by the planning staff but with extensive input from
the local community.
The vision statement leads to a series of goals and objectives. The goals should be realistic and achievable within the
timeframe of the comprehensive plan. The goals statements
are usually general and align with the overall community
vision. They set the parameters for the quality and character for the future development of the jurisdiction. They
should also maintain a balance among the interests of the
governing institutions: the community, the land owners, and
developers.
Typically, goals of the comprehensive plan focus on
•• Land use
•• Transportation
•• Population
•• Housing
•• Employment
•• Economic activity
Greenfield | Vision Statement
2050 Plan
Growth
• Investment in infrastructure that attracts new
business and employment opportunities.
Sustainability
• Continued focus toward our environmental
resources and policies that protect these
resources.
Community
• Neighborhood development that promotes
healthy lifestyles to grow new relationships.
Diversity
• Opportunities for all income ranges, ages, and
lifestyles.
F i g u r e 2 . 2 A Sample vision statment.
•• Retail activity
•• Industrial output
•• Energy
•• Recreation and open space
•• Natural and historic resources
•• Public utilities, services, and facilities
As an example of community goals, Kirkland, Washington, adopted their comprehensive plan in 2015 to envision the community in 2035. This jurisdiction includes a
paragraph as their vision statement. The following guiding principles
express the fundamental goals for guiding growth and
development in Kirkland over the 20-year horizon of the
Comprehensive Plan. They are based on and provide an
extension of the aspirations and values embodied in the
Vision Statement. The principles address a wide range of
topics and form the foundation of the goals and policies
contained in the elements of the Comprehensive Plan. They
strive to make Kirkland in 2035 an attractive, vibrant and
inviting place to live, work and visit.
See Figure 2.2A for a generic sample of a vision statement.
The vision and goals in a comprehensive plan are supported by data that contains a detailed inventory of existing
conditions and predictions of future trends. Then, comprehensive plans contain elements that lay out the strategy to
achieve a specific goal or objective.
Inventory and Trends. The inventory and trends section
provides a report of the overall existing community and projections on the future of the community. It shows statistics
such as age distribution, household formation, income, labor
02_Land_CH02_p017-124.indd 32
force, home, and automobile ownership of the citizens. The
inventory from previous years can provide information for
a forecast of population growth of the region. An inventory
analysis can also describe land use characteristics, including
a breakdown of number and type of housing units, structural
condition, and land and building area of nonresidential uses.
Economic data, such as retail sales and manufacturing output
is often reported and analyzed to estimate future economic
development. Many plans contain information on real estate
absorption and conversion rates, property sales, and leasing activity, while others may offer statistics on commuting
patterns and transportation use. With the help of the land
development consultant, the developer uses the inventory
and trends information to determine the needs of residents
and business, and their financial capacities and limitations.
During the design phase of a project, the inventory and
trends data can also be used by the design team to make
development decisions. This can include recommendations
on development amenities, internal accessibility, and character of the development. For example, an area that is experiencing population growth from an age group over 65 years
old may put more of an emphasis on accessible design and
low impact recreational facilities. The design team and developer should discuss the specific character of the community
to provide a development that fits the current and projected
demographics of the area.
Elements Summary. The elements of a comprehensive
plan include specific strategies and recommendations to
implement and achieve the community’s vision and goals.
The elements may be referred to as chapters, subsections,
or simply as plans within the comprehensive plan, depending on the jurisdiction. The type and number of elements
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2.2
included in a specific comprehensive plan vary by jurisdiction and the priorities of that locality.
For example, Table 2.2A shows that Clark County, Nevada,
includes 8 elements in their comprehensive plan, while
Burlington, Vermont, includes 10. Kirkland, Washington,
includes 11 elements. Longmont, Colorado, on the other
hand, decides against specific element plans and instead
includes chapters focused on policy, growth, and implementation to achieve their vision and goals.
Many elements of a comprehensive plan are focused on
development and infrastructure goals, but the elements will
often include policies on education, safety, and economic
development. While some policies might not directly identify infrastructure requirements, they may serve to inform
the jurisdiction’s decisions on what conditions are placed on
development plans. For example, the comprehensive plan may
identify the need for new schools because of overcrowding—
this goal may prevent a developer from gaining approval
for increased density in a residential development until the
schools are constructed. It is important to be aware of all
elements within the comprehensive plans to know how they
may affect a site. Each element’s strategies and recommendations overlay and work in conjunction with the underlying
plan, other elements, and parcel-specific findings.
Each element usually consists of a plan that includes
text, graphics, and maps related to that element. It is important to be aware of each of these plans to know how they
may affect a project. These element plans usually contain
six major parts:
1. Goals and objectives for the element to achieve the
overall community vision
2.
Inventories of existing characteristics, features,
resources, land uses, and/or facilities related to the
element
3. Projections of trends expected within the life of the
element
TA BL E 2 . 2 A
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Comprehensive Planning 33
4. Policies to be applied to achieve the element’s goals
5. Maps and text depicting and discussing the community, showing current and future land use, the
location of future public facilities, environmental
resources, and other features
6. Implementation plan how the community intends to
carry out the goals of the element
The goals and objectives are more detailed for each specific element. These may be similar to the overall community
goals and objectives of the comprehensive plan, but within
each element they will be more focused. The inventory and
trends are also more detailed to support the element’s recommendations. For example, the land use plan will include the
existing uses in the jurisdiction and predict future development patterns. An area with industrial uses that are underutilized by abandoned warehouses and old factories, may
be recommended for new residential uses. The transportation plan will summarize the existing infrastructure and
project future needs in the inventory and trends section. On
an over capacity road corridor, new transit may be recommended alongside to support future demands. The housing
plan will analyze existing housing stock and forecast future
housing trends. If the region is experiencing a large population growth in the younger age group, high-density housing
developments with retail within walking distance may be
recommended.
The policies and maps will also be relevant to each specific
element. Different regulations may exist for different areas
within a locality depending on where a site is located. For
example, an urban area may require smaller vehicular travel
lanes and larger sidewalks to promote pedestrian circulation, as recommended in the transportation plan, but roads
within the rural area of a jurisdiction will have different
design requirements. An area near a natural body of water
may be subject to more stringent environmental requirements because it is near a protected waterway, but another
Elements Included within Comprehensive Plans
Clark County, Nevada Elements
Burlington, Vermont Plans
Kirkland, Washington General Elements
Conservation element
Historic preservation element
Housing element
Land use element
Public facilities and services element
Recreation and open space element​
Safety element
Transportation element​
Land use plan
Natural environment
Built environment
Historic preservation
Transportation plan
Economic development plan
Community facilities and services plan
Energy plan
Housing plan
Education plan
Community character
Natural environment
Land use
Housing
Economic development
Transportation
Parks, recreation, and open space utilities
Public services
Human services
Capital facilities
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waterway may not have the same classification and may not
have the same requirements.
The implementation is important to ensure success of the
overall comprehensive plan and to produce results. Through
these implementation recommendations, the overall community vision is achieved. The section may include a list of
items that need to be implemented. A matrix may be included
that doles out work for each item, labels responsibility, sets a
date for implementation, and includes a cost.
As mentioned, comprehensive plans are regularly updated,
and this includes the elements within. A state government
may require this at set intervals of time, or goals may need to
be changed to reflect new community priorities, large-scale
transportation improvements, new economic development,
or new trends in urban design and development. More information about comprehensive plan updates is discussed later
in this chapter.
Again, a jurisdiction’s size and the community vision
will determine how many and what kind of elements they
have included in their comprehensive plan. It is important
to remember that comprehensive plans are all unique and a
representation of the local community, so the elements will
be different depending on the jurisdiction. The most common elements that are important for the site engineer that
will be discussed within this section are land use plans, transportation plans, and utility plans.
Land Use Plan. The land use plan is one of the most important elements found within a comprehensive plan. It projects
the future use of land in accordance with the jurisdiction’s
vision and community goals. The land use plan also serves
as a guide for the jurisdiction as they adopt or amend zoning regulations. If a developer investigates the opportunity
to change the zoning (through rezoning), the land use plan
will identify whether the change in use or density will match
the jurisdiction’s expectations. Refer to Chapter 2.3 for more
information on zoning and rezoning.
The land use strategies in the land use plan can help to
spur growth in a community. For instance, the plan may
identify an area as high density residential because the area is
desirable next to an existing commercial center. Or the land
use map may maintain low densities in an existing residential neighborhood that is not anticipating future growth.
Accompanying the land use plan text is the proposed land
use map. Often color-coded or shaded, the generalized land
use maps of the community represent a graphic depiction of
future land uses and facilities. The map divides geographic
areas into desired and projected uses and intensity. These
areas usually represent the broad categories of land use, such
as residential, commercial, and industrial. Subcategories of
development intensity show the gradation of land use patterns
from high to low densities. The map defines the boundaries
of the area, and provides a range of relative densities.
The map frequently shows the proposed location of significant facilities, such as transit centers, proposed freeway
interchanges, regional shopping malls, and new schools. It
02_Land_CH02_p017-124.indd 34
offers a projection of what the community would like to be at
a distant, but determined, point in time. Figure 2.2B provides
an example of a land use map.
The mapping of land use at a large scale often generates
considerable controversy. The land use map appears to fix
geographic location for the proposed facilities it recommends. People may be concerned if these locations are near
their homes. Consequently, it is not uncommon for residents
to view the plan as a certain future rather than a recommendation for development.
Often, the final land development does not conform to
the finite categories and specific locations shown in the land
use map. Some properties may undergo several use changes
awaiting conditions favorable to the ultimate land use; and
some, toward the end of the life of the plan, may wait for
changes proposed by a new plan. To eliminate some of the
ambiguity created by scale and time, many communities
undertake a multitiered and multiphased planning effort
through a sector plan, which is discussed in the next section.
It is important to remember that land use maps are projected future uses recommended by the comprehensive plan.
The zoning maps depict the existing and approved land uses.
Therefore, the zoning map depictions, supported by the zoning regulations, are the legal and enforceable use of a site that
must be complied with, not the comprehensive plan’s land
use map.
Transportation Plan. Transportation plans are another element commonly found in comprehensive plans. This plan
shows existing transportation infrastructure and facilities,
as well as future transportation needs. This future transportation may include new roads, widening of existing roads,
sidewalk improvements, future transit lines, and more.
These improvements are determined through extensive
studies that project the adequacy of existing facilities and
necessary improvements to support future growth. A map
usually accompanies the text. Figure 2.2C shows an example
of a transportation plan map.
The transportation plan can allow the development team
to identify desirable parcels of land based on future access
improvements—but the funding and schedule of transportation projects are typically uncertain as they often rely on
state or federal funding sources, so the transportation plan
should be used as a guide. A transportation plan may also
require the design team to consider how a future road widening at the front of a parcel may impact the proposed design.
Similarly, a new roadway that is proposed through a parcel of
land may require the developer to dedicate the land or construct the roadway.
Utility Plan. A utility plan may be included as an element
or within an element in the comprehensive plan. The utility plan identifies existing utility infrastructure and ensures
utility delivery in the future. The design team should review
the content regarding available public utilities such as water
and sewer. The existence of such utilities, system capacities,
and timing of future improvements is important where these
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Comprehensive Planning 35
MERRIFIELD AVENUE
ORCHARD ROAD
E
IV
R
K
D
O
HOLT STREET
LO
R
VE
O
GREENVILLE
ROAD
SKYLINE ROAD
THIRD ST
AD
O
E
LL
VI
JESSE LANE
D
BLVD
OA
ER
IN
KYL
EN
E
R
G
R
BRUI
SER
S
COUNTY
LAND USE PLAN
Adopted 2025
Residential
Commercial
Other
1 - 2 DU/AC
Office
Industrial
2 - 4 DU/AC
Retail and Other
Public Facilities,
Commercial Uses
Governmental and
Institutional
4 - 6 DU/AC
Parcel Boundary
Stream Corridor
Road Centerline
6 - 8 DU/AC
Figure 2.2B
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Example of a land use plan.
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MERRIFIELD AVENUE
ORCHARD ROAD
E
IV
R
K
D
O
HOLT STREET
LO
R
VE
O
GREENVILLE
ROAD
SKYLINE ROAD
THIRD ST
AD
O
E
LL
VI
JESSE LANE
BRUI
SER
S
BLVD
D
OA
ER
IN
KYL
EN
E
R
G
R
.
COUNTY
TRANSPORTATION PLAN
Adopted 2015
.
Figure 2.2C
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Arterial Road (Four Lanes Divided)
Collector Road (Three Lanes)
Local Street (Two Lanes)
Widen or Improve Collector Road to Three Lanes
Construct Two Lane Collector Road on New Location
Planned County Trail Network
Parcel Boundary
Stream Corridor
Example of a transportation plan.
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2.2
entities are needed to support the proposed development; in
some localities the adequacy and phasing of public facilities
controls development.
Even when the comprehensive plan and actual parcel zoning agree on land use and density, the sewer and water master plan dictates lot size and project density. Consequently,
the final project may be significantly less intense than envisioned, unless the developer can improve the system or defer
the project to a consistent time frame with the adopted comprehensive plan. Policy language in the plan often prohibits
the developer from constructing needed improvements to
advance project development. Some localities may use this
approach to discourage premature development in areas
where growth is not anticipated or desired.
Sector Plans. Many large localities will segment their planning focus into several planning areas or sectors, and other
communities may identify smaller areas that warrant special
study. These planning areas and studies utilize sector plans
to offer a more detailed analysis at a micro level to achieve
the community’s vision and goals. These sector plans may be
referred to as small-area plans, district plans, or master plans
(master plan can also be in reference to the comprehensive
plan itself or a large-scale development), depending on the
jurisdiction. This effort focuses the community vision, often
at several intermediate steps, to examine smaller subsections
of the jurisdiction. At its extreme, the plan makes parcel-byparcel recommendations on land use, density, compatibility,
and the facilities that serve them. When this happens, the
sector plan begins to resemble the zoning ordinance more
than a plan to guide future development. However, even at
this scale, the plan still serves only as recommendation for
future development, albeit a strong one.
Sector plans can be thought of as a comprehensive plan
within a comprehensive plan. The sector plan outlines the
vision and goals for the smaller area, includes more detailed
inventory and trends for that area, and incorporates multiple
elements to provide specific strategies and detailed recommendations for implementation. The text provides extensive
discussion of sector-wide policies, issues, and opportunities,
in addition to the jurisdiction-wide seen in the rest of the
comprehensive plan. Specific recommendations in the sector
plan achieve the overall vision and goals of the comprehensive plan.
In addition to having a large geographic area that requires
several smaller planning areas, a community may require a
sector plan and more focused detailed planning for other reasons. The jurisdiction may try to anticipate growth around new
infrastructure, such as a new sports stadium. Or the jurisdiction may simply be trying to revitalize a declining region, such
as an old manufacturing facility. Examples of areas that require
a more detailed plan include business districts, highway corridors, transit centers, areas of important institutional use, or
other significant concentrations of population and employment. Such areas often have unique circumstances deserving
closer examination due to their presence in the community.
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Comprehensive Planning 37
The sector plan (or a group of sector plans) may be
included within the comprehensive plan itself. Or, it is common for the sector plan to be a separate document(s) released
after the comprehensive plan is adopted. This will spread
the planning efforts across multiple documents that may
be adopted at different times after the comprehensive plan.
This will ensure the local jurisdiction to remain up to date,
while continually meeting community needs and potentially
responding to market changes.
Figure 2.2D shows the general land use plan for the
Ballston region in Arlington County, Virginia. Arlington
adopted sector plans for the areas around each metro station
throughout the county. Along with the accompanying plan
text, the Ballston Sector Plan has helped to guide development in this area.
Plan Implementation. Although the goals statements represent the community’s vision of quality of life and future
development, the plans for implementation are the key to that
vision’s realization. Despite its detail and breadth of coverage,
including all the elements and sector plans, the comprehensive plan is only a guide in most jurisdictions. The governing
body should strongly consider the plan’s recommendations
when making land use or fiscal decisions. It is through the
capital improvements plan and local zoning that the comprehensive plan is ultimately achieved.
The recommendations of the comprehensive plan will
help the local jurisdiction to adopt a capital improvements
plan (CIP). The CIP is a short-range plan which identifies
capital projects, provides a planning and implementation
schedule, and strategies for financing the projects. This
may include details about the location of each project and
recommended timing for implementation. Capital projects
refer to major public expenditures of the local jurisdiction.
This includes parks, schools, public buildings, transportation infrastructure, and public utilities such as water and
sewer. These capital projects can be new projects or maintenance, upgrades, or replacements of existing infrastructure.
The CIP ranks each capital project by priority and
includes a year-by-year schedule of expected project funding, estimates of project cost, and potential financing
sources. This is listed in tables and matrixes to reflect all
of the proposed improvements. The elements of a comprehensive plan will help the jurisdiction to make decisions
about each capital project. Specific recommendations of
the comprehensive plan can be achieved through the CIP.
As an example, if a new road is proposed in the transportation plan, the CIP may denote it as a capital project and
schedule its construction for the near future. The CIP is
regularly updated to reflect changing community needs,
priorities, and funding opportunities, so it is important to
stay up to date.
The link between the comprehensive plan and zoning
ordinance is important to remember. Again, most communities rely on the findings in the comprehensive plan when
considering zoning and site plan applications. The zoning
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Figure 2.2D
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Example of a sector plan.
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ordinance may mandate that the developer consider the
detailed provisions of the plan and address those issues in
the site plan proposal. For example, if the transportation
plan recommends a trail network through a site, approval of
that project may require the trail network to be constructed.
In some areas, sometimes referred to as “plan conformance
jurisdictions,” the comprehensive plan is a mandatory directive for land use regulations and, by law, zoning decisions
must conform to the plan’s mandates.
The existing zoning of a parcel takes precedence over the
recommendations of the comprehensive plan in almost all
cases. This is true even where the use and intensity provisions
of the comprehensive plan are more restrictive than those in
the zoning ordinance. If a parcel is zoned for an industrial
use then a developer can propose a project conforming to
the industrial use by right, even if the comprehensive plan
identifies that the parcel is recommended for residential
development. For this reason, an important role of the comprehensive plan is to recommend the adoption of changes to
ordinances that regulate land use and development.
The comprehensive plan represents an invaluable resource
in the data and direction it offers, yet the language in the text
is often vague and subject to interpretation, which can make
implementation challenging. Frequently, the comprehensive
plan content leads to varying interpretation by regulatory
officials, citizens, or the developer. This adds to the considerable challenge facing the development team in preparing a
project development application that conforms to the comprehensive plan. Assumptions may have to be made based
on the interpretation of the comprehensive plan that may be
revised during the review and approval process.
Comprehensive Plan Updates. Typically encountered
updates to the local comprehensive plan include supplemental comprehensive plan additions, comprehensive plan
amendments, and new comprehensive plans. A new comprehensive plan, or other major updates, may occur at a set 5- or
20-year interval depending on the jurisdiction. Additions and
amendments are possible during this time, but are not frequent occurrences. It is important to review the local comprehensive plan and see if any updates have been included.
Supplemental Comprehensive Plan Additions. Planning
staff respond to the community needs and continue to refine
the goals and objectives even after the adoption of a comprehensive plan. As mentioned, sector plans provide a more
detailed analysis, but they may be released sometime after
the initial comprehensive plan. This is to ensure that the plan
is up to date and meeting current community needs, while
potentially responding to market changes.
Like these sector plan updates, there are sometimes supplemental comprehensive plan additions that update the initial comprehensive plan. These supplement additions may
support specific areas and new growth potential; update
the elements within the existing comprehensive plan; or be
in response to new strategies the community is trying to
implement.
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Comprehensive Planning 39
Comprehensive Plan Amendments. Sometimes the comprehensive plan is outdated and the planning staff may believe
that it no longer accurately reflects the community vision or
good planning principles. This often occurs when a significant
amount of time has passed since the adoption of the existing
plan or to reflect conditions that have changed since its adoption. A comprehensive plan amendment would be warranted
to update the comprehensive plan. This could update a certain
part of an element, an entire element, or multiple elements
within the comprehensive plan. The plan amendment process
varies greatly among jurisdictions; nonetheless, the opportunity to amend the plan is typically available in some form.
Specific authorization to undertake a comprehensive
plan amendment might require legislative action. This can
be initiated by the jurisdiction itself, or sometimes a developer may propose an amendment to achieve more favorable
development conditions. In undertaking a comprehensive
plan amendment, the local jurisdiction undergoes a comprehensive study. When the proposed amendment is prepared,
notice is given to the community through mailings, advertisements, and/or posted signs. The draft comprehensive
plan amendment is reviewed by the professional staff and
will be the subject of meetings and public hearings before
the community’s planning commission and governing body.
This process is lengthy and uncertain, and can potentially
add as much as a year or more to the development process.
For example, a new mass transit line has the potential
to spur new growth near the station locations. To respond
to this opportunity, the jurisdiction can initiate a comprehensive plan amendment to their comprehensive plan. This
amendment will recommend policy updates, in a specific
area surrounding the new transit stations, to support growth
and new development. These include changes to many of
the comprehensive plan elements including plans for higher
densities in the land use plan, and new roads around the
station in the transportation plan. The comprehensive plan
amendment may change some specific elements or it may
change all of the elements, depending on the update.
Too many amendments can make the comprehensive plan
cumbersome to read and difficult to understand. It creates a
long and confusing document that is ineffective. Therefore, a
new plan may be required.
New Comprehensive Plan. If the entire comprehensive
plan is outdated, the community may initiate the creation of
a new comprehensive plan. This usually happens after the
span of many years and is a significant effort for the community to undertake. It requires a great deal of time, generally
several years, to develop a new plan and will be very costly
for the local jurisdiction.
It is important to understand that the comprehensive plan
is a “fluid” document that changes with time. The plan may
change or have additions implemented every few years as
the community grows. The design team, including the site
engineer, should be aware of the comprehensive plan and its
changes in each jurisdiction they work in.
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2.2.4. How to Use the Comprehensive Plan
Depending on the size of the community and its planning and zoning authority, the comprehensive plan may
consist of several documents and maps. The development
team must be aware of the current comprehensive plan, all
amendments, and pending updates. Local governments
periodically revise their comprehensive plans, both in their
entirety and on a piecemeal basis. Minor small-area amendments or sector plans may have been adopted in response
to new information, changing circumstances, or by another
developer’s proposal. The governing body frequently acts
on amendments apart from a more extensive plan review
process, so updates may be separate of the most current
versions of the plan.
All documents and maps that relate to jurisdiction-wide
issues should be assembled by the land development team,
and analyzed for possible impact on a specific parcel planned
for development. It is important to read all texts and understand all the maps within a comprehensive plan.
Table 2.2B shows all of the plan that Montgomery County
has adopted. Each has a different function and purpose for
different areas within the county. It would be important to
understand each document if working in this jurisdiction.
The development team should identify the techniques
and procedures that the local government uses to accomplish
plan recommendations. These procedures may range from
new or revised regulations or disincentives that discourage
development to incentives that provide greater economic
TA BL E 2 . 2 B
return to the developer in exchange for features that provide
public benefit. This is important to understand as a project
gets started, but ultimately the developer will have to make
their own decision regarding fiscal impacts of these recommendations to determine initial project feasibility.
The development team should be aware of how the community prepares its land use maps and the intent of their
graphic representations. For example, a symbol proposing
a regional shopping center site on the comprehensive plan’s
land use map may not necessarily fix its location. The text
may refer to specific circumstances or conditions that are
favorable to another site in the general area of the symbol.
Furthermore, specific land use maps may make explicit recommendations for specific parcels.
Land uses different from those shown on the maps may
be permitted, and sometimes even encouraged, depending
on circumstances and conditions identified in the text. For
example, the map may show a broad geographic area identified as low-density residential; however, text may explain
that nonresidential uses, such as neighborhood shopping
or service facilities, are appropriate. In such cases, the plan
specifies conditions that the land developer must consider in
developing such uses.
Again, it is important to remember that the comprehensive plan is simply a guide for development. Zoning is based
off the recommendations of the land use plan and other elements within the comprehensive plan. Zoning is the legally
binding regulation.
Listing of Montgomery County, Maryland General, Master, Sector, and Functional Plans
The type and number of plans available from various public agencies that can aid in the early design stages of a project vary
widely according to the municipality. For example, this list describes the types of plans available for Montgomery County,
Maryland.
General Plan:
Identifies, in broad terms, those areas suitable for various types of land uses—residential, commercial, agricultural, open
space, etc.
Local Area Master Plan:
Are available for each of the county’s 27 planning areas. The local area master plans include a statement of planning policies,
goals, and objectives, and a description of the planning area. Plans also include maps outlining recommended land uses, zoning, transportation facilities, and recommended general locations for public facilities, and possibly even include recommendations for scheduling financial capital improvements.
Sector Plans:
Are comprehensive plans for a portion of the Master Plan area. They describe the relationship of various land uses to transportation, services, and amenities. Plans may include maps or other graphics, text, and design guidelines.
Functional Master Plan:
Shows details of a specific system such as major highways, drainage, and stormwater management systems.
02_Land_CH02_p017-124.indd 40
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Chapter 2.3
Zoning
2.3.1. Introduction
Despite the effort involved in adopting a comprehensive plan,
legally, in most jurisdictions, it serves only as an advisory recommendation. In these majority jurisdictions, the plan does
not have the force of law. The enforcement of development
regulations falls to the community’s zoning ordinance. The
zoning ordinance is the legislative means by which a community sets detailed regulations for all aspects of the use of land.
The zoning regulations include use, layout, and intensity of
each parcel of ground within the jurisdiction as well as height,
setbacks, parking requirements, open space, and others.
The zoning ordinance is law and must be followed—the
zoning regulations must be met to obtain project approval
and permits. The approval process for rezoning cases is
described in greater detail in this chapter, and the review
process for a jurisdiction is introduced in Chapter 2.4. Therefore, it is important to understand the zoning regulations and
ensure that it is followed early in the planning stages of a
project. Zoning will affect all aspects of the project as design
work begins and plans are produced, which is described
throughout Chapters 4 and 5.
The comprehensive plan is used as the primary guide for
making zoning decisions and writing a zoning ordinance.
A decision-making body may disregard the recommendations of the comprehensive plan, but it does so at the risk
of losing regularity and uniformity in its zoning practices.
This will also undermine the integrity of the comprehensive plan. In most cases, all of the different elements in the
comprehensive plan shape the zoning regulations. As is discussed later in this chapter, the land use plan usually guides
the zoning district adoptions.
This chapter addresses the most typical zoning ordinances, concepts, and procedures. It is neither practical nor
possible to address all variations that exist throughout the
nation. However, there are basic similarities in the structure
of each type of regulation. This text focuses on their differing
impacts on land development and the way regulations or
procedures are implemented.
2.3.2. Zoning
The zoning ordinance is a legislative text that is adopted by
a local jurisdiction to regulate their land. It is usually a separately published title or chapter in the jurisdiction’s code of
laws. Per the Zoning Enabling Act of 1924, zoning is based on
the recommendations of the comprehensive plan. The comprehensive plan outlines strategies to achieve community
goals at a macro level. These recommendations then must
be drafted into the zoning ordinance to be implemented
through zoning regulations. The zoning defines the micro
level detail that the comprehensive plan outlines.
The land use plan of the comprehensive plan, which
offers a broad land use strategy for the jurisdiction, is usually
the starting point for zoning. The zoning will assign more
detailed zoning districts to each parcel. These zoning districts can be identified on the zoning map, which accompanies the zoning ordinance. The comprehensive plan is only a
guide and serves as an advisory planning tool. Zoning is law
and the zoning ordinance is enforceable. The existing zoning of a parcel takes precedence over the recommendations
of the comprehensive plan, even if there is a newer version
of the comprehensive plan. Updating a zoning ordinance to
align with the comprehensive plan will sometimes be initiated by the jurisdiction itself or can be achieved through the
rezoning process, which is described later in this chapter.
In its simplest form, zoning separates different or incompatible land uses and serves as a “nuisance-prohibiting”
device. Its purpose is to reduce the likelihood of one use creating a nuisance or having an undesirable impact on occupants
or future occupants of adjoining property (i.e., ensuring compatible uses). Provisions for zoning ordinances come from the
state’s police powers, and are necessary to protect the health
and safety of the public. Such zoning prevents overcrowding,
41
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establishes appropriate sanitary regulations, provides for a
more efficient transportation system and protects quiet residential areas.
For instance, the operation of a business such as a grocery
store requires the installation of signs, lighting, parking, and
trash disposal facilities and generates traffic. If that grocery
store operates next to a residence, the adjoining residents’
use and enjoyment of their property is diminished. The traffic, noise, activity, and lighting are likely to become a nuisance that infringes upon the residential character expected
by the homeowner. As a residence, its location has become
less desirable than others in the neighborhood. However,
if development conditions are regulated then the nuisances
of the grocery development could be mitigated, which may
increase the value of the residential property because of
retail convenience. Zoning can regulate the impact of one
use upon another. Conventional zoning prevents conflicts by
providing a greater separation between incompatible uses.
Where greater distance cannot be achieved between incompatible uses, the zoning ordinance requires other techniques
to reduce impacts such as a different architectural solution, smaller signs, a larger yard between the building and
property lines, fencing, landscaping, or restricted hours of
operation.
Each parcel within a community is typically zoned for
specific uses (residential, commercial, industrial, etc.), which
specifies the character and use of the site. If a developer proposes a project with a different use or character from what
is allowed in the current zoning the developer will need to
seek special use permits, rezoning, or both. The developer
may reference the comprehensive plan for initial insight into
potential support for rezoning approval. If the comprehensive plan indicates that the parcel is better served through
a different zoning condition, there is a higher probability of
support for the rezoning case. If the proposed project meets
the use requirement of the parcel zoning, the developer must
adhere to the conditions of the ordinance but would not be
required to process a rezoning case or special use permit. A
project that follows all existing zoning conditions is often
referred to as by-right development.
In some jurisdictions the only requirement for development is compliance with the zoning ordinance and building
codes. Some projects, however, have intricacies that require
more review, like those that subdivide a parcel. In most jurisdictions, detailed site plan reviews are required for all projects.
In addition to determining the allowable use of a parcel,
the zoning ordinance regulates building and dimensional
standards. This determines the building’s square footage,
density, heights, setbacks, parking, open space requirements,
and more. This will dictate the size and shape of buildings
and their location on a property. Therefore, the zoning can
also be used to establish aesthetic goals.
There are four main types of zoning that are common to
find in a zoning ordinance. Some jurisdictions prefer one
over the other or use a combination of these zoning concepts.
It is important to understand each type to know how to read
02_Land_CH02_p017-124.indd 42
the zoning ordinance and what to consider at the beginning
of a project.
Euclidean zoning: Conventional rigid zoning that separates uses
Negotiated zoning: Negotiation of regulations and
standards
Performance zoning: Zoning based upon set performance
standards
Form-based codes: Zoning regulations primarily based on
geometric conditions
Euclidean Zoning. Euclidean zoning, also known as single-use zoning, is the traditional type of zoning that most
people think of—it is the oldest type of zoning in the United
States. This rigid zoning concept creates several zoning districts within the jurisdiction, specifies allowable uses, and
applies development standards for each parcel. These zoning
districts include everything from residential to commercial,
and groups similar uses together.
Euclidean zoning is a very straightforward process
because it allows a property to be developed to any land use
that is allowed by right (provided that the development meets
zoning standards and complies with other development policies and regulations). There are several other options for
developers to pursue if the project is not by right, including
special exceptions, variances, and rezoning (this introduces
opportunity but also complications). These will be discussed
in the following sections and will show the challenges of
Euclidean zoning.
The allowable uses for each zone are determined differently, as is the subsequent process of development review by
the jurisdiction. The uses are usually related to the land use
plan in the comprehensive plan. Situations sometimes arise
where the zoning is not in conformance with the comprehensive plan. Strategies to overcome these differences in the
plans are described later in this chapter. The variations of
uses in a zone dictate what land use options exist for a site
and how project design can be undertaken. This will affect
the way in which local government approves development
projects and the durability of that approval. The advantage of
this type of zoning is greater certainty in both product and
project approval, however, the end result might be rather
uniform.
The Euclidean approach to zoning does not allow for much
flexibility and this may limit the developer. This may make it
difficult to address site concerns or respond to shifting market
activity or preferences. For example, a developer may plan for
a new residential community with a by-right condition that
allows for single family homes. But if a new light-rail system
is constructed near the site then it may be more appropriate
for a higher density development based on the new character of the region. Under a Euclidian zoning, the developer is
still restricted by the original zoning and the associated conditions of development. If the developer chooses to pursue
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2.3
a higher density they will be required to apply for rezoning,
which requires time and resources.
It should be noted that the exhaustive set of development
criteria, including the allowable uses and lists of standards
for each zone, can result in a lengthy ordinance. Euclidean
zoning often contains the longest text compared to other
types of zoning. The format of a zoning ordinance will
vary across jurisdictions and the definitions of terms used
within the ordinance may differ as well. Terminology such as
■
Zoning 43
building height, open space, and yard setback can have different definitions in each jurisdiction and should be studied
by the development team. Figure 2.3A provides the example
text from the Fairfax County Zoning Ordinance for one of
several zoning classifications; C-2 Commercial.
Zoning Districts. The zoning ordinance for Euclidean
zoning begins by dividing the jurisdiction into zoning
districts. The zoning ordinance ensures compatible uses
and achieves separation in the community by defining
F i g u r e 2 . 3 A Example of zoning text. (Source: Fairfax County Zoning Ordinance.)
02_Land_CH02_p017-124.indd 43
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F i g u r e 2 . 3 A (Continued )
categories of land use for each zone. Most zone categories
relate to residential, commercial, industrial, and agricultural uses. Sometimes they also include mixed-use or specialty zones.
The ordinance is usually organized in a hierarchy for each
zone that reflects intensity and operating impacts on the
community. The broad zoning categories are broken down
into more detailed zoning districts. For instance, under the
broad category of residential, an ordinance might specify
02_Land_CH02_p017-124.indd 44
districts for low-, medium-, and high-density zones—these
zones may limit building types to single family, townhomes,
or multifamily.
The text associated with these zoning districts will usually
describe the purpose of the zone, allowable uses, and development standards. These represent the differing types and
intensity of uses possible within each zoning district. This
ensures the intended magnitude and character of the development is appropriate for the community.
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2.3
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Zoning 45
F i g u r e 2 . 3 A (Continued )
Many jurisdictions draw zoning district boundaries in a step-down fashion. This pattern seeks to provide
gradation between two disparate uses, usually residential
and commercial or office. This reduces the most significant conflicts between the more extreme uses. Figure 2.3B
represents a zoning map with property divided into several districts between the most intense residential zones
and least dense residential zones. The permitted intensity
of commercial use reduces in each of several districts as
02_Land_CH02_p017-124.indd 45
distance from the most intense commercial area increases.
Similarly, the residential density will decrease as the distance from the most intense commercial area increases
(the highest density residential is located nearest the most
intense commercial zone).
This can be seen in a more traditional sense when considering town centers and attempting to provide gradation
between high and low densities within a jurisdiction. The
highest density will be in the urban center and then will
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F i g u r e 2 . 3 A (Continued )
gradually step-down to the less dense suburbs. For example,
transit-oriented developments will have the highest density
near the transit station. The allowable densities will then
taper down as the distance from the station increases. Highrise towers at the station may taper down to midrise buildings, then townhouses, and ultimately single-family homes
on the periphery.
02_Land_CH02_p017-124.indd 46
Some jurisdictions establish districts that provide for flexibility as well as a mix of uses and appearance where zoning
classifications meet. At urban centers with high-density residential and more intense commercial, it is common to find
mixed use zones that allow for more flexible designs.
As an example, the table in Figure 2.3C is from the City of
Lafayette, Louisiana’s Unified Development Code, the local
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02_Land_CH02_p017-124.indd 47
47
Figure 2.3B
Zoning map showing zoning transition, modified from Fairfax County zoning maps.
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Figure 2.3C
Layfayette, Louisiana, zoning districts.
zoning ordinance. This table lists all zoning districts within
the jurisdiction. It also correlates each district to the future
land use designated in PlanLafayette, their comprehensive
plan. It is important to see that the zoning is connected to
and achieving the comprehensive plan.
Zoning Map. Zoning districts for each parcel can be located
on the accompanying zoning map. This is included with the
zoning ordinance and shows the division of the entire jurisdiction into each zoning district that is defined in that text.
These zoning districts reflect the actual zoning of the land.
Each individual property in the jurisdiction is associated with
a zoning district.
02_Land_CH02_p017-124.indd 48
A zoning map provides a graphic depiction of zoning
areas to reflect density transitions and geographic relationships between parcels, transportation, and natural
resources. Each district is labeled with the name of the zone
that corresponds directly to the ordinance text. Frequently,
the district name is a shorthand indication of the use and
intensity of the district.
While the meaning of these labels is consistent within a
single jurisdiction, they often vary between jurisdictions. For
instance, a district labeled “R-5” might be a residential district with a permitted density of five dwelling units per acre.
Or, it may indicate a residential district permitting minimum
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2.3
lot sizes of 5,000 square feet. On the other hand, it may simply be the fifth in a hierarchy residential district.
As an example, in the City of St. Louis, Missouri, their
Official Zoning District Map disregards the previously
described shorthand indication altogether. Instead, each
zone is identified by a single letter from “A” through “L.”
“A” stands for the single-family dwelling district, “B” is for
the two-family dwelling district, and so on. Therefore, it is
important to be familiar with all local zoning documents and
naming conventions.
Overlay Districts. Zoning overlay districts may be identified on the zoning map. Jurisdictions use the overlay zones
to address areas with special conditions; unique opportunities due to local culture, historic land use, or location; and/or
unusual physical characteristics. Typical situations that suggest the use of overlay controls include downtown areas,
redevelopment opportunities, airports, highway corridors,
resource protection, floodplains, and historic areas.
Overlay districts are mapped on top of conventional district boundaries on the zoning map. These generally outline
an area on the map that has additional regulations or codes.
The specific criteria for each overlay district will be included
in the zoning ordinance, usually within a separate chapter or
subsection. Parcels in the overlay must conform to both sets
of zoning district text (the base zoning district ordinance and
the additional overlay district criteria), or the more restrictive if conflicts are apparent.
For example, Brooklyn, New York, has an overlay district
called the Special Downtown Brooklyn District (DB). This
overlay “establishes special height and setback regulations
and urban design guidelines to promote and support the
continued growth of Downtown Brooklyn as a unique mixed
use area.” The Special Downtown Brooklyn District has a
separate chapter within New York City’s zoning ordinance
that has additional provisions that must be followed for parcels within the overlay.
Additional examples of overlays for environmental and
historical preservation are included in Chapter 2.5.
Map Creation. How local jurisdictions create their zoning
map varies. Many jurisdictions adopted the first zoning map
simply as a graphic representation of land use patterns that
existed at the time. Areas of vacant land were frequently zoned
to a district similar to the surrounding area. In some communities, the adoption of the comprehensive plan is followed
by a jurisdictional-wide rezoning to match the recommendations of the comprehensive plan. In such cases, the zoning
map closely mirrors the land use patterns recommended in
the land use plan of the comprehensive plan.
For jurisdictions that do not automatically rezone subsequent to comprehensive plan adoption, rezoning may be
required to be in conformance to the comprehensive plan.
Any more intense use generally requires that the developer
seek a rezoning to a higher classification. The rezoning process, discussed later in this chapter, addresses rezoning and
the map amendment process in more detail. Therefore, it is
02_Land_CH02_p017-124.indd 49
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Zoning 49
crucial to understand what a property is zoned as, and how it
relates to the comprehensive plan.
The zoning map itself is often available in several forms
and scales, and usually includes a generalized zoning map
with a single-sheet depiction of the entire community. Most
jurisdictions publish the zoning map online, which allows for
dynamic navigation of the jurisdiction. The map shows major
roadways, landmarks, and natural features, with zoning district boundaries identified by various symbols or labels.
A large-scale map will frequently include parcel boundaries or may reference a tax map designation for each property. In some jurisdictions, the official map shows additional
information, such as previous and pending zoning, special
exception or special permit applications, as well as subdivision activity.
The map in Figure 2.3D is the zoning map for Portland,
Oregon. The map includes a key that lists the major zoning
classifications, effective dates of rezoning, and special requirements. The major zoning classifications are residential, commercial, and manufacturing districts, which are denoted by
“R,” “C,” and “M” letters on the map. Each zoning district is
described in the local zoning ordinance. An overlay district
can also be seen on the map as designated by the lowercase
letter. For example the “CX” represents central commercial
zoning and the “d” represents a design overlay district.
Allowable Uses. In Euclidean zoning, the regulations for
each zoning district specify what uses may or may not be
constructed or undertaken on parcels within its boundaries.
The most common zoning ordinances present an exhaustive
list of land uses permitted in each zoning district. This format regulates how much density and the overall intensity of
development that is acceptable on each property in a specific
zone. In the zoning ordinance, a typical listing of land uses
allowed in the zoning district includes:
•• Uses permitted by right
•• Uses permitted by right if certain specified conditions apply
•• Uses permitted by special exception
•• Uses permitted by special permit
•• Uses prohibited in a specific zone
By-Right Development. In undertaking a project where
the use is allowed by right, the developer need only show
compliance with the requirements of the zoning ordinance.
Plan review to secure approval from the jurisdiction is
accomplished through the normal administrative procedures and the building permit acquisition, which will be
discussed in the review procedures section of Chapter 2.4.
The site plan application will simply need to provide the
information necessary to confirm that zoning conditions
are met, such as a density calculation, building setback,
building height, and parking.
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3128
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S.W
50 C h a p t e r 2
300
07-09-2018
Comprehensive Plan designation where
there are other corresponding zones
BASEMAP ACQUIRED
Major Public Trails
LEGAL DESCRIPTION:
State ID Map Boundary
05 - 2018
NW 1/4 SEC.
03 - 1S - 1E
City of Portland, Bureau
of Planning and Sustainability
0
100
200
300
SCALE IN FEET
N
3129
\\bpsfile1\support$\gis\Publish_Data\Zoning\Maps\zone_quarter_section_maps.mxd
Figure 2.3D
City of Portland, Oregon, zoning map.
Zoning districts are organized in a hierarchy for each
zone, so therefore, the more intense zone will generally allow
for more uses then a less intense zone. For example, if the
low-density zoning district allows for single-family homes,
the medium-density zone may allow for single-family homes
and townhomes. There are exceptions, however, because an
ultrahigh-density zone may not allow for low-density uses.
A single-family home may not be allowed in an urban area
where zoning is established for multifamily residential.
Special Exception. A special exception, or conditional use,
is a use permissible within a zoning district subject to the
fulfillment of requirements dealing with compatibility and
site-specific design. A special exception is one that generates
02_Land_CH02_p017-124.indd 50
additional impacts to the surrounding neighborhood and
warrants special consideration. For instance, often private
schools or dormitories are considered a special exception use
in residentially zoned districts. Some jurisdictions may also
allow for a higher density of development within a site if a
special exception is granted. Despite the potential for negative
impacts, the principle underlying the classification of special
exception uses is that, if properly regulated, they are acceptable uses in the zoning districts where they are permitted.
Special exception review is most often focused on physical appearance and compatibility of the use within the existing neighborhood of the property. Special exception uses
must adhere to the character intended by the zoning district,
23/03/19 11:56 AM
2.3
as well as comply with the community’s comprehensive
plan. They may be desirable to the community and add to
its convenience; however, due to possible impacts such as
noise, traffic, or visual disruption, most zoning ordinances
establish conditions that apply in the development of special
exception uses. Such conditions may include a limitation on
the hours of operation; additional landscaping, buffers, and
barriers that exceed the standard specified for the underlying
zoning district; and a specification of the activities permitted
in conjunction with a special exception use.
In contrast to uses allowed by right, most ordinances
specify that uses needing special exception approval must
comply with the community’s comprehensive plan. These
requirements subject the development proposals needing a
special exception to the broad variety of policies and restrictions in the comprehensive plan that might not otherwise
apply. These could include density controls, requirements for
specific environmental measures, constraints on traffic generation, or desirable public improvements. Frequently, these
apply even to existing land uses proposed for redevelopment
or expansion of long-established uses.
The authority empowered to grant rezoning approval is
usually the same one that grants special exception approval;
this may be the governing body, a planning commission, or
zoning hearing officer. These bodies hold public hearings to
solicit input from community residents on upcoming cases
being processed for consideration. Special exception approvals are specific to the applicant with such approvals usually
remaining valid for a specific period within which construction must begin and be diligently pursued.
Special Permit. Some jurisdictions administer a separate special permit process for accessory uses. Projects that
involve uses listed as needing a special permit, or designated
as an accessory use, must go through an approval process not
unlike the special exception. However, the governing body
frequently delegates its authority to a board of appeals or
zoning staff administrator.
Approval of a special permit comes with conditions
attached that, like a special exception, help to reduce incompatibility between the use and adjoining neighborhood.
Typically, opportunities for public hearings are incorporated into the process, allowing affected neighbors to voice
support or opposition to the issuance of the permit. Within
a residential zoning district a special permit use may include
a home office, home child care, or church, but likely will not
permit a commercial use such as a retail establishment.
When the intended use requires review as a special permit,
approval to allow that use in the zoning district usually rests
with an elected or appointed body, or both. These are most
frequently the jurisdiction’s governing body, its planning
commission, a zoning appeals board, or a zoning hearing officer. In addition to a review by the professional planning staff,
the governing body usually holds public hearings to assess the
attitudes of neighbors and civic groups toward the project.
Using plans prepared by the design team, the developer must
justify the appropriateness of the proposed use. The approving
02_Land_CH02_p017-124.indd 51
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Zoning 51
authority may impose conditions to lessen potential impact.
Following approval, the normal administrative procedures for
subdivision and construction will apply.
Prohibited Uses. Uses prohibited in a specific zone are not
allowed at all in that zoning district. In most cases, if a jurisdiction does not list a use as allowable it is prohibited. In this
case, the developer can only attempt to change the zoning
through the rezoning process to be able to change the zone to
allow for a more favorable use. In some cases, the ordinance
may list specific limitations of permitted uses. For example,
a home office may be allowed in a residential district but is
prohibited from selling goods as a primary use.
For example, Lafayette, Louisiana, includes an extensive
use table in their zoning ordinance, as shown in Figure 2.3E.
This designates allowance for every use category type in
every zoning district in the jurisdiction. The use table lists
if it is allowed by right, as a conditional use, as an accessory
use, or not permitted.
The “P” stands for by right: The use is permitted if it meets
the standards established in the zoning district, and any
other applicable standards of this chapter.
The “C” stands for conditional use: The use requires a conditional use permit approved by the Planning and Zoning
Commission.
The “A” stands for accessory use: A use customarily incidental and subordinate to the principal use or building and
located on the same lot with the principal use or building.
Empty spaces are not permitted: A blank cell indicates that
the use is not allowed in the district. A property owner who
wants to establish the use may apply for a rezoning to a district that does allow that use.
Dimensional Standards. Euclidean zoning includes dimensional standards for many details of project design for each type
of land use. Typical criteria identified in the zoning district text
include lot size, density, lot coverage, and bulk requirements.
These requirements strive to protect and preserve the character of the community by preventing overcrowding, reducing conflicts between neighbors and neighboring properties,
guarding against encroachment on streets, and intrusion of
traffic noise. Examples of these are listed below:
•• Lot size
○○
Minimum and average lot size and lot width
•• Density
○○
Maximum density or intensity of development.
○○
Usually expressed in dwelling units per acre for
residential uses.
○○
Expressed as floor area ratio (FAR) for nonresidential uses. FAR is further defined later in this
chapter.
•• Maximum lot coverage
○○
Defined either by buildings or by impervious
surface, often expressed as a percentage
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52 C h a p t e r 2
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F i g u r e 2 . 3 E Lafayette, Louisiana, use table.
•• Minimum required open space
○○
Usually expressed as a percentage and referring to
parcel area unencumbered by buildings or parking.
○○
Often pedestrian-oriented hardscape is considered open space especially in urban areas.
•• Setbacks
○○
Minimum front, side, and rear yard requirements
that define the building envelope. These refer to
the distance of building setback from property
02_Land_CH02_p017-124.indd 52
lines, including street rights-of-way, utility corridors, transit lines (most commonly rail), waterfront or other environmentally sensitive areas.
○○
In some communities with waterfront property, the
yard facing the water is considered the front yard.
○○
The ordinance may specify when structures or
architectural features, such as porches and steps,
may extend beyond the setback line. The front
setback line is also called the building restriction
line (BRL).
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2.3
•• Minimum lot width
○○
If the lot is shaped so that its width at the normal setback line does not meet the minimum
standard, the building restriction line is forced to
be located farther from the front property line,
to a point where the width complies with the
standard.
•• Maximum or average building height
○○
Defined in feet and/or stories
•• Angle of bulk plane
○○
Relationship between building height and property line.
•• Functional requirements
○○
The dimensional standards in the zoning ordinance may include off-street parking, landscaping, screening, buffering, and tree preservation
requirements.
Other development standards that may be required by
the zoning ordinance can be found in the Development
Standards Manual within the subdivision ordinance. See
Chapter 2.4 for more information about the Development
Standards Manual.
Special exception or special permit uses also have
minimum dimensional standards and criteria listed separately in the text that identify criteria for size and intensity, proximity to other properties or facilities, landscaping,
Figure 2.3F
02_Land_CH02_p017-124.indd 53
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Zoning 53
screening, and similar conditions that mitigate impact to
adjacent properties.
On occasion, a property has limitations that hinder the
ability to meet the required development standards. For
example, due to limitations of the size and shape of the property, the site might not be able to meet the minimum setbacks or minimum lot widths. In this case, the developer can
apply for a variance or variation to deviate from the zoning
ordinance’s development standards.
Floor Area Ratio. Floor area ratio is the relationship
of building floor area to parcel size. For example, on a
40,000-square-foot parcel, if the zoning ordinance allows a
maximum FAR of 1.0, then it would be possible to construct
a building or buildings totaling 40,000 square feet of floor
area. This could result in a single-story building covering the
entire parcel, a two-story building covering half the parcel,
or some other configuration. Yard and height requirements
affect the actual parcel layout. A FAR of 0.5 for this same
parcel would allow 20,000 square feet of building area. See
Figure 2.3F for an illustration of FAR.
Bulk Plane. Bulk plane is a representation of the relationship of building height to property line. This protects
adjoining property views or access to sunlight and airflow.
Figure 2.3G illustrates angle of a bulk plane as applied to
building restriction lines.
Ordinance Example. Figure 2.3H shows the dimensional standards for an “RS” zone in Lafayette, Louisiana. This includes lot
requirements, setbacks, and bulk plane for each parcel. Other
development standards are included in the notes on that page,
as well as throughout the zoning ordinance text.
Graphic depiction of FAR.
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p
MBH
EBH
90ç
LL
G
MYR
KEY
p Angle of bulk plane
LL Lot line
MYR Minimum yard requirement
EBH Effective building height
MBH Maximum building height
G Grade (finished)
F i g u r e 2 . 3 G Graphic depiction of angle of bulk plane, modified
from Fairfax County Zoning Ordinance.
Variances and Variation. Two processes that allow deviation from the existing Euclidean zoning ordinance are variances and variations. Most variance applications address
building setbacks or similar technical requirements of the
ordinance, due primarily to physical characteristics of the
site under development. Some districts do allow usage variances as well if the existing land uses have been modified and
changes to the zoning ordinance make it difficult to adhere
strictly to the current ordinance.
To be allowed a variance, it must be proven that complying with newly adopted ordinances impose a hardship on the
property owner, who is otherwise unable to use the property
in the manner permitted prior to the change in the zoning
ordinance. The hardship must be unique to the property, and
the variance must be necessary to allow reasonable use of the
land. A purely economic hardship does not normally satisfy
this “hardship” test. In the case that development standards
are not limited by the physical site or usage hardship, but
the developer wants to change other zoning regulations, like
increasing the FAR or raising the maximum building height,
then the negotiated zoning process or a rezone should be
investigated.
A variance to zoning ordinance requirements may be
granted by the governing body, a subordinate body to whom
that power has been delegated, or, in some jurisdictions, a
quasi-judicial body appointed by judicial officials. As with
other zoning applications, opportunities for public comment
are normally provided. The approving authority may impose
conditions on development. In granting the variance, it must
ensure that the variance will not change the basic character
of the area, weaken enforcement of the ordinance, or provide
precedent for future requests for variances.
02_Land_CH02_p017-124.indd 54
As with variances, variations must not provide precedent
for future requests; however, in the case of variations “practical difficulties” may be a consideration in the review of the
application. In general, the findings must conclude:
1. The granting of the variation will not be detrimental
to the public safety, health, or welfare, or injurious to
other property.
2. The conditions on which the variation is based are
unique to the property for which the variation is
sought and are not applicable generally to other
properties.
3. The variation does not constitute a violation of any
other applicable law, ordinance, or regulation.
4. Because of the particular physical surroundings,
shape, or topography of the specific property
involved, a particular hardship to the owner would
result as distinguished from a mere inconvenience, if
the strict letter of these regulations is carried out.
Additionally, nonconformity may exist as to a lot, use, or
structure that does not fully comply with the requirements of
the zoning district in which it is located. This situation often
occurs with the adoption of a new zoning ordinance, text or
map amendment after a subdivision plat is recorded, or when
a land use is legally established. In most instances, the goal
of the community and governing body is to eliminate nonconforming uses and/or structures. Where that is not practical, most ordinances also include provisions that explain the
conditions under which such uses may continue to operate
or expand (i.e., to certify the nonconforming uses).
There are two types of nonconformities: nonconforming as to standards and nonconforming as to use. The first
case addresses uses permitted in a district whose development standards have since been revised in the ordinance.
This frequently occurs in older residential areas or established commercial areas when the lot size, density, parking,
or setback requirements have been changed in the zoning
ordinance. Most ordinances usually allow these uses to
continue, provided they are not expanded or enlarged, but
require that future additions comply with the new requirements. For instance, a new garage added to a house would
have to comply with new setback provisions, unless a variance is granted. A developer constructing a new department store addition to an existing strip shopping center
might be required to adhere to the newer off-street parking requirements. In a zoning district requiring lot sizes of
one acre, the zoning ordinance will usually allow a house
to be built on a smaller existing lot of record; however, the
structure must otherwise fulfill all other requirements of
the ordinance.
Sometimes, the actual use of a property becomes nonconforming due to a recent zoning text amendment eliminating
the once-permitted use. Typical ordinance provisions allow
the use to remain, but prohibit any expansion or enlargement.
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Zoning 55
F i g u r e 2 . 3 H Lafayette, Louisiana, dimensional standards.
02_Land_CH02_p017-124.indd 55
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Repairs and limited reconstruction could be allowed. Some
communities also adopt a schedule to enforce the removal
and/or amortization of nonconforming uses that they consider undesirable, such as billboards or other signage.
Rezoning Process. The Euclidean zoning ordinance provides
details on the administrative and legislative approval procedures for uses requiring approval of a rezoning. These specify
the required information needed for the preparation of plans
and supporting documentation for rezoning and similar applications. Ordinance provisions also provide the methodology
for amendments to the zoning ordinance text. Text amendments can change development potential as much as map
amendments by (for example) allowing a use previously not
permitted in a zone. Both amendment procedures may be initiated by either the governing body or individual appellant. In
most cases the request for a change in zoning is evaluated with
the proposed use identified in the comprehensive plan. More
information about rezoning is provided later in this chapter.
Negotiated Zoning. The next type of zoning is the negotiated
zoning, also known as incentive zoning. This zoning maintains
all aspects of the Euclidean zoning including the ordinance and
maps, with allowable uses and dimensional standards. The difference is in the procedure during the approval process. While
Euclidean zoning is a simple yes or no, that a project does or
does not meet the specific criteria to allow for development,
negotiated zoning allows for more flexibility.
This negotiation can occur on a project by project basis,
or through a large-scale development project. Some jurisdictions choose to offer incentives to developers if they meet certain criteria. For example, the zoning may allow for an 8-story
office building, but the jurisdiction will allow a 10-story building if the developer includes a public park, meets green building standards, and/or donates to the local affordable housing
fund. This situation can be beneficial for both the developer
(higher returns even if there are additional up-front costs) and
the local jurisdiction (benefitting and providing for the public
good). Proffers are another way that an individual developer
may work (or must work) with the local jurisdiction to achieve
approval. Planned unit developments (PUDs), on the other
hand, may allow more flexibility for larger projects by working
with the local jurisdiction upfront.
Understanding the local jurisdiction is important to know
if they allow negotiation or offer incentives during the zoning process. Some explicitly require it while others may allow
it as an option. It will depend on the project and the goals of
the development program to determine if negotiation should
be pursued.
Proffers. A proffer (pro offering) mandates that the certain
conditions are volunteered in writing by the developer prior
to action on the zoning request. The proffers bind development of the property to a specific development plan or set
of textual conditions. In addition, the developer sometimes
offers to construct improvements to public and private facilities that minimize impacts generated by the development.
The developer may also provide operational and aesthetic
incentives to the community as a condition of approval. The
02_Land_CH02_p017-124.indd 56
final plan and proffers are usually the result of extensive negotiation with community residents and the governing body.
The proffered conditions must be in writing, be signed,
must relate to the zoning itself, and should be in accord
with the comprehensive plan as well. If the governing body
accepts the proffers and approves the zoning, the proffered
conditions then run with the land. Thus, both current and
future owners or developers are bound to the approved plan
and conditions agreed to during proffer negotiations. Any
change to the development plan or the actual proffers must
be accomplished through a subsequent rezoning.
The development team should work closely with the
developer and business team when writing proffers. In many
cases, the land use attorney will author the proffers, but may
rely on the expertise of the design team to determine realistic proffer requirements. If a proffer is not written well, it
may delay the project approval because interpretations are
required to understand the original intent. It is common
that different regulatory groups may have conflicting recommendations for proffers. For example, the city may want
the developer to install a new traffic signal at the project
entrance, but the state department of transportation may not
permit the installation—any conflicts should be discussed
between all stakeholders to ensure compliance is feasible.
Some examples of proffers are
•• Land reservation for a new park or school
•• Dedication of land for new street right-of-way
•• Installing new traffic signals at warranted intersections
•• Meeting green building standards
•• Contributing funds for local capital improvements or
affordable housing
Planned Unit Development. A planned unit development (PUD) is an example of a negotiated zoning process
that attempts to enhance both flexibility and functional relationships between land uses. With conventional zoning patterns, the developer cannot easily respond to the changing
market demand associated with shifts in the economy of the
region. Nor can the developer address consumer preferences
for other than a single lot size or housing type. Individual
projects, undertaken in a small geographic area, by different developers sometimes lack cohesiveness, causing community identity to suffer. In addition, the extreme separation
of land uses, which results from conventional zoning, often
places unnecessary stress on community facilities.
The PUD allows for a mix of land uses, building types,
and intensities that can be constructed within a single project. It is most often applied in larger tracts of land by a single
land developer. Many jurisdictions will set a minimum parcel
size that can be considered for a PUD. Typically, a PUD can
include single family detached, single family attached, multifamily housing, and light neighborhood-serving commercial
retail uses. In larger projects, local zoning regulations may
23/03/19 11:56 AM
2.3
permit other more intense uses like mid-rise residential, highrise residential, commercial office, and commercial retail.
PUD allows the design team the opportunity to consider the project design in a comprehensive manner, rather
than treat each use individually and separately. PUD also
presents design opportunities to address broad functional
relationships between land use and facilities. These include
entrances to the community, vehicular and pedestrian traffic, stormwater management, and other infrastructure systems. The design team has greater control over placement of
uses, their design, and access between them.
A PUD offers the opportunity to interrelate uses in a common and controlled setting providing more flexibility than
achieved under conventional zones. The team addresses elements that affect community appearance and identity with a
unified approach including architectural style, landscaping,
street furnishings, open space, and signage. This comprehensive approach to design and construction of PUDs may allow
for a more efficient use of the land. In addition to a greater
efficiency in the use of the land, the developer is in greater
control of product marketing and the business environment.
Due to this design flexibility, cost savings in streets and utilities design, construction, and maintenance may be a result to
benefit both the developer and public agencies. PUD processes
are more intense for the developer and the team, demanding
much more up-front work, more community outreach and a
lengthier time frame; the trade-off is increased flexibility and,
often, increased density or intensity of development.
Some communities delineate PUD districts on their zoning maps. Ordinance provisions set use and intensity conditions that reflect the district’s relationship to other uses.
Other communities have the zone “float,” and it becomes
fixed during the rezoning process, frequently where the
applicant has assembled several properties and the project
may take on the general characteristics of the underlying
zones. A PUD in an otherwise residential area may still be
required to incorporate primarily residential uses. However,
housing type and arrangement may deviate from conventional district requirements. Often, limited neighborhood
serving retail areas will also be allowed. PUD provisions may
also address areas intended to be primarily shopping centers,
industrial, and office parks. Some jurisdictions allow PUD as
a special exception, which most closely resembles the floating zone, with the processing and administration differing
among jurisdictions.
The flexibility associated with PUD often means that
the final development may be viewed as less predictable to the
community. Consequently, PUD approvals often involve
extensive review procedures, public outreach, and negotiated
designs that might discourage their application. Considerable
up-front design costs may be encountered by the developer,
with no guarantee of project approval. Frequently, PUD provisions also require the developer to provide additional onsite amenities, such as open space, recreational facilities, and
land for public facilities such as schools or fire stations. This
may be documented in writing during the proffer process.
02_Land_CH02_p017-124.indd 57
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Zoning 57
Performance Zoning. Performance zoning is another type
of zoning that may be encountered in a jurisdiction. Performance zoning provides flexibility by regulating the effects or
impact of land uses on surrounding properties through performance standards. Types of performance standards may
include number of vehicle trips, density, or noise levels of
the development. Performance zoning may result in greater
expense in up-front analysis, both for the community and
the developer. To community residents, it sometimes does
not provide the desired clarity and certainty with respect to
expected development. It is in performance zoning where
greater conflicts may arise in the interpretation and establishment of requirements for development with differing views of
development from the public and private sectors.
Form-Based Code. A form-based code is a land development regulation that fosters predictable built results and a
high-quality public realm by using physical form (rather than
separation of uses) as the organizing principle for the code. A
form-based code is a regulation, not a mere guideline, adopted
into city, town, or county law (https://formbasedcodes.org/
definition/.).
Form-based codes focus on the geometric conditions of
the form, placement, and design of the development more
than the use of the land. They are less concerned with
specific separation of uses for a property, like the Euclidean
zoning that enforces a segregation of uses. Form-based codes
instead allow for a mix of uses, such as residential on the
same block as commercial, to create holistic neighborhoods
and encourage walkability. This allows for more flexibility in
design by focusing on the arrangement of the building and
the coordination with public spaces.
Prescriptive in nature, form-based codes regulate development through specific standards dealing with the relationships
of building massing, proximity to streets, height, scale, and
other elements that help define the character of a community.
This provides greater predictability about the look and feel of
development and how it functions as a place. This also allows
developers a clearer understanding of what the community
seeks. Regulations and form-based code standards are represented in both diagrams and words, and are coded to a regulating plan that illustrates the intentions of the community.
This determines which standards apply to which properties.
The Transect is often utilized to organize the form-based
code. An illustrative example of the transect system is provided in Figure 2.3I. The Transect relies on a categorization
system that organizes all elements of the urban environment on a scale from rural to urban. This focuses first on the
intended character and type of place of each neighborhood,
and second on the specific mix of uses within. The Transect
has six categories, moving from rural to urban. It begins with
two that are entirely rural in character: Natural (protected
areas in perpetuity) and rural (areas of high environmental
or scenic quality that are not currently preserved).
Next are two transition categories. First is the sub-urban,
which is the most purely residential zone, with some mixeduse (such as civic buildings). Next is general urban, the largest
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RURAL
TRANSECT
RURAL ZONES
T1 NATURAL
Figure 2.3I
T2 RURAL
T3 SUBURBAN
T4 GENERAL URBAN
T5 URBAN CENTER
T6 URBAN CORE
Illustration of the Transect system. (Modified for color, courtesy Duany Plater-Zyberk & Company.)
category in most neighborhoods. General Urban is primarily
residential, but more urban in character (somewhat higher
density with a mix of housing types and a slightly greater mix
of uses allowed).
At the urban end of the spectrum are two categories which
are primarily mixed use: urban center (this can be a small
neighborhood center or a larger town center, the latter serving more than one neighborhood) and urban core (serving
the region—typically a central business district). The urban
core is the most urban zone on the Transect.
The form-based code document is similar to the Euclidean
zoning document. Both ordinances are legislative texts that
are a separately published title or chapter in the jurisdiction’s code of laws. If a jurisdiction has both Euclidean and
form-based zoning, these documents will usually be located
together in the zoning ordinance. The biggest differences in
the documents, however, is that the Euclidean zoning ordinance is mostly text and in a longer document, while the
form-based code is highly illustrative and usually is a shorter
document.
Form-based codes generally include the regulating plan,
public space standards, building form standards, administration, definitions, and may include other standards.
Regulating Plan. The regulating plan graphically shows,
applies, and places the regulations and standards established
in a form-based code. It provides a public space master plan
with specific information on development parameters for
each parcel within. The regulating plan is the map for the
area that depicts the zones where the different form-based
code standards apply. The area may be the entire jurisdiction, or it is common for jurisdictions to utilize form-based
codes for specific areas, such as a downtown. The regulating
plan may include subareas within the overall area. Different
subareas may have different form-based code standards and
specific regulations for each subarea. In addition, there may
be a more detailed regulating plan for each subarea.
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URBAN
URBAN ZONES
The regulating plan is similar to a Euclidean zoning map.
The regulating plan identifies each parcel with a classification
similar to Euclidean’s zoning districts. The difference, though,
is in the form-based code standards. The regulating plan zones
correspond to the Transect and are organized around the categories of the Transect. Sometimes instead of a zone, the regulating plan may depict a frontage type on the map and include
setback lines. The frontage type then corresponds to the categories of the Transect. The form-based code standards have
specific regulations that apply to each zone or frontage type.
In addition to providing specific information on the development parameters for each parcel, the overall regulating plan
shows how each lot or project relates to the surrounding neighborhood. The regulating plan may identify additional regulations and/or special circumstances for specific locations. The
overlay districts, which were previously defined in this chapter, may be included on a form-based code regulating plan. It
is common for an overlay, such as a historic district overlay, to
be included on top of the zones on the regulating plan.
For example, Figure 2.3J is the regulating plan for Flagstaff, Arizona. This plan utilizes Transect categories as zoning
classifications, shown as T3 to T6. Each zoning district is further described in the city’s form-based code. The subsequent
map, in Figure 2.3K, is the street regulating plan for Beaufort,
South Carolina. This plan prescribes the allowable street section. Geometric requirements for each street section focus on
character, lane widths, sidewalk elements, necessity of bike
lanes and other elements. An example of street sections from
the Beaufort, SC plan are provided in Figure 2.3L.
Public Space Standards. The public space standards are the
first form-based code standards that apply to the zones (or
frontage types) denoted on the regulating plan. These standards help to define the streetscape, or street-space, for the
form-based code area. This ensures a coherent streetscape
throughout and assists developers to understand the relationship between the public space and their own building.
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Zoning 59
Downtown Regulating Plan
Figure 2.3J
Flagstaff, Arizona, regulating plan.
The public space standards establish the rules, standards, and recommendations for the public realm, especially streets and sidewalks. This identifies the basic
configurations and street type specifications by addressing vehicular traffic lane widths, curb radii, sidewalk
dimensions, and on-street parking configurations. This
also defines the parameters for the streetscape including
the placement of street trees, sidewalks, and other amenities or furnishings.
More information about open spaces, or civic spaces,
identified on the regulating plan will be included in the public space standards.
02_Land_CH02_p017-124.indd 59
Building Form Standards. The building form standards are
the next form-based code standards that apply to the zones
(or frontage types) denoted on the regulating plan. These standards shape the streetscape through placement and form controls on buildings. They are intended to ensure that proposed
development is compatible with existing and future development on neighboring properties and produces an environment of desirable character. The standards establish basic
parameters governing building form, including the envelope
for building placement, and certain permitted or required
building elements as they frame the streetscape, such as windows, doors, stoops, balconies, front porches and street walls.
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BOUNDARY STREET
BLADENST
NORTH STREET
DE
P
RO
OT
AD
RIBA
UT R
OA
HERMITAG E ROAD
Historic District
D
LEGEND
Major Thoroughfare
Boulevard
Avenue
Neighborhood Street 1 - General
Main Street 1 - Primary
Main Street 2 - Limited
Commercial Alley
Neighborhood Street 2 - Yield
Neighborhood Street 3 - Lane
Figure 2.3K
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Rear Lane
Parkway
Low Impact Road
Rural Road
Military Roads
Other (specific street section)
Beaufort, South Carolina, street regulating plan.
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C.6.7
Specific Applicability
Curb Type
Right-of-Way Width
Traffic Lanes
(Pavement Width)
Movement
(Design Speed)
Parking Lanes (Width)
Bike Facilities
Sidewalk (Width)
Planter Type (Width)
Street Trees
Specific Applicability
Curb Type
Right-of-Way Width
Traffic Lanes
(Pavement Width)
Movement
(Design Speed)
Parking Lanes (Width)
Bike Facilities
Sidewalk (Width)
Planter Type (Width)
Street Trees
Zoning 61
Boundary St. 2B East of Ribaut Road
(ST: 66 ft - 76 ft)
• Boundary Street (from Ribaut Road to Carteret Street) intended to be applied as a final Phase 2 condition in the
redevelopment of the Boundary Street corridor. The preliminary
phase condition is illustrated in “Boundary St. 2A” above.
Curb
66 to 76 feet
40’
2 lanes (10 to 14 feet each)
Slow (25 MPH)
2 sides parallel parking (7 to 8 feet each)
Sharrows (10 to 14 feet shared lanes)
2 sides (16 feet each)
20’
5x5 PLANTER
SIDEWALK
Tree wells (5 feet by 5 feet)
DECORATIVE
STREET LIGHT
40 feet on-center max., overstory;
24 feet on-center max., understory or palms
16’
C.6.8
■
6’
7’-8’ 10’-14’ 10’-14’ 7’-8’
66’-76’
6’
16’
Burton Hill Rd.
(BLVD: 100 ft)
• Burton Hill Road - entire length; This ambitious street
section was designed due to the purpose this road
serves. It links residential neighborhoods to higher
MULTI-USE
PATH
intensity commercial and industrial development. A
private developer will be required to install the entire
street section only if developing a whole block bound by perpendicular streets on both the north and PARALLEL
PARKING
south edge of the development - along Burton Hill
Road. If developing less than one block, the only
portion required to be installed, per table 7.1.3, are
the portions of the street section behind the curb,
on whichever side of the road the development is
located on.
Curb
100 feet
40’
MEDIAN
SIDEWALK
20’
2 lanes (11 feet each)
Slow (25 MPH)
PLANTING
STRIP
2 sides parallel parking (8 feet)
Multi-use path (one side only)
Sidewalk on west side (5 feet),
Multi-use path on east side (10 feet)
Planting strip (8 feet) & Planted median (16 feet)
40 feet on-center average
7’ 5’
8’
8’
11’
16’
100’
11’
8’
8’
10’
7’
F i g u r e 2 . 3 L Example specific street sections from the street regulating plan of Beaufort, SC.
02_Land_CH02_p017-124.indd 61
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Building form standards also ensure that the buildings
cooperate to create a functioning, sustainable, block structure.
The established boundaries (the building envelope) within
which development can occur ensures that the buildings
relate to each other and form a functioning and consistent
block structure. This may include information on allowed
building types, frontage style standards, and block standards,
depending on the jurisdiction. These standards aim for the
minimum level of control necessary to meet those goals.
The building form standards may include rules for development and redevelopment on private lots. This includes
regulation of private frontages, the components of a building
that provide an important transition and interface between
the public realm (streetscape) and the private realm (yard or
building). An example of building form standards for a fuel
Figure 2.3M
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station is shown in Figure 2.3M. This building, for example,
establishes requirements for visibility of fuel stations and
maximum number of pumps.
More information about parking, allowable uses, signage,
and other building standards may be included within the
building form standards.
Administration and Definitions. An administration section
is commonly included within the form-based code. This
includes information on the jurisdiction’s application process
and the typical review process for plan approval. More specific
details about the form-based code process are also included.
A definitions section is included at the end of a form-based
code with a list of key terms to help understand the code.
Form-based codes are intended to be easy to understand and
easy to follow (compared to Euclidean zoning ordinances).
Beaufort, SC example of building form standards.
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2.3
Other Standards. Other form-based code standards may
be included in a form-based code that apply to the zones (or
frontage types) denoted on the regulating plan. These are
more detailed standards that support the code to achieve the
community goals. It depends on the jurisdiction what standards they include in their form-based code. Additional common standards include architectural standards, landscaping
standards, signage standards, and environmental standards.
Architectural standards: Architectural standards are used
to achieve a coherent and high-quality building design
throughout the form-based code area. These standards
govern a building’s exterior elements and set the parameters for allowable materials, configurations, and techniques. This may include information on massing, facade
composition, windows and doors.
Landscaping standards: Landscape standards control
landscape design and vegetation materials on property as
they impact the streetscape. Some jurisdictions will prescribe allowable tree species and quantity of trees required.
Signage standards: Signage standards ensure that signs
reinforce the existing and envisioned character of the
community. They control allowable signage sizes, materials, illumination, and placement.
Environmental standards: Environmental standards
control issues such as stormwater runoff, steep slopes,
tree protection, conservation and preservation,
viewsheds, and others.
Implementation. Form-based codes can be implemented
in a jurisdiction by being mandatory, optional, or floating. It
is important to understand and follow the requirements of
the local jurisdiction.
Mandatory: Mandatory form-based codes are required
to be followed. The existing Euclidean zoning has been
removed and replaced by the form-based code.
Optional: Optional form-based codes, also known as
parallel codes, exist alongside the Euclidean zoning. In this
case, the form-based code may be an overlay in a specific
area of the jurisdiction. But in that area, both types of zoning are available. The developer is able to decide if they want
to follow the Euclidean zoning or the form-based code.
Floating: Floating form-based codes are available but
only become fixed through the rezoning process.
2.3.3. Zoning Techniques
It is important to note that the types and techniques to administer zoning are used differently by jurisdictions, and sometimes
are different within a community itself. There may even be multiple types of zoning within a jurisdiction. Euclidean zoning may
exist throughout the community, but form-based codes may
be along one specific corridor that is in redevelopment. Some
jurisdictions may offer incentives, allow for negotiations, and/or
require proffers. In addition, some jurisdictions simply use
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these zoning types to separate uses and ensure compatible uses,
while others use zoning as a specific tool to promote growth.
Zoning techniques are different types of tools that can be
useful in addressing different goals with the community. A
comprehensive plan may recommend, through the land use
plan, to allow for higher density and the zoning ordinance will
be utilized to achieve this goal. For example, a jurisdiction with
a new bus rapid transit line may allow for the revitalization of
an old, low-density industrial park. Their comprehensive plan
will recommend transit oriented developments at that location, and the zoning can then adopt this recommendation into
the ordinance. The jurisdiction can achieve new development
and the community will grow because of the cooperation
between the comprehensive plan and zoning. Every situation
is different so it is important to be aware of the comprehensive
plan and to understand the local jurisdiction’s zoning.
2.3.4. Rezoning Overview
The development program of a project frequently differs from
the land use allowed by the zoning district in which the property is located. Consequently, the property may be rezoned to
achieve a more favorable zone during the entitlement process,
which is introduced in Chapter 2.4. The rezoning will occur
early in the land development design process before any project plans can be produced and development can proceed.
To accomplish a rezone, the developer must demonstrate,
through analytic and legislative procedures, that the proposed
zoning is suitable for the property and that the proposed use is
appropriate for the community. This usually requires conformance with the jurisdiction’s comprehensive plan. Even when
the comprehensive plan recommendations support the zoning
change, the developer must earn the support of the public for
a project that might dramatically alter their community, which
can be a formidable task. The site engineer should anticipate,
respect, and accommodate community input.
From the developer’s perspective, the most prevalent reason for rezoning is to increase the land use intensity for higher
yield and greater profit potential or change the use to meet
development goals. Developing a tract to the maximum density permitted by its existing zoning does not necessarily mean
profits to the developer. In fact, in some instances, development at existing zoning is not economically feasible. In a typical economic market, the highest and best use of a property is
a critical factor in setting land value and prices. This land value
is established based on the scarcity of developable land and the
demand for property of similar potential.
The community’s comprehensive plan and zoning ordinance play an important role in determining and subsequently
obtaining the highest and best use. For example, a community’s comprehensive plan might suggest that a 20-acre parcel
of land located near a commercial area should be developed
at a multifamily residential density of 15 dwelling units to the
acre. Although existing zoning allows one dwelling per acre,
based on the comprehensive plan recommendation, the land’s
highest and best use becomes multifamily; this instance represents a great case for rezoning. The land use value is based
on the demand for multifamily development, the scarcity of
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multifamily zoned land and the potential yield of the land.
The developer’s successful rezoning effort is essential to the
subsequent purchase of the land.
Other motivations exist for seeking rezoning approval. As
noted in Chapter 2.2, comprehensive plans often set a range of
densities and uses, rather than set a finite density. In evaluating
the plan to determine potential uses, the developer’s feasibility
analysis may suggest land uses for which there is considerable
market demand in the area. The situation may exist where an
intended use may be more compatible to the surrounding area
than existing uses or uses permitted by existing zoning. As
an example, a suburban office park with high vacancies may
be better suited for residential development. A rezone from
commercial office to residential would allow for more opportunities of development. Further, the developer may wish to
rezone because of a preference or bias for a particular type of
development. For instance, a residential home builder would
not be interested in a commercial property, but a rezone to residential would allow for the construction of new townhomes.
Success in the rezoning process depends in part on the technical analyses performed by the developer’s team. Consideration of the rezoning proposal, however, frequently takes place
in a politically and emotionally charged atmosphere. Private
sector motivations, including the development and business
community, and public sector policies often come into conflict
with each other and local citizens’ concerns and desires.
It is important to note that the design team will become
involved with the interpretation and application of zoning
ordinances and comprehensive plans. These interpretations
are central to evaluation of the development for approval. The
rezoning process represents a major up-front expense to the
land developer and comes with few guarantees that the goals
of that investment will be realized. In view of the legal requirements of rezoning, the development team relies heavily on its
legal consultants throughout the process. The attorneys in the
legal team are an essential component of the development team.
With the advice of an experienced land use/zoning attorney, a
developer ensures that technical procedures are adhered to and
that government actions are consistent with the requirements
of local, state, and federal laws. During the rezoning process, it
is typical for the legal advisors to take the lead.
Rezoning Process. A rezoning application will be provided
by the local jurisdiction. After that is submitted, initial public
presentations may be required to inform the public about the
project. Finally, a formal public hearing with the local planning commission is required where a decision (or recommendation to board of supervisors) on the rezoning will be made.
Review of the Application Submittal. The review procedures of the rezoning application vary, depending on state
enabling legislation, local development ordinances, and customs and practices. In general, applications are prepared in
accordance with specific development ordinance standards
that specify graphic information that must be included and
the format for presenting the proposal. Such information usually includes extensive analysis and studies addressing possible impacts of the new development on the community and
the demand for additional public services that it will generate.
02_Land_CH02_p017-124.indd 64
Standard application forms and information checklists
are usually available from the municipality’s planning or
zoning office. Public notice may be required by way of certified mailings and posting signs at the property. The notices
generally contain information regarding the proposed use,
scheduled public hearings, and the responsible staff person
of the coordinating agency.
The applicant must follow legislated administrative procedure to the letter as adversary groups can delay or terminate
projects where proper notice has not been given. State legislation generally mandates that action of the decision-making
body must be completed within a certain specified time
period including the holding of public hearings required and
the specific findings relating to the application.
Public Presentations. The development team may make
initial public presentations at civic association or neighborhood group meetings. These presentations will educate the
public on the project and the rezoning, and gain their confidence and support in the project proposal. The development
team is there to listen to the support and concerns from the
citizens and to devise solutions for the negotiation details.
Facts, figures, and graphics should be prepared to support the statements made at all meetings. In view of the various levels of understanding of the project and local process of
the audience, graphics should be simple and easy to read. The
development team should be flexible and, consequently, presentation graphics should imply that flexibility. The graphics
must be prepared and reflect honesty, and they must be drawn
in such a manner as to imply accommodation to change.
The development team should be prepared with answers
to issues that are increasingly common. Perhaps the most frequent issue is neighboring residents’ concern regarding the
compatibility of the proposed project with existing community. Concerns about traffic congestion are also high on the
list of neighborhood objections to new projects. Sometimes,
those objections are withdrawn when commitments are made
to change the proposal or provide concessions to the community. For example, a developer may proffer the construction of
a new traffic signal to accommodate the increased traffic volumes of a new retail site. As with all design modifications, the
cost of the proposed improvement must be balanced against
expected return or investment. However, if incorporating the
changes means that the plan is more likely to win the support
of the community and can be made more marketable at the
same time, rezoning approval is more likely to be granted.
After initial meetings, the development team should evaluate comments. Analysis will include the practical and financial costs of modifying the plan proposal. All parties will not
be satisfied with the final plan. However, if the majority of
the people can be satisfied and the development team has
made a legitimate attempt to accommodate outstanding
issues, the project stands a reasonable chance for approval.
2.3.5. The Public Hearing
The public hearing scheduled by the planning commission,
zoning hearing examiner, and/or decision-making body is
often the only formal opportunity for the development team
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2.3
to directly address the decision-making body to discuss their
intentions and the merits of the application. In addition, this
forum provides citizens with an opportunity to express their
opinions about the project.
The meeting sessions during which development application-related public hearings are held are lengthy in many
jurisdictions. For the governing body, the public hearings
may be part of an even longer agenda reflecting their role in
managing the municipality. These public hearings frequently
run for hours, as participants in each case present testimony
reflecting their position on the application.
Members of the commission or governing body usually
are given a packet of materials prior to their meeting reflecting the day’s agenda. These information packets are often
lengthy, with endless photocopies of development applications, maps and plans, engineering analyses, traffic studies,
staff analyses, and recommendations as well as letters of
support and opposition. Members often engage in lengthy
debate when discussing a rezoning application.
The legislative or governing body normally renders its
final decision at the close of the hearing. Their decision
will be based on the information presented by the applicant, advice from the staff, and commissions and testimony
from the citizens. If it is believed that continued negotiations
between the developer and the opposing groups can achieve
greater accord, the public hearing or decision may be continued or deferred to a later date.
The outcome of the vote is contingent upon the response
of the citizens and how sympathetic the board or zoning
administrator is toward the project. It is imperative that the
development team proceed through the various stages by the
book, for example, all applications at the state level and local
level filled out and properly submitted and proper notification given. The negotiation and hearing process for a rezone
may take several months and possibly several years depending on the complexity of the project and the number of citizen
groups involved. The developer should anticipate this time
frame and establish the purchase contract accordingly.
The governing body often suggests or imposes conditions
or modifications that the developer must incorporate into
its proposal subsequent to final certification of the rezoning
application. Further, it may direct the applicant to establish
communication with neighborhood groups to resolve outstanding differences, deferring its decision until the proposal
is modified. Zoning ordinances usually establish procedures
for anyone aggrieved by the decision of the governing body
to appeal that decision in the court system. A waiting period
is often established after an approval is granted in order to
allow appeals to be filed. Once this statutory period passes,
the rezoning stands final.
Comprehensive Plan Amendment. The rezoning must be
consistent with the local jurisdiction’s existing comprehensive
plan. If not, then a comprehensive plan amendment may be
required before the rezoning process can be initiated. A comprehensive plan amendment, as described in Chapter 2.2, is a
change to the comprehensive plan that a developer can utilize
to achieve a more favorable condition. This can be a lengthy
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Zoning 65
process and can significantly delay a project, compounding
the lengthy rezoning process. The combined amendment and
rezone may prove to be extremely costly to a developer.
For instance, consider a site that is currently zoned as low
density and the comprehensive plan recommends a medium
density, but the developer wants a high density. The developer will have to initiate a comprehensive plan amendment
to change the comprehensive plan from medium density to
allow for a high density. Then the developer would have to
rezone the property from low density to high density. Each
of these steps can take several months or years to obtain
approval, and this must be completed before any project
plans can be produced.
2.3.6. How to Use the Zoning Ordinance and Map
Most zoning ordinances can be found online, as well as their
amendments, at the local jurisdiction’s website. The accompanying zoning map can also be found online, usually with
the zoning ordinance link.
Many communities adopt text amendments on a continuing basis as the government responds to development trends
or constituents’ issues. If an amendment exists for the zoning
ordinance, they may be published separately from the primary
document. The development team must use the most current
version of the zoning ordinance and should be familiar with
the terminology used by the jurisdiction.
The land development team must be familiar with the
ordinance and terminology of the region where a project is
planned. Most zoning ordinances will have a section for definitions, where key terms are defined by the jurisdiction. Again,
it is important to remember the terminology may vary across
localities, such as the term “building height” may be a measure
from the lowest point of the building in one jurisdiction but
might be measured from average grade around the building in
another jurisdiction. The definitions of a use may also differ
across jurisdictions, and may be subject to interpretation.
Additionally, interpretations of the ordinance may need
to be documented. An interpretation provides clarification
of any text that was noted as ambiguous. For example, the
code may require that one tree is planted along a road for
every 50 feet of new road centerline. This could mean one
tree on each side of the road every 50 feet, or an average of
one tree per 50 feet of roadway (100 feet separation on each
side). An interpretation of the ordinance would serve to document the resolution of the ambiguity so that it is enforced
consistently. Ideally, the zoning ordinance is later clarified to
remove the ambiguity.
Development Program. When considering sites during
the due diligence of the site selection process, it is crucial to
understand what each property is zoned as. This will require
reading the zoning ordinance to see what kind of zone it is,
what the allowable uses are, zoning dimensional standards,
and other regulations contained in the ordinance. This will
define the development potential of the site.
The development program must be considered with the
property’s zoning. A favorable zoning will allow for an easier approval process. Choosing a residential property for a
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commercial development, on the other hand, would require
a rezone that could prove costly for the developer. Likewise,
choosing a site zoned under a form-based code or initiating
a PUD, may allow for more flexibility for the project. This
could be great for the development program, as long as the
developer is prepared for additional up-front costs. Each
zone must be thoroughly considered during due diligence to
best achieve the development program.
Jurisdictional Challenges. It is common for a developer
and their team to operate in several jurisdictions. Consequently, they will encounter a broad variety of plans, ordinances, regulations, policies, and procedures that all can
affect project design, economics, and plan approval.
Figure 2.3N, depicts the Washington, D.C., metropolitan area surrounded by two states: Maryland and Virginia.
Each state operates under a different legislative process and
constitutional mandate. Within those two states, there are 13
adjacent or nearby counties, 4 major cities, and a significant
number of independent towns and villages of varying sizes.
Each of the jurisdictions of the region is unique in its economic base, its citizens’ attitudes, and its government’s priorities. Perhaps more to the point, each of these jurisdictions,
within the metropolitan area, has its own unique set of comprehensive plans and zoning regulations. The breadth and
expansion of the Washington market is not unique; similar
situations exist in numerous metropolitan areas.
Differences in a local government’s size, budget, and staff
also have broad implications for the development team.
These differences not only affect the number of public officials and agencies the development team must encounter, but
also the level of scrutiny given to the proposed rezoning documents or development plans. As the government’s size and
structure grows so does the potential for conflict between its
administrative and elected officials. Each official or department manager reviews rezoning and development proposals
with a different constituency or agenda in mind. Each has
competing needs and generates competing conclusions. The
environmental staff may not agree with the transportation
staff as they review a plan that widens a roadway by impacting undeveloped land. These conflicts bear directly on the
time it takes to secure zoning approval.
As a jurisdiction matures, political or civic leaders often
demand more exacting standards for development and
construction. The government responds by tailoring zoning regulations and procedures to address new priorities.
Sometimes, these new rules and standards take effect even
while a proposed zoning project is well underway. These new
standards and/or regulations, coupled with increased review
time and sometimes circuitous approval procedures, can
increase the future land development costs dramatically. The
land development team must frequently adjust design and
construction budgets during the rezoning process.
Given these differences, the site engineer must become
familiar with the comprehensive plan, and also the zoning
documents and other regulations, in effect in each jurisdiction within which the professional operates; otherwise plans
and/or proposals may not win government approval. It is the
site engineer to whom the developer looks to be an expert on
local policies and the controls applicable to each new project.
Even site engineers with prior local government work experience must keep track of newly evolving priorities, policies,
regulations, and standards.
As government workload increases and operating funds
decrease, development review staffs are forced to limit their
efforts to technical review. They are unable to devote considerable time or energy to shepherd applicants through the
jurisdiction’s regulations or review procedures. This places
a heavier burden on the site engineer to develop independence and expertise in the local land development review
and approval process. The plan reviewers are not responsible
for plan conformance—the site engineer and applicant bear
that responsibility. If noncompliance or nonconformance is
identified late in the design process, it can have a detrimental
impact on project schedule, design, and cost.
Developing a clear understanding of local plans, policies, and requirements may seem a monumental task. It is
impractical for one person to be sufficiently familiar with all
local regulatory programs in the region. Acquiring an overall
familiarity with the basic structure of government function
and regulations makes it easier to assimilate this information.
REFERENCE
F i g u r e 2 . 3 N Jurisdictions in the Washington, D.C., metropolitan area.
02_Land_CH02_p017-124.indd 66
Form Based Code Institute website, http://www.formbasedcodes
.org/, accessed October 2007.
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Chapter 2.4
Subdivision Ordinance, Review
Process, Building Codes, and
Development Costs
2.4.1. Introduction
In addition to the comprehensive plan and zoning ordinance, development is subject to other local regulations.
These should be evaluated during the pre-design stage to
ensure project success. First is the subdivision ordinance,
which is similar to the zoning ordinance by establishing a
legal requirement for conformance. A primary focus of the
subdivision ordinance is the modification to parcels through
subdividing a large parcel into smaller parcels or by consolidating multiple parcels. Therefore, when considering the
feasibility of a project, it is important to first determine if a
subdivision or consolidation of the property will be required
to achieve the project goals. The subdivision ordinance also
provides additional requirements to guide development
in the jurisdiction. In addition, the subdivision ordinance
defines the review process of site plans for the jurisdiction.
Other regulations to consider that could affect a project are
building codes and associated development costs imposed by
a jurisdiction.
A site plan, the development design document, demonstrates conformance with subdivision ordinance and other
development regulations. The site plan must be reviewed
and approved as part of the subdivision process. Its form
and purpose are usually prescribed in the various ordinances administered in the community, and the level of
detail required may vary among ordinances even within a
single locality.
Together, site plans and building codes play an important role in assuring proper review of proposed development
projects and ensuring the safety and structural integrity of
proposed buildings. In many jurisdictions, site plans are synonymous with subdivision plans and require only an administrative review. In contrast, some jurisdictions require site
plans to undergo greater scrutiny of not only an administrative staff review, but also the governing body, with public
hearings as part of a very formal approval process.
The underlying purposes of land use ordinances discussed within this chapter are to protect the community at
large from negative impacts of a forthcoming development
and to ensure that time tested standards are honored in the
construction of new structures. Like comprehensive plans
and zoning documents, a great deal of effort is required to
implement and enforce subdivision ordinances, site plan
approval, and building permits. Local governments are
invested in this effort because they retain a vested interest
in knowing that its residents and business operators are provided with safe, durable, and efficient developments. Land
is an exhaustible resource and its development is a primary
generator of revenues needed to provide public services and
maintain fiscal health. Government, therefore, assumes an
important role in protecting that resource and ensuring that
development projects remain marketable and a continued
community asset.
2.4.2. Subdivision Ordinance
Like the zoning ordinance, the subdivision ordinance is a
legislative text that is adopted by a local jurisdiction and is
included within the jurisdiction’s code of laws. Sometimes the
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subdivision ordinance is included alongside or with the zoning
ordinance. It is important to note the difference between zoning and subdivision ordinances: whereas zoning ordinances
regulate the permitted use of the land and the spatial relationships of those uses, subdivision ordinances specify the policies, procedures, and standards by which the infrastructure
systems necessary to support the use are physically created.
In 1928, the U.S. Department of Commerce created the
Standard City Planning Enabling Act and granted local
municipalities the right to regulate the subdivision of land
to “provide for the proper arrangement of streets in relation
to other existing or planned streets and to the master plan,
for adequate and convenient open spaces for traffic, utilities,
access of fire-fighting apparatus, recreation, light and air, and
for the avoidance of congestion of population, including minimum width and area of lots” (Department of Commerce).
Items such as grading, utility construction, and bonding were
also established as a part of the act and were permitted to be
conditions precedent for subdivision plat approval.
As a condition of plat approval, municipalities use these
subdivision ordinances to control street patterns, clustering
of housing, placement of public infrastructure, and availability of open space. As subdivision ordinances evolved, approvals for land subdivision included exaction requirements and
finally the addition of impact fees. These are discussed later
in this chapter with development costs. Subdivision approvals are also tied to adequate public facility requirements for
schools and water, sewer, police, and fire-protection services.
Finally, the latest addition to many subdivision requirements
is the allocation of perpetual easements that set aside natural
resource areas for public benefit.
Subdivision Plan. The subdivision of land, as defined by
most local subdivision ordinances, is the division of land
into two or more parcels. In addition, most ordinances also
cover the consolidation of parcels into larger areas. Jurisdictions may include condominium and cooperative ownership as forms of division where both horizontal and vertical
divisions may take place. In these cases, both the land and
building may be subdivided. Long-term leaseholds are also
frequently included in the definition as well.
As an example, a developer may be interested in purchasing a large tract to construct a new residential community. The
zoning may allow for the proposed use (single-family homes),
but as a single tract of land the property is suited for only one
home. A developer may subdivide the land so that it still conforms with the zoning but establishes additional lots for more
homes. The zoning ordinance may identify the minimum lot
size for the use, but it will be the subdivision ordinance that
will set requirements on the geometric configuration of the
new lots, the design standards for roads within the subdivision, tree preservation requirements, utility requirements for
the community, and other design conditions.
Local governing bodies impose standards and procedures
for the subdivision of land with the adoption of regulations in
the subdivision ordinance. Like the zoning regulations, subdivision regulations are an exercise of the local government’s
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police powers. They are intended to protect the health, safety,
and welfare of its citizens and facilitate community growth
in a controlled manner. Subdivision regulations are an
important mechanism to ensure that proposed development
complies with the requirements of the zoning ordinance. In
addition, the subdivision ordinances provide protection to
purchasers and users by ensuring that subdivided lots are
suitable for their intended purpose.
A subdivision plan must be submitted and approved by
the jurisdiction to allow for the subdivision or consolidation.
This plan includes a plat that shows the original boundary
survey of the parcel (to be discussed in Chapter 3.2) and
the proposed subdivided (or consolidated) parcel. Any open
space parcels, requisite easements, or right-of-way dedications for public infrastructure improvements are also shown
in this plan. The production of a subdivision plat is usually
performed by a surveyor in coordination with the site engineer. This occurs after schematic designs are produced but
before final construction documents are completed (as are
described in Chapters 4 and 5). Figure 2.4A shows an original boundary survey for a site and then a proposed subdivision plat.
Development Standards Manual. The Development Standards Manual is often included with the subdivision ordinance. This manual may be a section contained in the
ordinance or an independent/separate document. The
manual may be referred to as a Design Standards Manual,
Engineering Standards Manual, Facilities Standards Manual,
Manual of Standard Specifications, Manual of Practice, or
some other variation depending on the jurisdiction. The
Development Standards Manual is a supplement to the subdivision ordinance to guide development in the jurisdiction.
In addition to being required for projects under subdivision plan review, some zoning ordinances require compliance with the Development Standards Manual as well (more
information is provided with site plan review later in this
chapter). It provides policies, standards, and design details
for the various methods and requirements for construction
within the jurisdiction.
The Development Standards Manual performs several
specific functions. Of paramount importance is the adoption of
design, construction, and material standards for facilities that
serve and support the project and continued development of
the larger community. In many cases, the Development Standards Manual will reference national or state design regulations
and details. Although jurisdictions vary on the listing of
systems addressed by their development standards, the most
typical features, systems, and facilities are listed below:
•• Street design
•• Parking geometry
•• Lot size and geometry
•• Utility design and distribution
•• Sidewalk and trail design
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(a)
(b)
F i g u r e 2 . 4 A The above plats show (a ) the original boundary survey of the parcel containing a single dwelling unit and (b ) the subdivision
plan including eight lots and three (open space) parcels as well as requisite easements and ROW dedications for public infrastructure improvements.
Note: Plats have been modified for content and are intended as illustrative of the subdivision concept.
•• Easement requirements
•• Signage
•• Well and septic systems
•• Survey standards
•• Landscape and buffer requirements
•• Open space requirements
•• Protection of environmental and historic features
•• Erosion and sediment control
The Development Standards Manual requirements
attempt to offer protection to the consumer, who has a perception of the quality, design, and functionality of a project.
It provides assurance that systems will be adequate and
working at the time of purchase and that the investment is
sound relative to public infrastructure. Standards provide for
safe and proper design of transportation facilities and other
critical public facilities, such as adequate and safe water supply and sewer capacity.
The jurisdiction desires protection because it will own,
operate, and maintain many of the infrastructure facilities
provided by the developer. By setting minimum standards,
the jurisdiction can better predict lifecycle maintenance
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costs and avoid costly reconstruction of otherwise less durable facilities. Maintenance and repair will be made easier
because the ordinances provide for a record of the design of
roads and drainage facilities. Final construction documents
record the location of most underground utilities. In addition, the development standards help achieve continuity and
compatibility of systems. Since a residential or commercial
subdivision is part of a larger neighborhood and community, the infrastructure and public facilities must coincide
efficiently with the entire pattern. Not only must new development connect with the existing community, it must anticipate, and in some cases serve, future development.
Public infrastructure design criteria and construction
standards ensure that the public facilities and transportation network are compatible from one project to the next.
Provisions in the Development Standards Manual address
most components of those systems to ensure that individual projects do not disrupt the community. For instance,
improperly designed and located driveway entrances and
intersections can create hazardous driving situations, which
may severely impair the efficiency of the transportation network. Similarly, stormwater management systems can cause
flooding and damage on both upstream and downstream
properties if those properties are not considered in the
design. Every component of a development project has an
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effect on neighboring properties and the surrounding community. During the construction of one project, uncoordinated development on adjoining properties can have severe
consequences.
Related Development Ordinances. All levels of the development community (local, national, and even global) have
experienced a heightened concern with respect to the effect
of land use on the environment. To respond to this concern,
stormwater management ordinances, grading ordinances,
erosion and sediment control ordinances, and resource
ordinances have been developed to reduce the impact on
the environment. These related development ordinances are
either included as a part of the subdivision ordinance, within
the Development Standards Manual, or adopted as an independent ordinance. The provisions within require greater
care in both design and construction of site facilities and
must be considered early in a project.
Local, state, and federal legislators focus on the impacts of
land use on both ground and surface water quality and quantity. Water quality has been found to be degraded not only
by sediment runoff, but by the many chemical and metal
byproducts of both agricultural and urban activities. Related
to water quantity, continued attention has been focused on
the flooding caused by construction activities, including the
clearing, grading, and subsequent paving required to develop
land, and the inadequacy of design efforts and regulatory
practices for stormwater and site runoff.
Early storm drainage requirements led to the design of
systems intended to collect and discharge site runoff quickly,
without regard to the downstream property. Recognition of
the negative effects of this type of system for many applications has led to the adoption of stormwater management
ordinances requiring on-site collection, trapping, and slow
release or infiltration of storm runoff. Stormwater management facilities have become a common feature in land development, particularly in ecologically sensitive areas or coastal
watersheds. Coupled with greater interest in the use of natural features, such as wetlands to filter runoff, greater investigation on environmental effect, including water quality
and quantity, is now required by many communities. These
effects are further discussed in Chapter 2.5.
Concern about sediment damage and sediment runoff
from construction activities, other land disturbing activities, and agriculture has led to the adoption of grading
ordinances and erosion and sediment control ordinances.
Many jurisdictions also have tree protection, stream preservation, or other similar resource ordinances. For example, the Mississippi River has an intercounty conservation
regulation for water protection. These resource ordinances
include water quality regulations that are designed to protect existing natural resources by requiring preservation
of existing river buffer areas. Additionally, the ordinances
may require the creation of new buffers if little or no forest
currently exists on a property. Minimum vegetative coverage thresholds have been established based on the land use
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designation of the area to be developed. The regulations
allow for the categorization of existing and proposed forest
areas, which allows for the protection or enhancement of
higher priority areas over the areas that are deemed lower
priority. The result is that considerable forest areas have
been protected or reestablished as a part of the subdivision
process. These types of preservation ordinances are further
discussed in Chapter 2.5.
These issues and other development ordinances have led
to an evolution in site planning practices. Sensitive environmental features now play a more central role in the project
layout. Where they must be preserved, either by regulation
or by good design, environmental features are a key factor
in decisions about project feasibility. Additionally, the site
engineer must consider the size and placement of permanent stormwater facilities during the design process. Rather
than being viewed simply as constraints, creative development teams have begun to view environmental features as
an opportunity to enhance project appeal and marketability.
In some regions, the link between land use and the environment has been made stronger where the latter’s degradation has been shown to threaten a significant economic
resource. Comprehensive programs to protect these features seek to balance private property development rights
with broad public and economic concerns. Two examples of
this are in Texas, where protection of groundwater recharge
areas is a high priority, and in the mid-Atlantic’s Chesapeake
Bay region. In the latter case, administrative subdivision
reviews are being supplemented, and in some cases, supplanted with more extensive discretionary reviews. The goal
is to ensure that land disturbing activities are kept to absolute minimums. The land development team must design
protective devices with greater efficiency to prevent pollutant and sediment damage.
How to Use Subdivision Ordinances. It is necessary to
obtain the most current version of the subdivision ordinance. In addition, it is important to identify and compile
other ordinances, related regulations, or supplemental documents referenced in the subdivision ordinances that control
the physical development of land, including the Development Standards Manual and all related development ordinances. The community amends all of these ordinances and
documents from time to time, either by administrative or
legislative action with the latter being preferable.
Note the effective dates of ordinances and amendments,
as they may have different applications or may exempt preexisting conditions. Policies concerning existing recorded plats
for otherwise undeveloped subdivisions vary. These plats
sometimes predate even the earliest version of ordinances, or
more restrictive ordinances that might be in effect. Whether
or not a project can proceed and what required public
improvements apply varies depending on the jurisdiction. In
some jurisdictions, plat approvals have an expiration date if
no development takes place. Grandfathering is an exemption
granted to projects from later ordinance changes.
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It is important to identify possible exemptions for certain
types of developments. Among those often excluded are socalled “minor” subdivisions that sever one or two parcels from
a larger tract. An ordinance or a local policy may define a tract
using the parcel boundaries as they existed as of a certain date.
Family conveyance provisions may allow for a certain number of gift lots to be given to immediate family members. The
ordinance may exempt large lot subdivisions and permit estate
or agricultural use of these properties. Boundary adjustments
between adjoining properties and divisions caused by government condemnation are also frequently excluded from full
compliance with subdivision ordinances. Simple plat preparation and recordation procedures often apply to these exempted
divisions of property; however, minimum standards to protect
public health and safety may also apply.
When reviewing the subdivision ordinance, determine
the procedures for administrative waivers of certain requirements, appeals, and judicial challenges to administrative
or legislative actions by agency personnel or the governing body. Identify provisions for execution of performance
bonds and bond release procedures. These include preparation of as-built drawings.
The subdivision ordinance outlines permit processing
requirements that should be verified. Identify sections of the
ordinance that address site planning and design criteria, construction standards, and permit application requirements.
Required submission dates, schedules, and fees should also
be documented. More information about permit processing
is discussed during the review procedures in this chapter as
well as in Chapter 6.
2.4.3. Review Process
The review procedures for a development project in a jurisdiction are usually included with the subdivision ordinance
or another development ordinance. In most municipalities,
five general stages of review and approval can be expected as a
part of the development review process. It is important for the
developer and development team to recognize that the timing
of the stages can take several months to several years of processing depending on the nature of the project and the administrative requirements of the jurisdiction. Understanding the
development review and approval process a municipality uses
cannot be overstated. While the names of the stages are different in various parts of the country, the underlying intent and
staging is fairly common and proceeds as follows:
Entitlement review includes the jurisdictional review
of concept plans (optional in some jurisdictions) and
preliminary plans for the purpose of processing one
or more of the following: subdivision plan, comprehensive plan amendments, and/or rezoning efforts.
This could also include review of special exceptions
and variances under Euclidean zoning, planned unit
development approval under negotiated zoning, a
form-based code review, and more. Approvals at this
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stage establish yield, floor area ratio, type, size and
location of structures, streets, lots, parking, stormwater facilities, utilities, open space, and landscaping,
and the provision of exactions or proffers for public
improvements (see Development Costs later in this
chapter). Entitlement approvals are not a guarantee of
site plan approval, but designate the literal intent or
spirit of what is to be expected during all subsequent
stages of a project review. The concept and preliminary plans for the entitlement review are developed
during the conceptual and schematic design stage. See
Chapter 4 for more information about the production
of these plans.
Site plan review includes the jurisdictional review
of final site plans and/or final subdivision plans (if
applicable). A subdivision review may be required
separately. This book focuses on the site plans, which
include detailed engineering, landscape architecture, and surveying plans. These plans are based on
the approvals received during the entitlement review
process. The purpose of this stage is to finalize how
each of the development components relates to each
other and what will be constructed. The entitlement
stage is predominantly planning intensive, while the
site plan stage is engineering intensive. See Chapter 5
for more information about producing the final site
plan design documents.
Plat preparation, approval, and recordation in the land
records occur once development review agencies are
comfortable that all subdivision requirements have been
appropriately documented in accordance with local
development ordinances. More information about these
required plats is given in Chapter 5.1.
Permit acquisition is the stage in which environmental,
building, and other permits are issued. An approved
final site plan is necessary to acquire these permits. In
addition, construction cost estimates are approved, and
letters of credit or bonding requirements are fulfilled.
Permits are discussed in Chapter 6.1.
Closeout is the final stage of development and entails
as-built drawings, final construction inspection and
approvals, public street acceptance, and the return of
letters of credit or bond release. See Chapter 6.2 for more
information about closeout.
Entitlement Review. Some development projects can
take place without the necessity for an entitlement review.
Often, though, the entitlement process precedes the
detailed final design of a site plan. It should be understood,
that approval of an entitlement review is insufficient to initiate construction. Detailed design and structural analyses
are rarely incorporated to a sufficient degree in the entitlement process.
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The engineering analysis performed for a site plan in final
design often reveals that the assumptions used in the entitlement submittal were insufficient, sometimes are imprecise,
and in some cases incorrect for final design. This is because
of the differing level of detail each type of approval requires.
Because of the risk and expense involved in achieving this
level of accuracy in the entitlement stage with the more speculative legislative rezoning application, many land developers are reluctant to incur the costs associated with greater
exactness. Some jurisdiction may accept basic plan submissions for entitlement review and processing, while others
seek a formal commitment on the site design. Modifications
to the site layout or use of the site, after entitlement approval,
may require reprocessing the entitlement application.
The entitlement process is not intended to be the approval
process to begin construction. The entitlement process is
merely intended to designate the allowed use of the land
under the circumstances portrayed in the plans and other
documents submitted in support of that process. Compared
to final site plans, the design information included in the
entitlement process is rudimentary. Much more detailed
site plans are necessary for permit and construction. These
detailed engineering site plans must also be approved by public agencies prior to the issuance of a permit, as the site plans
must contain instructions and requirements to be adhered to
and met by the contractor to ensure protection of the public.
Each jurisdiction has administrative requirements and
procedures that govern the various site plan submittals, which
are usually contained within the subdivision ordinance. Failure to follow these requirements can at best result in a delay
in the site plan’s approval. It is possible for the site plan to be
disapproved if it fails to adhere to the administrative requirements that a jurisdiction desires to see in the design information. Most jurisdictions welcome a pre-application meeting
with staff so that the types of site plans needed and the application process are understood. More information about the
submission process is given in Chapter 4.1.
The land development design process is often lengthy and
arduous. This is especially true in developing urban areas.
Some minor development plans take as long or longer to
follow the review process than it has taken to develop the
design. The time schedule for the review process needs to be
considered when planning a land development project. This
will avoid costly surprises and provide a realistic timeframe
under which the client can anticipate approvals and the ability to procure required permits.
Site Plan Review. After the entitlement review, the site
plan review is required by the jurisdiction for permit issuance. This review ensures that projects are accorded the
same level of review and are subject to the same standards
of performance as subdivision plans and meet jurisdictional
requirements. These plan reviews are intended to verify conformance, and not meant to provide a formal constructability
review—that responsibility falls on the professional engineer.
In communities without site plan review authority,
the only requirement for development may be to show
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compliance with the zoning ordinance and building codes
(to be discussed later in this chapter), assuming that there
is no entitlement review required with the project. Absent
controlling regulations seen in subdivision ordinances, onsite private facilities are designed at the discretion of the land
development team, with only a cursory review during the
building permit stage (as is the case in Houston, Texas).
The site plan review may require compliance with subdivision regulations, including the Development Standards
Manual. The site plan ordinance for each jurisdiction
describes the information that must be shown and the parameters within which it must be reviewed. In these jurisdictions,
the site plan review has a special role in securing development approval. It is neither a technical subdivision plan-type
document given an administrative staff review nor an elaborate rezoning application given extensive public hearing by
the governing body. It is somewhere in between, and usually
applied in circumstances demanding greater control to guard
against incompatibility.
In some cases, developments are subject to both the subdivision and site plan review processes, which may result in
repetitive reviews. In such cases, the subdivision review processing is aimed at ensuring compliance with the zoning ordinance, producing durable records of ownership, and proper
consideration of fees and exactions. The site plan review process provides control of on- and off-site infrastructure, access,
and other features to ensure compatibility and connection
with the larger environment.
In Montgomery County, Maryland, for example, the site
plan review is used when the zoning ordinance specifies some
discretionary authority in the physical arrangement of a project. One of the county’s ordinances, for instance, calls for the
mandatory provision of affordable housing in projects with
more than 50 dwelling units with an automatic density bonus.
Normally, a site with residential zoning would only require
subdivision and construction plans, approved administratively.
However, the increased density triggers the requirement for a
site plan review. This gives the professional staff and the county’s
Planning Board greater leeway to consider a project’s internal
physical arrangement and relationship to its neighbors.
The Montgomery County, Maryland, process contrasts
with nearby Arlington County, Virginia. In Arlington County,
as part of the approval process, both the Planning Commission
and County Board review the site plans and hold public hearings. Although the county is limited in the off-site improvements that it can require from the developer, site plans are
given much the same public scrutiny as a rezoning application.
Again, it depends on the local jurisdiction and their
review process if they require and how they administer the
site plan review. Some municipalities even require several levels of review within their site plan review process. This book
assumes that the site plan review is required. However, in general, this book refers to “site plans” as the design documents,
not the review process. Site plan review will be explicitly stated
when being discussed. More information about producing site plan design documents is given in Chapter 5. More
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information about submitting these plans for final review is
given in Chapter 5.1.
Benefits to the Development Team. Development ordinances, including the zoning and subdivision ordinances,
impose considerable cost and time constraints on the developer; however, when consistently applied by local officials,
the ordinances can also offer protection to land developers.
Land development is an extremely competitive industry in
which many firms and individuals operate. Development
ordinances establish a uniform standard of plan processing,
design, and construction to which all developers must adhere.
In addition, the ordinances offer a predictable framework for
allocating development costs. Nearly every jurisdiction has
its own nuances to zoning and subdivision ordinances that
should be evaluated prior to starting a project.
Most subdivision ordinances also set maximum time limits
in the review procedures within which the local government
must act to approve or disapprove the submittal. Where properly applied, these ordinances can guard against costly project
delays and affirms that plans will be reviewed in a timely and
predictable manner. Such provisions also ensure that jurisdictions maintain adequate manpower levels based on projected
workload volumes. Mandatory time limits also prevent unfair
treatment of developers, since reviewing authorities cannot
set aside a plan viewed to be unfavorable for an indefinite
period as a tactic to discourage or delay a project.
For the site engineer and other members of the design
team, the development ordinances define essential parameters that help to guide the project site design. In some
regards, they eliminate the need to perform original research
and analysis, expediting project design and budgeting. Information concerning plan processing also guides in the preparation of project work programs and budgets.
2.4.4. Unified Development Codes
Unified Development Codes, or Community Development
Codes, consolidate all development-related ordinances and
regulations within a jurisdiction. This document will include
all components of the zoning ordinance, subdivision ordinance, Development Standards Manual, and review procedures. By having everything contained in one document, the
Unified Development Code helps to streamline the development process by removing overlaps and having a more convenient document for all applicable regulations. It is important
to be aware if a jurisdiction has a Unified Development Code
or separate development ordinances.
2.4.5. Building Codes
Building codes are important to consider during pre-design.
These codes must be followed to ensure that development is
viable and the buildings themselves can be occupied. Following these codes will be required to obtain building permits,
which are discussed in Chapter 6.1.
Building codes are adopted to protect the lives of building occupants and to guarantee safe, habitable, durable
structures. The review processes for building codes and
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building plans have been established nationwide to ensure
the health, safety, and welfare of future occupants of a
structure. There are a number of nationwide building
codes that are routinely used as the foundation for local
jurisdictions to augment, in order to meet and reflect local
concerns and issues.
While most jurisdictions adopt and modify one of the
national model codes published by long-established building
code organizations, some develop extensive amendments to
reflect local concerns and experiences.
With a few important exceptions, the building codes play
little role in horizontal land development planning. Their
primary focus is on structural integrity, fire prevention,
control and safety, suitability of materials, and support system operations (the vertical design of a building). However,
building codes do affect site design in three ways.
First, to control the spread of fire, building codes place
limitations on the proximity of buildings to each other
and to adjoining property lines. These restrictions become
important considerations in the design of clustered singlefamily detached homes, townhouses, and multiple building
complexes (these product types are described in Chapter 4).
When locating a building, the proximity to a property line
may increase fire-protection requirements and add to construction costs—it is important to coordinate fire-protection
requirements with the building design team.
In addition, to improve building occupants’ chances of
surviving a fire, building codes specify maximum building
height, the distance a person must travel to exit a building,
and the number of building or unit exits. Both height and
distance are often expressed as the number of stories above
grade—the height measurement within the building code
may be different from the height measurement defined by a
local jurisdiction. The height restrictions may vary depending on type of construction and the presence of automatic
fire suppression systems. However, the site engineer must
determine final grade elevations during the preliminary and
final design phases of a project to ensure these regulations
are satisfied.
Secondly, building codes and site design requirements have
an interface point—the location where the site connects to
the building. Building utilities are subject to different requirements than site utilities, but both requirements must be considered when connection points are designed. The building
code requirements for allowable materials, minimum slopes,
and geometric requirements must be considered when connecting the site to building utilities. In addition, the location
of building entrances affects site grading and the placement of
walkways, parking, lighting, and landscaping.
Lastly, federal regulations concerning access for persons
with various types of disabilities play a considerable role in
site grading and development. These regulations are incorporated into building codes, which specify that ground floor
units of all apartment buildings as well as places of public
accommodation and commercial facilities must be accessible.
Accessible, according to the ADA Standards for Accessible
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Design (Excerpt from 28 CFR Part 36), describes a site,
building, facility, or portion thereof that complies with these
guidelines (referring to the Standards). Taking this one step
further and applying it to a site, the primary consideration
from an engineering and design standpoint is the provision of accessible routes. An accessible route is “a continuous unobstructed path connecting all accessible spaces of a
building or facility. … Exterior accessible routes may include
parking access aisles, curb ramps, crosswalks at vehicular
ways, walks, ramps, and lifts” (28 CFR Part 36).
Accessible route provisions specify maximum grades and
minimum widths for walkways as well as grading allowances,
configuration recommendations, and surface treatments for
ramps and crosswalks. It is important to note than an accessible route is required between designated accessible spaces;
this does not mean that every possible route must be accessible, only the one designated as such. In addition to accessible
routes, the standards detail handicap parking requirements,
including the minimum number, acceptable location, signage, and dimensions for the spaces and passenger loading
areas. For each site, the site engineer should identify whether
the accessibility standards apply, and from there determine
how to best incorporate and account for handicap accessibility. This may include preparation of an accessible routes map,
special details for ramps, walks, and signs, detailed or largescale grading plans for certain areas of the site, commonly
those surrounding building entrances, and enhanced coordination efforts with the project architects. More information
about accessible routes is provided in Chapter 3.
2.4.6. Development Costs
Development costs associated with the approval of a project
must be considered and accounted for an early in-land development design process. This includes costs attributed to the
approval process (often referred to as a soft costs) and the
construction of the project, as well as other requirements by
jurisdictions to provide public facilities. Both must be understood and accounted for each project.
Public facilities range from subdivision streets to major
commuter highways, tot lots to regional parks, water lines
to fire stations, and sewer lines to treatment plants. As these
systems reach their capacities, their ability to function efficiently or accommodate new growth is diminished. The
consequences range from government leaders’ frustration to
dissatisfied constituencies or hazardous, even life-threatening, conditions created by inadequate facilities.
Historically, it has been the responsibility of the government to provide and improve many public facilities. Reductions in federal funding and corresponding local budget
impacts paired with increased citizen expectation for efficient service and fiscal responsibility have led many local
governments to find alternative, even innovative, ways to
pay for growth. Frequently, these efforts become the subject
of lengthy court challenges, as important public goals and
policies often conflict with private property rights. While
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substantial public obligation remains, the private sector
has been given an increased share of the burden. Development regulations play a critical role in allocating the costs of
basic public infrastructure between private developers and
the community’s taxpayers and prospective consumers in an
effort to fairly distribute the cost of growth.
Guaranteed Performance. Applicable development ordinances and other regulations may not only establish design
requirements, but protect the public from developers that are
unable to complete a project (usually due to financial problems). Subdivision ordinances usually include provisions
requiring the execution of performance and maintenance guarantees, such as bonds, letters of credit, cash escrow accounts,
or similar financial instruments to underwrite the installation
of public infrastructure. These guarantee construction of public improvements in accordance with approved plans. If the
developer is unable to complete the project, the municipality
uses this money to construct the improvements. More information about bonds is provided in Chapter 6.1.
Exactions, Infrastructure Enhancements, and Fees. In
recent decades, a significant portion of the responsibility
for improving the public infrastructure systems has been
transferred from federal, state, and local governments to
the land developer. This allocation of responsibility varies
widely among jurisdictions. Significant differences exist in
the form of the applicable regulations, timing of contributions, and the magnitude of financial commitments that
must be made. In most cases, the zoning ordinance, subdivision ordinance, or site plan regulations identify much of the
developer’s obligations, which are typically limited to on-site
improvements. Some jurisdictions have adopted separate
ordinances that relate development projects to existing community public facilities by tying required improvements to
existing capacity thresholds or deficiency levels for systems
beyond the boundaries of the site that serve the development. Typically, requirements are determined during the
project approval negotiation process, usually at the rezoning
or entitlement phase.
The land development team must closely evaluate the laws,
ordinances, and standards of the locality in which they are
proposing a development, to determine what exactions, infrastructure enhancements, and fees can be expected. The type,
form, and number of such requests or mandates for public
improvements are numerous and varied in their nature and
can represent a considerable expense. Depending on the size
and impact of a development proposal, they can range from
simple road frontage improvements to dedication of land for
schools or parks. Nevertheless, whatever dedications, reservations, fees, or other monetary retributions are required, it
is important that they are identified early in the process so
that design and pro-forma economic analysis (performed by
the developer and business team) can account for these exactions. This section highlights the most common methods by
which infrastructure improvements, fees, and other exactions
become part of the development process.
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Exaction is the term applied to the provision that the land
developers establish public, communal, and consumer benefits in exchange for and as a condition of development authorization. Depending on the jurisdiction and the type of project
or plan submission, exactions may be referred to as proffers
(as described in Chapter 2.3), development conditions, or
concessions. More specifically, exaction may include the following activities (adapted from Frank and Rhodes, 1983, p. 3):
•• Reservation, preservation, or dedication of land for
public or common facilities and use.
•• Construction, dedication, and/or operation of public
or common facilities and services.
•• Purchase of land, facilities, goods, and services for
public or common use.
•• Payment of monies to the jurisdiction to defray the
cost of its purchase of land, facilities, equipment, and
services.
•• Restriction of development potential or provision
of features and facilities intended to achieve social,
aesthetic, or economic goals or other public benefits.
Exactions add both direct and indirect costs, which must
be identified in preparing project budgets. Direct costs
include those costs associated with design and engineering, hard construction, direct cash payments, operating, and
plan review. Indirect costs may include lost project yield and
interest-carrying charges incurred in review and delay of
approval. These costs will be passed on to the consumer by
way of purchase price or leasing fees.
The cost of providing the exaction should be weighed
against the direct and indirect benefits to the occupants of
the proposed development as well as the less tangible value
that may be brought to the project. Indirect benefits address
important public goals or mitigate an undesired impact created
by the project. While the developer may incur an initial economic loss to fulfill the exaction, it is possible to recoup value,
both from approval of the project and improved marketability.
Land Reservation. Many communities have the authority to require the land developer to “reserve” private land
that may be needed for public use, including parks, schools,
or major road rights-of-way. Typically, the need and location of these facilities will be identified in the jurisdiction’s
comprehensive plan or capital improvement projects budget
(as described in Chapter 2.2).
Notice of the reservation usually comes during the earliest stages of project review of entitlement. Depending on the
size of the project, the reservation may be for an entire parcel
or a small portion of it. Such a reservation does not immediately involve the transfer of ownership. It does, however,
obligate the developer to keep the reserved area free from
construction for a specific period, during which the local
government considers acquisition of the property. The reservation is typically not a gift, as the jurisdiction will pay the
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owner the land’s fair market value, if it takes title to the property. Sometimes, the jurisdiction will release the reservation
after considering the cost of the property, available funding
sources, or alternative sites for the intended public use. This
allows the developer to proceed with site development activities unencumbered by a reservation.
Dedications. Mandatory dedication is a mechanism common to most jurisdictions, and is usually among the provisions contained in the subdivision regulations. Mandatory
dedication requires the developer to transfer ownership or
property rights of certain privately held lands to the governing body at no cost. Mandatory dedications, however, must
not violate the Fifth Amendment’s ban on taking of property without just compensation. Land most often subject to
mandatory dedication includes rights-of-way for streets and
public utilities, floodplains, easements, park and recreation
facilities, and similar uses. Such dedications usually are made
a condition of plan approval.
Although the dedication is made and recorded on the
subdivision record plat, the government rarely accepts the
land immediately. Instantaneous acceptance could result in
the transfer of maintenance obligations to the jurisdiction
before construction of required improvements is completed.
Subsequent construction vehicle damage would become the
jurisdiction’s liability. In addition, the governing body must
determine if the property will serve the public good and provide economic benefit. The cost of long-term maintenance of
the dedicated parcel is a primary factor in the government’s
decision whether or not to accept the property. Still another
consideration is the fact that the transfer to public ownership also reduces the land area subject to real property taxes.
Most jurisdictions advance density credit when a dedication
is made.
Transfer of ownership in some cases reduces maintenance
and other obligations that may be incurred by the developer
or by occupants of the project. In addition to a reduction of
operating and maintenance costs, dedicated features can add
to the market appeal of certain projects. Often, by setting
aside land for schools, libraries, parks, recreational facilities, and fire and rescue, the project becomes more attractive
to potential buyers, and the developer enhances its “good
neighbor” image within the permitting jurisdiction.
Perhaps the most frequent dedication is that of additional
right-of-way along roadways bordering a project. The local
government intends this widening to bring existing roads
up to current standards and accommodate the new traffic
movements associated with the project. Whether construction of improvements is performed by the jurisdiction or
required of the developer, the added right-of-way improves
the efficiency of the transportation system and enhances the
safety and convenience of site access.
Preservation. Community residents usually place a high
priority on natural, cultural, or man-made features that
add to the health, beauty, or character of the community.
Often, these features become threatened by a proposed
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development that prompts the community’s desire to protect them. However, the local government may not have the
financial resources or legal authority needed to acquire and
maintain them through reservation or dedication. To fill this
void, some communities are empowered to preserve these
features as an exaction in the development process.
Environmental preservation is often a goal contained in a
jurisdiction’s comprehensive plan. Plan language can recommend that an important environmental or cultural property
be set aside in lots or private open space with covenants and
restrictions that prohibit future disturbance. This language
is implemented as a proffer or development condition usually tacked on to and approved as part of rezoning or special
exception applications.
Environmental and historic protection can also be
achieved with the adoption of preservation ordinances that
allow reasonable use of the property while ensuring that the
desired feature is protected. Most common among these are
historical or archaeological preservation ordinances, which
may require a developer to pursue adaptive reuse or preservation of the sensitive portion of the site. In some instances,
developers have been required to restore historic facades as
part of a larger construction effort.
Recently, communities have sought to adopt tree preservation ordinances, which limit clearing and grading on portions of the site. Although preserved areas remain in private
or communal ownership, covenants and restrictions provide
permanent protection against future disturbance by builders
or lot owners. Such ordinances often limit the development
potential of a site or may involve added costs to accommodate their provisions. These restrictions often present the
developer and consultant with unique design opportunities
that can enhance a project’s appearance and marketability if
accounted and planned for early in the process and incorporated into the overall site concept.
Payments in Lieu. As noted in the prior discussion of subdivision ordinances and site plan regulations, many jurisdictions commonly require that the developer construct
improvements that connect to and/or directly serve the
project. Most jurisdictions tie the required improvements
to a specific “trigger event” within the development process,
such as a required permit or compliance timeframe. As an
example, depending on local policy and industry practices,
improvements should be completed prior to the issuance
of occupancy permits or release of performance bonds.
The required improvement and its trigger conditions are
typically specified in the proffer, development condition,
or exaction that binds it to the development program (see
Table 2.4A). Specific trigger conditions allow jurisdictions
to more thoroughly track required improvements and
contributions.
In some instances, the jurisdiction may not require the
developer to construct the improvements proposed for dedicated lands. Typically, this happens when the improvement
is of little immediate value, such as the widening of an abutting roadway section where there are no existing or proposed
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road improvements on either side of the subject parcel. A
needed facility may be located offsite, such as a major road
or regional stormwater management facility that is not under
the developer’s control or serves a broader population. In
such cases, a cash payment representing actual construction
cost, prorated share, or a flat fee may be required of the developer rather than actual construction of the improvement.
These various forms of payments in lieu, if allowed in the
jurisdiction and acceptable for the required improvement,
are typically specified as an alternative within the approved
proffer or exaction. The jurisdiction may wait until sufficient funds are collected and available in conjunction with
other developments before it constructs the facilities. The
funds usually are earmarked for improvements that serve
the project or an improvement in proximity to the project.
The jurisdiction may also require or allow one developer to
construct the improvements, and direct subsequent developers to reimburse their prorated share of the costs.
If the project is small, the jurisdiction may also waive the
requirement for land dedication. This occurs in instances
where the dedication of land for facilities, such as a recreation area, is disproportionately indexed to the size of the
project. If the resulting parcel will be of insignificant size, the
jurisdiction may offer the option of cash payment in lieu of
the land dedication. In this case, the payment represents the
project users’ impact on existing or future facilities that will
be provided by the municipality.
Impact Fees. An increasing number of jurisdictions are
adopting impact fees. This is a direct payment by a developer
to a jurisdiction and is intended to reimburse the jurisdiction’s actual capital costs of expanding public infrastructure
and facilities to service new development. The facilities are
not necessarily located on site and may not be used exclusively by the development that pays the fee.
Some communities have a longstanding, limited form of
impact fee relating to the provision of water and sewer systems. In this case, a residential or commercial user is assessed
a “tap” fee, also called a connection or availability charge,
usually at the issuance of a building permit. The actual fees
are derived through an elaborate analysis that projects the
capital costs of expanding treatment plants, pump stations,
and trunklines to serve new growth.
However, more recently, impact fees are being considered
for such items as roads, storm sewers, schools, libraries, and
similar services. What may distinguish sewer and water
services from these other facilities is, perhaps, the degree of
certainty with which the demand from and benefit to new
users can be isolated and quantified. This may be determined
by an adequate public facilities (APF) ordinance. An APF is
adopted by the local jurisdiction, and sometimes approved
by a special legislative authority. In communities with an
APF, a project cannot be approved if its demand will exceed
the available capacity.
An APF ordinance requires deference to a project if
the projected demand on public services exceeds the total
capacity or funding approval of any of the systems. The
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TA BL E 2 . 4 A
Sample Exactions
Exaction Type
Sample Language: Infrastructure Provision And Trigger Event
Land
Reservation
Future Road Alignment. The Applicant shall reserve an area of the site as depicted on the CDP/CDPA/FDP for
future right-of-way (ROW) for an interchange. Upon demand by the Board of Supervisors (BOS), the Applicant
shall convey said ROW area in fee simple to the BOS, as generally shown on the CDP/CDPA/FDP.
However, if said interchange is not funded for construction within 15 years from the date of these proffers or
if said interchange is deleted from the County’s Comprehensive Plan, whichever event first occurs, the Applicant’s obligation under this proffer shall terminate and cease and the Applicant will be entitled to use said
ROW area in any manner permitted by law.
Dedication
Urban Park. The Applicant shall construct the improvements in the Urban Park generally as detailed on the
CDP/FDP as may be modified by coordination with the Fairfax County Park Authority and following construction, dedicate to the FCPA the Urban Park in fee simple, prior to final bond release.
Preservation
Tree Preservation. The Applicant shall preserve trees on the Property as shown on the Tree Preservation Plan
prepared by Dewberry dated March 14, 1997. . . . The Applicant shall record conservation easements for the
Daniel’s Run stream valley, the northern boundary adjacent to the trail, and the 25 ′ wide area along the Property’s eastern boundary to ensure perpetual conservation of these tree preservation areas. Said easements
shall be recorded with the record plat for each phase of construction contiguous to that section of trail and/or
conservation area.
Payments-inLieu
Utilities. The Applicant agrees to contribute the sum of $553,000 to the City of Fairfax to be used for underground placement of existing overhead utility lines along portions of the Property’s frontage on Main Street and
Old Lee Highway. Payments shall be made on a pro rata basis at the time of the issuance of building permits
for each dwelling unit. These moneys shall be placed in escrow by the City in an interest bearing account to
be used solely to underground such existing overhead utilities within seven (7) years from the date of issuance
of the final building permit. At that time, the City shall either designate the construction of other improvements
which will directly benefit the Property, subject to the Applicant’s concurrence or the escrowed funds and
accumulated interest shall be returned to the Applicant.
Impact Fees
School Contributions. Prior to approval of the first Building Permit for the approved development, the Applicant
shall provide documentation to DPWES that the Applicant has donated the sum of $127,500.00 to the BOS for
the Fairhill Elementary School. . . . Prior to approval of the first residential use permit (RUP) the Applicant shall
provide documentation to DPWES that the Applicant has donated the sum of $22,500.00 to the BOS for the
Luther Jackson Middle School and the sum of $60,000.00 to the BOS for Falls Church High School.
Linkage
Maximum Density and Permitted Uses. A maximum of 270 multiple family dwelling units may be provided in 2
buildings, which will also include approximately 65,136 sf of retail uses to be located on the first and second
floor(s) of the North building and approximately 40,364 sf dedicated to retail uses to be located on the first
and second floor(s) of the South Building. The site shall not exceed 1.35 FAR and as depicted in the tabulations on Sheet 2 of the CDP/FDP the FAR increase over 1.2 shall consist of ADU’s and related bonus density
units as defined in Part 8 of Article 2 of the Zoning Ordinance. Project density as specified in this proffer shall
be reviewed and approved as part of the site plan approval process.
typical systems covered by an adequate facilities ordinance
include transportation, water, sewer, and school systems.
More recently in some communities, fire and police availability and response times have been included in the APF
ordinances, as fire and police service and effectiveness can
be seen as directly tied to the quality and effectiveness of the
infrastructure systems.
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Total capacity of public systems is derived by adding
to the capacity available in existing systems those facility
improvements that are planned, budgeted, or funded within
a specified period. For some systems, capacity can be determined by direct measurement, such as by measuring design
capacity for sewage or water treatment. For others, a policy
must be established concerning the level of service before
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the calculation of projected use is made. For instance, the
governing body must determine the number of students
per classroom that is acceptable to the community before
determining the total capacity of the school system. Similarly, while determining the capacity of the transportation
system, consider the level of service and congestion that will
be tolerated by the community. The jurisdiction must then
determine the existing use of those systems, again either by
direct measurement or estimated usage. In estimating current demand, the jurisdiction might determine the average
use generated by each residence and equivalent nonresidential operation. It is possible to calculate per unit demand by
analyzing historical data, field research, and demographic
information such as housing type or number of bedrooms.
The community must also estimate the projected demand
for projects that are committed, but not completed. These
are projects that are approved but not built, or projects over
which the government has no control because a discretionary approval, such as rezoning, is not required. These are
called projects in the “pipeline.” The final step is determining
the available capacity and system adequacy. If, after subtracting current and committed demand from the total system
capacity, capacity remains, then the facility is considered
adequate. However, each development proposal must demonstrate that its demand on systems will not exceed that
available capacity. Again, this is done by determining per
unit demand, such as average gallons per day of water use, or
number of vehicle trips per day of traffic generation.
When the APF ordinance restricts the approval of a project, the developer may be allowed to finance improvements
that will increase system capacity, thus allowing the project
to meet the adequacy test, or pay into a fund through impact
fees to be used by the jurisdiction to make the improvements
necessary to meet the public facility needs.
Despite the controversy and concern that the impact fees
can add dramatic costs to home ownership and businesses,
impact fees for services other than water and sewer are proliferating. Even in those communities not authorized to collect impact fees for infrastructure improvements, they have,
in many cases, imposed a “voluntary” fee to finance certain
major facilities. The jurisdiction bases the fee calculation
on the estimated cost of improvements needed to support
the total development projected to occur within a specific
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service area. This total dollar figure is converted to a unit
cost, using the projected number of dwelling units or nonresidential building areas. Thus, the fee applicable to each
development project is based on project size. Typically, the
fee is offered in conjunction with a discretionary rezoning
process, with payment being one of the proffers or conditions attached to the approval.
Linkage. Previously, the discussion of affordable housing
focused on the adoption of inclusionary zoning ordinances.
These ordinances require that residential developers provide
affordable, price-controlled housing as a condition of development approval. As an extension of that trend, nonresidential developers may also be required to provide affordable
housing. A growing number of jurisdictions argue that new
job creation in the community generates demand for affordable workforce housing—housing for office workers and
other service employees. Due to high land and construction
costs, it is increasingly difficult for these workers to find suitable housing in price ranges they can afford. In jurisdictions
where this “linkage” is required, the developer must mitigate
this impact, either by building affordable housing units or by
paying, on a per square-footage basis, into a housing trust
fund established by the jurisdiction. Money deposited in this
fund is used by the jurisdiction or its designee to purchase or
construct affordable housing.
Review and Processing Fees. Most state zoning and subdivision enabling acts authorize jurisdictions to collect
development review fees from the developer. These fees
offset jurisdiction expenditures for review personnel. In
some jurisdictions, this fee represents only a nominal fixed
charge. Urban jurisdictions that perform extensive reviews
often index their fees according to project size or the estimated cost of public improvements. These fees may represent a substantial expense. This enables the government to
maintain an adequate work force of qualified personnel. It
provides some assurance to the development team that project reviews will be timely and thorough. Some jurisdictions
also impose inspection fees to offset the costs associated
with that activity. These fees may be charged for inspection
of the public improvement construction, as well as for the
actual structure. Like review fees, these add to the cost of the
development project and must be considered during project
budgeting.
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Chapter 2.5
Environmental,
Geotechnical, and
Historical Considerations
2.5.1. Introduction
PART A—ENVIRONMENTAL CONSIDERATIONS
With an understanding of the local regulations, it is important to next investigate the potential environmental, geotechnical, and historical constraints that may exist on a site.
Through research and evaluation, which are discussed in this
chapter, the site engineer will be aware of and able to account
for these site constraints and additional opportunities.
An environmental, geotechnical, and historical review
should be conducted early in the pre-design stage to ensure
that appropriate measures can be taken (or a new site
selected). This review could determine that an entire site or
a portion of a site is not suitable for development. Most challenges that are identified can be overcome, and strategies will
be developed as designs are produced. However, if constraints
are identified and prove to not be resolvable, the site may not
be the best option for the developer. The challenge of environmental and historical site review lies with the ability to
identify the resources—areas such as floodplains, wetlands,
or areas of historical significance. These resources can often
be identified with a desktop review in the office; however, a
site visit by a qualified professional is typically required to
confirm the presence or absence of these resources. It is critical to have an understanding of the importance of the environmental and historical considerations while continuing to
work through the site selection.
This chapter is separated into three parts: (A) Environmental Considerations, (B) Geotechnical Considerations,
and (C) Historical Considerations.
Part A of this chapter focuses on the environmental impacts
of a project—what to consider and understand as a project
begins and moves through pre-design. This includes a
review of environmental regulations, an overview of natural
resources, local preservation efforts, required environmental
due diligence, and sustainability practices.
2.5.2. Environmental Policy and Regulations
This section reviews environmental policy and how regulations protect natural resources. Localities continually update
their development ordinances and add new regulations to
protect the environment and help reduce the effects of climate change. In many jurisdictions, environmental regulations are now an integral part of the development process.
As an example, conservation regulations that require mitigation for loss of woodlands, wetlands, or wildlife habitats are
increasingly common and have a substantial impact on the
development process. The preparation and formal submission of environmental inventories that document existing
conditions and identify environmental problems in need of
remediation is also regularly required. Sustainability practices are commonly included within these regulations as well.
Environmental Policy. Land development is faced with
the issue of meeting the demands of human needs, while
also protecting the environment and natural resources. Our
past actions as a society have resulted in significant impacts
to the environment due to human consumption and the
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mismanagement of our natural resources and hazardous
waste. To minimize degradation of the environment, environmental regulations were established to ensure that everyone acts responsibly toward the protection and conservation
of the environment.
Land development processes involve many activities that
have environmental impacts. Site selection, planning, grading, construction, and landscaping all require many decisions
that shape neighborhoods, towns, and regions in profound
ways. These decisions result in changes to the existing environment, which are long term (in human scale) and commit
natural resources in a way that affects future options. As these
decisions became more and more complex, natural ecosystems may not be able to accommodate the impacts of development. Consequently, government environmental policies
and regulations have been developed to preserve a quality of
life through vital natural resource protection, preservation,
and/or mitigation strategies. Regulations have allowed standards to be set avoiding ad hoc approaches with uneven consequences. This has allowed for land development to proceed
with managed consequences while minimizing impacts to
overall environmental conditions.
The environmental policy and regulations mentioned in
this text are not intended to be exhaustive or all-inclusive as
there are regional, state, and even local policies that shape
development projects from conception to actualization.
The site engineer must bear this in mind and become aware
of the agencies and regulations at all levels that may affect
development in a particular locale.
Environmental Regulations. Environmental regulations
are the mechanism for implementing the intent of the various
acts and statutes developed to protect human health and the
environment. The challenge for developers is that these regulations are diverse; they are also continually being updated
and revised. Therefore, the development team must take the
necessary steps to ensure compliance in order to minimize
liability, and avoid unanticipated costs and delays associated
with the purchase and/or development of a property.
A brief summary of important environmental regulations
from over the past few decades can help put into perspective
the progression of environmental regulations and their future
focus. The regulations most likely to have implications on a
project are the National Environmental Policy Act (NEPA)
that establishes the need for and extent of environmental
assessments; the Clean Water Act (CWA) that regulates wetland impacts and sets land disturbance permit and stormwater
treatment requirements; the Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA) that
focuses on response to existing environmental conditions and
required remediation; and the National Flood Insurance Program (NFIP) that encourages responsible development practices in flood-prone areas and establishes floodplains.
National Environmental Policy Act. In the 1960s, as a
result of deteriorating environmental conditions and serious health effects associated with those conditions, the time
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was right for the national discussions of how to effectively
address environmental problems. The National Environmental Policy Act (NEPA) was signed into law in January
of 1970, and it became the catalyst for the environmental
regulations to follow. NEPA is the declaration of the U.S.
policy on the environment. This national policy takes into
consideration balancing the preservation of the natural environment with human needs caused by population increases,
high-density urbanization, and industrial expansion, taking
into account economical and technical considerations.
Apart from being the declaration of national environmental policy, NEPA establishes procedures to ensure that
appropriate actions are taken to protect, restore, and enhance
the environment. NEPA provides a systematic means of dealing with environmental concerns and the associated costs. It
requires an environmental impact review for actions of federal and nonfederal agencies that use federal funds or require
federal approval or permits, such as a National Pollutant
Discharge Elimination (NPDES) permit, which is discussed
later in this chapter with the Clean Water Act. One of the
unique components of NEPA is that it provides a mechanism
for public participation.
The NEPA process is intended to help public officials
make decisions that are based on understanding the environmental consequences associated with a proposed action
by a federal agency as well as the public feedback. This perspective influences decisions made in land development
projects. NEPA requirements are invoked when federal
development activities are proposed, such as highways, airports, government buildings, military complexes, parkland
purchases, and others. In addition to federal projects, NEPA
requirements and environmental reviews can be required
for private developments in federally protected wetlands or
projects on/adjacent to federally controlled property, such as
limited access highways or military bases. This also includes
federal funding and federal permitting activities, so that any
involvement by the federal government triggers the NEPA
process. Many local jurisdictions have a similar environmental analysis process for state, local, and private actions.
Following the requirements of NEPA and prior to implementing a proposed action, an environmental review must
be performed to identify and address:
•• Any environmental impacts of the proposed action.
•• Any adverse environmental effects that cannot be
avoided.
•• Feasible alternatives to the proposed action.
•• The relationship between local, short-term uses
of built environment and the maintenance and
enhancement of environmental functions and values.
•• Any irreversible and irretrievable commitments of
resources that would be involved in the proposed
action should it be implemented.
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2.5
There are three levels of environmental review in the
NEPA process: (1) categorical exclusion (CE), (2) environmental assessment (EA), and (3) environmental impact
statement (EIS). An EIS is a major undertaking and requires
a significant investment of time and money. Environmental
specialists with experience in the NEPA process are typically required to perform these kinds of environmental
reviews.
Through NEPA, the Environmental Protection Agency
(EPA) was established in December of 1970 to set standards
and enforce environmental regulations. While NEPA sets
policy goals, the regulations of the EPA are the prescriptions
and methods to achieve those goals. The EPA is charged with
the responsibility of federal laws and rulemaking to implement U.S. environmental policy.
Clean Water Act. Originally known as the Federal
Water Pollution Control Act Amendments of 1972, the act
was amended in 1977 and renamed the Clean Water Act
(CWA). The objective of the CWA is to restore and maintain the chemical, physical, and biological integrity of U.S.
water resources. One of the major focuses of this program
is to control point source discharge of pollutants to water
through use of total maximum daily loads (TMDL). Point
sources are discharges that come from a single point, like a
pipe, ditch, or even a smokestack. Nonpoint sources (NPS)
are discharges that come from many diffuse sources, like
storm water runoff from agricultural fields, chemicals from
urban runoff and energy production, and sediment from
improperly managed construction sites. TMDLs regulate
the amount of pollutants permitted to be discharged to U.S.
waters.
National Pollutant Discharge Elimination System. Mandated
by Congress under the CWA, the National Pollutant Discharge
Elimination System (NPDES) program is a national program
that addresses the non-agricultural sources of storm water discharges that adversely affect the quality of U.S. waters (additional detail is provided in Chapter 3.5). The CWA requires
an approved NPDES permit to discharge pollutants through
a point source to surface waters. Again, point sources are discrete conveyances such as pipes or human-made ditches. The
permit specifies limits on what is being discharged and spells
out monitoring and reporting requirements, as well as other
provisions. Generally, the NPDES permit program is administered by authorized states.
Construction activities are considered an industrial
activity and, as such, will typically require an NPDES permit (the actual permit may be called something different depending on the state). Usually, an NPDES permit
is required if the construction activities result in the disturbance of more than 1 acre of total land area. However,
some jurisdictions may have more stringent requirements
based on regional or local environmental concerns. For
instance, in the Chesapeake Bay watershed, 2500 square
feet of land disturbance triggers the requirement for an erosion and sediment control permit. Construction activities
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Environmental, Geotechnical, and Historical Considerations 81
that require a permit may include clearing and grubbing,
grading, and excavation. In some states, a permit may not
be required if the runoff does not discharge into a waterway, as in the case where it evaporates from a catch basin
or similar isolated water body. As discussed previously
under NEPA requirements, the NPDES permit may trigger
a NEPA environmental review.
Additionally, as a result of the NPDES permit program,
construction sites will be required to have a Stormwater
Pollution Prevention Plan (SWPPP) prior to commencement of any construction activities. Stormwater runoff
from construction sites can cause significant harm to
downstream bodies of water. The goal of the SWPPP is
to maximize the benefits of pollution prevention and erosion and sediment control practices through the use of best
management practices during the construction process.
Since the primary focus of this program is on controlling
pollutants in any storm water discharge, incorporation of
well-thought-out and carefully implemented erosion and
sediment control measures is more important than ever.
Erosion and sediment control measures are discussed in
Chapter 5.7.
Waters of the United States. Waters of the United States
(WOTUS) are defined in the 1972 CWA as “navigable
waters.” The CWA regulates the discharge of dredged or fill
material into these WOTUS. This authority was extended
to non-navigable waters and wetlands, as well, but this continues to be challenged in courts regarding the actual jurisdictional reach intended by the CWA. Many development
projects affect stream channels or wetlands that constitute
WOTUS and, as such, are regulated by the EPA and the U.S.
Army Corps of Engineers (Corps) under the CWA. A permit
from the Corps is required before dredged or fill material,
related to construction activities, may be discarded into the
WOTUS.
Each state has a role in the federal permitting process
because each state must certify that the granting of a permit
by the Corps will not violate state water quality standards.
Many states have initiated programs to protect wetlands and
other waters beyond the range of WOTUS. Typically, state
wetland protection laws regulate draining, channelization,
or clearing of vegetation. In addition, some states have gone
further to protect those areas by requiring buffer areas adjacent to wetlands to prevent damage to the resource. Many
states have used the authority to issue or deny water quality certification of the CWA to regulate some fill impacts to
wetlands and other WOTUS, including intermittent streams.
A state can deny water quality certification of a proposed
impact, under the premise that the state’s water quality standards are not being met.
More information about wetlands and natural waters is
discussed later in this chapter. When considering a project
that involves potential impacts to WOTUS or wetland areas,
additional time and fees will likely be required for preparing
permit application(s) to the Corps and/or the local state.
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Comprehensive Environmental Response, Compensation, and Liability Act. Approved in 1980, the primary
purpose of the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA) is to protect
human health and the environment from the dangers of
hazardous waste. CERCLA focuses on response to existing
environmental conditions of a property and the liability of
the responsible parties. CERCLA created a revolving fund,
commonly referred to as “superfund,” that is utilized by
EPA, state, and local governments to investigate and remediate hazardous waste sites that have been listed by the EPA
on the National Priorities List (NPL). One of the unique
features of CERCLA is that governmental and private
parties who are not responsible for the investigation and
remediation of a property can perform the work using the
superfund and seek reimbursement from the responsible
parties. CERCLA covers all environmental media, including air, surface water, groundwater, soil, and sediment and
can apply to any type of facility. If cleanup is conducted onsite under the CERCLA program, no federal, state, or local
permit is required.
A CERCLA hazardous substance includes any substance
that the EPA has designated for special considerations
under the CWA, Resource Conservation and Recovery Act
(RCRA), Clean Air Act (CAA), Toxic Substances Control
Act (TSCA), etc., and any substance that presents a substantial danger to human health and the environment. Exclusions
from CERCLA’s hazardous substance list include petroleum,
natural gas, or synthetic gas used for fuel. Due diligence and
liability assessments fall under CERCLA regulations and
amendments, which are the most applicable regulations to
land development. This is generally only a concern for projects that have potential environmental concerns and require
remediation, such as brownfield sites or other redevelopment projects.
National Flood Insurance Program. Historically, the federal government responded to development in and around
the floodplain by constructing flood-control projects (dams,
levees, and seawalls) and providing emergency disaster relief
funding to both communities and individuals. However, this
approach did little to discourage unwise development, and
in some cases, may have encouraged development in floodprone areas. In response to escalating flood-related losses
and the burgeoning cost to the taxpayer, in 1968, the U.S.
Congress created the National Flood Insurance Program
(NFIP). The purpose of the NFIP is to encourage responsible
development practices in flood-prone areas, and to protect
property owners through an insurance mechanism that is
funded by those that are most at risk of flooding.
The NFIP, first established in 1968 with the passage of the
National Flood Insurance Act, was broadened and strengthened by the Flood Disaster Protection Act of 1973, as well as
the National Flood Insurance Reform Act of 1994. The NFIP
was administered by the Department of Housing and Urban
Development (HUD) until 1979, when it was absorbed by
the newly created Federal Emergency Management Agency
02_Land_CH02_p017-124.indd 82
(FEMA). Currently, FEMA administers the NFIP primarily
through two of its branches, the Federal Insurance Administration (FIA) and the Mitigation Directorate. The FIA is
responsible for administering the insurance aspects of the
program, while the Mitigation Directorate is responsible
for administering the floodplain management aspects of the
program.
The NFIP is a community-based program. A community’s participation in the NFIP is voluntary (although
some states require participation as part of a state-wide
floodplain management program). It is the responsibility
of each community to assess their flood risks and determine whether they would benefit from the flood insurance
and floodplain management assistance provided through
the program. Over 20,000 communities participate in the
program nationwide.
During the early years of the NFIP, to get as many communities in the program as quickly as possible, FEMA
established the “emergency phase” of the program, which
was designed to provide limited amounts of insurance coverage at less than actuarial rates. In general, no detailed
floodplain studies were conducted for communities in the
emergency program, and the communities are required to
adopt only limited floodplain management ordinances to
control future use of its floodplains. Only about 1% of the
communities participating in the NFIP are in the emergency phase.
The remaining participating communities are in the “regular phase” of the NFIP. These communities are generally
provided detailed studies of their flood-prone areas and are
required to adopt more comprehensive floodplain management ordinances in exchange for higher amounts of flood
insurance coverage.
Role of Municipal Governments and Lenders in Floodplain
Management Under the NFIP. When a community agrees
to participate in the NFIP, it receives flood hazard maps
prepared by FEMA and its residents become eligible for
flood insurance. In turn, the community agrees to adopt
and enforce minimum floodplain management regulations
within the Special Flood Hazard Area (SFHA) as depicted
on the flood hazard maps. The SFHA is defined as the land
area that would be inundated by a flood having a 1% chance
of occurring in any given year; this flood is referred to as the
“base” or “100-year” flood.
The minimum regulations that the community is required
to enforce depends on the type of flood hazards present in
the community and the level of detail with which the hazards
have been studied. At a minimum, a community must ensure
structures are built above the 100-year, or base flood elevation (BFE).
Generally, obstructions such as buildings or other structures in riverine floodplains inhibit the flow of floodwaters downstream and result in an increase of the upstream
flood elevations. Therefore, for many waterways, FEMA
has identified a floodway. The floodway is composed of
the actual stream channel plus the portion of the overbank
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2.5
area that must be kept free from encroachment in order
to convey the 1% annual chance flood without increasing
the BFE by more than a specified amount, or surcharge.
To ensure that development in the floodplain does not
result in unacceptable increases in the BFE, as a condition of participating in the NFIP, the community typically
adopts the floodway as part of its ordinance and stipulates
no structures are built within it that increase flood elevations. Figure 2.5A provides a graphic depiction of a floodway and floodway fringe zone.
Under the NFIP, the allowable increase, or surcharge,
within the floodway is 1 foot; however, many communities
and states have adopted more stringent floodway surcharge
limits. For example, New Jersey has established a maximum
surcharge limit of 0.2 foot, resulting in a floodway that is
wider than that needed to convey the 1% annual chance
flood with a 1.0 foot surcharge. Some conservative communities have even set a zero surcharge limit, effectively defining the entire floodplain as the floodway.
It is important to note that many municipalities have regulations and/or permit requirements governing construction
in floodplains that go above and beyond the FEMA NFIP
requirements. Therefore, it is important to become thoroughly familiar with the local requirements at the outset of
any project that may impact the floodplain.
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Environmental, Geotechnical, and Historical Considerations 83
In addition to the building regulations that local municipalities must enforce, mortgage lenders also have a role to
play in floodplain management. Under the NFIP, residential
mortgages that are federally backed (which almost all are)
must have flood insurance if the property is located within
the SFHA. Accordingly, all lenders are required to determine
prior to approving a mortgage whether the property is in
a SFHA, and if so, require the borrower to purchase flood
insurance as a condition of the loan.
FEMA Flood Map Products. One of the primary tools in
administering the NFIP are the flood hazard maps that identify flood-prone areas. Because the NFIP was established as a
community-based program, the maps were produced for individual communities. However, since the early 1990s, FEMA
has been converting the maps to a county-wide format.
FEMA has produced different types of flood hazard maps
over the history of the NFIP. During the early years in the
program, FEMA produced flood hazard boundary maps
(FHBMs), which show floodplain boundaries based upon
approximate data and limited analyses. They were typically
issued during the emergency phase of the NFIP, and generally have limited information regarding the flood-prone
areas. While there are still some communities with FHBMs,
most have been replaced with flood insurance rate maps
(FIRMs).
100 YEAR FLOOD PLAIN
FLOODWAY
FLOODWAY
FRINGE
STREAM
CHANNEL
FLOOD ELEVATION
WHEN CONFINED
WITHIN FLOODWAY
ENCROACHMENT
ENCROACHMENT
SURCHARGE1
C
D
B
A
AREA OF FLOOD PLAIN THAT
COULD BE USED FOR DEVELOPMENT
BY RAISING GROUND
FLOOD ELEVATION BEFORE
ENCROACHMENT ON FLOOD PLAIN
Line A - B is the flood elevation before encroachment
Line C - D is the flood elevation after encroachment
1Surcharge
not to exceed 1.0 foot (FEMA Requirement) or lesser
amount of specified by state.
F i g u r e 2 . 5 A Floodway schematic showing floodway and a floodway fringe.
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Communities that are in the regular phase of the NFIP
are provided with FIRMs. FIRMs are generally based upon
detailed studies of the flood-prone areas, and show more
precise information regarding the flood-prone areas than
the FHBM. A sample of a FIRM is provided in Figure 2.5B.
To identify the SFHA and other areas on the FIRMs, different zone designations are used. Noncoastal areas within
the SFHA are designated as Zone A, AE, AO, AH, A1-A30,
or A99. Each of these zone designations represent a different
type or risk class of flooding associated with the 1% annual
chance (100-year) flood. Similarly, for coastal areas, where
wave action is a concern, the SFHA is designated as V, VE,
or V1-V30. Areas outside the SFHA include Zones B, C, D,
and X. Because each zone designation represents different
classes of risk, the insurance premium for a property is based
upon the zone designation.
The floodway is typically shown on a community’s FIRM.
However, for a period between the mid-1970s and mid1980s, for some communities, FEMA did not show floodways on the FIRMS, but generated separate maps, referred to
as flood boundary and floodway maps (FBFMs), that show
only the floodways. As with the FHBMs, FEMA is in the
process of phasing out these maps and replacing them with
updated FIRMs.
Accompanying each FIRM is a report, referred to as the
flood insurance study (FIS) report, containing a large amount
of supporting information and data such as technical details
on how the flood studies were performed, tables containing
modeling parameters and results, charts containing floodway information, and graphs showing flood elevation profiles for each stream studied in detail.
In addition to producing hardcopy flood hazard maps,
FEMA has been producing and distributing FIRM data in
digital format. Referred to as the digital FIRM (DFIRM)
database, it consists of spatial (GIS) data in several formats,
nonspatial data tables, metadata, and a digital copy of the FIS
report. The GIS data typically includes
•• SFHA boundaries, floodway boundaries, and base
flood elevations
•• Transportation data and/or aerial photography
•• Stream centerline and coastal shoreline data
•• Model cross-sections (riverine analyses) and transects (coastal analyses)
•• Political boundaries
•• Benchmarks
•• FIRM panels
FEMA technical standards for all of these products are
compiled in “FEMA’s Guidelines and Specification for Flood
Hazard Mapping Partners.” FEMA’s FIRMs and related map
products can be obtained from the FEMA Flood Map Service Center online. For more information about updates to
02_Land_CH02_p017-124.indd 84
the National Flood Insurance Program and the Flood Insurance Rate Maps, refer to the Appendix in Chapter 7.4.
Environmental Regulations Considerations. From the
perspective of the developer and the development team,
these environmental regulations impact real estate transactions, demolition work, restoration efforts, the development
process, and overall construction of a project. Therefore,
understanding the basis and objectives of environmental
issues and integrating appropriate actions in planning and
schedules of development projects can prevent unanticipated delays and conflicts. Environmental issues must be
proactively addressed and dealt with, often with agencies,
procedures, and individuals outside the traditional development process. The earlier in the development process that
environmental issues are identified, the better, since some of
these issues can involve liability or even affect the purchase
or development potential of a property. Other environmental impacts may affect the design and scope of the project,
affecting development efficiencies and appropriate use of
resources.
2.5.3. Natural Resources Overview and
Preservation Efforts
As the built environment expands, it is imperative that land
development activities do not compromise the long-term
quality of our natural environment. The responsibility of land
stewardship is shared among everyone. Bodies of water, wetlands, buffer areas, forests, and other forms of open space are
a vital part of our economic future. Sustainability and green
building rating programs, to be discussed later in this chapter,
have brought increased attention to the value of open space,
habitat, and native vegetation not only in an environmental
context, but in terms of human health and happiness.
As such, these environmental features have become
increasingly regulated at the federal, state, and local levels
through laws, ordinances, and formal recommendations
included in comprehensive plans—linking the natural and
built environments. While wetlands and natural waters are
primarily regulated at the federal and state level; trees, landscape, and open space preservation regulations have become
commonplace in many localities. This section discusses
the applicable regulatory framework surrounding natural
resources and the considerations that should be included
during the due diligence of a project.
Beyond understanding the requirements, it is increasingly
important for the land development team and all stakeholders involved in a land development project to truly understand the value of the various functions natural resources
perform. From aesthetics to security to pollution prevention/remediation and climate control, natural resources are
a critical component of land development projects that warrant consideration throughout the design process. Therefore,
any individual who participates in the land development
design process should understand and appreciate the important functions and values provided by natural resources in
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2.5
Figure 2.5B
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Environmental, Geotechnical, and Historical Considerations 85
Sample flood insurance rate map.
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climate, air, and water quality preservation. Land development consultants are uniquely positioned, given their technical background and practical experience, to cultivate an
appropriate balance between preservation and construction
efforts.
The following sections discuss the importance of natural
waters, identifying wetlands, soil classifications, the value of
open space and vegetation, and preservation efforts to protect these natural resources.
Natural Waters and Wetlands. One of the primary constraints to land development is the presence of natural
waters. Waters of the United States, including stream channels, ponds, and lakes, are usually easy to identify. Floodplains have been effectively mapped all across the United
States by FEMA. Wetlands, on the other hand, are harder
to identify and require more attention. Natural waters have
historically been looked upon as being in conflict with development and property rights. However, coastal flooding and
erosion, as well as water quality issues associated with drinking water supplies, have increased public awareness to the
need to maintain and protect these critical resources.
More than 50% of the wetlands in the continental United
States have been destroyed in the last 200 years, according to
the U.S. Fish and Wildlife Service. Between the mid-1950s
and mid-1970s, there was an increase in wetland losses per
year with a net loss of 9 million acres of wetland (U.S. EPA).
Drainage of wetlands for agriculture was responsible for 85%
of losses, with development causing approximately 13% of
the losses.
Not only are these losses staggering, but many waterways
throughout the United States are also heavily polluted. The
Clean Water Act has helped to remediate and better control point source discharges and impacts from construction activities. NPS discharges continue to be a challenge to
manage. These sources include agricultural runoff (fertilizers, herbicides, and other agricultural chemicals that wash
into the streams) and urban runoff (oil, grease, and other
toxic chemicals that end up in the rivers). More information
about types of NPS pollutants are discussed in Chapter 5.5.
It is no surprise that polluted waterways are bad for the environment, including plants, animals, and humans as well.
Besides water quality, water quantity is another issue that
land development can affect.
Water quantity relates to the amount of water runoff produced in a storm event. In undeveloped land, during a storm
event, water naturally permeates through the soil surface
into the ground and slowly reaches downstream waterways.
Urbanization decreases the amount of permeable surfaces (by
constructing impervious surfaces like asphalt and concrete),
which increases the runoff. This runoff is not only too much
for the waterways to handle, it is also moving more quickly
and can be heavily polluted. This can lead to erosion and
dangerous flooding events. More information about water
quantity and quality concerns is discussed in Chapter 3.5.
Jurisdictions are commonly adopting preservation ordinances to better protect local waterways. In addition, land
02_Land_CH02_p017-124.indd 86
development projects are also focusing more on the environmental aspects of a site to restore and enhance waterways,
and in some cases create new wetlands. Wetlands are a vital
resource and can even help to repair the environment and
manage stormwater events.
As defined by the U.S. Army Corps of Engineers, wetlands
are “those areas that are inundated or saturated by surface
or ground water at a frequency and duration sufficient to
support, and that under normal circumstances do support,
a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps,
marshes, bogs, and similar areas.”
Wetlands represent a varied resource with many names,
usually associated with the prevalent vegetation. When wetlands share the same type of vegetation, they can take the
same name. For example, forested wetlands can be called
swamps and those dominated by grasses are marshes. No two
wetlands have identical plant species diversity, or the same
hydrologic and soil conditions. Generally, what all wetlands
have in common is soil saturation or inundation for extended
periods (i.e., wetland hydrology) and vegetation adapted to
these soil conditions (i.e., hydrophytic vegetation). The use
of such familiar terms as marsh, bog, or swamp may mean
different things in different regions of the country.
It is important to understand that while wetland areas are
generally described as swamps and bogs, a small site may still
have localized wetland areas of just a few hundred square feet.
The U.S. Fish and Wildlife Service Wetlands Mapper is an
efficient way to evaluate the potential presence of wetlands.
The following sections discuss wetland functions and
wetland delineations.
Wetland Functions. After a long history of wetland
destruction, wetlands are increasingly recognized as providing desirable functions to society. Wetlands provide many
functions related to improving water quality, flood management, and providing wildlife habitat.
Water Quality. Wetlands are known to act as chemical sinks
or transformers that can improve water quality. They act as
“kidneys of the landscape” (Mitsch and Gosselink, 1986) by
removing excess nutrients from rivers and streams.
As water purifiers, wetlands act to reduce nutrients, chemical wastes, and turbidity. Wetland plants remove nutrients,
such as phosphorus and nitrogen, from surface water and
utilize them for growth. Removing the nutrients from surface waters not only prevents algal blooms, but reduces the
potential for fish kill by preventing the decreases in dissolved
oxygen levels caused by the decomposing algal masses. A
wetland’s ability to absorb nutrients varies between individual wetlands and wetland types. Furthermore, a wetland can
be overloaded with nutrients and thus be unable to assimilate the entire nutrient load.
The effective removal of nutrients has been demonstrated on numerous projects by allowing nutrient laden
effluent (i.e., sewage or wastewater) to flow through wetlands. Organic compounds are decomposed and nitrogen
is converted to its gaseous form and removed (Mitsch and
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2.5
Gosselink, 1986). Heavy metals adsorbed to sediments are
allowed to settle out in the diffuse, low velocity environment
of a wetland. As sediments fall out of the water column, turbidity is reduced allowing sunlight to penetrate to support
growth of submerged aquatic vegetation (SAV).
Water Quantity. Wetlands within a watershed can reduce
flooding through the detention of floodwaters during a
storm. The water is then slowly released afterward into
streams, reducing peak flows. Water being released from the
wetlands can even recharge or discharge aquifers depending
upon their topographical position on the landscape. Therefore, a wetland may either replenish the aquifer or contribute
to base flow in streams. The relative ability of a wetland to
alter flood flows depends on several variables, including its
size relative to the size of the watershed, its relationship to
other wetlands in the watershed, and the amount of urbanization in the watershed. In many respects, wetlands serve as
a natural form of stormwater management.
Coastal wetlands provide similar benefits in terms of
reducing the potential flood damages associated with tropical
storms, hurricanes, northeasters, and the like. The wetland
vegetation dissipates wave energy, thus reducing potential
wave heights and storm surge. Coastal wetlands, such as barrier islands, serve as a buffer to protect shoreline areas from
wave-induced erosion. When located along streams and
bays, wetlands can buffer erosive forces and hold sediments,
thereby preventing loss of shoreline.
Fish and Wildlife Habitat. Wetlands are among the most
productive ecosystems in the world in terms of producing energy from the sun via photosynthesis and recycling
nutrients. They provide food and habitat for an array of
wildlife and are critical in the nesting, migration, and wintering of waterfowl. Although wetlands do not occupy a
high percentage of our total land area, wetlands are critical
to the life history of over one-third of our nation’s threatened and endangered plant and animal species. Coastal
wetlands provide spawning habitat, as well as nursery
and feeding areas, for much of our commercially important fish and shellfish. Additionally, wetlands are critical
feeding, resting, breeding and nesting areas for migratory
waterfowl.
As an example, consider the role of vegetation in an
estuary such as the Chesapeake Bay. Historically, vast
numbers of waterfowl (primarily redhead and canvasback
ducks) wintered in the freshwater areas of the bay, where
wild celery beds were abundant. Declines in this vegetation in the bay and its rivers have led to a decline in the
number of canvasbacks and redheads overwintering in
the area. In addition, commercially important fish species
such as striped bass, shad, and menhaden are dependent
upon the bay’s SAV for spawning, feeding, and nursery
habitat. Though likely not the sole reason for their diminished numbers, as SAV declined in the bay, the commercial
and sport fishery for striped bass was reduced to the point
where all fishing for the species was banned for several
years in the 1980s.
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Environmental, Geotechnical, and Historical Considerations 87
Wetland Delineation Criteria and Methodology. The
methodology for determining the presence of a wetland is
based upon three criteria: soils, vegetation, and hydrology. For
an area to be identified as a jurisdictional wetland, the area
must contain these three wetland parameters: hydric soils,
hydrophytic vegetation, and wetland hydrology, or evidence
thereof in disturbed areas. Wetlands are located somewhere
along a “natural wetness continuum” that exists on the landscape. Locating the boundary between wetlands and uplands
(non-wetland areas) on this continuum is the objective of
wetland delineation manuals. The definition of what constitutes a jurisdictional wetland has proven controversial with
there being many different versions of a single delineation
manual available. Field indicators of the three-wetland criteria are provided in the various available delineation manuals
and are discussed below. When field indicators of each of the
criteria are observed, the area is determined to be a wetland.
Soils. Hydric soils are defined as “soils that formed under
conditions of saturation, flooding, or ponding long enough
during the growing season to develop anaerobic conditions
in the upper part of the soil” (Federal Register, July 13, 1994).
Organic hydric soils are largely an accumulation of nondecomposed, organic remains (peat). These soils are easily
recognized as hydric soil by a thick organic surface layer that
is saturated for most of the year. Wetland identification in
problem areas, such as sandy soils, may take slightly longer
to identify in the field and may require additional detail by
the environmental specialist.
Vegetation. Hydrophytic vegetation is defined as plant life
growing in water or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content (Environmental Laboratory, 1987). Many plants have
adapted to these conditions to create the great diversity of
plants found in wetland environments. About 8000 plants
species are included on the Fish & Wildlife Services’ National
Wetland Plant List: 2016.
Hydrology. The hydrology of a wetland is the most important element that defines the wetland type and function.
Water that creates wetlands comes from several sources
including groundwater discharge, out-of-bank stream flow
(flooding), surface runoff of precipitation or snow melt,
direct precipitation, or tidal flooding. The roles of groundwater and surface water in creating wetlands are illustrated
in Figure 2.5C. As expressed in the figure, wetlands are typically located between the seldom flooded upland and perennial waters of creeks, rivers, and bays.
The presence or absence of wetland hydrology is determined using recorded data and field indicators. Recorded
hydrologic data, such as stream, lake, or tidal gauge records
can provide information on the duration of flooding in areas
adjacent to the water body. In groundwater driven wetlands,
the soil survey of a locality can be useful in determining
duration of soil saturation.
Wetland hydrology in the form of surface water or saturated soils is readily apparent during certain times of the year
in most wetlands. Evidence of this could be standing surface
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Figure 2.5C
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Schematic diagram showing wetlands, deep-water habitats, and uplands on landscape. (Adapted from Tiner, 1984.)
water or muddy ground surface underfoot; however, during
certain times of the year, primarily late spring to early fall
or during drought events, when surface water is not evident,
wetland hydrology may not be as obvious.
Field Delineation. In the field, wetlands professionals utilize the approved U.S. Army Corps of Engineers Wetland
Delineation Manual to evaluate the three primary characteristics of a wetland. The current Corps delineation methodology requires positive indicators of all three wetland criteria
(soils, hydrophytic vegetation, and hydrology) to delineate
an area as a wetland. The wetland-upland boundary is determined where one or more of the criteria are not present. The
only exception to this is the situation where one or more of
the criteria are not present due to a disturbance to the area.
A common example of a disturbed condition is the situation
where hydrophytic vegetation is not present because of farming activities that have temporarily replaced the natural vegetation with planted crops or pasture.
Wetland hydrology, generally the most difficult of the
three criteria to verify in the field, is normally confirmed
through the use of additional field indicators. Other than a
direct indicator of water, evidence of hydrology can be supported by the presence of hydric soils.
A wetland delineation is usually completed in two
stages: the preliminary investigation, to determine whether
wetlands may exist and their approximate limits; and the
detailed investigation, where the jurisdictional area for
preliminarily identified wetlands or WOTUS is accurately
identified based on an enhanced level of data collection and
field review.
The accuracy of a preliminary wetlands investigation is
lessened when the topographic relief is low and the wetland is not confined to distinct stream valleys. A lack of
02_Land_CH02_p017-124.indd 88
landmarks in areas of low relief also limits the accuracy
of the preliminary study because sketching the wetland
boundary in the field is an approximation and landmarks
aid in placing the wetlands/uplands boundary correctly.
All of these factors influence the confidence level of the
preliminary wetlands investigation and necessitate a more
formal, detailed investigation for accurate delineation.
Sources of readily available information that provide preliminary wetland delineations are discussed later in this
chapter.
A detailed wetland investigation offers the most accurate
delineation and measurement of the jurisdictional area on
the site. This level of effort is necessary when the confidence
level of the preliminary study is not sufficient or the exact
location of the boundary and/or total wetland acreage must
be accurately known for permitting purposes. A detailed
wetland boundary would need to be accurately known during final design (Chapter 5) to ensure avoidance or minimization of wetland impacts.
If the preliminary investigation of a site shows that it
may contain wetlands, it would be prudent for an experienced environmental specialist to conduct a detailed
wetland delineation. Due to the variability of regional conditions, the environmental specialist should be familiar
with the region and have a rapport with the local regulatory
authorities. The best strategy is to avoid wetlands and other
WOTUS in regard to impacts from land development.
Therefore, it is extremely important to identify these constraints that might hinder a project early in due diligence
stage. If the delineated wetland constitutes a majority of
the site, and the development program cannot be achieved,
then a new site may be required. More information about
preliminary wetland identification for a project is discussed
later in this chapter.
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2.5
Open Space and Vegetation. Open space and existing vegetation of a property is another important natural resource
to evaluate. Open space preservation in many cases is not
limited to the preservation of individual trees, but rather,
generally applies to the preservation of existing trees and
other associated vegetation across a large area that is maintained in a natural state. Preservation in this manner and at a
reasonable scale (i.e., in contiguous areas or connected in an
open space or greenway corridor) maintains the values and
functions of the resource. Within the context of sustainable
design and green building rating programs, open space is
prioritized to protect or restore habitat, promote biodiversity, and expand opportunities for interacting positively with
the environment.
The Values and Benefits. Understanding the significance
of open space and tree preservation requires knowledge of
the values and benefits provided by landscape elements and
open space in general. Figure 2.5D illustrates the functional
uses of plant material, from which the following values and
benefits are derived:
•• Aesthetics
•• Space definition and articulation
•• Screening undesirable views
•• Complementing or softening architecture
•• Creating a sense of unity among inharmonious buildings
•• Providing textural and pattern variety
•• Buffering incompatible land uses
•• Attracting wildlife
Trees affect the microclimate of an area by moderating the
effects of sun, wind, temperature, and precipitation. Such
climatological values and benefits include
•• Intercepting, filtering, or blocking unwanted solar
radiation
•• Blocking undesirable wind by obstruction
•• Directing wind flow by deflection
•• Reducing wind velocities by filtration
•• Moderating temperature changes (although this is
more directly a function of solar radiation interception)
The values and benefits of trees and open space relative to
site development and engineering are equally as important.
These include
•• Decreasing stormwater runoff directly through interception of rainfall and water uptake through the root
system, as well as filtering pollutants contained in
runoff from the adjacent watershed
•• Stabilization of soils
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Environmental, Geotechnical, and Historical Considerations 89
•• Reducing the glare and reflection characteristically
generated by the combination of buildings and/or
roadways and natural and/or artificial light
•• Acting as noise attenuators
•• Interacting with the particulate matter and gasses
known to cause air pollution to significantly reduce
the concentrations of these pollutants
•• Adding oxygen to the atmosphere
•• Recreational opportunities (i.e., gardening, ballfields,
hiking trails)
•• Educational opportunities
•• Nonpolluting transportation (i.e., bike trails)
•• Tourism
•• Flood protection
All of the architectural, aesthetic, climatological, and
engineering uses give value to trees that can be described
in both economic and legal terms (Miller, 1988). Because of
these measurable assets, trees and open space contribute to
and enhance property values. The resulting sum of values
equates to an improvement in the quality of life for everyone.
This concept is emphasized by the green building rating programs, which is discussed later in this chapter, where credits
are commonly awarded for habitat preservation or creation,
and open space optimization.
With many values and benefits, open space on a site,
including trees and vegetation, is important to preserve. Open
space consisting of old fields is primarily made up of a mix of
perennial flowers, grasses, and what some would classify as
weeds. Many wildlife species rely on these old fields for foraging and reproduction; in fact, many song bird species rely
strictly on old fields as their primary habitat. Old field areas
are accustomed to some manipulation, such as annual bush
hogging, grazing, or fire. Therefore, provided that these areas
are not graded, and the seedbank is protected, old fields tend
to be more resilient than forested areas during construction
activities. However, the long-term development plan for such
an area should recognize that some type of manipulation will
likely be necessary in the long term to maintain these areas
as open fields.
When considering tree preservation, the root zone area is the
critical and limiting factor of success. Plants depend upon roots
for water and mineral uptake, storage of food reserves, and the
synthesis of needed organic compounds and anchorage. Generally, tree roots are located in the upper 36 inches of the soil,
with the majority of the roots (85%) in the upper 18 inches.
Research has found that tree roots can extend far beyond a tree’s
drip line (outermost extent of tree canopy), typically branching
out from the trunk a distance of 1 to 1.5 times the tree’s height.
It is critical to protect the roots of a tree during construction.
Protection efforts during construction is described in erosion
and sediment control methods of Chapter 5.7.
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Figure 2.5D
Functional uses of plant material which have measurable values and benefits. (From International Society of Arboriculture: Guide
to Plant Appraisal, 8th ed., 1992.)
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Local Preservation Efforts. All natural resources are considered valuable environmental assets to local communities,
and the incremental but steady loss of these resources has
prompted many jurisdictions to enact legislation. Local regulations have been adopted to preserve and protect natural
waterways, riparian corridors (vegetation growing along a
body of water), forested areas, farmlands, and unique natural resources such as prairies, meadows, and deserts. In
addition, increasingly poor water quality and urbanization,
has led communities to protect more than just the natural
resources, including entire watersheds and even viewsheds
as well.
Regulations to preserve and protect natural resources and
environmental features generally begin with the local comprehensive plan. Specific recommendations and guidelines
may be adopted, in line with the community’s vision and
goals, dedicated to these preservation efforts. This could be
included within an environmental element of the comprehensive plan, as described in Chapter 2.2, or in a separate
environmental plan. Fairfax County, Virginia, for example,
has a Tree Action Plan that includes strategies for “conservation and management of the county’s tree resources.” This
includes visions and goals for the urban forest, core recommendations, and implementation strategies, similar to a comprehensive plan. Through the recommendations of the Tree
Action Plan, the county adopted a 30-year tree canopy goal to
increase Fairfax County’s tree cover to 45% by the year 2037.
The jurisdiction may implement these recommendations and environmental plans through the adoption of
preservation ordinances or other requirements in the local
zoning ordinance, subdivision ordinance, and/or a separate
environmental or landscape ordinance. Open space requirements, landscaping requirements, and other regulations may
be found in dimensional standards within the zoning text, as
described in Chapter 2.3, or Development Standards Manual, as described in Chapter 2.4.
Overlay districts, as described in Chapter 2.3, are commonly included on zoning maps and in the zoning ordinance
to protect natural resources. San Antonio, Texas, for example,
has River Improvement Overlay (RIO) Districts. “Its purpose
is to establish regulations to protect, preserve and enhance
the San Antonio River and its improvements by establishing
design standards and guidelines for properties located near
the river. The San Antonio River is a unique and precious
natural, cultural and historic resource that provides a physical connection through San Antonio by linking a variety of
neighborhoods, cultural sites, public parks and destinations.”
San Antonio’s Unified Development Code includes a chapter
for the zoning overlay districts that includes the RIO Districts. The text within outlines the purpose of the district,
through the required provisions and regulations, to “prevent
the negative impacts caused by incompatible and insensitive
development and promote new compatible development.”
The RIO Districts also “maintain the openness and natural
habitat of the river, access to its trails and provide safety for
its users” amongst other objectives.
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Brooklyn, New York, is another example that has a viewshed overlay. This overlay district can be seen in city zoning
maps as a shaded area that is designated “SV-1.” “The Special
Scenic View District (SV) is intended to prevent obstruction
of outstanding scenic views as seen from a public park, esplanade or mapped public place. No buildings or structures are
allowed to penetrate a scenic view plane except by special
permit of the City Planning Commission. The Brooklyn
Heights Scenic View District (SV-1) extends over an area
west of the Brooklyn Heights Promenade to protect the
views of the Lower Manhattan skyline, Governors Island, the
Statue of Liberty and the Brooklyn Bridge.”
Many of these preservation ordinances have been created
to act as guides for responsible or appropriate land development activities. These ordinances and regulations are generally not intended to prohibit development activity, merely to
cultivate a balance between the natural and built environment that is beneficial to all those participating in the development process. Land development and preservation need
not be mutually exclusive. In fact, open space preservation
has been demonstrated to prevent flood damage, attract
investment, revitalize cities, and boost tourism, along with
preserving the environment (Lerner and Poole, 1999).
Since communities differ in their natural environment,
political structures, cultural traditions, and legal framework, preservation ordinances vary between jurisdictions.
All jurisdictions do have similar goals of protecting natural
and cultural resources, views, and floodplains; so their ordinances and regulations will be specific to the local community desires and economic needs.
Regulations within preservation ordinances may also
vary widely between jurisdictions. For instance, some
localities are quite specific as to the percentage of existing
vegetation that must be retained, and have designed methodologies for determining the extent of existing vegetation
to be targeted for retention. Other localities simply offer
general guidelines, requiring that all attempts possible be
made to retain existing vegetation. Most preservation ordinances will establish the definitions, procedures, penalties,
and appeals process specific to the individual locality for
preservation and protection. In some instances, preservation ordinances are developed to protect entire ecosystems,
such as those areas that are known habitats of rare, threatened or endangered species, or watersheds that drain into a
public water supply.
Since variation between preservation ordinances can be
great, and all ordinances are specifically reflective of the local
policies, circumstances, and needs of a particular area, it is
more critical here to understand the general premise for an
ordinance, rather than the details specific to many localities.
Specific preservation ordinances exist in a majority of the
towns, cities, and urbanized areas nationwide, and they may
be separate or included as part of larger ordinances. It is the
responsibility of the site engineer and the development team
to be cognizant of the requirements particular to the jurisdiction in which a specific project is located.
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2.5.4. Environmental Due Diligence
With an understanding of environmental regulations and
local preservation efforts, and an appreciation of natural
resources, it is necessary to understand how to evaluate the
environmental conditions of a site during the due diligence
review of the site selection process. This section discusses
various environmental assessments that should be considered when evaluating a site for development purposes.
When determining the development potential of a particular piece of land, the land development team should first
evaluate the land’s opportunities and constraints, both of
which may be influenced by environmental issues. Therefore, a preliminary environmental review of a site should be
completed in the early stages of the land development design
process. This review is typically a two-step approach:
1. Perform a desktop review, or an office data review,
to identify environmental considerations that may
warrant closer field inspection. This review includes
a cursory investigation and assessment of available
wetland, floodplains, soils, wildlife, hazardous material, and natural hazards data for the site. This will
be discussed within this section of this chapter.
2. Complete a site inspection, also known as a site visit
or walkover, to confirm the information garnered
in the desktop review and ensure that current conditions (which may not be entirely or accurately
reflected in the desktop review) are identified and
assessed. This is discussed in Chapter 3.1.
These environmental assessments are an important component of the due diligence process because some environmental factors such as existing wetlands, habitats, or
hazardous materials could significantly alter development
potential and the overall development program. Additionally, environmental conditions could adversely affect the cost
and schedule of a development project. These assessments
may be required when producing a feasibility study (to be
introduced in Chapter 3.1) to ensure the viability of a site.
The discussions in the section are intended as an overview
to familiarize the development team with the typical environmental assessments related to the project. Each project is
unique, so additional assessments and more detailed reviews
may be required.
Additional attention will be required if the project constitutes a state and/or federal action. State actions will require
compliance with the applicable state regulations. Any undertaking that receives federal funding, permitting, and/or approvals will require conformance with NEPA, CWA, CERCLA, as
well as other applicable federal laws.
More information about each of these federal programs
can be found online. Some of them have interactive tools to
help perform necessary research. For NEPA, the EPA provides
a NEPAssist tool that “facilitates the environmental review
process and project planning in relation to environmental
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considerations. The web-based application draws environmental data dynamically from EPA Geographic Information System databases and web services and provides
immediate screening of environmental assessment indicators
for a user-defined area of interest. These features contribute to
a streamlined review process that potentially raises important
environmental issues at the earliest stages of project development.” NEPAssist provides information on EPA facilities
and jurisdictional content as well as natural waters, wetlands,
FEMA flood data, soils, critical habitats, land cover, and more.
It should be noted that these reviews at this stage are preliminary and for planning purposes only. They are intended
to help the design team identify any constraints inherent to
the site itself that may prohibit or substantially derail (either
because of time or cost) a proposed development project.
Early identification of environmental site conditions can
significantly enhance programming and planning, as well as
future design and construction efforts, and facilitate a realistic project schedule. Additional investigations will likely be
required during final design (Chapter 5), depending on the
existing environmental features.
More information about wetland assessments, floodplain
studies, soils investigations, wildlife habitat reviews, contamination investigations, and natural hazard risk assessments is
introduced within this section.
Wetland Assessment. The wetland desktop review consists of researching existing information concerning the site.
As mentioned previously, wetlands have important functions for the environment, and along with WOTUS, impacts
should be avoided and/or minimized as much as possible
during development. Therefore, it is important to first identify if any natural waters and/or wetlands exist on the site.
Typically, this process begins in the office as an evaluation of
available information. A desktop review will determine the
likelihood of the presence of WOTUS or wetlands on the site.
Examples of readily available information are discussed
below. This information provides a preliminary indication of
potential presence of wetlands on a site. It should be noted
that these resources are mass-produced at a small scale and
frequently are outdated or missing data. Therefore, a desktop
review of wetland data should not be considered sufficient to
exclude the presence of wetlands or WOTUS on a site. During the site investigation, an additional preliminary wetland
investigation may be required. If the preliminary information suggests that a wetland is present on the site, then a
detailed wetland delineation will be required for final design.
U.S. Geological Survey 7.5 Minute Topographic Series Maps.
These maps, which are also known as topographic quadrangles, can be obtained directly from the U.S. Geological
Survey (USGS). In addition, various websites also have topographic maps available online. Typically, there is a fee associated with downloading a map from these websites. These
maps can identify such features as marshes or lakes, ponds,
rivers, streams, and other water bodies that may be present
on the site. In addition, these maps can be used to determine drainage swales or low-lying areas that may exhibit
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2.5
wetland characteristics and should be evaluated during the
site inspection (to be introduced in Chapter 3.1). The largest
scale these maps are available in is 1:24,000.
Wetlands Mapping. Much of the U.S. has been mapped by
the U.S. Fish and Wildlife Service (USFWS) to produce the
National Wetland Inventory (NWI). These maps are available online at the USFWS Wetland Mapper website. The
NWI map is prepared from aerial photos and wetlands are
designated by the USFWS. If the NWI map indicates wetlands on the site, there is a high probability (more than 90%)
that they are jurisdictional wetlands. State and local wetland
programs may have also prepared wetlands mapping that
includes the site area. If available, these maps can typically be
found on the local county or state GIS webpage.
Web Soil Surveys. The USDA, through the Natural
Resources Conservation Service (NRCS), provides soil information for most jurisdictions online via the Web Soil Survey.
If there are hydric soil map units or any soil map units with
hydric soil inclusions indicated, then one could assume
that these areas are potential wetlands, and are areas which
should be field verified. Greater confidence in that determination would be obtained if the NWI map also showed wetland in this area. If no hydric soils or hydric soil inclusions
are indicated in the study area, it will likely lack the required
soil to meet the wetland criteria. However, the site should be
field-checked for verification purposes because soil conditions can change and development around a site can potentially affect the site hydrology.
Aerial Photographs Review. Aerial photographs can provide information on vegetation and hydrology and should be
consulted as part of a wetland data review. There are many
sources for aerial imagery, including commercial online
sources. Another example is the USGS TerraServer USA
webpage, which provides aerial photograph coverage for
much of the nation. Other possible sources for aerial photographs are county or city mapping or planning offices, the
USDA, Agricultural Stabilization and Conservation Service
(ASCS), and NRCS offices located in each county. Numerous
counties also have recent aerial photographs available on the
county’s GIS webpage.
Many counties, especially those with growing populations, will regularly commission aerial photo surveys to
update county land-use mapping. Color infrared (CIR) photography is the most useful when identifying wetland areas
from the desktop with aerial photographs as it can highlight
differences in vegetation and soil moisture.
Floodplain Study. A floodplain study graphically depicts
an engineering estimate of the water surface elevation
expected along a length of a stream for some specified design
storm. Typically, floodplain studies are conducted as a design
tool as well as a regulatory requirement. As a design tool,
they are often used to determine the limits of inundation, for
some specified recurrence interval, to be assured that new
developments will lie beyond the floodplain. Similarly, they
may be used to evaluate the changes in flood elevations due
to changes or additions to structures within the floodplain.
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A floodplain study may also be conducted to evaluate the
effectiveness of channel improvements or modifications.
Even if a floodplain study is not necessary for design, it
is often a regulatory requirement. It is common for local or
state governments to require that a floodplain study be performed and submitted in support of a site plan. Regulations
often contain a provision that a floodplain study is required
for a reach of stream if the drainage area is greater than some
minimum value (100 acres may be typical). It is common to
require a floodplain study to ensure that new construction
does not increase the water surface elevation more than the
allowable surcharge above the base flood elevation.
Preliminary Investigation. The first step in any flood
study is to determine the limits of the stream to be studied.
The limits will be determined based on the desired results of
the study. If the reason for performing a floodplain study is
due to a regulatory requirement related to the land development project, the municipality may require the entire length
of stream through, and possibly beyond, the project limits to
be studied. If the reason for performing the study is to determine the floodplain boundary in a specific location, or to
determine the necessity for channel improvements, etc., then
the limits of study must be determined such that the portion
of the stream impacting the development is studied.
Also, the desired precision of the results must be determined. This will determine the accuracy of the topographic
data needed to map the floodplain, as well as the hydrologic
and hydraulic methods used for the study. As expected,
higher accuracy generally comes at a higher price, since
more detailed hydrologic and hydraulic methods can cost
orders of magnitude more than simpler methods.
The appropriate recurrence interval(s) must be determined in order to provide the level of risk or protection
desired. The 100-year floodplain is the most common for
analysis, because of regulatory requirements from local jurisdictions and from FEMA.
Data Acquisition. Before beginning a floodplain study,
an information search should be conducted to determine
if previous studies on the subject stream exist, or if other
studies have been performed in close proximity that may
provide some of the data needed for the study. Some federal
agencies perform floodplain studies as an aid to identifying
flood prone areas. These agencies include the USGS, the U.S.
Army Corps of Engineers (Corps), and FEMA. In addition,
some local or state organizations may have drainage plans
or floodplain studies already established. Local zoning maps
may show existing floodplain limits. These limits could
have been established from studies performed by the local
municipality, as part of another development project along
the stream, or as a study done by the USGS.
If a development project is near a previously or concurrently developed site that is also affected by the subject
stream, a floodplain study may have been developed in conjunction with the nearby development. These studies should
be identified and examined to determine if they may be used
outright, or if some pieces of information may be useful.
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These reports and maps establishing the floodplain limits
should be studied to determine the underlying assumptions,
such as the land use conditions for determining the runoff.
This information will then be incorporated into the new
floodplain study or be used to justify the need for establishing new limits based on current conditions. The delineated
floodplain limits on the maps may be graphically depicted
on the project base map (to be introduced in Chapter 3.2).
It may then be the responsibility of the site engineer to further define the limits through metes and bounds or other
methods. If a new floodplain study is conducted, the new
limits are compared to the preliminary delineated limits.
Any discrepancies must be justified with the public review
agencies.
When performing a new study, a large amount of data is
required. Much of this data may have already been created
for other purposes and it is simply a matter of collecting it.
Types of data that are typically collected include
•• Topographic data and aerial photographs for the
entire study area
•• Hydrologic data such as stream gage data, and, if
detailed hydrologic modeling is required, rainfall
data, soils types, and land use coverages
•• Plans/information for structures in the floodplain,
such as bridges and culverts
•• Transportation and planimetric data layers to serve
as reference features on floodplain maps
•• Supplemental field survey at critical cross sections
•• Photographs or other field verification of surface
conditions (used for hydrology computations)
Topographic and aerial photographic data is often available
from local and state agencies, the USGS, and land development
project plans. Topographic mapping and aerial photographs
should cover the area to be studied, with plenty of additional
coverage beyond the actual floodplain limits. More information about topographic maps is introduced in Chapter 3.
Commonly, hydrologic data is obtained from the USGS,
although local and state agencies, particularly engineering departments, frequently maintain this type of data as
well. Bridge and culvert “as-built” plans can sometimes
be obtained from state departments of transportation or
local public works departments. If not available from these
sources, it may be necessary to have survey crews collect this
information as part of their field work.
Last, many states and local communities have GIS departments where digital planimetric data can be obtained, such
as transportation, parcel, and corporate limit data layers.
Producing a new floodplain study will generally occur
with the final design of a project. This work may be completed by a floodplain specialist or other consultant. For
more information about preparing a floodplain study, refer
to Chapter 7.4.
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2.5.5. Wildlife Habitat Review
It is important to review habitats that may be present on
the site. Habitat loss is the primary threat to most imperiled
species and critical habitat zones are essential to protecting threatened or endangered species. While the regulatory
aspect of critical habitat under the Endangered Species Act
does not apply directly to private and other nonfederal landowners, large-scale development on private and state land
typically requires a federal permit for either stormwater
discharges (NPDES) or impacts to natural waters; thus, the
project becomes subject to critical habitat regulations. Federal permitting agencies are required to coordinate with the
USFWS to ensure that projects they authorize are not likely
to jeopardize the existence of the listed species or result in the
destruction or modification of their habitat that is declared
to be critical.
As a result of this requirement, it is advisable to complete
a review of any potential development site to determine if
there is a likelihood to impact a threatened or endangered
species or their habitat as a result of the land development
process. This review can be accomplished by contacting
either the USFWS or the state equivalent. Some states or
local jurisdictions have this information available online.
2.5.6. Contamination Investigation
Contaminated sites may exist because of past or present
land use activities. Environmental regulations, as previously described, require the assessment of the type and
extent of contamination potentially present at a site, as
well as the development of adequate remediation measures
to address the contamination identified. Hazardous waste
refers generally to discarded waste materials from institutions, commercial establishments and residences that poses
an unacceptable risk to human health and safety, property
values, and the environment. Typical sources of contamination include surface impoundments, landfills, spills, tanks,
septic tanks, agriculture, urban runoff, deep well injection,
and illegal dumping.
Information about potential contamination affecting
a site can be gathered from state and local environmental
offices. In addition, the EPA has a website called Enforcement and Compliance History Online (ECHO) which also
provides information related to compliance, violations, and
enforcement actions. A comprehensive regulatory database
review report can be purchased from several companies that
maintain updated regulatory databases. Should the database
review indicate possible sources or presence of contamination, it is important to properly prepare for the site investigation by taking the necessary safety precautions to protect
field personnel.
The regulatory database review and site investigation
provide a preliminary understanding of potential contamination presence at a given site and surrounding properties
and help determine whether additional detailed investigations are warranted.
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2.5.7. Natural Hazard Risk Assessment
Several natural hazards could be associated with a project
depending on the geographic location of the site. Possible
natural hazards include flooding, wildfires, landslides, earthquakes, hurricanes, erosion, tornadoes, tsunamis, typhoons,
droughts, volcanic eruptions, and other severe events related
specifically to the geology or climate of a particular site.
Additional information about these hazards can be found on
the FEMA website.
Although natural hazards are relatively unpredictable,
risk assessment at the early stages of a project can result in
more informed and responsive site selection, planning, engineering, architectural and other design related decisions.
Specific design strategies can be utilized during the development process so as to attempt to mitigate natural hazards to
reduce the impacts of natural disasters at a given site. Simple
measures such as two means of ingress/egress from a development, reasonable shoreline setbacks for coastal development, adequate communications infrastructure, appropriate
material selections for buildings and infrastructure, and code
compliant construction can greatly enhance the built environment’s response to natural hazards and the protection of
communities both in terms of population and property loss.
2.5.8. Sustainability Practices
With an understanding of environmental regulations and
design considerations required, it is important to explore sustainability practices which are becoming more popular and
regulated in today’s market. The “green building” movement
has created a new industry. Cities as large as Chicago are
embracing sustainability by developing green roof projects
and implementing ordinances and special economic incentives that encourage public-private partnerships. Tax increment financing (TIF) districts, for example, utilize property
taxes within a specific area to fund community improvement
projects and encourage development within that district.
Denver, Colorado, has a green roof requirement for all
projects that submit a site development plan for any new
building with a gross floor area of 25,000 square feet or more
or a building addition that causes the building to become
25,000 square feet or more, and any existing building over
25,000 square feet that is seeking to do a roof replacement.
The required coverage of available roof space is determined
by the gross floor area of the building.
The City of San Francisco, California, created the Central SoMa Eco-District to encourage innovative district-scale
sustainable development projects. “An Eco-District calls for
a new model of public-private partnership that emphasizes
district-scale organization between the City, utility providers
and community stakeholders and the rigorous application
of integrated sustainability performance metrics to guide
investments in the areas of building development, infrastructure and community action and program delivery.” This
ecodistrict compliments the Central SoMa Area Plan (also
known as a sector plan as described in Chapter 2.2). The
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Central SoMa Area Plan proposes a development impact-fee
program (as described in Chapter 2.4) that will fund ecodistrict projects. This will achieve the vision and goals of the
city and the local residents.
The value of building green can be highlighted by financial savings and benefits of new technologies. Incentives
from municipalities are continuing to spur the development
of green buildings and the expansion of urban open spaces.
Other jurisdictions, such as Montgomery County, Maryland,
and Arlington County, Virginia, are developing regulations
to make green building certification a requirement for proposed developments.
Dallas, Texas, also has comprehensive green building
standards for both new residential and commercial construction. These standards are regulated by the City of Dallas
Green Ordinance. “Dallas recognizes the fundamental link
between the building code’s intent of ‘safeguarding the public
health, safety and general welfare,’ and preserving a safe and
healthy natural environment. Incorporating sustainability
through energy efficiency, water conservation and resource
reuse and reduction translates into a stronger economy and
area growth.”
As part of the green legislation being considered and
implemented by governing bodies, some are considering
issues closely related to climate change, such as reduction of
carbon emissions though thoughtful planning and site selection, and through new technologies as they come into the
marketplace.
2.5.9. Green Building Rating Programs
As jurisdictions continue to adopt green building ordinances
or to “strongly recommend” green building practices through
their comprehensive plans, zoning ordinances, and other
regulatory language, it is often important for the site engineer to consider sustainability practices early in the design
process. It may also be important to resolve or design open
space and landscaping as a separate infrastructure component subject to the same level of increasing detail afforded to
roads, utilities, and grading.
Several organizations currently provide green building rating programs with design guidelines and third party evaluation/certification of designs in accordance with their respective
criteria or rating system. These organizations include
•• United States Green Building Council (USGBC)—
Leadership in Energy and Environmental Design
(LEED) Rating Systems
•• National Association of Homebuilders (NAHB)—
Model Green Home Building Guidelines
•• Green Building Initiative (GBI)—Green Globes
•• National Institute of Building Sciences—Whole
Building Design Guide
•• Enterprise Community Partners—Green
Communities
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•• Building Research Establishment (BRE) Limited—
Environmental Assessment Method (BREEAM)
•• Institute for Sustainable Infrastructure
(ISI)—Envision
Many of the green building rating systems include open
space preservation and restoration as a component part of
the site development section. First and foremost, through
avoidance, the rating systems encourage selecting a site or
placing site development outside of environmentally sensitive areas.
This combination of credits related to open space and natural resource preservation emphasizes the front-end importance of decisions related to site selection and appropriate
consideration of environmental and natural resources from
the start of the project throughout the design and development process. These credits are intended to be attainable for
any site whether rural, suburban, or urban.
Selection of the appropriate design guidelines and rating
system is dependent upon the type of development proposed,
jurisdictional requirements, and developer priorities. The site
engineer should be familiar with the various third party certifying entities and their rating systems or guidelines in order
to advise clients appropriately for their specific application.
The USGBC—LEED program is a common green building rating system. The USGBC offers several different green
building rating systems covering nearly every market sector
of the land development industry.
Each site and building is different and the green building
guidelines, in their various forms, seek to accommodate this
uniqueness while still ultimately producing a sustainable project. To obtain LEED certification, a development must obtain
TA BL E 2 . 5 A
a minimum amount of points or credits as prescribed in the
applicable rating system, in addition to all prerequisites. Additional levels of certification, such as silver, gold and platinum,
corresponding to increasing levels of sustainability or environmentally friendly design, can be achieved by accumulating
credits above the minimum necessary for certification.
Many projects now have one, if not more, LEED Accredited Professional (LEED AP) as members of the design team
to facilitate a more integrated design process including
educating team members as to the credit requirements and
documenting the design efforts in a manner conducive to
certifying the project.
Knowing the established criteria, a determination
should be made by the design team, with input from the
client/developer, as to the desired green building design
goals for the project. Care should then be taken during
preparation of the engineering feasibility study and environmental impact study to include evaluation of the characteristics of the site that do or do not comply with the
criteria of the applicable rating system and the established
goals (Table 2.5A).
By considering the applicable rating system criteria during the initial stages of site planning, such as the due diligence stage, projects can more easily reach the desired green
building goals by simply selecting sites that meet all or some
of the existing location and condition criteria previously discussed. The design team will also have the opportunity early
on to determine if some of the noncompliant characteristics
of the selected site can be overcome by incorporating techniques for sustainable development into the project design,
thereby gaining credits necessary for achieving the preferred
level of certification.
Integrating Sustainability into the Land Development Design Process
Design Phase
Action
Feasibility & Site Analysis
Establish Goals, Identify Constraints and Opportunities
Concept Design
Think Big, Innovate and Develop Sustainable Design Strategies
Schematic Design
Refine Goals, Implement Design Strategies, Build Baseline Models
Final Design
Integrate and Detail Design Strategies in Construction Plans and Specifications
Plan Approval/Permitting
Submit for Certification, Revise as necessary
Construction
Follow Through and Coordinate with contractor
Postconstruction
Monitor and Maintain Sustainable Systems; Train/Educate Users
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Environmental, Geotechnical, and Historical Considerations 97
POINCIANA PARKWAY
Case Study: In 2017, the Osceola County Expressway Authority (OCX) and the residents of Poinciana, Florida, celebrated the completion of the new, 10-mile-long Poinciana Parkway. The parkway is a limited-access toll road that
connects north-central Osceola County to Polk County, and enables the community of more than 50,000 to link to I-4,
which runs west to Tampa, northeast through Orlando, and ultimately to Daytona Beach. The parkway has become part
of a vital roadway network for residents commuting to Orlando or visiting the Walt Disney World Resort, and will serve
as Poinciana’s primary hurricane evacuation route while also improving emergency-response services to the community.
LIFE IN THE SMALL CITY
First developed in the 1960s, Poinciana lies south of Orlando in Osceola County. The community is located on an “island”
surrounded by Reedy Creek Swamp and other wetlands. Over the past 20 years, Poinciana has been among Florida’s
fastest-growing areas, and the increased population had resulted in one of the worst small-city commutes in the nation.
The developer originally planned to construct a parkway in the 1990s, but the project was postponed due to a lack of
funding and a number of environmental challenges. In 2010, the Florida legislature created the OCX with the purpose of
developing a limited-access toll road network in Osceola County. The Poinciana Parkway would serve as the first critical
link in the regional network. OCX selected the design-build team of Dewberry and Jr. Davis Construction/United Infrastructure Group—Poinciana Parkway LLC—to design and construct the parkway. The team divided the project into
four segments, which allowed the parkway to open in phases. This approach enabled OCX to generate revenue earlier by
collecting tolls on the completed segments as they opened.
The parkway extends from Cypress Parkway northwest to U.S. 17/92, with at-grade intersections at each terminus. The
alignment traverses the Poinciana development with grade-separated interchanges at Koa Street and Marigold Avenue, both important collector roads providing access to the parkway. Construction included three bridge structures: a
62-span, 6169-ft-long low-level bridge through the Reedy Creek Mitigation Bank and two single-span roadway overpasses. The final design allows for a future six-lane divided roadway, with the initial project constructing two lanes.
OCX had two main objectives that led to the decision to use the design-build delivery method. First, the authority
sought to complete the project as quickly as possible in order to alleviate traffic issues, improve access to the Poinciana
development, and begin to capture revenue from the tolls as soon as possible—helping to fund future projects and validate the authority’s long-termmaster plan. Second, OCX placed a priority on addressing environmental challenges in a
responsible manner, includingthe complicated crossing through the mitigation bank.
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PROTECTING THE ECOSYSTEM
In 2013, shortly before design work began on the parkway, 3520 acres of Reedy Creek Swamp were reclaimed as a mitigation bank. The preserve is a rich and diverse natural habitat consisting of woods, low-lying wetlands, swamps and
marshes that provide a home to many species such as bears, panthers and alligators; a wide variety of fish and birds,
including a blue heron rookery; and native flora ranging from old-growth cypresses to butterfly orchids. Reedy Creek
also serves as one of the northernmost sources of water for the Everglades. The swamp is a popular destination for nature
lovers who enjoy the visitor center and the network of trails in the area.
Dewberry addressed many environmental concerns during the design, permitting and construction process for the
parkway. The team assessed a variety of aspects including the roadways, bridges, drainage, utilities and fiber networks to
determine the extent of impacts on the local ecosystem, including the swamp. While an earlier proposal had suggested
two 2000-ft-long bridges separated by a long earthen causeway through the swamp, Dewberry engineers proposed a
single 6169-ft-long bridge over the environmentally sensitive area, eliminating the earthen plug. This approach maximized hydraulic flows and expanded the wildlife corridor.
The bridge is arched to provide a minimum 8-ft and maximum 24-ft clearance over the swamp to allow wildlife to walk
or swim underneath. The arched profile facilitates the collection of stormwater runoff from the roadway into a scupper
system that discharges into stormwater treatment ponds at both ends, eliminating direct discharge into the sensitive
wetlands.
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Environmental, Geotechnical, and Historical Considerations 99
The Jr. Davis Construction/United infrastructure team, with an extensive background in local projects and a strong
working knowledge of soil and drainage conditions in the area, also focused on minimizing environmental disturbances
during construction. One critical measure involved the use of low-pressure segmented barges to facilitate access as the
bridge was constructed over the swamp. The barges served as material staging platforms, equipment work platforms, and
marsh mats for the areas of soft soils and inadequate flotation.
The use of the barges enabled the team to remove existing trees from the ground up, while preserving the natural vegetative root mat, which allowed for rapid regrowth and added stability to the work site. Rather than sinking into the root
mat and wet soil, the sectional barges floated when necessary, barely leaving a footprint behind.
The bridge was constructed using driven concrete piles with precast beams. The choice of precast allowed for longer,
nearly 100-ft spans, reducing the number of bents in the swamp. The spans were still short enough to allow for easy
delivery and were light enough to handle from the cranes set on the barges. The superstructure required 7700 cu yd
of concrete, with an additional 1000 cu yd for the substructure. The abutments and interior bents were constructed
with piles driven from the ground topped by cast-in-place concrete cap beams. The girders were Florida-I 45 beams at
45 inches deep and 99.25 feet long.
Numerous agencies and stakeholders were involved in the effort, with Dewberry coordinating with OCX as well as
the Florida Department of Transportation-Turnpike Enterprise, the Central Florida Expressway Authority, the Florida
Department of Transportation-District One, the Florida Department of Environmental Protection, the U.S. Army Corps
of Engineers, the South Florida Water Management District, Osceola County, Polk County, the Reedy Creek Mitigation
Bank and numerous utility companies. The first phase opened in April 2016 and the entire $68.8 million project was
completed in January 2017—3 months ahead of schedule.
Contribution by Kevin Knudsen, P.E.
Featured in the September 2017 issue of Road&Bridges Magazine
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PART B—GEOTECHNICAL CONSIDERATIONS
This part focuses on subsurface conditions of a site. The
geotechnical considerations can have a significant influence
on the cost to develop a site. The presence of poor soils or
shallow rock may make the site cost-prohibitive to develop.
During the pre-design phase of a project it is likely that geotechnical information will come from public sources and will
be supplemented with site-specific information during later
phases of the project.
Many public agencies require subsurface information
before the project is approved. Early investigations can
provide information to the development team on potential
issues. The extent of required information collected depends
on the nature of the subsurface conditions and the project.
Where foundation and subsurface conditions are relatively
straight forward, the information may be adequately covered
by a short soils report. For complex projects with unusual
or difficult conditions (problem soils), the required information may be quite extensive. Local ordinances will generally
dictate the type of information needed.
All investigations should begin by researching available
information, followed by a thorough surface reconnaissance.
Available information that may be helpful include published
geologic reports, NRCS county soil reports, stereo air photos
used for photogrammetric mapping, flood maps, hydrology
maps, information and experience at nearby sites, etc. Based
on an assessment of this information, a subsurface investigation program can be planned to obtain adequate data for
analysis and design of the project. Specific objectives of the
investigation normally include
•• Location and depth of soil layers, bedrock and groundwater within the depths of significant stress increases
expected under a proposed structure; as well as other
features that may affect the project, such as subsurface
cavities, hazardous waste, and contaminated soil
•• Samples of all significant materials for visual examination and classification or laboratory testing if
necessary
•• Field data to determine the density or compactness
of all materials
•• Any other special requirements based on project
plans or subsurface conditions
Due to the inherent uncertainties of soil and rock behavior, the
detail of the subsurface investigation is carried out to a level
that balances both design requirements and cost with risk.
The type of project may dictate the extent of the geotechnical
report and when it is initiated. For development projects such
as commercial and retail projects, the subsurface investigation is begun early in the planning phase. Whereas, residential
development (lot and street layout) may lend itself to use of
publicly available soil survey to provide sufficient preliminary information to begin the project and defer a thorough
02_Land_CH02_p017-124.indd 100
subsurface investigation until the specific site details are finalized and the scope of the investigation refined.
2.5.10. Soils
Decisions for the planning and design of a project depend
highly on subsurface conditions at the site. Not only is geologic and soil information necessary for design, but information on what is contained below the surface affects various
project decisions as well. The subsurface investigation aids
the development team and helps the developer in making
economic and financial decisions for the project. A description of geotechnical considerations is included in Part B of
this subchapter. Knowledge of subsurface conditions, such as
contaminated groundwater, presence of hazardous materials
(natural or manufactured) and extremely poor soil and rock
conditions at the early planning stages can save the developer
time and money. For example, conditions may be so egregious that the developer may elect to abandon the project at
this site in favor of another where development costs may be
more predictable or feasible. Although performing the subsurface exploration may be an investment up front (subsurface exploration costs can be around 0.5% to 1.0% of the total
construction costs), the long-term savings often warrant such
an expense.
From an engineering perspective, subsurface soils are
the organic and inorganic materials of the earth’s surface
that are capable of being displaced by shovel or nominal
mechanical efforts and may provide the foundation and
support of the proposed project. Soil is an integral part of
the design analysis and construction activities and is often
used as a foundation in its natural undisturbed state or as
a construction material for fill, backfill, and embankments.
The behavior of soil may vary depending on the situation.
Many structural failures have been attributed to not properly addressing subsurface conditions or predicting soil
behavior. The site engineer should be concerned with what
types of soils lie below the ground surface and their associated properties and characteristics including strength,
compressibility, permeability, grain size distribution, and
moisture content.
Besides those methods which describe and indicate soil
properties and parameters for engineering purposes, other
methods of describing, categorizing, and identifying soils
based on the interests of other professions are available to
assist the engineer. The soil scientist (agronomist) is interested in the origin, distribution, and classification of soil for
agricultural purposes. Geology is the study of the origin, history, and structure of the earth with a focus on rock and rock
formations. While the geologist typically considers soil as the
weathering product of rocks, the agronomist’s interest in soil
is obviously related to agriculture. Soils and their properties
are often described by terms specific to these two disciplines,
and it is helpful if the engineer can relate their terms to engineering interests. The geotechnical engineer is looking at the
strength, compressibility, and permeability of the soils at the
site in order to make assessments about the soils stability,
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2.5
settlement, and drainage characteristics, as related to the
proposed construction.
Strength and compressibility of the soils are the primary
characteristics that are important in foundations of structures (buildings, retaining walls, and headwalls). Permeability and strength are soil characteristics that are important
when designing stormwater management facilities, engineered slopes, and changing the topography at the site. The
strength characteristics of soils are important to predict slope
behavior and prevent slope failures. Erodability is another
soil characteristic that is important for evaluation of erosion
potential and determination of soil erosion control measures.
Buildings founded on weak soils can result in catastrophic
structural failures. Buildings founded on compressible soils
can experience excessive settlement resulting in expensive
repairs. The success of infiltration facilities, common in low
impact development approaches to site design, hinges on the
permeability of underlying soil stratas. A thorough understanding of soil properties—strength, compressibility, and
permeability—will allow the land development engineer to
accurately assess the feasibility of various design options and
ultimately, to accurately design, detail, and specify site features. For more information about soil properties and these
characteristics refer to Chapter 7.3.
Geotechnical engineering is the branch of civil engineering that involves the design and use of soil and rock in
engineered construction; it is involved in all types of construction in, on, or of the earth. With specific knowledge of
the soil properties of a site, the geotechnical engineer can
design soil support and fill projects, while predicting their
behavior comparable to other engineering materials such as
concrete or steel. While some projects may require careful
consideration of rock and groundwater conditions, the geotechnical engineer also needs to understand the geology,
hydrology, and local problem soils in the vicinity of the site.
Typically the developer will hire a geotechnical engineer that
has experience with similar developments, is familiar with
the project region, and accustomed to the requirements of
the permitting jurisdiction.
Prior to developing a site, the developer will need to be
familiar with conditions both above and below the surface of
the site. Will it be necessary to cut or fill the site, or import a
significant amount of soil to grade the site for the proposed
project? If the soils below the surface are not investigated significantly and foundations designed properly, excessive and
damaging differential settlements may occur.
Soil Classification. From an engineering perspective, the
primary interest in soil is as a construction medium; that is,
whether a soil will be effective as a foundation base, provide a
stable embankment or cut slope, or serve as adequate backfill
material. The effectiveness of a soil for these purposes may
initially be determined by its classification using properties
such as particle size distribution and plasticity.
Particle size distribution, that is the gradation of a soil sample, is a form of textural classification. In the textural sense soil
is classed as gravel, sand, silt and clay. These textural categories
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F i g u r e 2 . 5 E Soil textural triangle.
separate the soil according to grain size. Although these textural terms are commonly used by various organizations and
agencies, the size limits for each particle class vary. Using the
size limits of the U.S. Department of Agriculture (USDA)
(Sand: 2.0 to 0.05 mm, silt: 0.05 to 0.002 mm, clay:
<0.002 mm), Figure 2.5E can be used to classify the soil once
the relative weight (in percent) of each constituent is known.
Textural classification relates the physical properties of
soils, particle size, shape, and soil composition. The particle
size distribution method is most effective for categorizing
cohesionless (i.e., coarse grained) soils whereas cohesive soil
(e.g., clays) characteristics are best described by properties
related to their plasticity.
There are several standardized soil classification methodologies commonly in use today that take into account
both the particle size distribution and plasticity, including
the unified soil classification (USC) system, the American
Association of State Highway and Transportation Officials
(AASHTO) System for roads, the Natural Resource Conservation Service system, and others (Table 2.5B shows differences in particle size limits for several selected agencies).
Geotechnical engineers will typically use the USC classification in describing soils for land development purposes.
More information about the USC system is included in
Chapter 7.3.
Although soil classification provides a description for
evaluating how the soil will likely behave, based on its
engineering properties, this alone cannot be the sole criterion for measuring the soil’s behavior. A detailed investigation of the soil, using subsurface exploration methods and
other detailed soil investigations and laboratory tests, as
appropriate, is needed to supplement the predicted behavior
based on generalized classification. Specific laboratory and
field tests provide reasonable assurance of the anticipated
performance of the soil as a construction medium.
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TA BL E 2 . 5 B
Particle-Size Limits for Selected Agencies
(Reprinted with permission from Fang, Hsai-Yang, ed. 1991. Foundation Engineering Handbook, 2nd ed., New York: Van Nostrand Reinhold.)
Types of Soil and Other Subsurface Materials. Gravels
and sands, silts and clays, bedrock, and limestone are introduced within this section.
Gravels and Sands. Gravels and sands are products of
weathered rock. Gravel ranges from a maximum size of
3 inches (7.62 cm) to 0.2 inches (4.76 mm). Crushed stone,
bank-run gravel or pea gravel are just some of the names
given to particular types of gravel, depending on origin and
gradation. If the gravel is dense and composed of sound rock
fragments, it can provide a good support for the building
foundation. However, if the rock fragments are weak, loose,
very rounded or overlie softer layers, they may not be suitable for foundations without ground improvement.
Sand materials are smaller than gravel ranging from 0.074
to 4.76 mm by the USC system. Sand can be described in
three sizes: fine from 0.074 to 0.42 mm; medium from 0.42
to 2 mm; and coarse from 2 to 4.76 mm. Both gravel and
sand are described as cohesionless materials. Sands and gravels are also described in relative density terms as very loose,
medium dense, or very dense. Sands and gravels together
can make excellent foundation materials if sufficiently dense
or compact. However, loose or poorly compacted sands
may yield excessive settlement. Depending on the situation,
sands by themselves may not be suitable for foundations,
embankments, and dams. Where subject to scour, the sand
may erode beneath the foundation or from an embankment,
02_Land_CH02_p017-124.indd 102
resulting in failure of the structure. Caution is necessary
when excavating in sands and gravels as these materials are
cohesionless and their slopes are not stable. Numerous utility
trenches have failed resulting in significant injury or death
to construction workers. High groundwater can particularly
affect the stability of sandy subsoils during excavation creating a running sand or a quick condition causing rapid flow
or collapse of the sand stratum.
Silts and Clays. Silts and clays are described as fine-grained
soils; both will pass through a very fine sieve (200 openings
per inch also known as a number 200 sieve). Both silts and
clays are classified by particle size and plasticity. Plasticity
and consistency are measured by ASTM D423 (liquid limit)
and D424 (plastic limits). The liquid limit (LL) and plastic
limit (PL) are boundary soil moisture contents wherein the
soil begins to behave as a viscous liquid or a moldable plastic material, respectively. When the clay’s moisture content is
below the PL, the clay behaves like a brittle solid. When the
moisture content is between the PL and LL, the clay behaves
like a moldable plastic material, and when it is above the LL
the clay behaves like a viscous liquid. Silts and clays can perform satisfactorily or very poorly under foundation loads,
depending on their moisture contents. Therefore because of
this sensitivity to moisture, when bearing foundations are
planned on silts or clays, it is critical during construction to
prevent these soils from becoming wet and absorbing too
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2.5
much moisture. Silts and clays are also described in consistency terms as being very soft, soft, firm, stiff, very stiff, or
hard. Silt particle size is barely perceptible to the human eye;
it is smaller, but still much coarser than clay. Clay is much
finer that silt, with the primary method of determining the
difference between clay and silt conducted by timing settlement of the particles in a column of water (hydrometer) in
the laboratory.
Bedrock. Bedrock is solid material comprising unweathered rock typically lying beneath surface deposits of soil such
as sand, clay, or gravel. The three classes of rock based on
geologic origin are igneous, sedimentary, and metamorphic.
Igneous rocks are formed by solidification of molten material. They are generally uniform in structure and lack stratification and cleavage planes. Examples of igneous rock are
granite, diorite, gabbro, basalt, and diabase.
Sedimentary rocks are products of weathering and disintegration of rock, then erosion and sedimentation, chiefly
by water. These rocks are formed by mechanical cementation, chemical precipitation, and pressure. Examples of
sedimentary rock are sandstone, limestone, dolomite, shale,
and chert. Sedimentary rocks typically have rounded grains,
stratifications, inclination of bedding planes, and abrupt
color changes between layers.
Metamorphic rock is formed by the alteration of igneous
or sedimentary rocks by heat and pressure. Typical metamorphic rock includes quartzite, marble, slate, gneiss, and
schist. Some features include the ease with which parallel
layers break into slabs. In general, harder and more sound
rock is less susceptible to scour or crushing.
Usually natural bedrock is regarded as the best bearing material for structural foundations; however, there are
conditions, such as sinkholes, close joints and fractures,
weathering and soil-filled seams, which can present problems. Bridge foundation failures have occurred due to scour
of rock or rock-like materials. Building foundation failures
have occurred due to sinkholes. Slope failures have occurred
due to unfavorable orientation of clay filled joints within an
otherwise sound rock.
The ultimate bearing pressure of rocks can be taken conservatively as the average compression strength of unconfined rock core samples. Commonly, the core samples
obtained during the subsurface investigation reveal that the
rock across the site has imperfections and fractures which
have a significant influence on rock behavior when external
loads are applied. The spacing of discontinuities is an indication of overall rock quality. The allowable bearing pressure of
rock varies depending on the quality of the rock and can be
arrived at by applying a factor of safety to the ultimate bearing
pressure appropriate to the quality of the rock. For example,
for a good quality rock with widely spaced joints and fractures and no soil seams the factor of safety may be as low as
2 or 3. For poor quality rock with very close joints, but
no soil seams, the factor of safety may be as high as 10 or
more. Rocks with soil seams or open joints require special
considerations.
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Environmental, Geotechnical, and Historical Considerations 103
Limestone. Limestone and dolomite contain substantial
amounts of carbonate minerals (CO3). Gypsum and anhydrite are salts composed of calcium sulfate (CaSO4). All of
these sedimentary minerals are highly soluble in water and,
consequently, the presence and development of subsurface cavities is a consideration, particularly where there is
groundwater movement through joints and fractures in the
carbonate tock. Marl and chalk are other similar types of
materials typically found near shorelines.
The surface collapse of soil overlying a subsurface cavity
is usually referred to as a sinkhole, particularly in limestone
and dolomite terrain. Where sinkholes are abundant the
terrain is known as karst topography. Open solution voids
in the bedrock allow the erosion of overlying soil into the
void by groundwater movement. This action, in turn, forms
a soil cavity at the bedrock surface, which will continue to
enlarge in soil above the bedrock surface until the soil can
no longer support itself and it collapses, forming a sinkhole at the ground surface. These conditions emphasize the
importance of positive surface and ground water control in
karst regions. An example of surface collapse and preparation of the bedrock surface for remedial measures is shown
in Figure 2.5F.
(a)
(b)
Figure 2.5F
dial measures.
(a) Surface collapse, (b) preparing bedrock for reme-
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F i g u r e 2 . 5 G Example of the water cycle from a sink hole
through an aquifer to a spring.
Sinkholes (Figure 2.5G) can occur where the rock below
the land surface is limestone, carbonate rock, or salt beds.
These rocks can be naturally dissolved by groundwater circulating through them. This process can dissolve the rock
creating, voids, spaces, and caverns underground. The
overburden soil usually remains intact until the underground spaces grow too large and the surrounding rock
weakens. If there is not enough support for the overburden above the spaces then a sudden dramatic failure of the
land surface can occur. These collapses can occur without
warning and, in urban developed areas, can have significant consequences.
The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and
Pennsylvania. See Figure 2.5H.
Problem Soils. Certain types of soils require special attention because of their poor or irregular performance characteristics. The areal extent of such soils varies and specific
Evaporite Rocks - Salt and Gypsum
Karst from Evaporite Rock
Karst from Carbonate Rock
(Modified from Davies and Legrand, 1972)
F i g u r e 2 . 5 H Rock formation distribution in the United States
(modified from Davies and Legrand, 1972).
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problem soils are indigenous to selected areas of the country.
The site may contain several isolated pockets of such soils
or may be a major deposit area. Identification and location
of problem soils early in the design process alleviates costly
delays for redesign or unanticipated costs for additional
excavation and reinforced foundation design. Additionally, provisions in local ordinances may require additional
explorations, testing, and special designs. In extreme cases,
the provisions may even preclude certain design elements
of the project.
Expansive clays, frost susceptible soils, metastable soils,
organic soils, dispersive clays, and normally consolidated
clays, overconsolidated clays, and underconsolidated clays
are introduced in this section.
Expansive Clays. Certain clays undergo severe volume
changes without any removal or application of external
loads. Such shrink and swell phenomena are the result of
changing moisture conditions. Soils that are most susceptible to such moisture induced volume changes are commonly
included in problem soils categories. Typically, these soils
have a high plasticity index (PI is the mathematical difference between liquid limit and plastic limit) and are predominant in the south and southwest United States. Local names
for such soils are gumbo, adobe, black cotton, black jack,
and others. Swelling of these soils can create high stresses on
foundations, walls, and slabs, even to the extent of distressing and cracking basement walls and lifting light structures.
Environmental factors greatly affect the behavior of expansive soils. Such factors as surrounding and underlying soils,
hydrology, slope angle and length, and vegetation can influence the behavior of these expansive soils.
Damage to structures on expansive soils may not become
evident for years after construction is completed. Shrinking of
the soil during dry periods causes gaps to form under footings and foundations thereby reducing the bearing support of
the soil. This results in uneven structure settlement causing
cracks in the structure. When the soil becomes moist again,
the expanding soil causes the foundation to rebound to near
original position. After several cycles of shrink and swell,
the structure does not return to its full original position; the
building deteriorates further, and damage becomes apparent.
Conversely, damage can be caused by excessive heave of the
structure: if the foundation floor slabs are constructed on dry
expansive clay, that is, during drought, the clay may later take
on moisture during a wet period or as a result of leakage from
a broken water or sewer line resulting in significant swelling.
Buildings do not have to be directly situated on expansive soils to develop problems. Experience in Fairfax County,
Virginia, shows that some houses have settlement problems
with footings located on soils 3 feet above the expansive soil
strata (Figure 2.5I).
Using expansive soils as backfill around basement walls is
not a recommended practice. Buildup of excessive pressures
against the wall is caused after the soil settles and moisture
is absorbed (Figure 2.5J). During dry periods, additional
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Figure 2.5I
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Environmental, Geotechnical, and Historical Considerations 105
Expansive soils underlying stable soil.
soil particles fall into the cracks of the soil. When the soil
expands again, the added soil coupled with the volume
increase creates significant pressures on the wall. The high
pressures and cyclic stress loadings may eventually cause the
wall to fail. Besides being a disruption and financial burden
to the homeowner, the resulting wall movement can cause
damage to sewer, gas, and water pipes, and precipitate secondary damages from those failures.
As illustrated in Figure 2.5K, large trees in expansive soils
near buildings can exacerbate the damaging effects of shrink
and swell phenomena. During a dry period the tree’s demand
for water can cause localized shrinking of the soil. The
decrease in volume in a small area coupled with increased
volume elsewhere near the house creates a wavy, uneven
ground surface around the structure. Large elm, poplar, and
willow trees can contribute to such failure; however, small
diameter trees (e.g., less than 12 inches) and shallow roots
result in significantly less failures. A recommendation is that
trees be planted more than one-half their expected mature
height away from the foundation.
When construction cannot be avoided on expansive soils,
there are remedies that can mitigate damage potential, some
of which can be costly. Such remedies include (Brown, 1988):
Figure 2.5K
Tree influence on expansive soils.
•• Excavate expansive soils and replace with granular
soil or other suitable material.
•• Construct the foundation below the shrink-swell zone
or near the water table (which then requires other
special design considerations for foundation design).
•• Design the foundation so that the dead loads totally
counteract the heave.
•• Use blankets of impervious soil adjacent to foundations and grade the areas away from the foundations
to prevent surface water infiltration.
•• Locate water and drainage lines to direct water
around and away from foundations.
Figure 2.5J
02_Land_CH02_p017-124.indd 105
Foundation wall damage from expansive soils.
Frost Susceptible Soils. Frost heave is a natural phenomenon and will occur wherever water in soil is exposed to sustained freezing temperatures. When the water in the voids
freezes, it expands about 10% which in itself does not create frost heave problems. Soil heaving due to freezing occurs
when saturated fine-grained soils, within the capillary zone,
above the water table form ice lenses or layers parallel to the
ground surface. Ground heaving increases as the ice lenses
accumulate and grow to thickness of several inches.
Frost heave is particularly damaging to pavement and
slabs due to the erratic distribution of the ice lenses, which
results in differential heaving and cracking. Figure 2.5L
shows the damage caused by frost heave of a ground level
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F i g u r e 2 . 5 L Photo of frost heave damage to a concrete floor slab.
floor slab. Notice the deformed metal wall studs buckling
from the upward displacement of the ground floor slab. The
formation of ice lenses is illustrated in Figure 2.5M (Sowers,
1979, pg. 138).
Nonfrost susceptible soils are those that cannot readily
support the capillary rise of water and therefore prevent the
formation of ice lenses growth. These soils are typically “free
draining soils” like clean coarse sands, gravel, and crushed
stone.
Typically, silty soils and very fine sands are prone to
objectionable frost heave because they facilitate capillary
Figure 2.5M
02_Land_CH02_p017-124.indd 106
rise. Frost susceptible soils are typically nonuniform soils
containing more than 3% particles finer than 0.02 mm and
uniform soil if more than 10% is finer than 0.02 mm.
Prevention of frost heave in footings is most easily
achieved by following the recommendations of the local
building codes, for example, planning footings at depths
below the local frost line. Controlling heave in slabs, walls,
pavements, utilities, and other construction components can
be achieved by removing frost-susceptible soils throughout
the depth of frost penetration and replacing it with nonfrost-susceptible soils. Alternatively, for pavements and
slabs a horizontal blanket of coarse-grained-free draining
sand, gravel, or crushed stone placed above the water level
will break the capillary tension forces. Care should be taken
when designing these blankets, as improper drainage or
inadequate thickness could aggravate rather than prevent
frost heave (Sowers, 1979, pg. 141). Figure 2.5N shows common frost penetration depths in the United States. Frost penetration depth also effects most utility designs, specifically
water, sewer, and storm. Minimum cover over these pipes is
typically greater than or equal to the frost penetration depth
to prevent freezing of the conveyed fluids.
Metastable Soils. Collapsing soils are those that decrease in
volume when they become saturated or when subjected to
vibration after saturation. These are called metastable soils
and are associated with fine sand-silt-clay materials deposited
along the base of mountains and along alluvial fan deposits
in arid and semiarid regions. Similar to expansive soils, collapsing soils are generally stable until the moisture content
changes. Additional moisture disrupts the clay and water
bonding that maintains the soil structure in its metastable
condition, collapsing the soil into a more compact and stable
condition often resulting in rapid and severe settlement.
Loess, a soil found in sections of the Midwest and Western United States, is the most common type of metastable
soil. Compacted loess is a satisfactory foundation material
for spread footings or mat foundations as long as the dry
unit weight is around 99 lb/ft3 or more. For shallow deposits (3 to 6 feet deep), compaction is necessary for the full
depth of the deposit or above the permanent water table
to ensure adequate performance. For loess with dry unit
Formation of ice lenses.
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Environmental, Geotechnical, and Historical Considerations 107
F i g u r e 2 . 5 N Max frost penetration.
weights less than 99 lb/ft3 pile foundations driven to underlying acceptable soil strata should be considered.
As previously mentioned, collapsing soils are typically
found in desert arid and semi-arid environments, alluvial
valleys, and playas. Gypsum and anhydrite are often present in or around such soils. In general, these soils are very
moisture sensitive and exhibit greatly reduced strength when
wet. Differential settlement occurs if the soils are spread over
a wide area or the structure is partially founded on pockets
of metastable soils.
Organic Soils. Those soils that contain significant amounts
of decayed and decaying vegetation matter are organic soils.
These soils have very little cohesive or friction strength and are
highly compressible, even under light loads. Aside from high
initial compression settlement, organic soils can be expected
to compress further over very long periods of time due to continual decay of the organic constituent parts. Organic soils are
characteristic of estuarine, lacustrine, and floodplain areas.
Peat, a common organic soil, is fibrous, partially decomposed organic matter or a soil containing 80% or more
fibrous organic matter. Peat soils are dark brown or black,
loose and extremely compressible. Muskeg is a type of peat
indigenous to Northwest Canada and Alaska.
Dispersive Clays. Certain types of soils will go into suspension and become highly erodible in the presence of water
02_Land_CH02_p017-124.indd 107
(flowing or static) despite being compacted. Such soils are
known as dispersive clays and often contain montmorillonite
or illite minerals. These dispersive clays are not uncommon
and are typically found in floodplain deposits, slope wash,
lake beds, and loess.
Normally Consolidated Clays, Overconsolidated Clays, and
Underconsolidated Clays. A soil is normally consolidated if
the maximum pressure on an element of soil mass is equal
to the present soil column pressure and the soil has fully
consolidated under that pressure. That is, the compression
state of the soil is due to the existing overburden weight.
Otherwise, if the soil mass compression behavior is due to
pressures larger than the present overburden pressure, the
soil is overconsolidated. Such overconsolidated clay strata
are common where significant removal of overburden has
occurred over time, such as from the retreat of glaciers, longterm erosion, land movement, or lowering of the groundwater table. Underconsolidated soils are those soils that are still
consolidating under their own weight.
Normally consolidated clays are subject to additional
long-term settlements under loads greater than the existing overburden pressure. Such long-term settlements can be
reasonably estimated from tests performed on undisturbed
samples (ASTM D2435). Under consolidated soils are soils
that are still settling under the current overburden pressure.
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2.5.11. Preliminary Investigation
Soil survey reports are often available from the Natural
Resources Conservation Service (NRCS). These reports
delineate and describe the soils of a locality generally within
the upper 6 feet. The very early NRCS soil reports were compiled mainly for agricultural purposes by the USDA. Current
reports now contain additional information usable in land
planning and engineering design. This information can be
accessed through the USDA’s NRCS website. The reports
describe the pedological soil series found in the locality, provide a map showing their location, and provide tables that
equate the soil series to engineering index properties. Other
tables provide data relating the soil series to wildlife habitat,
building site development, sanitary facilities, construction
materials, water management, physical and chemical properties, and hydrologic characteristics.
The NRCS soil survey report is an invaluable document for any development project. The information can be
used throughout various stages of the development’s design
process. As part of the preliminary investigation, the soils
should be mapped onto a scaled work drawing of the site. An
example of a soils map is shown in Figure 2.5O. Soils or other
subsurface conditions that are anticipated to present problems for the project (e.g., high water table, poor bearing support, highly erodible silt) are identified on the map. These
questionable areas need to be flagged in the field. The soils
and subsurface conditions in the questionable areas can then
be confirmed with supplemental data such as existing geotechnical reports for nearby projects or geotechnical investigations at the site. Although the NRCS soils report should
not be used as the sole source of subsurface conditions for
the project, it may allow the design of the project to proceed
expediently.
This preliminary soils investigation should be completed
during the desktop review. A preliminary subsurface exploration program is sometimes advantageous to gather sufficient data to perform preliminary calculations for size and
cost for foundations of major structures and to determine
if there are any subsurface conditions that will significantly
impact the project. The amount of data gathered in the
field during a preliminary program depends on the extent
of existing data and the need for additional information for
preliminary design purposes. Unanticipated field conditions
may warrant extra test pits, bore holes, and additional soil
samples. As details of the proposed project develop during
design, additional subsurface exploration may be required to
supplement the previously obtained data.
Field exploration programs should be carefully planned
and laid out to obtain adequate data for design of the project. While no set rules apply to methodically laying out
exploration points that cover all conceivable conditions,
sound judgment should supersede any generalized rulesof-thumb exploration techniques. The number and location of the exploration points depend on the prevailing soil
conditions, the variability of the soils, and proposed project
details. Highly variable soil conditions warrant an increase
in the number of exploration points to determine the areal
extent of the various soil conditions, particularly, problem
soils that may be encountered. The increased number of
exploration points, however, does not necessarily require
an increase in the number of tests for determination of the
soil’s physical properties and parameters. The number of
different types of soil within the stressed zone influences
the number and types of laboratory tests performed. The
following guidelines may be helpful in establishing acceptable boring locations:
•• Buildings: A boring at each corner and one in the
middle as a minimum. For larger building groups the
objective might be to locate and space the borings to
establish subsurface cross sections in perpendicular
directions.
Figure 2.5O
02_Land_CH02_p017-124.indd 108
Example of a soil map.
•• Dams: The objective is to establish geologic profiles
across the valley at the longitudinal axis, at the
downstream and upstream toes, and at all major
hydraulic structures such as spillways, outlet works,
and large culverts/conduits. Borings within the
impoundment area itself may help identify the
suitability of excavated soils for the dam embankment and the seepage/infiltration characteristics of
the area.
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•• Roads: Borings are located to obtain data for four different criteria:
○○
○○
○○
Shallow borings along the alignment are spaced
to identify and verify areas of similar soils as
anticipated from other data sources such as soil
maps and aerial photographs.
Borings are taken in the vicinity of major
structures such as bridge abutments, piers, and
retaining walls. The number of borings for abutments or piers typically is one or two. Borings
for retaining walls are placed to give longitudinal
and transverse subsurface profiles.
Borings are taken in cut areas to determine the
difficulty of excavation and slope stability. These
borings are more closely spaced and deeper than
the roadway alignment borings.
TA BL E 2 . 5 C
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Environmental, Geotechnical, and Historical Considerations 109
○○
Borings for borrow areas provide data on the
suitability of the soil to be used as fill material.
Depending on the size of the projected borrow
area, a grid pattern with borings at 200- to 400foot intervals may suffice.
Again it is emphasized that the actual location of the borings
should depend on project details, anticipated site conditions,
and what is actually encountered during the drilling. The
preceding generalizations are only to provide initial guidance. Other guidelines are presented in Table 2.5C.
2.5.12. Depth of Exploration
The depth of test borings depends on two factors; first, the
magnitude and distribution of the imposed loading and second, the subsurface characteristics. Since the basic objectives
of any boring program are to determine a subsurface profile and to identify engineering properties of the materials,
Guidelines for Boring Layout
Areas for Investigation
Boring Layout
New site of wide extent
Space preliminary borings 200–500 ft apart so that area between any four borings includes
approximately 10% of total area. In detailed exploration, add borings to establish geological
sections at the most useful orientations.
Development of site on soft
compressible strata
Space borings 100–200 ft at possible building locations. Add intermediate borings when
building sites are determined.
Large structure with separate
closely spaced footings
Space borings approximately 50 ft in both directions, including borings at possible exterior
foundation walls at machinery or elevator pits, and to establish geologic sections at the most
useful orientations.
Low-­load warehouse building
and large area
Minimum of four borings at corners plus intermediate borings at interior foundations sufficient to define subsoil profile.
Isolated rigid foundation,
2500–10,000 ft 2 in area
Minimum of three borings around perimeter. Add interior borings depending on initial results.
Isolated rigid foundation, less
than 2500 ft 2 in area
Minimum of two borings at opposite corners. Add more for erratic conditions.
Major waterfront structures,
such as dry docks
If definite site is established, space borings generally not farther than 50 ft, adding intermediate borings at critical locations, such as deep pump-­well, gate seat, tunnel, or culverts.
Long bulkhead or wharf wall
Preliminary borings on line of wall at 200-­ft spacing. Add intermediate borings to decrease
spacing to 50 ft. Place certain intermediate borings inboard and outboard of wall line to
determine materials in scour zone at toe and in active wedge behind wall.
Slope stability, deep cuts, high
embankments
Provide three to five borings on line in the critical direction to provide geological section for
analysis. Number of geological sections depends on extent of stability problem. For an active
slide, place at least one boring upslope of sliding area.
Dams and water retention
structures
Space preliminary borings approximately 200 ft over foundation area. Decrease spacing
on centerline to 100 ft by intermediate borings. Include borings at location of cutoff, critical
spots in abutment, spillway, and outlet works.
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borings should extend to a depth that provides a reasonable
comfort level for design analysis purposes. As in selecting the
location of borings, this is a judgment decision to be made by
the geotechnical expert and the person designing the foundation and structure. A suggested rule of thumb is “to carry borings to such depth that the net increase in soil stress under the
weight of the structure is less than 10% of the average load
of the structure, or less than 5% of the effective stress in the
soil at that depth, whichever gives the lesser depth, unless
bedrock or dense soils known to lie on rock are encountered
first”(American Society of Civil Engineers, 1976).
The following is presented as a guide for determining
boring depths:
•• Extend borings through any unsuitable and questionable
material into firm stable soils that are capable of sustaining the imposed loads without excessive settlement.
•• In residential areas where houses have relatively
shallow foundations, the depth depends on the
nature of the subsurface soils. Typically the depth
of exploration will be 5 to 10 feet below the foundation. In most cases borings will not be needed at
every dwelling location.
•• For other buildings with substantial loadings, the
depth of exploration beneath the footing extends 1.5
to 2 times the least dimension of the structure. In the
case where the loading is carried on piles or caissons,
the exploration must extend a sufficient depth into a
competent bearing stratum to carry the imposed loading, and to be sure that underlying weaker material is
not present.
•• In borrow areas the depth of exploration extends to
the depth required to provide the amount of suitable
material needed.
•• Borings should extend a sufficient depth into any
apparent rock to ensure the existence of bedrock
rather than a boulder.
Other guidelines are presented in Table 2.5D.
•• Explorations along roads are carried to depths of
±5 feet below subgrade in areas of light cut and
fill where no adverse subsurface conditions exist.
In areas of deep cuts and large embankments the
depth of exploration depends on the topography
and nature of the underlying soils.
2.5.13. Subsurface Investigation
A more in-depth subsurface exploration will be necessary with the final design of a project and is performed by
geotechnical engineers. It is recommended that the developer contract separately with the geotechnical engineer to
TA BL E 2 . 5 D
Guidelines for Boring Depths
Areas for Investigation
Boring Layout
Large structure with separate
closely spaced footings
Extend to depth where increase in vertical stress for combined foundations is less than 10%
of effective overburden stress. Generally, all borings should extend no less than 30 ft below
lowest part of foundation unless rock is encountered at shallower depth.
Isolated rigid foundations
Extend to depth where vertical stress decreases to 10% of bearing pressure. Generally,
all borings should extend no less than 30 ft below lowest part of foundation unless rock is
encountered at shallower depth.
Long bulkhead or wharf wall
Extend to depth below dredge line between ¾ and 1½ times unbalanced height of wall. Where
stratification indicates possible deep stability problem, selected borings should reach top of
hard stratum.
Slope stability
Extend to an elevation below active or potential failure surface and into hard stratum, or to a
depth for which failure is unlikely because of geometry of cross section.
Deep cuts
Extend to depth between ¾ and 1 times base width of narrow cuts. Where cut is above
groundwater in stable materials, depth of 4–8 ft below base may suffice. Where base is below
groundwater, determine extent of pervious strata below base.
High embankments
Extend to depth between ½ and 1¼ times horizontal length of side slope in relatively homogeneous foundation. Where soft strata are encountered, borings should reach hard materials.
Dams and water retention
structures
Extend to depth of ½ base width of earth dams or 1–1½ times height of small concrete dams
in relatively homogeneous foundations. Borings may terminate after penetration of 10–20 ft in
hard and impervious stratum if continuity of this stratum is known from reconnaissance.
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2.5
perform the work. Depending on the contract arrangements
between the developer and the site engineer, provisions may
be made so that the site engineer can contract directly with
the geotechnical engineer. On the other hand, the site engineer may have geotechnical staff and resources capable of
doing the subsurface analysis. Additionally, and unless they
possess the ability to do so within their firm, quite often the
geotechnical engineer subcontracts with another company
to perform the drilling, test pits, and collection of samples,
under the engineer’s direction.
2.5.14. Geotechnical Planning
Proper planning is one of the most important aspects of a
project. Many projects that have failed or had significant cost
overruns can be attributed to poor initial planning or insufficient coordination between the developer, design team,
and geotechnical engineer. Under preferable circumstances,
a project should be developed as follows:
•• Prior to the field investigation and relatively early
in the development process (concept and schematic
design phase), a meeting between the developer, site
engineer, other members of the design team, and
the geotechnical engineer should be scheduled to
discuss relevant project information and establish
the scope for the field investigation. Talking points
in this meeting should include
○○
Size, shape, and exact location of the project.
○○
Review of a site plan showing existing topography versus proposed topography; existing
structures and roads versus proposed structures and roads (including buildings, retaining
walls, headwalls, and bridges); and proposed
stormwater management facilities. The plan
should also show finished floor elevations of
both the existing and proposed structures. A
preliminary or schematic plan is acceptable for
this purpose.
○○
Type of structure, number of floors, basements,
structural loads (both wall and column loads),
column spacing and settlement limits.
○○
Major infrastructure needs including roads, site
drainage and stormwater management facilities
particularly if they are infiltration-based designs,
water and sewer mains, and any on-site wastewater treatment facilities.
○○
Determination of any required tree or sensitive
ecosystem (wetlands, resource protection areas)
preservation areas. Limits of clearing and grading
should be noted and discussed so as to avoid field
investigation in restricted areas.
○○
Assessment of cut and fill requirements.
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2.5.15. Geotechnical Proposal
With the information in the prior (Geotechnical Planning)
section, the geotechnical engineer can develop a detailed
scope of work to investigate the site to determine the subsurface conditions. If the investigation is conducted early enough
in the development stage of the project, significant cost savings can be realized. Early investigation may indicate that the
development of the site—the proposed structures, foundations, and earth work—may be more costly than is feasible.
The field investigation can include one or all of the following: test pits; soil/auger or wash borings with standard
penetration testing (SPT); cone penetrometer test (CPT);
and geophysical surveys. The geotechnical engineer’s proposal will typically include
•• Time frame to complete the investigation (field work,
laboratory testing, and report preparation)
•• Number and estimated depths of test borings and/or
test pits that will be performed
•• Type and number of laboratory tests that can be
anticipated
•• Estimated budget
It is important that the developer selects a geotechnical engineer that is familiar with the geology, soils, and local problems in the project area.
The more information that the geotechnical engineer can
obtain from the design team and from the field, the better the
quality of the report and design recommendations that can
be provided. The following sections provide more detailed
information regarding the field investigation process.
2.5.16. The Soils Report
The geotechnical consultant provides the site engineer with
the geotechnical information in the form of a report. This
information impacts not only the preliminary design and layout of the development, but also the final engineering design.
Some of this information may be used in developing part of
the specifications, which will impact the contractor’s approach
to construction and his bid estimates. Data included in the
report will be of specific use for different aspects of design
and construction. For example, the land development engineer is interested in permeability rates and groundwater elevations in order to design SWM facilities, bearing capacity as
it relates to appropriate pavement sections, and slope stability
angles for purposes of detailed grading plans. From a construction perspective, the earthwork contractor is concerned
about the difficulty of excavation, existence of bedrock, the
“rippability” of the rock, suitability of excavated material for
reuse, and the presence of groundwater. The foundation contractor is interested in confirming that the soil is capable of
providing suitable bearing support, or learning if there will be
difficulty in driving piles or drilling shafts, as well as knowing
their expected depths.
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The geotechnical report is typically organized as follows,
but depending on the nature of the project some of the listed
topics may not be included:
•• Scope and purpose of the investigation
•• General description of the proposed project to
include column and wall load, settlement limits,
infrastructure improvements, and building characteristics, such as finished floor elevations
•• Geologic conditions of the site that describe the
physiographic region, the geologic formations, and
predominant soils
•• Drainage characteristics and facilities
•• Methods and details of exploration program
•• Types and results of laboratory tests performed on
the samples
•• Description of site including topography, ground cover,
structures (if present), and any unusual conditions
•• Details of subsurface conditions determined from the
exploration and testing program
•• Groundwater characteristics and any seasonal or
tidal influence that may occur at the site
02_Land_CH02_p017-124.indd 112
•• Details of the analysis used for design evaluations
•• Conclusions and detailed design and construction
recommendations for critical project components
including footings and foundations, pavement sections, retaining walls if required or as a suggested
design alternative, and construction recommendations for such requirements as dewatering and slope
stabilization
•• Plan showing the locations of borings and proposed
building layout
•• Logs (results) of the borings (see Figure 2.5P), as well
as subsurface profiles
•• Photographs of the site at the time of field
investigation
•• Other recommendations regarding the foundation,
pavement, retaining wall, and construction considerations (to be further discussed in Chapter 7.3)
The information contained within a soils report is not a
guarantee of soil performance and is not intended to define
every aspect of the site’s soil condition. The report is a summary of collected data and recommendations by the professional engineer.
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Figure 2.5P
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Environmental, Geotechnical, and Historical Considerations 113
Boring log.
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PROJECT SPIRIT—NEW FEDEX GROUND DISTRIBUTION HUB FACILITY
Location: Ocala, Florida
Client: FedEx Ground
Completion Date: Design—May 2013, Construction—February 2016
Case Study: Dewberry was selected to provide due diligence services, civil engineering/site design, surveying and
mapping, environmental, permitting, and construction administration services for a new FedEx Ground distribution hub facility on a 150-acre site in the northeast corner of the City of Ocala, Florida. The final construction
plans called for approximately 580,000 square foot (SF) facility with supporting infrastructure. The facility was
designed for an ultimate build-out of the center to approximately 780,000 SF of buildings and ancillary buildings
on 190 total acres.
FedEx Ground was interested in developing a regional distribution hub for the state of Florida. The specific site was
identified based on FedEx’s internal parameters including distances to their offices and access to the interstate system.
The site also had to be able to support the future expansion anticipated for the proposed facility. However, the site
selected is located in an area that has predominantly karst soils, which allow the formation of sinkholes. FedEx requires
continuous operation; therefore, Dewberry not only addressed mitigation of the karst soil issues, but also designed the
site to prevent or minimize disruptions to site operations due to volatile soil conditions.
Working with the geotechnical subconsultant and the rest of the design team, a plan was developed to construct the
buildings and below ground gas tanks on a foundation design comprised of piles. The pile supported foundation is
typical of bridge design in Florida but is very rare as a building foundation type. The use of a pile foundation was
unique and greatly increases the life cycle of the facility by reducing issues that might negatively affect the operation
of the site.
To prevent sinkholes from occurring, the dry ponds were designed to “heal” the sinkholes with a bottom layer of loose
sediment that carries the water into the sinkhole. This fills the sinkhole and minimizes the hole or preventing it from
occurring. The pond design with the extra loose sediment to help begin to fill sinkholes that occur in the ponds is a new
design that the Dewberry team developed.
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2.5
PART C—HISTORICAL CONSIDERATIONS
Part C of this chapter focuses on the historical considerations
of a project—site conditions that should be evaluated based
on historic properties. This includes a review of historic
preservation regulations, definition of historic properties,
local preservation efforts, required due diligence, and evaluating potential impacts.
Historic properties include both historic architectural and
archaeological resources. This may include buildings, objects,
landscapes, viewsheds, or other artifacts of historical or cultural
significance. Land development by its nature is the creation of
“places,” each of which has the opportunity to establish and reinforce history. With this understanding, each developer bears the
responsibility to respect the past and to plan for the future.
Preserving the artifacts of our history is a reasonable
response from a culture that seeks a sense of continuity while
living amid constant physical change. Many people and
communities feel a decline in continuity as they witness the
changes in the built environment. Steady and rapid change,
be it building construction, increased vehicular traffic,
and/or cultural integration, contribute to the psychological
need people feel to live with a “sense of place.”
The information in this section is organized to broaden the
understanding of historic preservation within the framework
of ongoing land development and societal change. In order to
appreciate the importance of preserving our past, this section
begins with an overview of the historic preservation movement
in our nation and early regulations. The sections that follow
provide a discussion of the basic steps involved in the evaluation and impact assessments conducted for historic properties.
2.5.17. Historic Preservation Movement and Regulations
One of the first records of the historic preservation movement
is reputed to be Fairfax County, Virginia. It began in 1858, when
the Mount Vernon Ladies Association succeeded in saving
Mount Vernon, the plantation home of President George Washington. Their efforts were enhanced when the Commonwealth
of Virginia passed legislation, empowering the association with
the duty to manage, maintain, and restore the property as a place
of national importance for public education and enjoyment.
Figure 2.5Q
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Environmental, Geotechnical, and Historical Considerations 115
Comparable efforts can be matched by works of many
other organized and individual endeavors nationwide. Restoration began in Sturbridge Village, Massachusetts in 1859;
in the historic district of El Pueblo de Los Angeles in 1920;
Monticello, the home of President Thomas Jefferson in 1923
(Figure 2.5Q); the historic core of Charleston, South Carolina in 1930; and Colonial Williamsburg, Virginia in 1930.
These are but a few examples.
Key legislative actions by the United States Congress have
fostered the growth and importance of historic preservation.
They include the Antiquities Act of 1906, which concentrated
national attention on the protection of specific building and
archeological sites, and was largely spurred by park and monument building efforts starting after the Civil War. In 1949,
Congress chartered the establishment of the National Trust for
Historic Preservation, a quasi-public organization created to
bridge the gap between private citizen efforts and government
objectives to identify and preserve qualifying properties and
sites of historic significance. Congress passed the Housing Act
of 1961, which included requirements for the Secretary and
Department of Housing and Urban Development (HUD) to
identify, assess, and aid in the protection of historic resources
within the guidelines for urban renewal activities.
Congress passed the National Historic Preservation Act of
1966 (NHPA), as amended, which became the principal federal
law dealing with historic preservation. The NHPA establishes
a national policy of preserving, restoring, and maintaining
cultural resources. NHPA established the National Register of
Historic Places (National Register) and National Historic Landmarks, which protects historic sites. NHPA also encouraged state
and local preservation programs, including the designation of a
State Historic Preservation Officer (SHPO). The SHPO administers the national historic preservation program at the state
level, reviews National Register nominations, maintains data on
historic properties that have been identified but not yet nominated, and consults with federal agencies during Section 106
review. A key statutory provision of the NHPA is Section 106,
which requires federal agencies to consider the effects of their
undertakings on historic properties. This includes requirements
of identifying historic properties, assessing adverse effect, and
resolving adverse effects. This requires coordination with the
Monticello, the home of Thomas Jefferson, Charlottesville, Virginia. (Photo Courtesy of Cody Pennetti.)
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SHPO to ensure that the projects they authorize are not likely to
jeopardize existing cultural resources.
Other regulations are included in Section 4(f) of the U.S.
Department of Transportation (USDOT) Act of 1966, which
requires a review of transportation project’s effect to historic
properties. Cultural assessments are also part of the formal
environmental analysis required under NEPA, as discussed
previously. New projects are evaluated based on potential
effects to historic architectural and archaeological resources;
this includes assessing both positive and negative impacts to
the character, scale, or style of historic buildings and districts.
2.5.18. Historic Properties and Preservation Efforts
The consideration of potential impacts to historic properties is an important factor in many land development projects. Generally, resources are categorized as either known or
potential historic properties.
Known Historic Properties. Known historic properties
are those that are officially recognized and include resources
designated as National Historic Landmarks, properties listed
in or determined eligible for listing in the National Register, properties listed in or determined eligible for listing in a
State Register of Historic Places, and/or properties that are
designated as local landmarks.
In order to determine whether a property is eligible for
inclusion in the National Register, the National Register Criteria for Evaluation are applied, as follows:
The quality of significance in American history, architecture, archeology, engineering, and culture is present
in districts, sites, buildings, structures, and objects that
possess integrity of location, design, setting, materials,
workmanship, feeling, and association, and
A. That are associated with events that have made a
significant contribution to the broad patterns of
our history
B. That are associated with the lives of persons significant in our past
C. That embody the distinctive characteristics of a
type, period, or method of construction, or that
represent the work of a master, or that possess
high artistic values, or that represent a significant
and distinguishable entity whose components
may lack individual distinction
D. That have yielded, or may be likely to yield, information important in prehistory or history
Many state and local landmark laws adopt the National Register criteria for evaluating and defining historic properties.
Potential Historic Properties. Potential historic properties are those that are not officially recognized, but based on
field inspections and background research, appear to satisfy
criteria for official designation. If a project requires review
of potential impacts to historic properties, it is important to
understand the criteria used to define a historic property.
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For compliance with local and state regulations, review of
the regulation text will be necessary in order to determine
the definition. For federal undertakings, a historic property
means a property listed in or eligible for inclusion in the
National Register.
Local Preservation Efforts. While federal regulations of
historic preservation have been introduced, it is important
to also recognize and understand local regulations that are
commonly encountered that preserve and protect historic
properties. Most historic preservation efforts at the local
level begin with the comprehensive plan. Specific historic
preservation recommendations and other guidelines may
be adopted in accordance with the community’s vision and
goals. This could be included within a historical or cultural element of the comprehensive plan, as described in
Chapter 2.2, or in a separate historic preservation plan.
The jurisdiction may implement these recommendations and historic preservation plans through the adoption
of preservation requirements in the local zoning ordinance,
subdivision ordinance, and/or a separate historic preservation ordinances. These local ordinances may identify known
historic properties, while others may describe the processes
to identify and classify potential resources.
As an example, consider Savannah, Georgia, in Chatham
County. Within the Chatham County-Savannah Comprehensive Plan is a chapter on quality of life that includes a subsection on Historic and Cultural Resources. This element of
the comprehensive plan describes the dozens of historic districts that have been established that are either listed in the
National Register or are a local historic district. It is common
for municipalities to designate entire districts that contain a
group of buildings, properties, and/or site of historical significance. The Historic and Cultural Resources subsection of
the Chatham County-Savannah Comprehensive Plan states
that “the preservation and revitalization of these historic and
cultural areas is a primary goal in Chatham County.”
Savannah zoning regulations include an overlay zone for
the Savannah Historic District, which covers the majority of
downtown Savannah and was designated as a National Historic Landmark in 1966. The purpose of the historic district,
as defined in the zoning regulations, is to “provide for the
preservation and protection of historic buildings, structures,
appurtenances and places that are of basic and vital importance for the development and maintenance of the community’s vacation-travel industry, its tourism, its culture, and for
the protection of property values because of their association
with history; their unique architectural details; or their being
a part of or related to a square, park, or area, the design or
general arrangement of which should be preserved and/or
developed according to a fixed plan based on economic, cultural, historical or architectural motives or purposes.”
Savannah zoning regulations also state that “the historic
district regulations are intended to preserve and protect historic or architecturally worthy buildings, structures, sites,
monuments, streetscapes, squares, and neighborhoods of the
historic district.” This includes specific regulations and requirements on visual compatibility factors and design standards that
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affect development within the overlay district. Conformance
is required by all projects to ensure that historic properties
are protected or other development is compatible with those
historic resources. In addition, the regulations state that “in all
zoning districts within the boundaries of the historic district,
the regulations for both the zoning district and the historic district shall apply. Whenever there is conflict between the regulations of the zoning district and the regulations of the historic
district, the regulations of the historic district shall apply.”
In addition to the standard review process and permits that
would be expected in a development project (as described in
Chapter 2.4), the historic district overlay in Savannah requires
a Certificate of Appropriateness for all exterior changes, new
construction, demolition, signage, etc. within the historic district boundaries visible from the public right of way. This is
another layer of regulation which the development team must
consider in design development.
2.5.19. Historical Due Diligence
An important factor that may be overlooked during the initial stages of a land development project is the consideration
of potential impacts to historic properties. Just as sites are
researched and evaluated for their potential to contain environmentally sensitive areas, such as contamination and/or
wetlands, potential impacts to historic properties must also
be considered early in the planning process.
The level of consideration will be dependent on the
undertaking and the regulatory climate. Questions to ask at
the beginning of the planning process include
•• Does the proposed project require compliance with
any local historic preservation laws and/or regulations?
•• Are there any local ordinances that deal with the
treatment of historic properties?
•• Are there any locally designated historic resources on
the project site or in close proximity to the project
site that could potentially be impacted?
After reviewing local laws, it must be determined if the project is a state and/or federal action. State actions will require
compliance with the applicable state regulations. Any undertaking that receives federal funding, permitting, and/or
approvals will require review under NEPA, Section 106 of
the NHPA, as well as other applicable federal laws.
Once the determination has been made as to whether a
project will be subject to review under a local, state, or federal regulation, the next step is to identify whether any historic properties (including both historic architectural and/or
archaeological resources) are located within the project study
area. This study area should extend beyond the project site
to ensure that the project will not have a negative impact on
surrounding properties. Both a desktop review and site investigation research may be required in order to identify any
known or potential historical properties.
Known Historic Properties. Each project should begin with
a desktop review to identify any known historic properties
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within the study area. This can be completed in the office by
online searches of the National Historic Landmarks program,
State and National Registers, and properties that are designated
as local landmarks. Additional attention should be spent to
review any locally designated historic properties. Many municipalities often list designated properties in their comprehensive
plan and have other information including their historic preservation laws on their local websites. To identify locally designated historic properties, consultation may be necessary with
the municipal historic preservation or planning department to
obtain information regarding local landmarks.
Additionally, to identify known resources, it may be necessary to consult with the local SHPO. This often requires
coordination with the SHPO’s office or website to research
the records and determine if any historic properties are
located within the project study area. Information on both
historic architectural and archaeological resources can be
obtained from the SHPO. Consultation with SHPO can also
indicate whether there are other state-operated repositories
for additional information on historic properties.
Potential Historic Properties. To determine if potential
historic properties exist on a site, a site investigation may be
required (to be further discussed in Chapter 3.1). An initial field
inspection can indicate whether any buildings or structures exist
on the project site or within the study area. General information
about the project site’s historic development, prior usage, and
topographic features (such as its proximity to water) can help
determine its potential to contain archaeological resources. In
addition, visible land elements can be observed during the field
inspection to help indicate whether a site may contain archaeologically relevant areas. Foundations, ruins, walls, wells, pits/
dumps, and/or unusual vegetation formations are just a few
examples of land elements that may indicate that a property warrants further investigation. Typically, this level of effort must be
completed by a trained architectural historian or archaeologist.
When potential resources are identified as part of a state
and/or federal undertaking, a survey is often conducted in
order to document the property. Generally, an architectural
survey involves the completion of survey forms that record
descriptive and historic information about the subject property. Many states have specific guidelines for the format of
such a survey; consultation with the SHPO would be necessary to determine the level of documentation required. Upon
completion of the architectural survey, the survey forms and
possibly a summary report would be submitted to the SHPO
for review. The SHPO would then comment on the property’s
eligibility for listing in the National Register. For local undertakings, it is recommended to consult the local law in order to
determine the appropriate level of survey and documentation
that would be required. Again, this level of effort must be completed by a trained architectural historian or archaeologist.
2.5.20. Impacts to Historic Properties
If known or potential historic properties have been identified on a site, the level of required regulatory review (i.e.,
federal, state, or local) will also guide any subsequent, formal
impact assessments. As part of an impact assessment, a full
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understanding of the proposed project is required, as well as
any alternatives developed during the project design. Although
some land development projects may result in adverse impacts
to historic properties (such as physical destruction or visual
intrusions), it is important to note that land development projects can also produce positive impacts by remedying an existing condition that is detrimental to the historic property (e.g.,
poor drainage, erosion, limited access, etc.).
The goal of this section is to provide guidance for the
assessment of potential impacts when a historic property
may be affected by a proposed land development project.
Further, the basic tenets of preservation are discussed in
terms of site planning and design strategies that make use of
unique historic architectural or archaeological resources as
opportunities for project enhancement.
The next section reviews the required assessment, defines
impacts, and explain strategies to treat impacts.
Impact Assessment. Many land development projects that
are subject to environmental review require an assessment
as to whether historic properties may be impacted by project
activities. The level of assessment will vary depending on the
regulations being followed; the impact assessment may be an
independent report or may be part of a larger study, such as an
Environmental Impact Statement (EIS) within the NEPA process, which is looking at potential impacts to several environmental disciplines. Generally, as part of an impact assessment,
an overview of the proposed project, including the project limits
and study area boundaries would be provided. The next step
in the assessment would be to define existing conditions. This
would include the identification of any historic properties that
are located within the study area. The impact assessment would
then look at whether the proposed project would impact the
identified historic properties located in the study area. If adverse
impacts are anticipated, often consultation with the local community and local officials, as well as any involved state and or
federal agencies would be required in order to resolve adverse
impacts and develop mitigation measures.
A well-documented impact assessment is a time-consuming,
detailed, fact-gathering process, which serves to clarify cultural,
material, architectural, landscape and physical characteristics of
a site, district, person, or location. This effort is well worthwhile
when it comes to public hearings and community meetings on
the land development project: preservation is an extremely personal issue for many citizens and government officials. Proper
documentation is illustrative of a developer’s commitment to
the community, the established protocol, and to a high-quality work product whether the resource is preserved, reused or
removed. Generally, this work is performed by a trained architectural historian or archaeologist.
Defining Impacts. When assessing potential impacts to
historic properties, both direct and indirect impacts are considered. Direct physical impacts include demolition, alteration, or damage from construction on nearby sites. Indirect,
contextual impacts include the isolation of a property from
its surrounding environment or the introduction of visual,
audible, or atmospheric elements that are out of character
with a property or that alter its setting.
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Many municipalities have specific laws or ordinances that
define the level of assessment that would be required at the
local level. Many states also have specific requirements that
must be followed for state actions.
In general, a proposed project is deemed to have an
adverse effect if it would alter a historic property in a manner that would diminish any of the characteristics of the
property that qualifies it for inclusion in the National Register. Adverse effects on historic properties include, but are
not limited to
•• Physical destruction, alteration, or damage to all or
part of the property
•• Removal of the property from its historic location
•• Change of the character of the property’s use or of
physical features within the property’s setting that
contribute to its historic significance
•• Introduction of visual, atmospheric, or audible elements that diminish the integrity of the property’s
significant historic features
•• Neglect of a property which causes its deterioration
The above definition of an adverse effect can be useful
when trying to determine whether a proposed project may
result in adverse impacts to historic properties.
Avoidance, Minimization, and Mitigation. If historic
properties are identified on or near a proposed project
site, a primary goal of the design should be to minimize
any adverse impact to the historic property. If avoidance
is not feasible, then the next step is to minimize potential
impacts. Depending on the federal, state, or local regulation being followed, consultation with the local municipality and/or SHPO may also be required. If after careful
consideration of all avoidance and minimization strategies
it is found that a proposed project would adversely impact
a historic property, mitigation measures may need to be
developed. Often, the finding of an adverse impact will not
stop a project from moving forward, but will add additional
consultation and review time—all these factors can also
lead to additional project costs and increased time for the
project schedule.
There are many types of mitigation strategies that may
be employed to resolve adverse impacts to historic properties. At the federal and the state level, consultation usually
results in a memorandum of agreement (MOA), which outlines agreed-upon measures to avoid, minimize, or mitigate adverse effects. Some common examples of mitigation
include the following:
•• Photographic documentation (i.e., photographs of historic resources or archaeological findings documenting
their current condition before project activities begin)
•• Historic research (e.g., researching the significance
and background of properties within an appropriate
historic context)
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•• Salvage and or reuse of historic materials (e.g.,
salvaging historic materials from properties slated for
demolition and reusing these materials on other historic buildings or other aspects of the project design)
•• Context sensitive design (i.e., developing an approach
to planning and design based on active and early
partnerships with communities)
•• Adaptive reuse (i.e., adapting buildings for new uses
while retaining their historic features)
•• Public outreach (e.g., open communication with the
public through announcements, public materials,
public meetings, etc.)
•• Public informational displays (e.g., display materials
housed at local libraries, community centers, or other
public space)
Treatment of Historic Properties. The Secretary of the
Interior’s Standards for the Treatment of Historic Properties
(the Standards) provides guidance to property owners and
preservation professionals on how to protect historic properties. Although adherence to the Standards may not be
required to satisfy local or state regulations, they are important principles in the preservation field and can provide valuable guidance regarding the treatment of historic properties
for any land development project. Following the Standards
can help to avoid or minimize adverse impacts and can also
be utilized to develop successful mitigation measures.
As summarized from the Standards, four treatment
approaches are recommended: preservation, rehabilitation, restoration, and reconstruction. These approaches are
defined below in hierarchical order.
Preservation: The act or process of applying measures
necessary to sustain the existing form, integrity, and
materials of an historic property. Work, including preliminary measures to protect and stabilize the property,
generally focuses upon the ongoing maintenance and
repair of historic materials and features rather than
extensive replacement and new construction. New
exterior additions are not within the scope of this treatment; however, the limited and sensitive upgrading of
mechanical, electrical, and plumbing systems and other
code-required work to make properties functional is
appropriate within a preservation project.
Rehabilitation: The act or process of making possible a
compatible use for a property through repair, alterations,
and additions while preserving those portions or features
which convey its historical, cultural, or architectural values.
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Restoration: The act or process of accurately depicting the form, features, and character of a property as it
appeared at a particular period of time by means of the
removal of features from other periods in its history
and reconstruction of missing features from the restoration period. The limited and sensitive upgrading of
mechanical, electrical, and plumbing systems and other
code-required work to make properties functional is
appropriate within a restoration project.
Reconstruction: The act or process of depicting, by means
of new construction, the form, features, and detailing
of a nonsurviving site, landscape, building, structure, or
object for the purpose of replicating its appearance at a
specific period of time and in its historic location.
It is important to note that choosing the most appropriate treatment requires careful decision making about a property’s relative importance in history, its physical condition,
the proposed use of the property as well as mandated code
requirements that would need to be followed as part of the
proposed project.
Incorporation. Consideration of historic properties can
contribute to successful design development and project
marketing. In each case, the identification of unique land
features, artifacts, and historic architectural or archeological resources helped shape the land development project
design response. Historic properties became a component
of the larger development program, or by virtue of association, became incorporated into a unique marketing tool, like
name recognition. The process of identifying and assessing
impacts to historic properties should challenge development
and design professionals to a higher level of design, marketing, and financial solutions.
REFERENCES
American Society of Civil Engineers, Subsurface Investigation
for Design and Construction of Foundations of Buildings,
1976.
Brown, Thomas L. 1988. Site Engineering for Developers and Builders. Washington, DC: National Association of Home Builders.
Environmental Laboratory. 1987. U.S. Army Corps of Engineers
Wetlands Delineation Manual. Vicksburg, MS: U.S. Army Engineer Waterways Experiment Station.
Lerner, Steve, and William Poole. 1999. The Economic Benefits of
Parks and Open Space. San Francisco, CA: Trust for Public Land.
Also see http://www.tpl.org.
Mitsch, W.J., and J.G. Gosselink. 1986. Wetlands. New York: Van
Nostrand Reinhold.
Sowers, George F. 1979. Introductory Soil Mechanics and Foundations: Geotechnical Engineering, 4th ed. New York: Macmillan.
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PETER J. BIONDI ROUTE 206 BYPASS—CONTRACT A
Location: Hillsborough, New Jersey
Client: New Jersey Department of Transportation
Completion Date: October 2013
Case Study: Dewberry designed the Peter J. Biondi Route 206 Bypass for the New Jersey Department of Transportation.
The bypass allows drivers to circumvent the Township of Hillsborough, New Jersey, thus decreasing congestion, improving safety, and diverting traffic from the center of town. Preliminary and final design of 3 miles of a new limited access
bypass section of Route 206 included many engineering challenges, such as designing the roadway through environmentally sensitive land, while securing the appropriate permits and approvals, and addressing underground pipes/utilities
and rights of way, and other obstacles.
A unique situation was encountered where the bypass crossed a railroad track. This railroad is in historic district. An
architectural treatment was required to satisfy State Historic Preservation Office requirements. Due to the historic nature
of the railroad property, special features were applied to the bridge fencing, abutment and retaining walls, including
simulated cut rock, vertical face parapets, balustrade detail and black iron period-style fencing to meet the requirements
and blend with the surrounding area.
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BELLMAWR PARK MUTUAL HOUSING HISTORIC DISTRICT
Case Study: The I-295/I-76/Route 42 interchange is one of the largest and most congested intersections in southern New
Jersey, carrying large volumes of commuter traffic destined to and from Philadelphia via the Walt Whitman Bridge. It is also
a connection via Route 42 and the Atlantic City Expressway to the shore areas for weekend trips. Presently, the I-295/I-76/
Route 42 interchange does not provide a direct connection for I-295 through traffic. The existing interchange requires
motorists to reduce speed in both directions of I-295 so that they can safely negotiate ramps with 35 mph speed limits.
Dewberry is responsible for each phase of this $900-million direct connection from the initial preparation of environmental documents to completion of final design and through construction. The purpose of this project is to improve safety
and reduce traffic congestion at the interchange of I-295/I-76/Route 42 while providing a direct connection for I-295
through traffic. The project addresses quality-of-life issues as they relate to motorists, residents, and the environment.
The project was federally funded, so the I-295/I-76/Route 42 Direct Connection project advanced in compliance with the
National Environmental Policy Act (NEPA) process, which required an Environmental Impact Statement (EIS) in order to
determine a preferred alternative. Initially, a Purpose and Need Statement was agreed upon with stakeholders, and then an
extensive list of alternatives was developed. There were 26 build alternatives that were investigated and ultimately shortlisted to
five through an interactive process with the public and environmental agencies. These five build alternatives were then advanced
from an engineering standpoint to fine-tune the roadway alignments and profiles. Two of the alternatives included a new bridge
carrying I-295 over I-76/Route 42, two alternatives included a stacked roadway where I-295 southbound was over the top of
I-295 northbound as it crossed over I-76/Route 42, and the fifth alternative carried I-295 below I-76/Route 42 in a tunnel section. Preliminary bridge and retaining wall limits, stormwater management needs, construction staging schemes, geotechnical
concerns, etc., were then evaluated for each of the alternatives. From these conceptual roadway designs, technical environmental
studies were performed. An alternative analysis was performed to select the preferred alternative, which will carry I-295 over
I-76/Route 42 on a six-lane structure. In all, 13 bridges, two culverts, 16 retaining walls and noise walls are proposed.
During the NEPA process, a unique historic district was identified. In addition to NEPA, analyses were required pursuant to
Section 106 of the National Historic Preservation Act, among others. As a first step in the due diligence process for historic
resources, an architectural survey was conducted to identify any unknown historic properties. The Bellmawr Park Mutual
Housing Historic District was identified as part of this process. It is interesting to note that at first glance, the buildings that
are located within this historic district would not seem important—they are predominantly early 1940s ranch-style dwellings,
containing two to four attached units, with very simple designs and a lack of ornamentation. But, the detailed background
research revealed that the Bellmawr Park Mutual Housing complex was historically significant as a mutual housing development. So, since the historic district was not significant for its architecture, it could have been overlooked if not for the process
of having trained professionals conduct the architectural survey and the associated background research.
The Bellmawr Park Mutual Housing Historic District was constructed in 1942 to house workers and their families
employed at the New York Shipbuilding Corporation. It is significant for its association with the development of
the mutual housing concept associated with World War II-era defense housing projects. It was designed by progressiveminded architects and planners who promoted the use of European-inspired architectural styles and progressive
community planning ideas. The Bellmawr Park Mutual Housing Historic District is also important for its association
with innovative housing design and construction methods first utilized on a large-scale basis for World War II-era
defense housing and subsequently popularized for modern-era housing. The historic district is distinguished by its
unified building types that convey a cohesive form of mid-20th-century worker housing. Bellmawr Park still functions
today as originally conceived—as a mutual home ownership development comprised of 500 residential housing units.
Typical type A building, Beechwood Place, view northwest.
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As part of the proposed roadway improvements, five buildings within the historic district needed to be demolished, thus
resulting in an adverse effect to the historic district. Extensive consultation with the local community and involved agencies as well as the New Jersey Historic Preservation Office was conducted in order to develop measures to minimize and
mitigate the adverse effect to the historic district. A memorandum of agreement was prepared that stipulated the specific
mitigation measures that would be undertaken. These measures included photographic documentation of the buildings
slated for demolition so that there would be a permanent record of their appearance within the historic district. This
work was conducted pursuant to the requirements of the Historic American Buildings Survey, which defines standards
for archival documentation and photography. A registration form was also prepared to officially list the Bellmawr Park
Mutual Housing Historic District in the National Register of Historic Places. In addition, a conservation plan was developed to assist the Bellmawr Park Mutual Housing Corporation in the storage and long-term conservation of historic
documentation and archival materials. Finally, since five new buildings would be constructed within the historic district
to replace the buildings that would be demolished, a feasibility assessment was conducted in order to identify suitable
locations within the historic district for the replacement housing. A set of design guidelines was also developed for the
new housing units so they would be in-keeping with the character of the historic district.
Typical type C building, Carter Avenue, view southeast.
The I-295/I-76/Route 42 Direct Connection project began in the summer of 2000. The EIS was completed in 2008 and in
March 2009, FHWA officials gave final environmental approval for the project. In the summer of 2010, Dewberry submitted
Right-of-Way and preliminary design documents. Construction of an advanced ITS Contract is complete. Contracts 1, 2,
and 3 are under construction, with Final Design for Contract 4 underway. The estimated construction completion is 2021.
Beechwood Place streetscape, view northwest.
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LITTLE MUNCY CREEK PRATT TRUSS
Location: SR 2069 Moreland Township, Lycoming County, PA
Client: Pennsylvania Department of Transportation District 3-0
Completion Date: December 2015
The Pennsylvania Department of Transportation’s (PennDOT) Engineering District 3-0 recently completed an unusual
bridge replacement project in the rural community of Moreland Township, Lycoming County, Pennsylvania. Known
locally as the Little Muncy Truss, the S.R. 2069, Section 001 bridge carries S.R. 2069 (Moreland Township Road) over
Little Muncy Creek, part of the Susquehanna River watershed. The bridge, originally built in 1904 by the Owego Bridge
Company of New York, is a contributing resource to the Smith/Wallis Gristmill Historic District within this predominantly agricultural community.
A valued resource
The Little Muncy Truss is considered a local landmark in the Moreland Township community. Surrounded by farmland, the
bridge lies along the western edge of the historic district, composed of 14 buildings that date as far back as the circa-1796 gristmill. The bridge is regularly used by residents, including farmers who rely on the structure for deliveries and crop transport.
The Little Muncy Truss is also notable for its rare structure type. The existing bridge was a single-lane, single-span,
113-foot-long steel Pratt truss, with the steel members held together by pins. A common construction approach in the
early 20th century, only two similar pin-connected Pratt truss bridges carry vehicular traffic in Pennsylvania today, making the rehabilitation project unique and somewhat challenging.
Following extensive deterioration over the years, including section loss along the lower eyebar chords, the bridge’s weight
limit had been downgraded to five tons. In replacing the structure, PennDOT sought to remove the bridge from the structurally deficient list and increase the load capacity to 17 tons. This would accommodate use by township vehicles, school buses,
emergency vehicles, and large farm delivery trucks that had been prevented from using the truss for several years. PennDOT
also placed a priority on retaining the bridge’s historic integrity, while minimizing long-term maintenance requirements.
INCREASING LOAD CAPACITY
PennDOT District 3-0 selected the consulting firm of Dewberry to design a six-panel, pin-connected truss, based on the
firm’s prior experience with truss rehabilitations and replacements. Initially, the team planned to complete the design and
detailing using as-built plans, old fabrication drawings, and prior inspection notes. During a site visit to gather samples,
however, the team noticed that the skew angle of the truss did not match the as-built plans or shop drawings. The team
completed new documentation with the correct structural geometry in order to proceed with the reconstruction design.
The Pennsylvania Historical and Museum Commission was kept informed of the design process because of the bridge’s
contribution to the historical district. The team investigated reuse of the existing members to retain as much of the original bridge as possible, but as a result of the widespread deterioration, only the decorative lattice railing could be salvaged.
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The replacement structure, which features a 110-foot single-span length and a 14-foot
clear roadway width, was designed to retain the original aesthetics while significantly
improving the load capacity. To increase the load ratings and as added safety precautions, the design included catch plates, additional counters, higher strength steel, and
an increase to the size of critical members. These measures allowed all load postings
to be removed and the bridge to rate for PennDOT’s design vehicles.
MINIMIZING DISRUPTION
With direct coordination between Dewberry’s engineers and the fabricator, the
bridge was erected on site with no preassembly at the fabrication yard to verify
fit-up, an approach that reduced time and cost. The entire truss was assembled on
a staging area beside the bridge abutments and swung into place by a single crane,
drawing a crowd of local residents interested in the construction. Dewberry provided a plan of the minimal essential members to be installed to swing the bridge
safely into place while maintaining its structural integrity. Minimizing the pick
weight of the crane by only installing critical members allowed for a more economical crane size, reducing both cost and environmental impact. This method
eliminated the need for an invasive causeway to be installed in the stream, which is
classified as both a Cold Water Fishery and a Migratory Fishes Stream.
The structure was designed to be above the 100-year floodplain and above the 1972
flood of record. The embankment was stabilized with large riprap to provide resilience to major flood events.
The new bridge retains the character of the historic structure, drawing appreciation from the many residents who carefully followed the design and construction process. With all posted load restrictions removed, the bridge is now able to
be used by emergency vehicles, school buses, and farm vehicles. The $1.5 million project received an “Excellent” quality
rating from PennDOT, earned an Achievement Award from the Association for Bridge Construction and Design, and is
a featured project for the PennDOT Connects initiative.
Contribution by Curtis D. Sanno, P.E.
Featured in the Road&Bridges Magazine
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Chapter 3
Site Analysis and Engineering
Fundamentals
Chapter 3.1 focuses on the site analysis of the project, the continuation of the land development design process from the due
diligence (see Chapter 2). The site analysis evaluates the physical characteristics of site while due diligence phase focuses on a
regulatory assessment. Additionally, the engineering feasibility
study, site inspection, and reading a plan set will be introduced.
Chapter 3.2 introduces the surveying and preparation of
base maps, which are used as the foundation of design work.
Prior to design work, a site diagram is often developed to
show the spatial relationship of proposed site features—this
process is introduced. Sources of data used in a base map are
identified in this chapter as well.
Chapter 3.3 introduces roadway design fundamentals,
starting with functional classifications of roadways. The
different parts of roadways are introduced so a designer
can understand which road systems are appropriate for the
design of a site. Considerations for road use, street patterns,
parking requirements, and other transportation systems for
a site are introduced as well.
Chapter 3.4 provides information on grading strategies,
beginning with an introduction to reading a topographic map
PRE-DESIGN
and understanding contour lines. Considerations for grading, and how earthwork can influence the project design, are
identified in this chapter. Fundamentals of grading provide
a foundation for design considerations for a site—grading is
more of an art than a science.
Chapter 3.5 includes information related to hydrologic
analysis and different methodologies used for determining stormwater runoff for a site. The runoff computations
are used when designing stormwater management systems,
which are introduced in this chapter (detailed descriptions of
different systems are identified in Appendix 7.2). Procedures
for preliminary sizing of stormwater systems are identified to
help with early design efforts.
Chapter 3.6 is separated into Part A, B, C, and D to reference storm drainage, sanitary sewer, water distribution, and
dry utility systems, respectively. This chapter identifies the
different materials and components that are used for utility
systems, as well as context for checking capacity and demand
of utility systems. Design fundamentals of each utility system
are introduced, which inform early design decisions of the
utility networks.
DESIGN
POST-DESIGN
Preliminary
Detailed
CHAPTER 2
CHAPTER 3
CHAPTER 4
CHAPTER 5
CHAPTER 6
Due Dilligence
Site Analysis
Conceptual &
Schematic Design
Final Design
Permits &
Construction
F i g u r e 3 . 1 A The land development design process.
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Chapter 3.1
Feasibility Study,
Site Inspection and
Plan Sheet Comprehension
3.1.1. Introduction
A project often begins with the development team asking,
“What is the highest and best use of this site?” This initial
question requires an investigation into the information
identified in Chapter 2 related to the parcel zoning, comprehensive plans, and other applicable site regulations. The
experienced site engineer must also recognize the unspoken
parts of those questions that are perhaps more indicative of
the level of investigation and commitment required: “Can I
make a profit?,” “Is the expense and effort worth the return?,”
“Can I complete the development in a reasonable amount of
time?,” “Will people buy what we have to offer in a predictable period of time?,” “Can I make a valuable contribution to
the community and my reputation?” Answering these questions requires extensive evaluation of all the components,
participants, and dynamics of land development that affect
the project and property.
The site analysis phase of a project is generally a predesign effort associated with the due diligence phase. The
site analysis provides context for the requirements of site
design. This will apply the contents of Chapter 2, including the local zoning documents and environmental regulations, to a specific project site to understand the limitations
and development potential. In addition, the site will be
further analyzed to determine if and how the property can
be developed with an emphasis on physical characteristics
(terrain, infrastructure, soils, etc.). The information gathered during this phase of work is critical because all future
design and development decisions will be based on the
information uncovered. At the end of the site analysis, a
developer should be able to choose whether site is viable for
the program proposed. A developer will likely continue to
review the viability of the proposed development program
through all design phases, but the site analysis is the first
step in moving a project forward.
Chapter 3 includes information related to all aspects of
site engineering and items to consider during the site analysis phase of work, but an expert understanding of site engineering is required to perform a site analysis. The content
from all other chapters within this book will inform the site
analysis. Much of the content introduced in Chapter 3 can
be used to determine if there are easily identifiable “deal
breakers” (or red flags) for the site. Poor traffic access, lack
of utilities, difficult terrain, poor soils, zoning challenges,
and other conditions should be evaluated early in the project. Some deal breakers can be identified during site analysis
but require a more detailed understanding of planning and
engineering issues that may arise. Historic knowledge and
regional context can also provide valuable insight into the
development opportunities for a site. The development team
has a lot to consider during the site analysis phase, often
with very little information available. This phase is also challenging (especially for technical professionals) because the
deliverables lack finite solutions and decisions must be made
from the available information.
One of the products of the site analysis phase is a feasibility study. The feasibility study documents all information
gathered during the site analysis and due diligence phases
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3.1
of work and should also identify any gaps in information or
assumptions that have been made. The study will likely be
referenced in later phases of the project as a basis of design.
Another product of the site analysis is the base map and
subsequent site diagram. The site diagram uses the base maps
(identified in Chapter 3.2) to illustrate the constraints and
opportunities that are identified during site analysis. The
diagram is a graphic depiction of the project’s site conditions
but is roughly developed such that is still considered a predesign effort.
After the site analysis is complete, the conceptual
designs can be produced with additional detail added during the schematic design phase (both to be discussed in
Chapter 4). Finally, after approval of the design (by the
developer or preliminary approval by the jurisdiction), the
final engineering design can commence (to be discussed in
Chapter 5). This entire process from the site analysis, into
the conceptual and schematic design, and eventually into
final engineering design is a cohesive effort but is an iterative process. Initial assumptions and preliminary designs
will be refined throughout this process. It is important to
understand that generally the design process is not linear,
and initial concept plans never make it to construction in
their entirety.
Chapter 3 introduces the content necessary to perform
a site analysis, including the fundamentals of all aspects of
site engineering. This level of introduction should enable
the site engineer to identify existing site features, and produce the feasibility study and site diagram. This can also
allow the site engineer to produce the conceptual designs
as well as early schematic designs (both to be introduced in
Chapter 4). More detailed information regarding information topics (that may be required for schematic designs) will
be introduced with final design in Chapter 5.
This text presents the design process required in a site
analysis and final design in a typical sequential order, by
first focusing on larger site features and then looking at
more detailed site elements. Understanding the whole process helps to produce the best design for a site. Therefore, an
iteration of designs may be required as the design process
is learned and conflicts are identified between different site
elements. With time, the designer can improve their work
by having greater experience with the process.
3.1.2. Engineering Feasibility Study
Land development is highly regulated at all levels of government. The developer may spend considerable amounts
of money to demonstrate that the development program
and the design comply with those regulations. This must be
done without any guarantees that construction will ever be
authorized or profit realized. In addition, because the design,
processing, and construction period can take several years,
even a well-conceived project may find no buyers. Economic
and demographic forces often change while the development
program remains essentially the same.
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In a competitive market where vacant land is in short
supply, decisions must be made in a relatively brief period
of time. Otherwise, the land may be lost to another buyer,
potentially a competitor. In periods of tight money supply
and slow economic growth, the decision to purchase must
be well reasoned. The developer must be assured that the
investment will provide economic return. This is particularly
true if rising land prices have not abated, despite weakness in
the real estate market. The developer must base the commitment of resources to purchase land on a determination that
the land will have future value and use.
Land development is a risky business. To help abate
that risk, an engineering feasibility study is often required
fairly early in the development process in order to identify
problems likely to be encountered during planning, design,
government review, and construction, as well as to more
resolutely determine potential uses for the land. It is this
engineering feasibility study that aids a developer in answering their basic questions and minimizing the risk incurred
in purchasing land with the intent to develop or redevelop.
Such questions include
•• What are the physical characteristics of the site?
Are they conducive to the type of land development
envisioned?
•• What regulations are applied?
•• What are the costs involved in providing infrastructure to the envisioned development?
•• What is the timing of the design and approval
processes?
An engineering feasibility study is ideally commissioned
and completed before the land is purchased. This study can
take place either before negotiations with a landowner or
during a purchase contract option period. If the study suggests that an unfavorable price has been asked for the property, the developer has an opportunity to reject the purchase
or renegotiate the price. Meanwhile, the land is protected
from purchase by other buyers. Failing to perform a thorough investigation can lead to costly mistakes and, in some
cases, expensive lawsuits. At their first meeting, the site engineer should advise the developer of the importance of performing an engineering feasibility study as part of the scope
of services.
To perform such studies, one must understand the utility,
value, and potential use of a tract of land. These are based
on a number of underlying principles concerning physical
and economic characteristics common to all land. Physical
characteristics of land include its immobility; that is, it has a
fixed location and cannot be moved to avoid or take advantage of other locational factors. In addition, from the government’s standpoint, its fixed location makes it easy to regulate,
tax, and attach. Land is unique, with no two properties being
identical in size, shape, elevation, view, or access to natural
and man-made resources. Notwithstanding certain natural
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forces that cause erosion, inundation, or landslide, land is indestructible. This reflects a stable investment. Economic characteristics include its scarcity, with no more land being created
and unused land in increasingly short supply. Land exists in
a discrete quantity. The nature of improvements affects not
only the value of the land but that of adjoining land as well.
Land represents a permanence of investment. While certain
improvements can be destroyed, the public infrastructure usually remains that the investment is characterized as long term
and stable. Perhaps most important are the public’s area preferences to location, which result in similar structures being valued differently in alternative locations (Fillmore et al., 1988).
The value of land is affected by government regulations that control its development and use. These laws in
many ways supersede and restrict the rights associated with
land ownership while protecting the public health, safety,
and welfare. Development potential may be severely constrained by environmental factors. As a consequence, site
development may not provide sufficient yield to return a
profit. The cost of securing development approval or complying with local, state, and federal regulations may be so
severe as to detract from the land’s value as a development
site. Changes in development regulations may also affect
parcel value.
Physical constraints and opportunities of the site need
to be identified and quantified to assist the developer in
evaluation of the property. In addition to these inherent
characteristics are those that affect its physical adaptability
to certain uses. For example, consider two parcels of waterfront property: one flat and the other steep. The likelihood
that the latter can be used for maritime purposes is limited.
In addition, practical limitations on the mobility of people,
automobiles, machinery, and equipment affect convenience
and ease of certain uses, especially when coupled with climatic differences. Roads, parking areas, and service drives
must be designed to accommodate these limitations. For
instance, if a road must be built on steeply sloped land, it
may not be easily negotiated by an automobile. In climates
where snow and ice are a factor, such a road may be rendered impassible for days or weeks at a time. The slope of
sidewalks and trails must accommodate human capabilities.
Long, steeply sloped walkways and stairways from streets
and parking areas can be a hindrance to occupants as well
as a market deterrent. In many instances, the law requires
that access by people with disabilities must be considered.
Graded slopes must safely accommodate maintenance
equipment, such as mowers. While site grading during construction attempts to eliminate practical conflicts, existing
topography and local ordinances sometimes make extensive regrading economically infeasible. The purpose of the
engineering feasibility study is to establish a framework for
making such determinations.
Land developers who routinely operate in most communities are likely to be familiar with the land development
process and expectations. The developer often is sensitive to
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the area’s real estate market and can visualize project layout.
Many are familiar with local attitudes and the political motivation of area leaders, but some developers may be unfamiliar with the locality or with development in general. Most
developers engage the development team during the land
acquisition (option) phase because of the need to assemble
information prepared by unbiased professionals. Even if it
was feasible to employ a large full-time staff of development
specialists, their judgment might be considered prejudicial
in negotiations with property owners. In addition, the site
engineer brings the benefit of the experience gained in working with many clients and projects. This cumulative knowledge about the process is valuable to even the sophisticated
developer.
In cases where rezoning is inevitable, there will be political and legal issues that will have to be addressed; an attorney
familiar with zoning should be retained for these purposes
(refer to Chapter 2.3 for more information on zoning). The
site engineer may act as technical advisor to the attorney
and developer; however, the engineer should refrain from
offering legal advice. It is important to note, though, that the
development team will become involved with the interpretation and application of zoning ordinances and comprehensive plans. These interpretations are usually as important as
the laws themselves and become central to evaluation of the
development for approval.
Often, the site engineer performs this analysis and prepares the feasibility study in coordination with other members of the design team as necessary. It is important for all
members of the design team, whether they are involved in
the production of the feasibility study or not, to understand
the investigation process and the study results.
An example of a preliminary engineering feasibility study
appears in the appendix in Chapter 7.6. Refer to this report
to understand the scope and process and information contained within the engineering feasibility study as described
in the following sections.
Scope and Process of the Study. The engineering feasibility study should evaluate the physical, environmental, regulatory, and/or other constraints that must be overcome or
accommodated in constructing the intended use. This study
is usually produced by the design team for the developer and
their business team. The results of this study often affect the
purchase price of a property, which is frequently based on
presumed development potential. Incorrect assumptions
about development potential frequently prove to be in error
because of physical, locational, or external characteristics not
properly considered.
The feasibility study is important in providing legal protection to the prospective owner or developer. As discussed
in the preceding chapters (Chapter 2.5), the undetected
presence of wetlands, endangered species habitat, hazardous waste, or other environmental concerns could subject
the owner to expensive cleanup operations or litigation
under federal laws. The engineering feasibility study is often
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3.1
performed concurrently with the previously described environmental studies and usually references or even includes
them as a component part or appendix.
The developer will use information from these studies to procure loans and begin the project’s “go, no-go”
decision-making process. For this reason, the feasibility
study should be completed before the actual purchase of
the land. The developer should insist on having a study
period established as a contingency clause in the purchase
contract. The purchase of the land may hinge on the information in the study as well as other contingencies. Time
and money are the major concerns of the developer. The
study is performed with developer’s funds. If the developer
elects not to purchase the land, this is the money that will
not be recouped from the project. For this reason, it is to
the developer’s benefit to incur as little expense as possible
at this early stage. Some purchases may be contingent on
whether a rezoning or other entitlement application (such
as special exception, variance, or subdivision) is approved,
as described in Chapter 2. Others may hinge on whether a
minimum number of “buildable” lots can be obtained,
which will be described with yield studies in Chapter 4. It
should be emphasized that, where there is a clear intent to
rezone a parcel, land purchase contracts under consideration by the developer should include zoning contingency
clauses such that if approval is not granted by the governing body, the developer is released from the obligations of
the contract.
Because the seller wants to obtain maximum price for
their land and sell it in a reasonable amount of time, the
study period specified in the contract may only be on the
order of weeks or months. The buyer may be able to negotiate a longer time period within which to conduct the analysis.
Normally, however, the buyer must be willing to compensate
the seller for extraordinary periods of time. A higher purchase offering may be needed to extend the buyer’s purchase
option period. Because of the financial risk, the client needs
accurate information in short period of time. The decision to
exercise a purchase option will be based on the information
in the feasibility study.
Most experienced developers will know the lowest
value of a cost per unit that renders the project uneconomical. In practice, the developer may study several
alternative uses based on the information compiled during the assessment period. This helps determine what uses
are economically feasible or whether the land development project can be profitable. Depending on the skill of
the design team, a seemingly poor site may be rendered
profitable. In some heavily developed areas, there may
be few options among alternative tracts of land—forcing land developers and design teams to more carefully
consider less desirable sites. Some sites are ignored during earlier stages of a community’s urbanization because
of undesirable characteristics, but in time the surrounding area may develop and make the site more valuable.
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Such sites may now be candidates for development if the
higher development costs are compensated for by greater
yield and current market prices.
The intent of the study is to identify development constraints, or “red flags,” along with options to minimize
or alleviate those constraints. The study does not always
specify preferred solutions. Those will be addressed later,
if and when the developer moves forward with the purchase and development of the tract. Some site constraints,
when recognized early in the process, can be accommodated by the plans and used advantageously in a variety of
ways—most commonly as site amenities or for marketing
and branding purposes. For example, a wetland could be
perceived as a major constraint, or it could be incorporated
into the development as a unique amenity with footbridges.
Whereas the reviewing agencies were initially opposed to
developments that impinged on the wetland area, incorporating it as an amenity could help convince the locality to
approve the project.
The study approach varies with the developer’s intentions,
preconceptions, and circumstances related to the property,
defined in the development program. If the developer knows
exactly what land use will be constructed and the zoning is
compatible with that use, the feasibility study will analyze
the site in accordance with that use and zoning. If the use is
uncertain, the study will identify land use options based on
the potential of the land. The potential of the land considers the ultimate density (derived from existing and master
planned zoning designation) in context to the existing and
planned infrastructure and public facilities.
The developer may wish to consider several options and
multiples sites; the development team may need to perform
several feasibility studies if there is a significant variation in
land use permitted by the possible zoning. Each will have
unique elements—location, topography, zoning, access,
and/or infrastructure systems—that separate it from other
sites. In this scenario, a site selection study should be performed to evaluate the sites based on similar metrics to
determine which site is best suited for the development program (to be discussed in Chapter 4.1).
Frequently, the client will require a cost estimate for the
construction of certain items in the study, such as utilities,
road, and other infrastructure improvements. Additionally,
unusual or extraordinary costs will be identified and estimated. This will help in assessing the economic feasibility
of the site. These preliminary cost estimates are discussed in
Chapter 4.
The feasibility study must be well organized and is usually supported by maps, photographs, and other graphics. It
is often, but not always, presented in report form, although
annotated base and topographic maps may suffice, depending on the client and the complexity of the project. Any
biases or opinions by the author should be included only
at the request of the client and stated as such. The document will serve as evidence should any discrepancies arise
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or if lawsuits are filed as a result of claims for incorrect
information.
Required Information. The engineering feasibility study
requires a comprehensive collection of all information that
could affect the site and its development. For purposes of this
section, the research and analysis associated with an engineering feasibility study are categorized into three types of
information:
1. Legal condition of the site such as easements, land
rights, and other property encumbrances
2. Regulatory concerns of the site such as applicable
master plans, zoning, ordinance requirements, possible citizen opposition, and governmental review
considerations
3. Physical condition of the site such as topography,
soils, utilities, drainage, and external influences created by neighboring properties and uses
Such items have a significant impact on how the land is
developed and how successful the project will be.
Legal. Title investigation must be performed by an attorney or title insurance company for the developer to ensure
that the landowner holds a fee estate in the subject property.
The following title and other property information should be
reviewed to determine the legal constraints and opportunities of the site:
Land ownership records, including property description
(using metes and bounds or government survey method).
•• Do these records match the scope of the project?
•• Will land be excluded from the project and do
subdivision regulations allow such exclusion?
•• Is the property contiguous or are there portions of the property separated by rights-of-way
(ROW) or other properties?
Chain of title traced back to the creation of the tract
boundaries or the adoption of local subdivision regulations, whichever is earlier. A title company usually performs a title search to ensure, and ultimately insure, that
the chain of title has not been broken. The purpose of
the land design team’s analysis is to determine the applicability of local development regulations.
•• Has the subject parcel been legally subdivided
from its parent tract (i.e., with government
approval if such approval was required at the
time the parent tract was split)?
Deed conditions, restrictions, or covenants that could
affect future use and enjoyment of the property.
•• Is the proposed use prohibited by deed?
•• What private deed restrictions are imposed on
the final land use, such as lot size, setback from
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property boundaries, architectural style, or building material?
•• Is any portion of the tract, such as a lot around an
existing dwelling, to be reserved for the existing
owner?
•• Is the property or a portion thereof restricted
from development through local, state, or federal programs such as Farmland Preservation,
Green Acres, or Open Space Preservation or
other Transferable Development Rights (TDR)
program?
Although federal programs do exist, many of the current preservation programs are managed by local or state
agencies. Typically, the preserving agency places a deed
restriction on the property limiting its use to specific
activities such as agricultural (as in the case of Farmland
Preservation) or active/passive recreation (Green Acres
or Open Space Preservation). Depending on the mandates of the preservation program, the deed restriction
may run for a limited duration or in perpetuity and is
binding to future owners of the property.
Prior recorded plats, including government takings,
boundary adjustments, and subdivision plats.
•• If there is an existing subdivision plat of record,
what is its status?
•• Can it be used advantageously; can the lots be
developed and sold?
•• What are the developer’s responsibilities concerning platted public improvements, such as streets
and storm drainage?
•• What other requirements would apply?
Identify local procedures for street, ROW, or easement
vacation and abandonment.
Records of easements appurtenant (usually providing
access to or through the property or adjoining properties) and in gross (usually energy or communications
transmission lines) showing purpose and holder of the
easement.
•• What rights are accorded the holder and what
limitations are placed on the developer’s use of
the easement?
•• Can easements be abandoned or relocated?
•• Are there potential instances of adverse possession or prescriptive easements on the property?
Refer to Chapter 2.1 for more detailed definitions of the
aforementioned legal terms to define a property.
Regulatory. Regulatory information must be included in
an engineering feasibility analysis in order to identify the
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3.1
appropriate processes to allow the proposed development.
These processes will influence the timing, cost, and extent of
community and public involvement in the development process. The following aspects—planning, zoning, and related
development information—of an engineering feasibility
study will aid the developer in the assessment of the regulatory aspects of a proposed project.
Relevant comprehensive plan, zoning maps, and texts: These
include growth management ordinances, such as adequate
public facilities ordinances, impact fees, and other construction limitations such as annual building permit caps. These
also include information on miscellaneous fees, such as
filing and processing, recreation, drainage, and others.
•• When must fees be paid, improvements installed?
Current property zoning, proposed zoning, including uses
permitted by right and those requiring special exception.
Describe the purpose of the zoning district.
•• Do zoning boundaries divide the subject
property?
•• What is the relationship of zoning district boundaries to the subject property?
Requirements for zoning overlay or special districts: Examples of such districts include highway corridor, historic,
transit area, central business district, transfer of development rights, resource protection, conservation, or other
management areas allowing “credits” for increases in the
permitted base density of development.
Comprehensive plan recommendations for the site, for
example, density or allowable land use.
•• Does the current comprehensive plan show any
roadway improvements that will impact the site?
Pertinent requirements of the zoning ordinance: These
include maximum density or floor area ratio, minimum
or average lot size, setbacks (from project boundaries, property lines, lot lines, ROW, railroads, highways,
waterfront, etc.), building height and bulk requirements,
maximum lot coverage and open space requirements,
off-street parking and loading, screening buffering and
landscaping requirements.
•• Is ROW of future roadways used for density credit?
Proposed or pending changes in comprehensive plans
or development regulations likely to be adopted within
the project’s lifespan.
•• What significant impacts will these changes have
on the property’s use or yield?
•• Will the change increase the time it takes to
secure construction authorization, or expose the
project to additional public hearings or government agencies’ review?
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Grandfathered or vested rights that may be jeopardized by
the proposed development.
•• Will the project be exposed to requirements far
beyond those needed for the additional construction or expanded use?
Development history of the property, including records
of previous submissions for rezoning, special exception,
subdivision, or building permits.
The likelihood of citizen opposition, and delays associated
with the development process. The experience of the
land design team is particularly valuable here. Familiarity with local issues and past encounters with civic
groups can help the development team prepare for future
conflicts.
Subdivision and other ordinance regulations specific to
the site, such as lighting or signing ordinances, tree or
historic preservation, or planting ordinances.
•• Can private streets be utilized?
•• Will there be any assessment by any local or state
agency for road construction?
•• Are there requirements for adjoining property
owners to contribute to the cost of constructing
streets to service their properties?
Requirements for “green” building design: In general
terms, green building refers to site development and
building design that promotes energy and resource
conservation and produces a healthy and productive
environment (internal and external to the building)
for people to work and live. Many federal, state, and
local jurisdictions have adopted certain (accredited)
third-party green building certification as a mandated
standard or require a project design to demonstrate the
ability to attain a specific level of certification. Several
third-party review agencies currently exist, each with
differing evaluation criteria. Determination as to what
guidelines, if any, will apply to the proposed development based on the regulatory climate and owner/
developer preference should be made in the early stages
of engineering feasibility. Further discussion of typical
elements of green building design and items to consider
during the engineering feasibility is presented at the conclusion of this chapter.
Refer to Chapter 2 for more information on the aforementioned regulations identified during the due diligence
stage.
Physical. Research and review of the site’s physical attributes and constraints includes the site location, access, topography, drainage, vegetation, soils, and utilities. The following
information must be assembled and reviewed to determine
the physical constraints and opportunities of the site:
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Location
Access
•• Configuration and site area from tax maps
and tax records. Is this mapping, site area, and
ownership information consistent with the title
information?
•• Presence of landlocked parcels and other properties adjoining the subject property. These may
require extensions of roads and utilities as part of
a development plan (Figure 3.1B).
•• Existing structures, paved or developed areas,
fences, and walls. Are all improvements contained within the site boundaries?
•• Public road frontage and property access information. Who is responsible for maintenance and
repairs of frontage and access roads? Is the property’s frontage on a public road sufficient for gaining
proper access to the future development? If there
is no frontage, are access ROW to public roads of
sufficiently short length and adequate width to
accommodate local street requirements. These
standards include width, grade, drainage, and
maximum cul-de-sac length. Is there sufficient
room for construction equipment to maneuver?
•• Adjacent properties, including information on
ownership, zoning, land uses, and their proximity
to property boundaries.
•• Encroachments from structures on adjoining
properties, including existing access ways that
may lead to claims of adverse possession or prescriptive easements.
Figure 3.1B
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Landlocked parcel, due to property lines, that could necessitate extension of public infrastructure by the developer.
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If not, can additional land or access be acquired?
Will there be adequate sight distance at proposed
entrances, as well as vertical and horizontal road
curves? Is there sufficient frontage to provide
proper spacing between road entrances?
•• Existing roads, both adjacent and across from subject property, and road conditions. Include ROW
width, pavement width, site distance at hills and
curves, sidewalks, curb and gutter, and drainage
swale information. Indicate conditions that may
hamper flexibility in site design. For example,
requirements for minimum spacing between intersections on the abutting roadway may limit potential entry points to the development. Large trees
at the edge of pavement may prevent widening or
draw public opposition. Will any existing streets or
ROW require abandonment or vacation?
•• Will existing and proposed highways in the vicinity of the site generate highway noise that should
be mitigated?
Topography
•• In performing a topographic analysis, identify
streams, swales, ridges, and similar landforms
and features. Identify steep slopes, where grading may be expensive or prohibited (greater than
15 and 25%), excessively flat areas (less than 2%)
where storm and sanitary sewer drainage may be
difficult or expensive to achieve. Show incremental breakdown of intermediate slopes to assist in
plan layout.
Storm drainage
•• Drainage basin and watershed within which the
property is located; unique restrictions or conditions applicable to development.
•• Description of on-site drainage patterns.
•• Location or plans for regional stormwater management facilities; timing of public
improvements.
•• Existing floodplains from local jurisdiction,
United States Geological Survey (USGS), or Federal Emergency Management Agency (FEMA)
reports.
•• Potential for floodplain when stream is present.
Small streams and swales not flowing continuously throughout the year may still need to be
analyzed for their flood capacity and status as
jurisdictional waters.
•• Probable locations and sizes of culvert and outfall improvements due to increased runoff from
development.
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•• Downstream problems with drainage; known
complaints.
•• Requirements for non-point pollution control
and “best management practices” (BMPs) during
construction and subsequent to final development, including performance requirements for
stormwater quantity, quality, and groundwater
recharge.
•• Stormwater management facility design constraints, including review of apparent seasonal
high groundwater elevations. Will the existing
groundwater table dictate the use of retention
basins (wet ponds) with permanent pools of
water or significant amounts of imported fill to
maintain dry detention basins?
•• Evaluation of adequate outfall, including presence and/or provision of necessary easements
for access and maintenance, current physical
condition and ownership of existing structures,
or potential site improvements necessary for
new outfall.
•• Location, size, depth, and condition of existing
pipes.
•• Overland relief constraints from downstream
properties and potential overland relief constraints of the subject site to upstream properties.
•• Location of wetlands and other sensitive environmental areas (from National Wetlands Inventory
maps or other available mapping).
Vegetation
•• The location of large (species) trees, areas of tree
cover, including a review of the quality and type
of existing trees should be determined.
Geotechnical
•• Soils information, including types and characteristics, bearing strength, stability, shrink/swell
potential, perched groundwater table, estimated
seasonal high water table, presence of naturally
occurring asbestos, radon potential or existence
of residual pesticides from historical agricultural
uses, and suitability with regard to building foundations, stormwater management facilities, culverts, utility trenching, and erodibility.
•• Consider soil percolation characteristics; do
soils indicate the need for extensive earth movement and placement of engineered foundations
on compacted fills? Where soils of questionable
suitability are identified, the site engineer should
recommend that further investigation and testing
be performed by a geologist or other expert.
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Sanitary sewer
•• Agency with ownership and approval authority.
•• Sewershed in which the property lies; available
capacity, projected demand, local restrictions
concerning sewer allocation.
•• Location, size, depth of, and distance to existing
lines.
•• The age and condition of existing lines in order
to evaluate whether current materials are compatible at connection points and whether the type
of pipe will be structurally adequate for proposed
uses over the line.
•• If not on site or adjacent, the distance to appropriate connections, means of access, and need for
easements.
•• Responsibility for extension and improvements
and current timing of public improvements;
potential for reimbursement from public or private funds, such as other developers.
•• Gravity versus pumped versus on-site package
treatment plant.
•• Interference of system construction with other
utilities/features.
•• Pro-rata shares/assessment fees.
•• On-site disposal issues, including treatment
method, soil suitability, drainfield and lot size
restrictions, impact on project density/lot size,
comprehensive plan and ordinance considerations, and depth to water table. Spot percolation
tests may be required.
Water distribution
•• On-site well information, including depth to
water table, groundwater quantity, water quality,
impact on project density, proximity to dwellings
and septic systems, and other ordinance requirements. Test wells may be required.
Other public or private utilities and services
•• Service options for energy and communications
utilities, such as electric, gas (natural or liquid
propane), cable, telephone, and fiberoptic. Are
there competing companies serving the same
area? If so, are rate structures and builder incentives comparable?
•• Current and projected levels of service. Are
improvements budgeted, is timing compatible
with project? Can timing be advanced?
•• Responsibility for design, upgrade, and
connection.
•• Consideration for required easements. Who will
obtain?
•• Connection fees, when must they be paid?
•• Electric, telephone, and cable undergrounding requirements, on-site, adjacent, and off-site
responsibilities and contributions.
•• Information regarding the provision of trash
removal and recycling (curb side pickup), street
cleaning, snow plowing, and similar services.
Are services public or private? Are there alternative providers? If trash removal is public, is
it available to condominiums and commercial
operations? What is the availability of private
contractors for these services?
•• Size, location, depth of, and distance to existing
water mains, means of access, and need for easements. Are offsite easements required to extend
service?
•• Location, proximity, and planned improvements of elementary and secondary schools,
means of access (pedestrian, school bus, and
public transit). Do sidewalks or trails exist
between site and schools? Will interior sidewalks be required in the development? Are
there school impact fees?
•• Water quality, quantity, pressure, and necessary
corrective measures.
•• Requirements and responsibility for installation
and maintenance of streetlights.
•• Agency with ownership and approval authority.
•• Responsibility for extensions and improvements,
associated fees; timing of public improvements.
•• Requirements for fire hydrants; water supply and
distribution requirements for fire flow.
•• The age and condition of existing lines in order
to evaluate whether current materials are compatible at connection points and whether the type
of pipe will be structurally adequate for proposed
uses over the line.
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Other aspects of the site such as the following should be
investigated
•• Availability, proximity, and planned improvements of emergency services, such as police, fire,
and rescue.
•• Aircraft flight patterns and noise contours.
•• Unusual on-site and adjacent features, such as
cemeteries, railroads, historic properties.
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•• Natural hazard (i.e., earthquake or flooding)
potential, prevailing weather patterns, and solar
exposure that could affect project design.
•• Research into previous uses, needed to determine
possible underground structures or contaminating conditions.
Chapters 3.2 through 3.6 provide more information on
the aforementioned physical information of a property that
will be identified during the site analysis.
Sources of Information. The design team must become
familiar with information resources and the local sources of
that information. These simplify the investigation of property conditions and local regulations and eliminate much of
the need for original research and testing. Throughout the
pre-design stage, sources must be documented with particular care, whether the source is available public documents or
conversations with public officials. This is especially important in preparing the final report document, where the site
engineer’s opinion must be separated from others’ opinions
or established facts.
Of particular value are existing public records. These
include published tax maps of the community, which depict
property boundaries, land area, and landowners, along with
references to recorded subdivision plats and deeds. An office
of land records, court clerk, or similar agency maintains copies of deeds, subdivision plats, and similar records relating to
property ownership.
The local planning, public works, building, or transportation department often maintains aerial photos of the
community, taken at various intervals. These provide both
historic reference and indications of recent or current use.
These photographs often are printed in conjunction with tax
maps of the community.
Some communities provide topographic maps of the
community based on aerial photography. Otherwise, these
are available from private sources or can be commissioned
for each project. In addition, the USGS quadrangle maps
are useful for identifying site topography, natural and
man-made features, perennial and intermittent streams,
and other items of interest to the site engineer. In many
communities, the agency publishes soil maps and related
information.
Recent building and development plans, permits, and
application materials often are kept on file in various
agencies, either in their original files with all supporting
documents or digital medium. Most are available for public inspection. These provide records of previous studies
that might apply to the subject property. Along with public
records of construction plans and as-built documents for
public facilities, these records are useful for information
relating to underground utilities. These files often provide
useful information concerning the experiences of previous
developers. The site engineer’s own in-house records of its
previous and ongoing projects near the development may
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contain recent studies or more current information than
available from other sources.
Copies of local plans, regulations, and ordinances are
available from these agencies. The community’s comprehensive plan may list local, state, and federal sources for information upon which the plan is based. The transportation
agency may have recent studies of existing traffic counts,
capacities, and level of service for the road network near the
project.
A local historic preservation agency or society may have
compiled a register of historic properties or sites, including
archeological information. The local economic development
authority or chamber of commerce provides useful market
area information. The local utility companies will provide
distribution maps of the service areas.
Many federal agencies provide maps of various utility
to the development team. In addition to USGS and FEMA,
these include United States Fish and Wildlife Service, Army
Corps of Engineers, Environmental Protection Agency, and
Natural Resource Conservation Service.
Most of the aforementioned local, state, and federal agencies provide data electronically, either for free or for a small
fee. A good portion of the available information can be
accessed directly from the internet via the agency’s website
and downloaded for use in reports and design documents.
Other independent sites may provide a compilation of data
such as ordinances, tax maps, zoning maps, aerial maps, or
other information contained in geographic information systems (GIS). However, care should be exercised when utilizing
data obtained via the internet as the available information
may not be current. Care should be taken to review the frequency at which any online source updates its information,
and a simple call to the local agency for data verification is
recommended. The “desktop” review is a critical component
of the feasibility study: information obtained in this manner
can help streamline the site visit and records review, allowing
consultants to focus on acquiring specific missing data and
confirming collected data.
Once all information is retrieved and compiled, the land
design team prepares base maps of the subject property at
a level of detail and accuracy commensurate with the time
and budget (the process of creating a base map is provided
in Chapter 3.2).
3.1.3. Site Inspection
A site inspection, also known as a field investigation, site
visit or walkover, is often required when completing an engineering feasibility study. Much of the information garnered
during the desktop review is outdated, reflecting only property conditions at a specific date in the past. Physical conditions change constantly through the action of both man and
nature, and maps may not accurately represent the actual
field conditions. In addition, contours and other information normally shown on aerial topographic maps may be
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unclear. The ground may have been obscured by foliage or
snow cover, depending on the season of the year, time of
day, or other conditions at the time the area was flown. A
site shown as wooded may, in fact, have subsequently been
cleared and graded. Illegal dumping or similar activity may
have taken place. Adjacent properties may have undergone
development since the maps were last produced. Certain
features, such as wetlands and small streams, may not readily be evident and often require field analysis to verify their
location.
For these reasons, it is imperative that a field inspection
be performed as part of the engineering feasibility study.
The visit is necessary both to verify and build upon information collected elsewhere. The person visiting the site is
looking for obvious contradictions with recorded information, as well as evidence that implies conditions not previously known. Of importance are conditions that may
render the land unusable or impose extreme costs in the
development.
Prior to visiting the site, the field personnel assembles and
prepares information in advance. In addition to the base map
(to be introduced in Chapter 3.2), a clipboard, writing paper,
tape measures, scales, waterproof pens and markers, and a
digital camera should be among the equipment brought to
the site.
It is imperative that the field visit team make arrangements with the developer, the owner, and any residents of
the site prior to the site visit. Agents of the developer may
not necessarily have right of entry if not spelled out in the
purchase contract. Unexpected visits may be unwelcome
by occupants, who may not even be aware that the property is being sold. The field team should also verify with
the developer and owner that no known environmental
hazards exist on-site to adequately prepare and protect
field personnel.
The team should be appropriately dressed for the visit,
anticipating the weather, brambles and dense brush, mud,
standing water, poison ivy, insects, snakes and other wildlife, and any predetermined environmental conditions. One
of the purposes of the field inspection is to determine the
site’s response to rain and runoff. Therefore, it is advisable
to schedule at least one visit to the site during or soon after
storms or spring thaws. The field team can observe ponding,
running water, and other surface conditions that otherwise
may not be visible.
Site Inspection Process. Upon arriving at the site, the
team should drive the boundaries of the site, noting landmarks such as fences, hills, swales, and curves in the roadway that have previously been recorded on the base map.
These will serve as points of reference while walking the
site and make it easier to record information about the visit.
A systematic walking tour of the site should be planned,
taking care to include critical natural and man-made features that were previously noted in the office review of site
information.
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On the base map, note and verify those areas that are
important to the development of the site, either as problems
or opportunities. Outline the apparent boundaries between
different topographical and geological conditions, such as
between improved and unimproved areas, stable and unstable slope areas, wetlands and dry ground, wooded areas and
open fields. Visualize property boundaries, particularly in
locations where topography or other circumstances appear
to create difficulties during development. Depending on the
relationship between the site boundaries and topography,
construction in these areas may necessitate the acquisition of
easements for drainage or equipment access across adjoining properties. Retaining walls may be necessary at property
boundaries if significant grade changes are required.
Areas of interest that should be recorded on base maps
during the field reconnaissance are described below:
•• Streams, swales, washes, and evidence of confined
running water and intermittent streams, such as
unusual patterns of fallen leaves, vegetation and
stones, soil erosion, uprooted or undercut trees, and
areas cleared of leaves.
•• Floodplains, often evidenced by high water marks on
shrubs, tree trunks, and low-hanging branches.
•• Ponds, lakes, and other impoundments, again, trying
to identify the limit of impoundment.
•• Marshes, swamps, wetlands, bogs, wet and soggy
areas, noting types of vegetation, areas of matted
leaves, or unusual soil coloration that may suggest
frequent or periodic inundation. If it is confirmed
during the site inspection that potential jurisdictional
areas are located onsite, a formal wetland delineation
would be required by an environmental specialist.
•• Ridges and obvious drainage divides with nearby
running or ponding water at lower elevations, indicating high water table, springs, and springheads.
•• Potential stormwater outfall points and conditions of
the outfall, including any visible erosion, continuous
flow within a channel, etc.
•• Evidence of pollution or sedimentation in running
and standing water, from on-site or off-site uses.
•• Condition of stream valleys, banks, and shorelines.
•• Areas and types of vegetation, boundaries of wooded
areas, and stands of trees that might serve as buffers
against adjoining properties, protected species, large
trees and other specimen trees, or mature ornamental landscape materials that may be preserved, either
in place or transplanted for subsequent reuse in the
development.
•• Presence of fish and wildlife and evidence of animal
habitats, such as beaver dams, and eagle aeries that
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must be considered and protected or that may even
preclude development of the property.
•• Cliffs and other unusual landforms indigenous to
certain areas of the country, such as coastal and
Great Lakes dunes, sinkholes.
•• Areas of steep slopes, noting vegetative cover.
•• Evidence and sources of erosion and slope instability,
such as leaning trees, poles, fences, broken pavement
at top of slope, softness at toe of slope, sharp vertical
drops suggesting landslides, and other signs of previous slope movement; exposed soil colors.
•• Locations where, based on visual inspection and soil
map data, additional subsurface explorations (e.g.,
auger boring, test pits) will be necessary.
•• Rock outcroppings, which may create problems in
site excavation for roads, utility trenching, well and
septic system suitability and foundations. Consider
their possible use as aesthetic features and points of
interest to enhance the development’s market appeal.
It may be possible to stockpile stone and rock for
subsequent use in erosion and sediment control or
for landscape material.
•• Evidence of strong prevailing winds, such as distorted plant and tree growth.
•• Location, use and structural condition of buildings,
paved areas, abandoned wells, and other man-made
features on the site, whether or not they are to be
preserved; evidence of flood damage, earth settlement, and movement in walls and foundations.
•• Outbuildings and storage areas noting any hazardous
materials signs (pesticides, herbicides, paints, solvents are often stored in these locations)
•• Existing pilings and retaining walls.
•• On waterfront property, piers, moorings, and other
marine uses, access points to the edge of water. Observe
conditions and uses of adjoining and opposing shorelines, maritime activity, and information concerning
water quality, depth, and bottom configuration.
•• Evidence of cemeteries, gravesites, burial grounds,
archaeological and historic sites, battlefields, old
foundations, and other unusual or unexpected existing or prior land uses on site that could limit development potential, incite community opposition, or
delay project approval.
•• Evidence of trespass and community use of the
property, such as footpaths, dirt bike trails, picnic
areas, and sports fields, which may be an indication of potential community opposition to the
development.
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•• Interesting views within and from the site, areas that
might be cleared to enhance views and views from
adjoining properties onto and over the site.
•• Character, condition and use of adjacent property,
and proximity of neighbors and site improvements.
Record evidence of access easements, encroachment
of fences, and structures.
•• Current construction activities on or near the site.
•• Evidence of noise, smoke, dust, odors, light intrusion,
or other activities from sources within the site or
nearby uses, such as from nearby industry, highways,
railroad crossings, racetracks, hospitals, fire and
rescue stations, schools, commercial areas, airports,
landfills, sewer lines, or sewage treatment plants.
These could affect the site’s value or market appeal.
Prevailing winds should be considered, evaluating
their impact.
•• Noise walls, landscape barriers, or special construction techniques and materials may be employed to
mitigate potential nuisance.
•• Evidence of significant trash, debris, chemical or oil
dumping, burial, and storage.
•• Evidence of unusual odors may suggest natural
decay, sensitive environmental features, or ground
contamination.
•• Sight distances at curves and hills adjacent to the
property and probable entrances.
•• Traffic congestion on adjoining roads and nearby
intersections.
•• Condition of surrounding roads and pavement,
including paving and shoulder stability and widths,
roadside swales, curbs and gutters, location of nearby
and opposing driveways.
•• Existing utilities including electric, water, gas, sewer,
stormwater, water wells, septic systems.
•• Condition, size, and location of culverts, outfall
channels, and any existing drainage pipes and
swales.
•• Locations of overhead utility and power transmission
lines.
•• Underground and aboveground storage tanks (vent
pipes and fill ports are indicators of underground
storage tanks).
•• Manholes, standpipes, vent pipes, signs, and other
evidence of underground tanks, sewers, and transmission pipes. Isolated areas with poor vegetation,
in contrast to its surroundings, may be indicative of
subsurface materials.
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Depending on size and scope of the project, several
individuals may be required to walk the site. In addition, it
may be necessary to revisit the site with other professionals
whose expertise is indicated by the findings or local requirements. Further analysis may be required where preliminary
investigations show the presence of unusual soils or wetland areas. Additional site visits will be needed if the project
moves beyond the feasibility study. Therefore, as thorough
an inventory as possible at this phase will simplify later work.
However, this must be balanced with efforts to control costs
at this phase. These costs must be controlled because of the
possibility of project abandonment.
During the site visit, extensive photographs should be
taken. The location and direction of each photo should
be noted on the base map for reference in the office; the
GPS receiver in most digital cameras can also be used to
document location. These should include photographs of
important views and significant features. A series of panoramic shots taken from the property boundaries is useful for setting points of reference. Including people or
other items of known size helps establish height, depth,
and width of features being photographed. As in all photography, lighting and shadows are important to adding
dimension. Photographs serve as valuable reminders when
the site inspection is studied back in the office. In addition, photographs assist other members of the development team who were not present in the field. Videos from
the site visit also provide a useful reference, allowing for
more interactive commentary about site features. It also
provides a clear record of site conditions prior to development. This is useful for comparison to conditions during
and after construction if legal or procedural conflicts arise.
Figure 3.1C shows how site photographs can be referenced
to the property.
F i g u r e 3 . 1 C Site photographs cross-referenced with their location on a map often help others assess potential constraints or liabilities. (a ) Refuse
site #5, (b ) Refuse site #7, and (c ) Refuse site #9.
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yield studies and beginning the design process should the
project advance to that phase. More information about base
maps is described in Chapter 3.2.
A site investigation report is helpful for documenting site
conditions. The report is often developed on a site plan sheet
with photographs referenced to different locations on the
project site. A formal report may be requested by the developer to document existing conditions and evaluate condition of the site. Photographs and notes are often referenced
throughout the project design phases to provide a different
perspective than what is shown on a topographic survey.
(a)
(b)
(c)
Figure 3.1C
(Continued )
3.1.4. Site Analysis Mapping and Report
Data from the site visit must be compared to other recorded
data. Any inconsistencies should be resolved to verify true
field conditions. Of particular importance are discrepancies
in property boundaries and topography. The site engineer
should transfer information to a clean base map, which will
be used to report results of the study to the developer. In
addition, the map becomes an important tool in performing
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3.1.5. Reading a Plan Sheet
As the engineering feasibility study and base maps are produced during site analysis, the site engineer must be familiar
with plan sheets. It is important to understand and be able to
read a plan sheet.
A typical site plan sheet depicts the site layout with an
orthogonal view looking down on the site, which is an unfamiliar perspective and can be difficult to understand. The
land development plan sheets will also show infrastructure
elements that are not visible in the built environment such
as underground utilities, which may further obfuscate the
plan sheets. To differentiate between various infrastructure
systems, most plans use symbols and different line styles, but
there is no universal format and plan sheets from different
engineering firms can look dramatically different.
Some organizations, such as the United States National
CAD Standards (NCS), provide guidance to design professionals for plan organization and styles to promote consistency in plan production within the industry. Unfortunately,
many design firms and other infrastructure groups may
develop their own set of styles and symbols, so it is necessary
to study the format used with each plan set.
Symbols. The symbols used on a plan sheet are meant to
depict an infrastructure component, such as a fire hydrant or
a street light. Symbols may be scaled to represent the physical
dimension of the infrastructure, or they are shown larger for
legibility. If a symbol does not represent the actual size of the
physical feature, the designer should be careful to consider
the actual dimensions during the design.
While there are some common symbols within the industry, there is no universal system and each plan set should
include a legend for the symbols used with the plan set. The
designer should be familiar with the symbology used for a
given project. In most cases, annotations are still appropriate
for labeling features on a site plan. Different line types are also
used on plan sheets and represent fences, streams, curbs, and
other linear features. Standard patterns, or hatches, are used
to identify different material types on a site, such as sidewalks
and planting areas.
When possible, it’s best to represent infrastructure features as accurately. It’s important to always consider the relationship between symbols on a plan sheet and the physical
feature. In many cases, a symbol is provided for legibility
and does not represent the physical dimensions of a feature.
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For example, a valve within a water supply system may only
be several inches in diameter and, when scaled on a plan
sheet, would only appear as an illegible dot—in this case, a
symbol is used at a scale larger than the physical feature, so
it is legible (refer to Figure 3.1D for example symbols in a
legend).
Plan Sheet Layout. Each plan sheet should always have the
following information:
1. Drawing and project title
2. Date of publish or issuance
3. Authors, which usually includes information on the
drafter, reviewer, and approver
4. Firm or designer contact information
5. North arrow, on plan sheets
6. Graphic scale
Plan sheets may have a different format when presented
as either an exhibit or a formal construction document. It
is important to have a clear description of the project title
and the date the plans are published (or issued for review or
construction). Providing the author information allows for
accountability, as does the firm name and contact information. A north arrow on the plan sheet provides a reference
for orientation and should include an annotation to reference
the project datum (refer to Chapter 3.2 for more information
on geographic datum). All plan sheets should be plotted to
scale, such that 1 inch on paper is equivalent to a physical
distance—common scales are 5, 10, 20, 25, 30, 40, 50, 60,
100, and larger values for vicinity maps. When depicting a
profile view with an exaggerated vertical scale, it is necessary
to show both the horizontal and the vertical scales. The scale
should be depicted with a graphic bar that helps to reference
the scale when sheets are plotted on different paper sizes
(Figure 3.1E). A sheet that does not have a plan view, such
as a sheet with only notes, narratives, or details, may exclude
the north arrow and scale.
While plan sheets are often referred to as blueprints,
the nomenclature is antiquated, and most plans are either
delivered digitally or printed to a large format printer (also
referred to as a plotter). Most printers can manage paper at
least 36 inches wide, which allows for common prints of arch
D, 24- × 36-inch paper size. The plan sheets are often printed
as grayscale, and legibility can be challenging because of the
amount of information that is shown. It’s important for the
plan sheet to be organized and stylized to ensure legibility.
It’s possible that a design is correct, but if it’s difficult to
comprehend, it may cause unnecessary errors and delays in
construction.
The design team should take care to provide the right
information in the right format for each plan sheet. Choosing the proper scale for a plan sheet allows the information
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to be shown at the right level of detail. Annotations on a
plan sheet should be clear and concise. When presenting
designs to various stakeholders, such as the developer or
community, the plans may require edits to make it easier
for nontechnical groups to understand what’s being proposed. The ability to communicate ideas through graphics
is a skill.
Plan Sheet Views. There are several views that are standard within a plan set. Most plan sets will include sheets that
have plan view, profile views, and section views (Figure 3.1F
provides a conceptual representation of these views). A single sheet may only show a plan view or could show multiple
views of the infrastructure. Examples of sheet formatting and
layouts are provided throughout Chapter 5.
Plan. The plan view is an orthogonal view looking down
at a site. A single project may require multiple plan views
that represent different infrastructure elements, such as one
for utilities and one for grading. Projects that have a large
geographic area may require multiple viewports of the project, where each viewport focuses on a different area of the
site. When multiple viewports are required, it is necessary
to have matchlines that identify how each sheet connects
to another. A key plan showing the entire site with each
sheet is often helpful (Figure 3.1G). When organizing the
sheets, it is important to try and maintain a constant viewing direction and to consider the best place to break a sheet.
For example, a sheet should limit the amount of negative
space (large empty areas) and elements such as a building or an intersection should be contained within a single
sheet, when possible.
Profile. A profile view, or longitudinal cross section, is a
plane that is cut along a road or utility alignment. The existing ground and proposed ground are depicted in the profile
view with pipe networks shown or vertical road geometry
shown. For a road project, it is common to see both road
and utility information shown in a single profile because the
utility network will generally follow the road alignment. Profiles are shown with both stations and elevations in a grid
format. The stations are provided for linear referencing or
measurement.
Stations are written as follows:
•• A station of 5000 feet is represented as 50 + 00.
•• A location that is 25 feet beyond 50 + 00 is written as
50 + 25.
•• A location that is 100 feet beyond 50 + 00 is written
as 51 + 00.
•• A location that is 125 feet beyond 50 + 00 is written
as 51 + 25.
A road profile will begin with the starting station of the
alignment and continue until the end of the roadway. The
starting station is generally recommended to be 10 + 00 as
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141
F i g u r e 3 . 1 D Example of a legend.
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F i g u r e 3 . 1 G Example of a key plan.
F i g u r e 3 . 1 E Example of scales.
opposed to 00 + 00 in case information needs to be shown at
a location before the start of the road (negative stations are
unconventional). For utility profiles, each segment of a utility will have a separate profile view.
Figure 3.1F
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Plan, profile, and section views concepts.
A vertical exaggeration is usually applied to a profile
view because of the scale of infrastructure elements. A
common exaggeration is 2, 5, or 10. This exaggeration
makes it easier to depict the vertical design of infrastructure on a profile view. For example, a profile with a horizontal scale of 50 could have a vertical exaggeration of
10, which creates a vertical scale of 5 within the profile
view. Figure 3.1H provides a comparison between the
actual vertical scale at a (1:1) and an exaggerated vertical
scale (10:1).
Section. A section view, or cross section, is a plane that is
cut perpendicular to a linear feature. Section views are generally shown for roadways and stormwater channels. Some
F i g u r e 3 . 1 H Example of a vertical exaggeration.
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F i g u r e 3 . 1 I Example of a section view.
road projects require section views to be shown at a given
increment, such as every 50 feet along a roadway, to depict
how the roadway varies along the alignment. For a natural
channel, a section is used to show different varying channel geometry and often identifies the water surface elevation
with each section.
The format of a section view is like a profile view—the
section is depicted in a grid format with stations and elevations, often with a vertical exaggeration. The one difference
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with a section view is that the stationing will be relative to the
centerline of the roadway or channel, such that there are negative and positive stations (or offsets) shown from the center.
Figure 3.1I provides an example section from a roadway.
REFERENCE
Fillmore W. Galaty, Wellington J. Allaway, and Robert C. Kyle,
Modern Real Estate Practice, 11th ed, Real Estate Education
Company, Chicago, 1988, p. 21.
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Chapter 3.2
Base Map and Site Diagram
3.2.1. Introduction
Good information begets good design. A base map, as
defined herein, is a graphic representation of the existing
site conditions and development constraints. Base maps are
an essential part of the site analysis because they are used to
depict a collection of relevant information applicable to the
site. This information will come from various sources such
as field survey, zoning maps, geographic information system
(GIS) databases, environmental assessments, and geotechnical studies. When the information is compiled, it provides a
comprehensive view of the site opportunities and constraints.
The base map provides a foundation for which design decisions are made. The information shown on a base map will
allow the design team to make programmatic decisions for the
project at early stages that carry forward through final design.
If information is not shown in the base map, such as a floodplain, the design team could end up with significant rework that
takes time and money to address. Additionally, if the information is inaccurate, such as the location of the property lines,
there will likely be significant consequences during final design.
The base map should include survey information for the
site’s physical features, topography, site boundary to identify
the property lines, existing easements, and zoning information. The base map should also include natural site amenities
and site constraints such as areas of vegetation, streams, ponds,
wetlands, soil conditions, and floodplains. Jurisdictional information such as zoning maps and comprehensive plans should
also be referenced into the base map to determine the impact of
future infrastructure. A complete list of information that should
be included in a base map is provided later in this chapter.
In some cases, the base map is prepared incrementally. Incremental development of the base map is common when a developer wants to limit the initial investment during a study period;
if the first base map identifies that the site is completely encumbered by wetlands, the developer may look for another opportunity. While the best source of base map information will come
from field survey and on-site investigations, the initial base map
may be compiled from readily available information such as GIS
data, desktop investigations, or record drawings.
A land development project may span across hundreds of
acres, but subcentimeter accuracy is required for design and
construction. For this reason, the source of data shown on a base
map should be identified and any uncertainty in the data should
be clearly noted. A property line from a GIS source may be
acceptable for concept design and site analysis, but if the information is not accurate (and GIS data usually is not accurate), it
will create problems during final design and construction.
The uncertainty of data used for the base map should be
clearly communicated across all members of the development
team and other stakeholders. The possibility of future changes
(based on information collected in later design phases) should
be acknowledged. The accuracy of a property line location
could change the building location, setbacks, landscape buffer, parking areas, and other site features that can change the
development program. Design contingencies should be used, as
appropriate, to consider inaccurate base map information.
The types of content shown on a base map during schematic design are often identical to those that will be shown
on the base map during final engineering; the difference is in
the level of detail. Whereas final engineering documents are
certified by professionals to document the source and accuracy of information, the preliminary engineering documents
are prepared with just enough information to meet early site
analysis and planning goals. This chapter provides information on how to prepare a base map that will serve as a source
of planning and design phases of a project.
The base mapping documents for a land development
project should always be geographically referenced; each
map will have a horizontal and vertical geodetic datum. A
geodetic datum is a coordinate system used for identifying
a specific location on the surface of the earth, that is, for
calculating the coordinates of points on the earth. North
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American Datum of 1983 (NAD83) values are examples of
a geodetic datum. The coordinate system provides a relation between geographic information from various sources
allowing for a single map to show all information.
A base map is established with an identified horizontal
and vertical datum. As other mapping information is compiled, such as a floodplain map or historic preservation map,
the coordinate systems can be used to reference the information into a single map based on location information.
Purpose and Content of a Base Map. A base map is a
graphic depiction of the project property with relevant information that may influence the design. As noted in Chapter 2,
there are restrictions on how land can be developed based on
environmental, historic, topographic, or other conditions of
the project site. For example, if a floodplain exists within the
project site, the development team must understand the limitations of development within, or adjacent to, the floodplain.
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In the early design phases of a project, the base map may
comprise only information that is published through easily
accessible sources, and in later phases of design, it may include
information gathered by the engineering and survey team.
Base maps are often separated into a series to clarify different development conditions. A base map showing topography
of the site can be used to identify rough grades and earthwork of
the site. A base map of utilities can identify possible utility connection points or show major utilities that should be considered
during early design phases. The design team can depict zoning
setbacks on a base map to show the true limitations of development. All the information on the various base maps helps the
development team design the project with a clear understanding of all relevant information. In all cases, the source and date
of the information should be documented for reference.
See Figure 3.2A, for an example of a base map that shows
site features and environmental conditions.
F i g u r e 3 . 2 A Example of a base map.
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3.2.2. Sample Base Map Content
The base map deliverable may contain one or more different
maps, depending on the complexity of the site. The following
list provides a reference to the main components of a base map
but is not intended to be inclusive of all relevant information.
1. Site descriptions: This content provides information
relevant to the location and ownership of the site as
well as adjacent parcels. The land use, types of road,
and owner information will be referenced throughout the duration of the project.
a. Site owner
b. Site address, coordinate location, datum
c. Major roads, intersections
d. Site area (based on tax map records)
2. Legal: The graphic depiction of the property defines
the physical space of the development. In addition
to the property line, it is important to understand
the location of existing easements and rights-of-way,
which can carry significant development rights and
restrictions.
a. Property boundaries
b. Lot lines
c. Public and private easements
d. Right-of-way
3. Physical features: The physical information of the site
is important even if the project will remove all existing features. The condition of soil within the site can
change structural requirements, and existing buildings and infrastructure may have specific demolition
requirements. Off-site physical features can also
influence the proposed site design based on connection or separation requirements.
a. Topography
b. Soil classifications
c. Buildings
d. Fences and walls
e. Roads (including classification and speed limits)
f. Driveways and parking spaces (or structures)
g. Transit centers, bus stops
h. Sidewalks, trails
i. Streams, ponds, lakes, rivers
j. Parks
4. Utilities: Information will come from the local
jurisdiction and utility providers. Sources vary, and
accuracy will vary (sewer systems are often easy to
identify from visible structures, whereas a waterline
may change direct and depth without any indications from the surface). When possible, the base
map should show location, size of utility, and other
physical characteristics like inverts and elevations.
a. Storm drains, stormwater management systems
b. Sanitary sewer, septic systems, pumps
c. Water distribution, wells, hydrants
d. Power (overhead or underground)
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e. Communication (overhead or underground)
f. Gas
5. Zoning and regulatory: This information is not visible
like the physical site features, but these site characteristics will govern the allowable development for the site,
as noted in Chapter 2.3. The type and number of zoning
and regulatory areas can be different across jurisdictions.
a. Zoning classification
b. Comprehensive plan classification
c. Special study areas
d. Floodplains
e. Airport areas
f. Conservation areas
g. School districts
h. Census tracts
i. Political districts
6. Environmental and historic: Some initial studies can
be performed to identify environmental and historic
site conditions. This work is often required during
transfer of land.
a. Wetlands
b. Water features (ponds, streams, etc.)
c. Stream buffers
d. Historic features and viewsheds
After compiling the information, the development team
should evaluate the information and identify site constraints
and opportunities. The effort spent on the development of a
base map should focus on potential red flags first. If there is
an indication of a possible red flag on the site (such as a floodplain occupying a large area of the site) the first information
collected should be floodplain maps. The team can save time
and money by prioritizing potential issues that may encourage
abandoning the site.
3.2.3. Data Formats
When compiling a base map, it is important to understand the
format types of data that can be utilized. Preliminary information will usually come from multiple sources with different data formats. Therefore, it is important that the source of
information should be documented with notes related to the
confidence of the information. For example, if the base map
is compiled from computer-aided design and drafting (CAD)
information produced for record drawings (field survey after
construction has been completed), the confidence level would
be high. Conversely, if information is sketched into the drawing based on information shown on hand-drawn design documents, the confidence would be much lower. For utilities, the
quality of information is generally represented with a notation of A, B, C, and D. The designation of “A” is the highest
level and indicates the utility was field located and surveyed,
whereas “D” means that the information could have come just
from a verbal representation or assumption.
This section is important because failure to understand
the different types of data could cause the wrong data to be
requested. If the wrong data is requested, or not enough data is
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requested, it may require additional expenses to correct miscommunication. The types of data to be discussed within this
section are raster data and vector data, descriptive data and
map annotations, planimetric maps and topographic maps,
digital elevation models, contours, and digital orthophotos.
Raster Data and Vector Data. Raster data consists of pixels
or grid cells of uniform resolution. Digital images are raster
data, as are maps or engineer drawings scanned at 500 or 1000
dots per inch (dpi), for example. A pixel (picture element) is
the smallest indivisible element of a digital image. One pixel
of a LANDSAT satellite multispectral image equates to a 30by 30-meter square area on the ground; 1 pixel of a standard
USGS digital orthophoto quarter quad (DOQQ) covers a 1 ×
1-meter square area, and 1 pixel of a high-resolution digital
orthophoto typically produced specifically for a land development project might cover a 6 × 6-inch area on the ground.
Such images are referred to as having 30-meter, 1-meter,
or 6-inch pixel resolution, respectively. However, users can
typically see features that are much smaller than the pixel
resolution; for example, road paint stripes, 4 inches wide,
can often be seen on DOQQs with 1-meter pixel resolution.
The resolution of scanned documents is referred to in terms
of dpi. Raster images can be displayed on the computer at
different scales, to the point where pixel breakdown occurs,
and individual pixels are visible, but zooming in does not
improve the accuracy of such raster data.
Somewhat different from a pixel, a raster grid cell is one
element of a more detailed image or surface, simplified for
cost and convenience using user-defined grid-spacing criteria. Square grids are commonly defined to reduce the computer file storage requirements for large geospatial datasets.
For example, whereas it might be desirable to utilize digital
orthophotos with 6-inch pixel resolution as base maps, larger
grid cells of 1, 10, 50 meters, and larger are often preferred to
display soil types, geology, vegetation classification, land use/
land cover, and natural features that do not need to be precisely depicted. Similarly with elevation data, whereas it might
be necessary to collect light detection and ranging (LiDAR)
data at 4 points per square meter (common when collecting
data in vegetated areas), the file sizes are very large for recording millions/billions of points, and the analysis/display software is more expensive. File sizes are much smaller when
using a digital elevation model (DEM) grid spacing of 1 meter
(are larger) for which x-y coordinates do not need to be individually stored and only the average z value is stored for each
square grid cell. Instead of complex horizontal coordinates,
grid cells are tracked by sequential rows and columns.
Vector data consists of 2D (x-y) or 3D (x-y-z) coordinates defining the locations of point, line, and area features.
Related terms include nodes, vertices, shape points, arcs,
degenerate lines, line strings, line chains, edges, polygons,
and other terms that have their own definitions. Vector data
may be displayed on a computer with different colors, line
styles (solid, dashed, dotted, etc.), line weights (thicknesses),
and symbols. Vector lines and curves are normally smooth,
while grid cell raster data have a stair-step appearance, especially when zoomed-in to view the raster data at a large scale.
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CAD drawings (and to some extent, PDFs) are a form of
vector data, as are survey data with points having 2D or 3D
coordinates, and lines having distances and bearings.
Merged raster/vector data is now common with modern
GISs. Raster images are more understandable to humans,
but attributed vector data is more intelligible to computers.
The merger of raster and vector data (normally the overlay
of vector data on top of raster images) allows the best of both
worlds. The raster data normally serves as a base map for
overlay of vector GIS data.
Descriptive Data and Map Annotations. Descriptive data
more fully describes the geospatial data. Annotations are provided on maps to label the features shown, and some labels can
be automatically created from information stored in a database
(such as a GIS database). Attributes describe the various point,
line, and area features, such as length, diameter, manufacturer,
model, serial number, material composition, burial depth, and
installation data of a utility feature. Design software, such as
CAD and GIS applications, can interpret this data to display
annotations or graphically scale an attribute. For example, a
line representing a pipe can vary in width based on the pipe
diameter attribute. Often, feature attribute codes are used to
describe diverse feature types. Alphanumeric textual data or
attributes are often stored in relational databases, and selected
items from the database may be displayed on maps. Descriptive databases can be managed separately from geospatial data;
for example, a tax assessor’s database may be linked to a county’s parcel maps but maintained separately and confidentially
from spatial databases available to the public.
Annotations are map symbols or alphanumeric labels
such as route numbers, elevations, or names of towns, streets,
rivers, or mountains placed on maps. On digital maps, annotations are normally key entered, then stored with coordinates for the beginning, center, or ending of the annotation
and/or with other rules for placement and orientation, text
fonts, and special characters and symbols of various sizes.
Planimetric Maps and Topographic Maps. Planimetric maps display the horizontal positions of natural and
man-made features and boundary lines. Planimetric data
is displayed in two dimensions only. If maps do not display
elevation data with contour lines or an alternative method,
they are planimetric maps. In a computer, planimetric data is
treated as 2D files having x-y coordinates. Digital orthophotos are a form of planimetric map where no elevation data is
presented. Planimetric maps and planimetric data normally
form the base maps for overlay of other data required for a
land development project.
When people refer to planimetrics, they normally mean
planimetric data that can be seen and mapped horizontally,
typically from stereo photogrammetry described later. Planimetrics include hydrographic features (e.g., rivers, lakes, and
shorelines), transportation features (e.g., road/highway edge
of pavement, bridges, railroad tracks, and airport runways
and taxiways), man-made features (e.g., building footprints,
transmission lines, fire hydrants), and other features that
can be seen from aerial photography. The term planimetrics
does not normally include boundary lines and underground
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utilities that cannot be mapped with stereo aerial photography, even though they can be surveyed and georeferenced on
planimetric maps. Similarly, a plan view shows the horizontal location of features as though looking straight down from
infinity (an overhead orthogonal view).
Topographic maps, on the other hand, display both the
horizontal positions of natural and man-made features and
boundary lines as well as elevation data (normally contour
lines). In a digital file, topographic data may be represented
with 3D objects having x-y-z coordinates for individual points
or lines. Topographic data and topographic surveys normally
end at the edge of water bodies, but they often include the elevation the water surface—when referencing the water surface
elevation, it is important to identify that the information for
the surface below the water may be unknown. This is important when infrastructure (such as a bridge or utility) crosses
a body of water. If information below the water surface is
required, a bathymetric survey may be performed.
Both planimetric data and topographic data are produced
by combining field surveying, photogrammetric mapping,
and/or remote sensing. Topographic data is also produced
by new remote sensing technologies, especially LiDAR, for
which the intensity images approximate an orthophoto image
and for which lidargrammetry can be used to generate 2D or
3D breaklines. Whether produced from photogrammetry or
lidargrammetry, breaklines are linear features that describe a
change in the smoothness or continuity of a surface. The two
most common forms of breaklines are as follows:
•• A soft breakline ensures that known z values along a
linear feature are maintained (e.g., elevations along a
pipeline, road centerline, or drainage ditch) and that
linear features and polygon edges are maintained in a
triangulated irregular network (TIN) surface model, by
enforcing the breaklines as TIN edges. They are generally synonymous with 3D breaklines because they are
depicted with series of x-y-z coordinates. Somewhat
rounded ridges (road crowns) or the trough of a drain
may be collected using soft breaklines.
•• A hard breakline defines interruptions in surface
smoothness, for example, to define streams, shorelines, dams, ridges, building footprints, and other
locations with abrupt surface changes. Although some
hard breaklines are 3D breaklines, they are often 2D
breaklines because features such as shorelines and
building footprints are normally depicted with series
of x-y coordinates only, often digitized from digital
orthophotos that include no elevation data.
Mass points are irregularly spaced points, each with x-y
location coordinates and z value, typically (but not always)
used to form a TIN. When generated manually (by a photogrammetrist, for example) mass points are ideally chosen to
depict the most significant variations in the slope or aspect of
TIN triangles. However, when generated automatically (e.g.,
by photogrammetric automated image correlation, LiDAR)
mass point spacing and pattern depend on the characteristics
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of the technologies used to acquire the data and postspacing
criteria selected by the operator.
More information about topographic surveys is discussed
later in this chapter.
Digital Elevation Models. Digital elevation models (DEMs)
have at least three different meanings to different users. For
some, DEM is a generic term for digital topographic and/or
bathymetric data in all its various forms. For the U.S. Geological Survey (USGS), a DEM is a standard form of elevation dataset at regularly spaced intervals in x and y directions
georeferenced in Universal Transverse Mercator (UTM)
coordinates (with uniform 30- or 10-meter grid spacing)
or geographic coordinates (with uniform 1-arc-second or
1/3-arc-second grid spacing); some data is now available in
1/9-arc-second, approximately 3-meter spacing at the equator. For others, a DEM has z values at regularly spaced intervals in x and y directions, but with alternative specifications,
such as narrower grid spacing and state plane coordinates.
DEMs always imply elevation of the terrain (bare-earth
z values) devoid of vegetation and man-made features, as
opposed to digital surface models (DSMs) that include the
elevations of treetops, rooftops, towers, and other features
raised above the terrain. In Europe, DEMs are considered to
be synonymous with digital terrain models (DTMs), but in the
United States DTMs include irregularly spaced mass points
and/or breaklines where the slope changes, thereby depicting
the true shape of the terrain more accurately than a gridded
DEM. For more information on this subject, see Digital Elevation Model Technologies and Applications: The DEM User’s
Manual, published in 2007 by the American Society for Photogrammetry and Remote Sensing (ASPRS). DEMs, DTMs,
and DSMs are efficiently used for computer analysis and display of the topographic surface.
Contours. Contours are lines of equal elevation. Contours
are shown on topographic maps. They are intended exclusively for human interpretation and have little if any value
for computer analyses of the terrain (contours should not
be used to create DTMs). Contours have traditionally been
produced by stereo photogrammetric compilation, where the
operator can see the breaklines (where the slope changes) and
can manually shape the contour lines so that they are aesthetically pleasing. When manually compiling maps using
photogrammetry, the compiler also shapes contours (according to established rules) where they cross streams and roads.
When contours are generated automatically from DEMs,
they are not as aesthetically pleasing. A DEM has no way of
knowing where a breakline exists between DEM points and,
therefore, cannot automatically shape the contours to depict
streams, roads, retaining walls, and so on, correctly. Breaklines are added to correct for this limitation.
More information about contours is provided in Chapter 3.5.
Digital Orthophotos. A digital orthophoto has the image
qualities of an aerial photograph but the metric properties of a map. A digital orthophoto is a digital image from
a perspective photo or image, corrected by an orthorectification process so as to remove tilt displacement (caused by
the roll, pitch, and yaw of the aircraft in flight) and relief
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displacement (caused by the perspective view of the aerial
photograph, which causes taller objects to appear larger and
closer than they really are to the camera). Digital images are
produced either by acquiring the aerial images with a digital
camera or by scanning aerial film. Processes for removing
tilt displacement and relief displacement are explained in the
section on photogrammetry later in this chapter.
3.2.4. Datums
When compiling a base map, it is important to understand the
different kinds of geodetic datum references that may be seen.
Preliminary information will usually have different datum references, or coordinate systems, so it is necessary to understand
the reference requirements of planning documents before the
project design begins. As multiple sources of information and
data are compiled, the datum provide a reference for any necessary translations (movement of data). The datum allow for
different sources of information to be correlated so that all
information is represented in the right location. North American Datum of 1983 (NAD83) is an example of a geodetic
datum. Within a given datum, two numeric values represent a
geographic point, like an x and y value of a graph. The datum
are also used during construction to identify property lines
and proposed infrastructure improvements.
All planning documents, base maps, design documents,
and site plans should include a reference to the horizontal and
the vertical datum of the project. Spatial data deals with location, shapes, and the relationships among features (topology).
Site drawings, survey drawings, and architectural drawings are
examples of spatial data normally compiled with CAD technology. Accuracies are typically depicted in relative terms—for
example, boundary surveys relative to survey corners for which
the geographic coordinates may be unknown in an absolute
sense. Pairs of x-y coordinates may be referenced to an arbitrary origin, and accuracies are typically relative—for example,
estimated as n parts per million of the distance surveyed from
one point to the next. The curvature of the earth is often a negligible factor, and the rules of plane geometry typically apply.
Geospatial data refers to spatial data for which geographic
coordinates are known in an absolute sense, that is, the spatial dataset is georeferenced to true ground coordinates. The
curvature of the earth is important, and the rules of spherical geometry typically apply. Positioning is relative to geodetic
data, using control surveys having geodetic network accuracy, rather than local accuracy relative to an arbitrary origin.
Geospatial data is georeferenced as 2D or 3D coordinates of
points on, above, or below a mathematical model of the earth
(ellipsoid). Horizontal positions may be expressed in terms of
geographic coordinates, that is, longitude east or west of the
Greenwich Meridian and latitude north or south of the equator.
For land development general planning purposes, horizontal positions are normally expressed as 2D rectangular coordinate pairs, that is, easting (x) and northing (y) coordinates
relative to a horizontal datum and coordinate system origin
(where x and y coordinates are zero) normally defined by the
State Plane Coordinate System (SPCS). For detailed planning
purposes, a z value is added to each x-y coordinate pair in
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order to define elevations relative to a vertical datum and origin where the elevation (z value) is zero. These 3D coordinates may be obtained from ground surveying (as described
in detail in Chapter 5.2) or aerial surveying and mapping, to
include photogrammetry and LiDAR. These 3D coordinates
could also be obtained from various forms of GIS.
Global positions system (GPS) measures heights above a
global mathematical surface model called an ellipsoid. In the
United States, the ellipsoid of choice currently is the World
Geodetic System 1984 (WGS84). The current version of the
global geoid used in the United States is GEOID12B. For GPS
operations, an elevation or orthometric height is the distance
from the GEOID to the point of interest on the ground.
The types of data to be discussed within this section are
horizontal control system coordinate data; vertical control
data; global positioning system; NGS ellipsoid models; and
geoid models and orthometric heights.
Horizontal Control System Coordinate Datums. The horizontal coordinate systems establish survey control for agencies. These are usually selected from one of the National
Geodetic Survey (NGS) State Plane Coordinate System
geodetic map projections (1927 or 1983) or NGS Universal
Transverse Mercator Zones. There is one or more specific
coordinate systems designed for each state, which has subsequently been incorporated into state law as the approved
state coordinate system by the state legislature.
The most current version of the national horizontal
datum is the NAD 83, with adjustments based on National
Spatial Reference System (currently NSRS 2011). The previous horizontal datum was NAD 27, which used different
conventions and measurements and also included very different coordinate values.
A point on a site is referenced by the easting and northing
value. Similar to how a point on a grid is defined by the X and
Y values, the easting and northing is a numeric definition for
the location of the point-based the State Plane Coordinate
System. These values define a specific point on a site that can
be referenced back to the design documents.
Vertical Control Datums. The vertical datum is frequently
defined as one of the NGS national (engineering and mapping focus versus tidal) vertical datums. Sometimes it is the
older 1929 National Geodetic Vertical Datum (NGVD29), but
increasingly it is the 1988 North American Vertical Datum
(NAVD88). These vertical datums are designed to span the
entire continent with a uniform vertical datum suitable for mapping and infrastructure design. It’s necessary to understand and
identify the vertical datum used on a project as not all jurisdictions use the same datum. The elevation value that’s identified
for a feature will be different if the vertical datums are different
(and the difference is not constant across the continent).
It is critical for the design team to understand that these
datums usually do not match local sea level on the coasts of
the seas, bays, and the Great Lakes. Corrections based on ties
of the engineering datum to local water height monitoring
stations must be made for coastal design work. In addition,
the design team should be aware that it is not uncommon to
find agency-specific assumed vertical datums, especially in
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older coastal cities, that are not a part of the national datum
definitions.
Vertical datum shifts, which can propagate with global
navigation satellite system (GNSS)-based survey control, are
an important source of error in development projects. This is
especially true when tying two different projects together or
when a subsequent project phase is started years later. Project
vertical datum errors are often economically catastrophic for
a development project and must be guarded against continually by the project surveyors. Projects with multiple datums
are best served by including a nomograph (diagram) on the
project plan set.
Global Positioning System. Most jurisdictions have created specifications for the accuracy of ties to GPS, and
almost all land development projects will make use of GIS
horizontal and vertical control, particularly in metropolitan
areas. Land development surveyors will most likely encounter this standard and specification if they are involved in the
establishment of geodetic control for the creation of a GIS.
The Federal Geodetic Control Subcommittee (FGCS)
has developed the Geometric Geodetic Accuracy Standards
and Specifications for Using GPS Relative Positioning Techniques. It created new survey categories, including AA, A,
and B order. The corresponding relative position accuracies for the new orders are 1:100,000,000, 1:10,000,000, and
1:1,000,000. The traditional first, second, and third orders
are now included in the C group in the FGCS document (as
C1, C2-I and C2-II, and C3), with the corresponding accuracies being 1:100,000, 1:50,000 (or 1:20,000), and 1:10,000.
Using first-order points as an example, network accuracy
in NAD 27 was approximately 10 m, while local accuracy
was 1 part in 100,000. With NAD 83, network accuracy was
reduced to 1 m, while local accuracy remained the same. But
with the addition of GPS into the High Accuracy Reference
Networks (HARN), the network accuracy improved to a tenth
of a meter, and local accuracy for B-order marks improved to
1 part in a million, while A-order marks improved to 1 in
10 million. Finally, with continuously operating reference
station, both network and local accuracies have improved to
1 cm. Both standards are relative accuracy measures.
Ellipsoidal/orthometric heights are also included in the
new standards. The difference between horizontal control,
discussed previously, and vertical control is that horizontal
accuracy is expressed as 2D (x, y) circular error, vertical control accuracy is expressed as 1D (z) linear error, and both
are expressed at a 95 percent confidence level. Additionally,
these standards meet the needs of GIS users.
NGS Ellipsoid Models. There are two NGS ellipsoids used
for the latitude and longitude determinations of the national
control networks. The 1927 datum is based on the 1866
Clarke Ellipsoid, which is a surface fit (mean sea level), continental geoid model. The 1983 datum is based on the GRS80
global fit (earth centric) ellipsoid, which at the time of its
formation was very nearly equal to the WGS84 ellipsoid. All
GNSS measurements in the United States are made in the
WGS84 definition.
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The older 1927 datum contains many distortions and there
is not a precise, known relationship, between the 1927 and 1983
datums. Free software developed by NGS and the U.S. Corps of
Engineers such as CORPSCON can perform mapping accuracy
transformations between the two systems, but the transformation accuracies are not suitable for precise surveys.
Geoid Models and Orthometric Heights. From the practical perspective of project survey requirements, the elevations
used in the project are equivalent to what the NGS terms the
orthometric height. Classical differential leveling is carried out
in the local gravity field. GPS uses a national gravity model
called the geoid model, which closely models the variation in
local gravity fields across the entire continent. This model corresponds closely to the national engineering datum NAVD88.
GNSS locates a position on the WGS84 ellipsoid, which
is a mathematical model of the earth and then uses the
geoid model assigned in the processing software along with
the height of the GNSS receiver antenna to develop a best
estimate of orthometric height (elevation) at the position
measured. There can be significant differences between the
GNSS elevation derived from its two models of the shape and
gravity of the earth and the differentially leveled elevation
measured on-site.
Land surveyors working on a development project should
expect to have to comply with local agency survey horizontal coordinate and vertical datum requirements. If these do
not exist, surveyors will use GNSS equipment and methods
to establish appropriate modern NAD83 State Plane coordinates and NAVD88 elevations for the project.
3.2.5. Accuracy Requirements
As information is compiled for base maps, it’s important to
document the source as well as the accuracy of the information shown. Absent of a reference to potential inaccuracies, the
information presented by a design professional is often considered accurate. Some sources of information will not have any
guarantee or reference to accuracy requirements, but information prepared by a surveyor should follow standard accuracy
requirements. According to the National Mediator Accreditation System (NMAS) (Bureau of the Budget, 1947), horizontal
and vertical accuracy of maps are defined as follows:
•• For maps produced at a scale of 1:20,000 and larger
(which includes most land development projects),
“not more than 10 percent of the points tested shall be
in error by more than 1/30 inch, measured on the publication scale…. These limits of accuracy shall apply
in all cases to positions of well-defined points only.
‘Well defined’ points are those that are easily visible
or recoverable on the ground, such as the following:
monuments or markers, such as bench marks, property boundary monuments, intersections of roads,
railroads, etc.; corners of large buildings or structures
(or center points of small buildings), etc. In general
what is ‘well defined’ will also be determined by what
is plottable on the scale of the map within 1/100 inch.”
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Circular Map Accuracy Standard (CMAS) equals
1/30th of an inch; at ground scale, the CMAS equals
1/30th of an inch divided by the map scale. For
example, DOQQs compiled at 1 in = 1000 feet have
a CMAS of 1/30th of an inch on the map or 33.3 feet
on the ground.
•• “Vertical accuracy, as applied to contour maps on all
publication scales, shall be such that not more than
10 percent of the elevations tested shall be in error
more than one-half the contour interval. In checking
elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal
displacement within the permissible horizontal error
for a map of that scale.” For example, the Vertical
Map Accuracy Standard (VMAS) equals one-half the
contour interval on the map, or 1.0 foot in ground
elevation if the contour interval is 2 feet.
Both the CMAS and the VMAS are standards at the 90 percent confidence level, but these NMAS errors do not necessarily follow a normal distribution—that is, there are no limits
to the size of the errors that exceed the CMAS or VMAS. The
NMAS is now obsolete for use with digital geospatial data.
ALTA/NSPS Standard. For commercial property, another
standard exists. Because there is such wide variation in the
standard of surveying between different locales, clients need
assurance about the degree of accuracy of the survey work.
Members of the American Land Title Association (ALTA),
in cooperation with the National Society of Professional
Surveyors (NSPS), realized that no national standard existed
to judge the accuracy of land title surveys. So, in 1962, the
ALTA/NSPS Minimum Standard Detail Requirements for
Land Title Surveys were developed. Some state boards of
registration have incorporated the ALTA/NSPS Standards
into the accuracy requirements of their minimum standards, and land development surveyors should check their
local and state requirements to see whether this condition
exists and applies to their projects.
The ALTA/NSPS standard has undergone many updates
after starting in 1962. The original rationale behind these
standards was that the title insurance industry needed to be
held responsible for the legal aspects of accuracy standards,
while the surveying community would be responsible for the
surveying and accuracy aspects. The title industry has generally not concerned itself with the accuracy portion of the
standards but instead has relied on NSPS to develop them.
With the 2016 changes, ALTA and NSPS separated the survey accuracy specifications from the rest of the standards to
allow for the adoption of necessary updates without having
to revise and readopt the entire document.
In the most recent version of the ALTA/NSPS Standards,
the organizations have reverted to a positional accuracy
standard. This new standard allows surveyors to employ
appropriate procedures and equipment if a maximum allowable amount of positional uncertainty in any corner location
is not exceeded.
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3.2.6. Typical Surveys
The typical surveys encountered on a land development project
are boundary and topographic surveys. These are necessary to
start a project and are an important aspect of defining the base
map. The developer may be reluctant to invest in actual survey
work during the site analysis phase, but survey information (as
opposed to GIS or other public information) is accurate, certified,
and significantly more detailed. There is a significant amount of
information that can only be obtained through survey work and
ordering a site survey is the only way to ensure the information is
current and certified by a professional. More information about
detailed surveys is discussed in chapter 5.2, but the survey types
and basic survey process are introduced in this chapter.
Boundary Survey. A boundary survey is a process to identify the property lines, true property corners, and easements
within a tract of land. In the land development process, the
determination of tract location and geometry is extremely
important. All future work related to development hinges on
the initial survey work. Should an error occur in the survey,
it will most likely manifest itself in later stages of the development process. It is conceivable that errors in the initial
survey could lead to improper location of streets and other
improvements and to incorrectly placed interior boundary
lines. Errors such as these could bring on project delays, possible project shutdown, and redesign which inevitably cost
both time and money. The results of poor judgment associated with boundary line location can be severe: clients lose
confidence and can even pull or rescind work as a result.
When the development involves one or more loans, the
lender typically requires certain assurances to be confident
that its investment is as risk-free as possible. One of the tools
to minimize risk is the ALTA/NSPS Land Title Survey. This
type of survey is defined by a set of standards adopted and
periodically revised by the ALTA and the NSPS.
The Minimum Standard Detail Requirements for ALTA/
NSPS Land Title Surveys outline the responsibilities of the
surveyor in conducting a survey that will be used by the title
company and lender in conjunction with the closing of a
commercial loan on real property. The standards also outline
the responsibility of the client or his or her representative
with respect to providing complete sets of documents from
the title research and as may otherwise be required by the
surveyor to conduct a proper and complete survey. In addition, the optional items allow the client or lender to request
additional information from the surveyor related to certain
specific issues such as availability of utilities.
The ALTA/NSPS Minimum Standards call for the surveyor to resolve the boundary of the property in addition to
providing comprehensive documentation of any facts on the
ground or in the provided records that may be evidence of
otherwise unknown or undisclosed title problems. Such facts
might include, for example, gaps or overlaps with adjoiners, and potential encroachments and uses of the property
by others. The disclosure of these facts allows the title company, lender, and buyer to weigh the associated risks and to
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Example of a boundary plat. (Reprinted by permission of Uniwest Construction, Inc.)
negotiate or formulate appropriate resolutions to any issues
that might have been found during the survey. An example
of a boundary plat is shown in Figure 3.2B.
Easements. An easement is an area of a parcel that grants
certain rights to an entity for a specific purpose but does not
include ownership rights (as introduced in Chapter 2.1).
An underground sewer pipe may have an easement
that allows the utility provider to access the site and maintain the pipe system. Many land and title features may be
located within easements. In addition, easements are often
associated with infrastructure systems; these include, but
are not limited to, easements for streets, wells, drainfields,
stormwater runoff, storm and sanitary sewers, gas lines,
and power lines. Easements do not have to be associated
with a physical infrastructure feature—an easement may
exist to provide access across a site, reserve an area of land
for future roadways, allow temporary construction activities by others, or provide other rights. Easements are usually recorded in the local courthouse and referenced in the
plans with a deed book and page number (often identified
as DB/PG).
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Two types of easements, namely express and implied,
can lead to land disputes. The law provides for express easements. The law also recognizes that easements may exist or
be created that are not express. These are implied and prescriptive easements (refer to Chapter 2.1 for more information on easement types).
It is important that the project surveyor identify whether
any easements exist at the time the boundary survey is being
conducted, since they may interfere with later development
plans. For example, an easement may exist for a utility system that prohibits development of a new structure within
the easement; some easements, such as electric transmission
lines, can be over a hundred feet wide. Therefore, locating
existing easements is an important part of any survey. Various
easements, such as storm, sanitary, ingress/egress, and public
access are shown in the subdivision plat in Figure 3.2C.
Topographic Survey. While the boundary survey is the
delineation of property lines and easements, topographic
survey is the delineation of physical features. A topographic
survey usually includes planimetric features, but Figure 3.2D
provides a graphic representation for how the information
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Figure 3.2C
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Example of easements on a subdivision plat with easements.
F i g u r e 3 . 2 D Example of a (a ) topographic map and (b ) planimetric (zoning) map.
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might vary between a topographic and planimetric map.
Although the topographic features of a property may have
some significance to project feasibility and site analysis, their
primary importance is initially realized in the early design
and engineering phase. The horizontal and vertical positions
of natural and man-made features play a critical role in determining how a site will be engineered to suit the intended use
of the property. Consideration must also be given to the way
these features relate to those lying on adjoining properties
and within street rights-of-way or easements. Determining
how utility services can be provided to the proposed development is among the first preliminary design tasks undertaken.
Topographic information for areas well outside of the project site boundaries is often needed to make preliminary decisions regarding design considerations such as storm drainage
patterns. When this is the case, the use of smaller-scale topographic maps, such as those available from the USGS with a
scale of 1 inch = 2000 feet or from local GIS data, is appropriate. As the design process begins to focus more on intricate
details, smaller-scale (e.g., 1 inch = 50 feet) maps are needed
to ensure that site engineering is as balanced as possible with
regard to earthmoving and the use of utilities.
The size and relative detail of a site often determine the
surveying methods employed. For large areas, aerial photography and photogrammetry are economical means of
providing detailed maps, as is LiDAR. These surveys have
become increasingly more accurate with improved technology, but they always need to be field verified. Moreover,
there may be site features that will need additional detail
such as areas obscured by tree cover. Field methods employing electronic survey systems may be used on other sites to
create quality maps in a timely fashion. For small sites, other
methods—such as radial conventional or RTK GPS (realtime kinematic global positioning system) surveys—may
prove more cost-effective.
The information collected for a topographic survey focuses
on elevations at points around a site. When the elevations are
field measured, the points selected are determined by the surveyor based on what information is required to accurately define
the site topography. The critical elevations include locations
around a building, utility structures, channel sections, vegetation, and other site features. In undeveloped parts of a site, such
as a large field or forested area, the elevations points are collected
on a grid. These elevations are then used to generate contour
maps for the site, which provides a more legible way to interpret
the topography (see Chapter 3.4 for more information).
With remote sensing processes, like LiDAR, the system
collects all visible information (often millions of points).
The collected points are often referred to as a point cloud,
which provides a detailed geospatial definition of all visible
points. The surveyor (or technician) will identify critical
elevations within the model to create the necessary contour map. In this case, the field time is significantly reduced
but so much data is collected from the LiDAR system that
it requires time to determine what information should be
shown in the plan. Figure 3.2E depicts the point cloud from
03_Land_CH03_p125-304.indd 154
F i g u r e 3 . 2 E Example of LiDAR data.
terrestrial LiDAR survey of a roadway and adjacent areas.
The visible features (buildings, road signs, utility poles) are
comprised of various points.
Because the topographic map is an integral part of the design
process, omission of valuable information in a topographic survey can lead to costly problems. Detailed description about
how survey work is performed is provided in Chapter 5.2.
3.2.7. Survey Process
A site survey, whether from terrestrial or aerial methods, is
required to accurately locate topographic, planimetric, and
subsurface utility features on the site. Field (terrestrial survey)
methods are necessary in land development when detailed
location of surface and subsurface utilities is required. For
large sites (often above 20 acres), an aerial survey may be
performed to collect topographic and planimetric features
(subsurface utilities would still require field work). There are
several different methods and tools for gathering the survey
data, as identified in Chapter 5.2.
General Procedures. Land development applications often
require a series of maps showing different data and they are
drawn at different scales. Normally, the site engineer will
specify a scale or contour interval and coverage required for
the mapping, as well as the final product required. When
defining the scope of work for survey, the preliminary design
information should be referenced. Both horizontal and vertical control datums are required.
Research of Boundary Survey. Research for boundary
surveys begins by gathering property descriptions, tax
maps, roadway plans, zoning maps, easements, and other
related information. Gathering of this information begins
when the developer enters negotiations for the future
development of a tract of land. An important depository
of research information is the local courthouse. Local utility companies often can provide valuable maps and information. State and federal agencies maintain valuable land
information. Title insurance companies can play a role
in providing deeds, easements, other record title information, and previous surveys of the subject property or
adjoining properties; however, title insurance companies
often do not research far enough back in time to return to
the original survey.
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3.2
Existing maps or plats of the land provide visual support
for reaching conclusions that ultimately result in re-creating
boundary locations. Maps may be found during the search
of land records and from the evidence found during the field
survey. Plotting individual deeds, patents, and grants provides pieces of a jigsaw puzzle. When the pieces of the puzzle
fit together and form a logical composite map, this may provide some clues to successfully interpret land ownership and
boundary location.
Two of the primary systems that form the basis of land
descriptions in the United States are the metes-and-bounds
system and the Public Land Survey (PLS) System (later called
the General Land Office, or GLO System). There are other
systems that have historically been used in different areas
within the United States; therefore, surveyors should familiarize themselves with the system that applies to the project.
These systems have an impact on interpretation of evidence
gathered in the research process.
Research of Land Records. The actual process of conducting research varies widely across different regions of
the United States. The project surveyor must seek out all
information from public and private records that may be of
benefit in performing the survey. Ideally this information is
gathered and evaluated prior to commencement of the fieldwork. If not, additional trips to the field will be required as
new pieces of evidence from the records dictate further field
investigation.
Research information comes from a variety of sources.
Typically, a visit to the local jurisdiction assessor and/or
auditor (the name and function of county offices differ in
various parts of the country) will help in the search for the
record descriptions of the subject property and its adjoiners.
Most tax maps are geographically inaccurate, but they are
useful in identifying the juxtaposition of subject and adjoining properties, roads, streets, and other features.
Once the names and property transfer dates are determined,
a copy of the actual transfer document can normally be found
Figure 3.2F
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in the registry of deeds, county recorder’s office, or clerk of the
court. This office of public record varies from state to state.
All states allow for the recording of deeds when the transfer of real property takes place, and almost all such transfers
by deed are recorded. These recordation statutes provide a
means of giving notice of ownership of the estates disclosed in
the recorded instrument. Unrecorded deeds or other instruments are generally valid only between the immediate parties.
These recordation statutes are not applicable in cases of title
acquired by unwritten means such as adverse possession.
Intentions as to boundary location must be obtained
from the information at the time the lines and corners were
created. Comparison of the deeds of the subject tract should
be made with those of the adjoining property to determine
whether conflicting calls exist.
The type or method of physical record keeping varies across jurisdictions. Many continue to maintain their
records as they have for 200 or more years, in transfer books,
grantee/grantor index books, and deed record books, while
many others have converted modern forms of recordkeeping. With GIS, many more jurisdictions will have records in
a digital format, which is often available online.
The actual process of checking the title begins with the
name of the present owner in the grantee index. The grantor
of this conveyance was the grantee of the previous conveyance.
Looking each previous conveyance up in this manner results
in the development of a chain of title for the property. Using
this chain, a person familiar with reading and interpreting title
documents can determine whether each person in the chain
took title to the property in a regular manner without defect.
The grantor’s name, the type of instrument, and the book and
page of the actual recorded document involved in the transaction is listed with each entry. In some instances, a brief
description and location of the property are included. A copy
of a partial page from a typical grantee index book is shown in
Figure 3.2F. A grantor book is identical except that the index is
on the name of the grantor rather than the grantee.
Example of a grantee index book.
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Visible evidence of potential encroachments of any kind
must be located and the history of the encroachment documented if possible. An encroachment may be defined as a
trespass or the commencement of a gradual taking of possession or rights of another. Encroachments may take the form
of walls, fences, buildings, or other structures representing
occupation or possession, which extend onto another’s title
rights. Encroachments may lead to claims of unwritten rights
such as adverse possession or acquiescence when the party
whose land is encroached upon allows the encroachment to
go unchallenged. Such an implied consent may cause loss of
title through certain doctrines.
Often land contemplated for development comprises several smaller tracts, which are joined together to form a larger,
more developable tract (consolidation). In such a situation,
it is important for the surveyor to confirm the contiguity of
the parcels involved or to identify the existence of any gaps
between the parcels. This is critical because the record title to
such a gap may lie in the hands of some former owner rather
than in the hands of the owner of the parcels being surveyed.
Gaps can be as troublesome as encroachments if not identified and quantified before development begins.
Boundary Discrepancies. The project surveyor must
check for consistency of identifying calls for monuments by
adjoining documents with those of the subject property. Any
differences should be noted and reconciled if possible.
Discrepancies in metes-and-bounds descriptions are
often the result of variation in magnetic north between the
time the legal description was constructed and the present
reference to north, as well as the instruments used to perform the survey. Variation in distances can also exist. Both
human error and improvements in technology are factors in
discrepancies between calls in different documents.
Frequently the surveyor must search the records for a
longer time than required for title purposes. There are two
reasons for this extended search. As previously discussed,
when senior rights are involved in the determination of
ownership, such a search may be mandated. The second situation is required because bearings are often copied from
older deeds. The origin of the bearing would be the date of
the survey that produced the bearing. The date of the origin of the bearing must be known so corrections for magnetic declination can be properly made. This is necessary in
the proper preparation of the composite map. Reference to
direction in different documents having various dates can
be adjusted to a common north system of reference. This
common reference could be true north, State Plane Coordinate Grid North, or some other local system that might
be prescribed.
Differences in distances can result from a failure to convert units used in original surveys to those of the adjoining
property or those in use at present. Reconstructing an old
survey is not a simple matter. The surveyor must be familiar
with the units of measurement used at the time the original
survey was made. Often these measurements had more than
one interpretation.
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Early land grants were often described using differing lengths of chains and varying sizes of acres. The best
example of this is given by the variation in the acre in different parts of England. In Devonshire and Somersetshire,
the acre was 160 perches of 15 feet, or 36,000 square feet. In
Cornwall, the perches were 18 feet and yielded 51,840 square
feet. In Lancashire, the perches were 21 feet and the acre was
70,560 square feet. When the early settlers of this United States
began to survey the land, they often used the measure with
which they were familiar. Today, an acre is 43,560 square feet.
Courts throughout the United States, particularly in the
western states, have found that many of the original government surveys of parts of the public lands were imprecise. The
reasons for these inaccuracies included low land value, difficulties encountered during the surveys, and, in some cases,
outright fraud. This phenomenon is also true for those early
surveys of the original grants in this country.
Project Control. The first step in planning for the topographic survey of a land development project is the completion of well-planned horizontal and vertical control
networks. These networks must be geometrically strong and
well monumented.
Data must be gathered and compiled in such a way that it
can be used for preliminary and final design. Electronic and
robotic total stations, GNSS in all its many configurations,
and electronic data collectors are common instruments for
field-run surveys. Photogrammetry, which uses aerial photographs combined with ground control points to generate
topographic maps, has become the preferred method for surveying large sites.
The photogrammetric survey method is not appropriate
for determining boundary lines, although it may serve in an
evidentiary capacity, allowing a broad overview of an area
not available at ground level. Small sites, particularly those
with extensive features, are still commonly surveyed using
field methods. Both the field survey data and the photogrammetric survey data can be compiled into digital topographic
maps to serve as the base layer for design files.
The number and location of the control points depends
on the nature of the project and on the surveying method
used. As a matter of convenience during the design and
construction phases of the project, control points should be
interspersed throughout the site. It may be cost effective to
use traverse points located in the boundary determination
as horizontal control stations for the topographic survey.
Establishing control points as part of the boundary traverse
loop and cross-ties through the middle of the site provide
a network convenient for use throughout the design and
construction phases of the project. To do this, adequate forethought on traverse location is needed at the time when the
traverse is being set for the boundary determination.
Traverse Survey. The traverse may be conducted using
conventional equipment and procedures, such as measuring angles and distances with a total station or theodolite
and an electronic distance-measuring (EDM) device, or the
surveyor may employ the use of global positioning system
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(GPS) receivers to establish locations using satellites. Frequently a combination of procedures and equipment is used.
Available personnel, terrain, site features, vegetation, and
traffic often dictate the most logical equipment and procedures to employ.
No matter what procedures and equipment are used, the
lines and points on the traverse should be located on the subject property when at all possible to prevent the need to work
off-site. Traverse stations on the survey should be placed at
the most advantageous locations and frequencies, considering they will subsequently be used to locate features on the
property and possibly for other functions later in the development process. These uses include topographical surveys,
construction stakeout work, and control surveys for the
development. If points are destroyed, the configuration of
the initial traverse should allow for easy replacement.
The traverse control points serve as the basis for locating
visible, found, or described corners, fences, tree lines, and
other features relevant to the title, lines, and corners in addition to those improvements called for in the state or ALTA/
NSPS standards or as may be required by the client.
Cross-ties between nonadjacent points of the traverse
should be made as frequently as is practical. This affords
checks on the survey work and allows for better survey
adjustment of results. These cross-ties also provide additional stations for control and stakeout in later stages of
development. Survey markers set on this traverse, such as
rebar, pipes, or concrete monuments, should be of a length
and diameter to be secure after installation or as required by
state minimum standards. Alternative objects such as “PK”
nails or scribe marks can be used when they are more suitable for the conditions.
All traverse stations should be referenced for ease of
future recovery. A minimum of three references is desirable.
The references should be accurately described and recorded
in the field notes. See the illustration in Figure 3.2G for an
example.
F i g u r e 3 . 2 G Methods to reference traverse stations (transverse ties).
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F i g u r e 3 . 2 H A boundary point is tied to the traverse using a mini-
mum of two references. Shown here is (a ) a measured angle and distance
directly to the point and (b ) measured distance to POL (point on line) and
offset distance to the point.
Field crews must be sure that the information to be in
relation to the boundary traverse is properly tied to the survey traverse line using methods that afford redundancy in
measurement to better ensure the integrity of the data. This
is particularly important when locating markers and monuments, which are integral to the resolution of the boundary.
See Figure 3.2H for an example of points to be tied to the
traverse.
Locating Features and Improvements. When physical
features are located with a survey they are depicted and often
annotated, as referenced in Figure 3.2I, for abbreviations
used in data collectors. Found iron pipes, concrete markers,
stakes, and other objects used to mark corners and points on
line are evidence and must be located. The locations of tree
lines, old fence lines, walls, ditches, and other features on or
near the division lines must be determined and described
in detail, including the material from which objects are
made and the condition of the objects. Photographs of the
evidence for future reference are helpful. Such photographs
can provide excellent supportive evidence should the survey
result in litigation. Points found during the survey and called
for in the deed should be used as traverse stations when they
are accessible.
Existing streets may have an impact on the boundary
line determination and later development. The locations
of pavement, right-of-way lines, and centerlines should be
recorded in the field notes. When no pavement exists, the
traveled way should be located. Access roads on or across
property, whether in use or not, should be located and
shown. Such roads might be used for access to surrounding
property and as such might be included in an easement.
Cuts or embankments, tree lines, or fencerows, which
might indicate long-abandoned roads, should be located
and shown.
Coordinating Field and Office Procedures. Before field
work begins, office procedures begin with a meeting between
the site engineer, field party chief and the surveyor of the
computing department. A lack of communication between
these two will usually result in error, frustration, and inefficiency. Most topographic surveys are done in concert with
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F i g u r e 3 . 2 I Codes and abbreviations used for data collectors.
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other field tasks such as boundary survey or soil borings. In
most instances, knowledge of foregoing survey operations
will reside within the files of the computing department.
The survey party chief, in collaboration with the computing department personnel, must establish a data format.
Each point located in the field must be uniquely identified as
to horizontal location, vertical elevation, and physical character. Depending on data collector and computer software
flexibility and compatibility, some or all these values may
be catalogued within the data collector for delivery to the
computer. Point numbers, subject-specific or layer-specific
numeric codes, and physical descriptions and notations are
often used. In most instances, a standard set of codes and
descriptive abbreviations alleviate problems with interpreting field data.
Before entering the field, the computing department may
download existing traverse control and other pertinent coordinate data into the data collector. Whether or not such coordinate data is available or used within the data collector, the
computer must be able to assign a unique identity to each
3D collection point. Most coordinate geometry software uses
point number identification.
The survey party chief should review the project with
the computing department surveyor to set survey priorities.
What datum will be used? What accuracy and precision are
acceptable under the scope of services? What will the horizontal scale and contour interval be? Will extra observations
be needed to demonstrate minute vertical detail (i.e., top of
curb, inlet grates, pond risers and pipe inverts, manhole rims
and inverts, slab elevations, height of buildings)? Is the project devoid of detail or is it covered with buildings and roads?
If there are numerous structures, have they been located previously or will a location survey be part of the task? How
will the data be collected to develop contours along the face
of buildings, walls, curblines, critical section ditches, and
streams? What will the topographic data be used for? Can
the data be collected in a random pattern, minimum grid
interval, or stationed baseline/offset cross section? All these
considerations can have immense impact on the suitability of
the finished topographic map.
Once all field data is collected, all points defining breaks
in the lay of the land must be connected within the digital
model. If an adequate coding system has been devised, the
connection of these points will automatically occur through
the software. These lines that connect the points are referred
to as breaklines. All contours are interpolated along these
lines. A contour of a given value will naturally break in a
certain direction as it runs between the interpolated points
of equal elevation. Examples of breaklines include top or
bottom of bank; top or bottom of curb; building/wall faces;
edge of pavement; centerline of stream, ditch, or road; or
any continuous linear or curvilinear break in the elevation
of the land. Every line segment will have two corresponding
coordinates defining the location and elevation of the ends of
the lines. Using interpolation, the computer can easily locate
where any given contour elevation falls.
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Having set up these known natural and man-made
control lines, the technician initiates a computer network
of similar breaklines, connecting all other miscellaneous
points within the file that are intended for topographic compilation. The technician protects the integrity of the input
data by making use of elevation, numeric, or proximity filters to eliminate the use of extraneous data points within the
coordinate file. No network line will cross another. Network
lines are constructed in such a manner that they form a
continuous pattern of triangular sections whose vertices are
composed of field points. The maximum allowable length
of the network lines (selected by the technician) dictates the
accuracy of the topography. The density of field observations, however, limits the length of the network lines. The
computer cannot create a network in areas where data collection is insufficient to accommodate a given line length.
No contours are obtainable in such an area. The technician
must then decide if more field data is necessary or if a longer
network line is acceptable.
When the entire network is complete, and all errors have
been dealt with, the technician has a good network file, or
TIN (triangulated irregular network) file from which contours of a given interval may be derived. Having selected an
interval and scale suitable for end users, the computer creates (by interpolation) the location of all points within the
network for each contour. Contours are then created within
the triangular network sections by graphical depiction on the
screen. The topographic survey map is then ready for plotting. The TIN file should be kept inviolate; it can be used
again and again to generate additional topographical maps of
varying contour intervals, as well as centerline profiles, grading scenarios, and earth takeoffs.
3.2.8. Site Diagram
The site diagram is often considered a pre-design effort that
focuses on the spatial relationship of different parts of the
existing site (the diagram may also be referred to as a bubble
plan). This diagram usually compliments the feasibility study
and is prepared as the base map content is collected during
the site analysis phase. The site diagram highlights the constraints and opportunities identified on the site. The site diagram can provide the most value to the design team when
survey information is used but in early phases the information may be collected from various sources with low confidence in accuracy.
Site constraints should be noted and labeled, using shading, heavy outline, and other techniques to identify areas of
the site that are totally unusable or usable with significant
corrections. These include floodplain areas, unstable or
erodible soils, or soils of poor bearing capacity, steep slopes,
wetlands, and other environmentally sensitive areas that
local state and federal regulations accord special treatment.
Area measurements of these encumbrances are recorded on
both the map and in tabular form in a report.
The site diagram should indicate those facilities and
improvements that will be necessary, because of function,
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industry practice, or government regulation, to develop the
site. These include probable vehicular access points, along
with an assessment of improvements to public roads that will
be needed to facilitate safe, convenient access.
Along with these site constraints, the map should reflect
significant site opportunities. These should stress references
to the recommendations and guidance of the community’s
comprehensive plan, an indication of the issues that the
development program must resolve. The map should suggest
potential amenities that could facilitate development and
enhance project marketing. Important views, stands of trees
at property boundaries, and similar characteristics should be
highlighted.
Site data is also compiled in a report form in addition to
the site diagram graphic. The report must be well organized
to present each category of information studied by the site
engineer. Sources of information should be clearly stated,
including public officials who have been interviewed during
the feasibility study. The site engineer should attach copies of
the comprehensive plan or zoning ordinance language that
have unique and direct bearing on the project. The site engineer should identify items for which administrative waivers
appear appropriate and likely to be granted.
Limitations on the reliability and scope of the information should be noted. Any opinions of the site engineer
included in the report should be clearly identified as such. In
addition, the report’s findings should be qualified as being
preliminary only, withholding any guarantees about potential problems that could not have been identified because
of the scope or depth of study. The report will be used by
the developer to determine future actions concerning the
property.
The report should clearly identify qualifications to its
conclusions. Sources and timeliness of information, limitations imposed by time and resources, and lack of applicability to subsequent projects should be firmly established. The
report will likely serve as a reference document in further
negotiations with the landowner and lender. If the developer
concludes that development is feasible, the documentation is
useful for those engaged to perform subsequent analysis and
design. In addition, the report serves as documentation of
the consultant’s findings, useful in case of subsequent claims
of error or omission.
Buildable Area. The content identified on the site diagram will help to identify the buildable area of a site. The
buildable area is simply the net site area suitable for development. Undevelopable and unbuildable land is subtracted
from the gross (total) land area of the site. Undevelopable land refers to site area that is prohibited from being
developed based on jurisdictional regulations, which could
include floodways, floodplains, wetlands, conservation
areas, and historic features. Unbuildable land may include
portions of a site characterized by physical characteristics
that make construction impractical, which includes problem soils and steep slopes.
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Once the area of encumbrances is subtracted from the
gross area, the resulting area is the buildable area. The buildable area may be a preliminary first step of design, because it
will be the starting point of conceptual designs (to be introduced in Chapter 4.1).
To define the buildable area, it is important to start with
the environmental and historic areas that were previously
described in Chapter 2.5. Floodplains, wetlands, problem
soils, preservation areas, and historic features should be
avoided. These are the first undevelopable and unbuildable
areas of a site. The site analysis will further identify additional constraints that may need to be avoided including
steep slopes. As these are added to the base map, the site diagram will form and the usable buildable area will be defined.
It is important to complete this during pre-design to ensure
the site is satisfactory for development. If the site analysis
finds too many constraints (not enough buildable area),
a new site may need to be chosen. While it is undesirable
and costly, it may be possible to use site area encumbered by
unbuildable areas, but the process and conditions should be
carefully evaluated.
A water feature, such as an existing stream, is likely an
unbuildable area. By subtracting the water features from
the property, as undevelopable parts of the site, it is possible to identify what part of the property is developable (the
buildable area). If a stream flows through a 5-acre property, and 2 acres are defined as stream and wetlands, only 3
acres are potentially developable. This analysis can quickly
change the perceived value of the site and the development
program.
Environmental/Historic Content. Environmental and historical features of a site that were identified during the preliminary investigation should be the first content included
on the base map. As discussed in Chapter 2.5, identifying
environmental and historic constraints is an important step
in the site selection process. Red flags, including impediments to the site or undevelopable areas, could prove too
costly or too limiting to develop upon.
Finding the best way to avoid impacts, minimize damages, or mitigate impacts is crucial to the success of a project.
The environment will be an important aspect of the entire
design process and through the construction as well. It is
critical to understand how to protect the natural resources
and the local environment when beginning to design a project. Design considerations for wetland and then open space
and vegetation is discussed below.
Wetlands. Once on-site wetlands and other bodies of
water have been accurately mapped, they can be overlaid on
a site plan. This will help to determine the buildable area
on the site (avoiding wetlands), or identify where wetlands
may be impacted. In order to obtain permits to impact wetlands, the project must demonstrate a process of avoidance,
minimization, and ultimately mitigation for impacts that are
unavoidable. All reasoning for not avoiding or minimizing
must be backed up with defensible justification, including
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costs. Costs detrimental to the development (i.e., lost buildable area) must be balanced with costs of mitigation and
the consequences of the impacts. If wetlands and waters are
present on-site, the final delineation may reduce the number of buildable lots or area originally envisioned.
Wetland mitigation costs vary from region to region and
are normally a product of the cost of the land. The monetary value for mitigation can be hundreds of thousands of
dollars per acre, and hundreds of dollars per foot. It should
also be noted that if the project impacts a forested wetland,
mitigation is normally required at a 2:1 ratio. If a project
is expected to impact a high-quality stream, as determined
by the environmental professional utilizing the approved
stream assessment methodology for the physiographic
region, the mitigation credit ratio could also be higher
than 1:1. Overall costs should be considered; for example, a
stream crossing utilizing culvert directly impacts the stream
and requires mitigation. A pre-cast concrete bridge will
likely have a higher material cost but does not impact the
stream directly and could then be a less expensive option
when factoring in the mitigation cost associated with the
stream impact from the culvert. Additionally, the bridge
option reduces the permit requirements and prevents disturbance of natural features.
For most projects, the best strategy is avoidance. In the
initial site planning effort, it is best to consider layouts where
development occurs in the uplands, while wetlands and
stream valley corridors are avoided.
Open Space and Vegetation. Preservation is best accomplished during the early planning or feasibility phase of site
development, as part of the site inventory and analysis. A
review of the existing site conditions from the feasibility
analysis and field investigation provides an overview of the
limits of the existing vegetation, the site topography (refer
to Chapter 3.4), soils locations and descriptions, open fields,
existing buildings, and other existing and/or natural features. These living and nonliving factors influence the species and quality of vegetation indigenous to the site. A review
of recent aerial photography of the site affords not only an
overview of the site but allows a characterization of existing
vegetative patterns on site as well.
The appropriate analysis of this collected data identifies
and prioritizes the quality of the existing vegetation and its
placement in the landscape. The prioritization of the existing
vegetation in turn helps to define the buildable area of the
property. Areas that should be preserved and avoided should
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be overlaid on a site plan to determine the buildable area of
a site. Together with the overlay of the wetlands and other
bodies of water, this site plan will be a good indication of the
development potential of the property. This in turn will influence the placement of roads, buildings, parking lots, utilities, and other development program features integrated into
the conceptual design of the site, which will be discussed in
Chapter 4. The objective is to place site improvements where
there will be a minimal impact on the desirable vegetation targeted for preservation. Note that land disturbance adjacent to
preserved areas can still impact the natural environment. For
example, trees have a critical root zone that extends beyond
the tree trunk—when impacted (even with minor land disturbance) the tree may eventually die. Proper tree preservation should be provided when establishing limits of work.
For open space and tree preservation to be successful,
it must be integrated into the early planning stages of land
development. It is nearly an impossible task to address these
types of preservation issues later in the site design stages
while still achieving the optimum balance between the built
and natural environments. Making the most of natural amenities is contingent upon early consideration of these spaces.
Retro-fitted open space and tree preservation has a greater
chance of failure and can create unwanted liabilities and
costs. Properly planned and implemented open space and
tree preservation can enhance the aesthetic, natural, and economic environments for all.
Example. Figures 3.2J through 3.2R show a base map,
series of site diagrams, and a composite map of constraints
and opportunities that may be produced for a project during
the site analysis stage of the land development design process. Identifying this information on a map is critical to accurately reflect the site conditions. More information about the
content on these maps was introduced in Chapter 2 or will
be introduced throughout Chapter 3. These site diagrams
will help the developer to make a decision regarding the site
selection. This is also important to have when design efforts
begin. The site diagram is the starting point for conceptual
designs (to be introduced in Chapter 4.2).
Another example of a series of site diagram graphics is
included with the preliminary engineering feasibility study
in the appendix, Chapter 7.6.
REFERENCE
Bureau of the Budget. 1947. National Map Accuracy Standards.
Washington, DC: Office of Management and Budget.
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Example of a base map.
Figure 3.2J
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Site diagram: area classifications.
163
Figure 3.2K
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F i g u r e 3 . 2 L Site diagram: zoning.
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165
F i g u r e 3 . 2 M Site diagram: resource protection areas.
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F i g u r e 3 . 2 N Site diagram: soils map.
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Site diagram: steep slopes.
Figure 3.2O
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F i g u r e 3 . 2 P Site diagram: drainage patterns.
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Site diagram: utilities.
169
Figure 3.2Q
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F i g u r e 3 . 2 R Site diagram composite map.
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Chapter 3.3
Transportation Fundamentals
3.3.1. Introduction
Transportation systems, such as roads, must be identified
and analyzed when considering a project site. This is critical to ensure that the access into the site is feasible and to
understand local transportation requirements (as defined in
the local comprehensive plan and development ordinances
as discussed in Chapter 2). New transportation systems will
also often establish the pattern for the site layoutgrids of
streets are common for urban areas, whereas meandering
streets are common for suburban communities. The design
of the streets, both in size and alignment, can establish the
character of the site. Narrow streets with sidewalks and
street trees will enhance the aesthetics of a neighborhood,
whereas wide multilane roads may be appropriate for an
industrial development.
This chapter begins by introducing typical roadway
characteristics that are important for an initial site analysis.
These characteristics are also the basis of design that will be
encountered as site design progresses. The second half of this
chapter focuses on preliminary engineering design concepts
that will shape the road design for a project. These design
concepts will be encountered and utilized during schematic
design (Chapter 4). The final design of roadways is then
described in Chapter 5.3.
When discussing a road, it is important to be familiar with
the road geometry. The geometry of a roadway is generally
defined in three parts: a cross section, alignment, and profile.
A roadway cross section refers to a detailed cut of a roadway
that’s taken perpendicular to the direction of travel and represents the typical elements of the roadway. The road alignment represents the horizontal geometry of the roadway,
including curves and tangents. The profile represents the vertical geometry of the roadway along the alignment. Together,
the profile and alignment define a three-dimensional polyline and the cross section is applied along this polyline.
The components of the road cross section are discussed
in this chapter when identifying the typical roadway characteristics. The road alignment and profile are introduced in
Chapter 5.3.
3.3.2. Functional Classifications of Roadways
Several different classification systems exist for grouping
highways and streets. These systems are used to establish a
common base according to an interest, function, or operation of the road. The classification is used by municipalities
to establish access and design criteria of the roadway. With
a land development project, the classification of the roadway can determine whether a turn lane is required into the
site, if a traffic signal can be installed, how far the entrance
needs to be spaced from another entrance, and the permissible movements in and out of the site. Access to a site will
greatly influence the desirability of the site and success of the
development.
The AASHTO (American Association of State Highway
and Transportation Officials) handbook A Policy on Geometric Design of Highways and Streets includes a detailed
discussion of functional highway systems (the text is often
referenced as the Green Book). AASHTO describes a typical trip as including the following six stages of travel movement or service to facilitate vehicular movement:
1. Main movement
2. Transition
3. Distribution
4. Collection
5. Access
6. Termination
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The roadway functions and traffic volumes are directly
related to the movement hierarchy served. Figure 3.3A illustrates the hierarchy of movement and depicts the use of
roadways within a typical trip. In many land development
projects most of the planning and design focus on the roads
pertaining to the latter parts of the travel, that is, terminal
access, collection, and occasionally distribution.
The highway and street network provide travel mobility
and access to property. At one end of the functional category
are the high-access, low-mobility local roads and streets.
The other extreme is the high-mobility, low-access arterial
highways such as freeways. By reducing the access and intersections to a highway, the mobility (design speed, geometry,
travel time) can be improved. Between these two extremes
of highways and local roads are collection and distribution
roads, which link the two functional extremes.
Functional classification is divided into rural and urban
systems, which possess different vehicle and traffic characteristics. The systems differ in travel patterns, the density
of the streets and highways within the network, and the
type of land served by the network. As might be inferred,
the population density sets the distinction. Comparatively
dense areas, as designated by state or local officials, with
populations of 5000 or more people are considered urban.
Population is used to further divide this category into two
subcategories: urbanized areas, those with populations of
50,000 and above, and small urban areas defined by a population between 5000 and 50,000. As shown in the AASHTO
handbook, the urban functional system divides streets
into principal arterial, minor arterial, collector, and local
functions.
F i g u r e 3 . 3 A Hierarchy of movement.
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Principal Arterial. A principal arterial is often referred to
as a freewayroads with controlled access interchanges and
high mobility across major population areas. The principal
arterials provide service to major centers of activity and are
the corridors that accommodate the highest traffic volume.
A high proportion of the rural or urban area travel occurs on
the principal arterial system. In the urbanized areas it is these
streets that provide linkage to the rural arterial roadways.
The principal arterial system is composed of several types of
roadways with full control of access, such as interstate highways and other freeways, and other principal arterial streets
with partial or no control of access.
Minor Arterial. Next in the hierarchy of the functional
system is the minor arterial street system, which is often
referred to as a highway. These distributor streets provide
greater access to adjoining land than that afforded by the
principal arterial system. Traffic movement, as compared to
the principal arterial system, is greatly impeded. Minor arterial streets provide travel between communities and possible
connections to collector roads. Typically, such roads provide
the boundaries of identifiable neighborhoods. Their intersection spacing relates to the density of the developed area.
Spacing ranges from ½ to 3 miles. In fully developed areas,
the spacing normally does not exceed 1 mile.
Collector. The collector street system provides for land
access and traffic flow in all land use areas. These streets link
the local street system with the principal and minor arterial street systems. Collector streets through predominantly
residential areas serve as the main access to the development
from the arterial or another collector. The residential collector street has average daily traffic (ADT) counts of 1000 to
3000 vehicles per day (VPD). Single-family dwelling units
may have direct access on these roads where traffic volumes
are on the lower end of this range. As the collector street
traffic volumes approach 3000 VPD, access to single-family
units becomes undesirable; however, access to small public
and local commercial/retail facilities would still be considered appropriate.
Local Street. Lowest in the hierarchy is the local street
system. The main function of the local street is access to
the land adjoining the roadway and linkage to streets mentioned previously that are higher in the order within the
functional system. Although collector streets are the main
routes that provide access to the development, local streets
provide access to the properties within the development. In
residential areas, the volume of local streets is usually less
than 1000 VPD.
Figure 3.3B schematically shows the functional classification of streets and the interconnection of the street hierarchy.
A summary of the characteristics of the AASHTO classification appears in Table 3.3A.
Jurisdictional Classifications. Ultimately for land development projects, street classification for new roads will fall
into the domain of the local controlling body. Some jurisdictions may provide additional classifications of roadways and
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3.3
Legend
Arterial street
Collector street
Commercial area
Local street
F i g u r e 3 . 3 B Suburban street network. (From A Policy on Geometric
Design of Highways and Streets, 2004, by the American Association of State
Highway and Transportation Officials, Washington, D.C. Used with permission).
assign design requirements related to bike lanes, sidewalks,
and parking requirements. Description and definition of the
street categories are usually found in local subdivision ordinances and standards. In all likelihood the categories will be
defined according to function and in part by anticipated traffic volumes. Regardless of the basis for grouping, classification should follow a pattern similar to those outlined in this
section.
Placing priority on function when designing local access
streets requires that the designer consider many related factors apart from ADT. Land development projects are only
pieces in the regional development scheme. Design of streets
within a project should, therefore, conform to the overall
street system for the entire neighborhood. Such factors as
TA BL E 3 . 3 A
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economy, efficiency, safety, and viability lead to functionally
well-designed streets.
Public and Private Roads. The road systems throughout
a region may be public or private. A public road is operated and maintained by a public agencytypically the state,
county, city, or town department of transportation (DOT).
A private road is owned, operated, and maintained by a private entity, such as a land owner or the operator of a retail
center. A private road may be open to the public through
express or implied agreements.
Public Streets. A public roadway is operated and maintained by a public agency and resides in a parcel of land
referred to as a right-of-way, which is owned by a public
agency (public roadways may also exist within easements).
In some cases, the land may be owned by a local authority,
such as the county, but the maintenance is performed by
state DOT. After a road is constructed, and inspected for
design and construction conformance, the DOT will accept
the roadway into their operation and maintenance system.
When public roads are constructed by a private developer,
the roads must be accepted into the public roadway system.
After acceptance, the developer is no longer responsible for
the roadway. To be accepted into the public system, the design
and construction of the road must follow specific requirements that are established by the governing agencythese
roads have little flexibility in unique designs and materials.
A public agency will usually identify whether a road should
be included in the public roadway system based on location
and use of the road.
Private Streets. A private street is constructed, operated,
and maintained by a private entity, but the roadway may be
open to the public through an easement or agreement. The
design and operation of the road is often important for the
local jurisdiction and therefore should follow jurisdictional
design and operation requirements. Private roads may have
less stringent design requirements, lower speeds (less than
25 mph), nonstandard signage, upgraded hardscape, or other
functional differences from a public roadway. Private roads
are often clearly labeled as private roads and are seen in residential developments, a college campus, industrial parks, or
commercial areas.
Summary of Characteristics of the Functional System
Relative Volume
LOS*/Operating
Speed
Access Control
Through movements; major activity centers
High
High
Full to partial
Minor arterial
Lesser activity centers
High to medium
High to moderate
Some limitation
Collector
Regional and some local land access
Medium to low
Moderate to low
None
Local
Neighborhood land access
Low
Low
None
Street
Classification
Service Area
Principle arterial
*
Level of service—a qualitative measure of the operating conditions.
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3.3.3. Roadway Components
The components of a road include pavement, travel lanes,
shoulders, medians, side slopes, curb and gutters, drainage
ditches (median and side ditches), sidewalks, and right-ofway widths. The cross section represents a template for the
roadway that’s associated with the road alignment, which is
usually identified in the typical section.
The specific elements included in the design of any road
depend on the function and the jurisdictional requirements
of the road, or may depend on the developer’s requirements
for private road networks. Whether a road is public or private, there are minimum operational standards for travel lanes
(vehicle, pedestrian, bike). These standards identify minimum
and maximum lane widths and grades, as well as lateral relationships between elements, such as landscaping adjacent to
a vehicular lane. These standards are different based on the
roadway classifications and operational requirements. Lower
speed roads may allow for narrower lanes, while highway lanes
that are meant for trucks will require large lane widths. These
cross-sectional elements and their dimensions are identified
on a detail drawing called the typical cross section.
Figure 3.3C
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It is possible for a project to require several typical cross
sections. One project may include construction of the turn
lane on a highway, an internal boulevard street, and neighborhood streets, which would each have a different cross
section. Several examples of variations in cross-sectional elements are indicated in Figure 3.3C, while an example of a
typical section drawing that might be included in a site plan
is shown in Figure 3.3D.
Travel Lanes. The local ordinances and traffic studies typically specify the minimum number of lanes and their widths
for a street, which is a function of the anticipated traffic volume and functional classification. The lane width is measured from the edge of the travel for the road, which may
be an edge of pavement at the gutter or a lane marking. A
standard vehicular lane width is 12 feet, which provides drivers with a sense of comfort as they travel adjacent to other
vehicles, especially at high speeds (45 mph and more). A
reduced lane width is often appropriate on lower speed and
lower volume local streets, and may be 10 or 11 feet in width.
The lesser width reduces driver comfort, but may encourage adherence to following lower speed limits and promote
Cross-sectional elements—nomenclature for urban streets.
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F i g u r e 3 . 3 D Typical section.
traffic calming. A turn lane is typically set to the same width
of the through lanes.
A “normal crown” section has a high point in the middle
(usually at the centerline) with the pavement cross slopes
extending toward the outer edges of the pavement. In most
conditions the cross slopes are uniform across the total width
of one direction of the pavement. However, for multilane
roads, the cross slope remains uniform across a lane width,
and any changes in the cross slope should occur at the edges
of a travel lane. For instance, a left-turn lane may cut into
the center median of the road and would maintain the cross
slope of the adjacent lanes.
For many local and collector streets, lanes on a normal
crown section are typically sloped at 1% to 2%, toward the
outer edge of pavement. This cross slope is sufficient to direct
stormwater runoff into the gutters or to the side ditches
and yet does not cause any significant driver discomfort
because of the slight tilt of the vehicle. Cross slopes change
when superelevation requirements are incorporated into the
design. Superelevation, the design of banking the roadway
beyond the typical cross slope, is more common with highways than land development roadways.
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Other Lane Categories. In addition to standard vehicular
lanes, there are other road features to consider. Street parking, bike lanes, and bus lanes are all common elements of a
street. On-street parking is usually limited to parallel parking
spaces. A parallel parking space is typically 8 feet in width,
measured from the edge of travel to the face of a curb, but has
been implemented with a 7-foot width as well.
An on-road bike lane may be either shared with a vehicular lane or be marked separately. If the bike travel is shared
with vehicular traffic there might be a shared lane that’s a
total of 14 feet wide. If the bike lane is adjacent to the vehicular lane it’s usually located on the right side of travel, adjacent to the curb or on-street parking. When the bike lane is
separate it’s typically 4 feet wide when there’s a buffer (e.g.,
an additional 2 feet from a gutter) or 5 feet for cases when
it passes between a through lane and a right-turn lane. In
some jurisdictions, like the City of Portland, the bike lane is
as wide as 6 feet with a 2-foot shy zone on each side.
Bus lanes and bus stops should be coordinated with the
public transportation authorities. In some areas, like New
York City, a dedicated bus lane exists within the streets and
is an inherent part of the road network. Other areas may just
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include a bus stop adjacent to a normal travel lane or could
have an area for the bus to pull off from the roadway.
Medians. Medians are used to increase the separation distance between opposing traffic and, in some cases, to provide
refuge for pedestrians. In urban and suburban areas, medians may be raised, concrete malls or painted areas, approximately 4 to 16 feet wide. Wider median strips are typically
grass strips with a ditch. In rural areas and high-speed roads,
medians can be 30 feet and wider. Medians are typically not
used on local streets and are only occasionally used on collector streets. Their use is mostly in commercial areas, where
the traffic volumes are much higher and turning movements
are more frequent and complex.
The minimum width of a median (typically 4 feet) is often
governed by a width necessary for road sign placement and
appropriate clearance of the signs. At this width, the median
is generally completely concrete to prevent the need for
maintenance and mowing. Larger medians can be grass and
planted with trees. When a median exists between opposing lanes of traffic, it is often set to be 16 or 28 feet wide,
which allows for one or two 12-foot turn lanes to extend into
the median while still maintaining a 4-foot median at the
intersection. A separation of 1 foot is usually recommended
between a median and an adjacent lane, so that the edge of
travel is designated by a yellow pavement marking set 1 foot
from the median edge.
Design considerations include the spacing of median
breaks. Median breaks allow traffic turning and crossover
movements into and out of entrances, essentially providing more accessibility. Median breaks are also necessary for
most intersecting streets. Guidelines for spacing between
median breaks are provided by the governing public agency.
These guidelines may only prescribe minimum distances for
ordinary conditions and may be modified by the reviewing
agency to accommodate the specific conditions. Each jurisdiction has minimum spacing requirements. This spacing
is subject to the traffic volume on the through street, traffic
volume making the turning movement, whether the turning movements are signalized and the design speed of the
through street. The design engineer must assess the impact
of the median breaks on the level of service (LOS) of the
through street. A high number of median breaks and crossovers can significantly reduce the LOS on a given street,
especially during peak hours.
The minimum spacing of median breaks can be viewed
as the sum of the dimensions of median features that are
required between two intersections. These elements include
setback of the median noses to allow for proper turning
radius at the intersections, length of each of the required
left-turn lanes (especially when the median is narrow, eliminating the possibility of overlap between opposing left-turn
lanes), and length of the median transition between the leftturn lanes. Figure 3.3E illustrates the typical median geometry between two intersections and how it can dictate the
minimum spacing between intersections/median breaks. In
some cases, where a median width is approximately 28 feet or
greater, the required left-turn lanes may be configured such
that they overlap each other for some distance, thus allowing
a reduction in the overall intersection spacing.
In any event, the minimum spacing dictated by the median
features discussed above must be compared to the local standard for intersection/median break spacing. The greater of
the two should then be used to govern design.
On commercial and retail projects, there will be a desire
to have numerous median breaks to allow for land access.
However, local conditions and the reviewing agency may
restrict the number of openings, which can affect the layout
design of the project. The site engineer needs to verify with
F i g u r e 3 . 3 E Median break spacing.
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Figure 3.3F
Transportation Fundamentals 177
Common types of curb and gutter.
the review agencies the location of the median breaks early
in the planning stage.
Another situation occasionally overlooked is when a site
has frontage on an existing road with a median. In developing the site, a median break might be anticipated to promote
site access, but the governing agency will have authority as
to whether a median break is permissible. Again, the site
engineer should verify such requirements with the reviewing
agency early in the planning stage.
Turn Lanes. Dedicated turn lanes may be required on a
highway for vehicles entering a site or located within the site
to accommodate internal traffic circulation. The turn lane is
comprised of the taper and the storage bay. The taper often
varies from a minimum of about 100 feet to a length determine
by the lane transition equation, as noted in Equation (3.3A).
The traffic study for the site will determine if a turn lane
is warranted for a site and would designate the required
length of a turn bay based on storage requirements. The
width of the turn lane is often required to match the width
of the main travel lane. Based on timing or available storage
length, there may be a requirement for multiple turn lanes
(dual left turns, dual right turns, etc.). In general, higher
volume highways will often warrant a turn lane into a site,
whereas a small local road may not require a turn lane for
traffic entering a site.
For 40 mph or less
L = S 2W ÷ 60
For 45 mph or greater
L=W×S
(3.3A)
where L = length of transition
S = design speed
W = width of offset on each side
Source: 2011 AASHTO Green Book, Page 3-134,
Equations 3-37 and 3-38.
Continuous two-way median left-turn lanes are often
used for lower speed arterial highways. The median of the
roadway effectively acts as a turn lane for traffic in both
directions to make a left turn. There are inherent dangers
in this condition, but the intent is to allow turning vehicles
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to move outside the travel lanes without required additional
road width.
Curb and Gutter. In most urban areas curb or combination curb and gutter streets are preferred. Curbs are not
typical on principal arterials or high-speed arterials. Their
purpose is to facilitate drainage, separate traffic lanes from
pedestrian walkways, contribute to the aesthetic appearance,
and reduce maintenance. Several different types of curbs are
often utilized depending on the locality and design speeds.
Figure 3.3F shows various types of curbs. A common curb
and gutter width is 2.5 feet, to accommodate a 2-foot gutter
and a 6-inch curb width.
Barrier curbs, with a vertical height of 6 to 8 inches, are
typically used for low-design speed roads. Roads with highdesign speeds utilize mountable curbs with 3 or 4 inches in
height. A barrier curb protects the adjacent off street areas,
such as sidewalks, from vehicles while also delineating edge
of travel and providing drainage conveyance. The mountable curb is often used in urban areas to allow for emergency vehicles to access an area, but most passenger vehicles
would still be challenged to cross over a mountable curb. The
mountable curbs are also used for high-speed roads because
a barrier curb may cause an unpredictable impact trajectory,
especially when located adjacent to a guardrail, bridge, or
other roadside obstruction.
A gutter is often incorporated into the curb, and may range
from 1 to 2 feet in width. The cross slope of a gutter is typically
larger than a road cross slope to provide additional drainage
capacity. Gutter cross slopes are typically set to 1:12, or 8.33%
as compared to the 1:48, or 2.08%, typical road cross slope.
Sidewalks and Utility Strips. Sidewalks may be located
adjacent to the curb in retail and residential areas, or a buffer
may be provided between the curb and the sidewalk. When
the sidewalk is located adjacent to a curb the sidewalk width
is measured from the back of curb. A typical buffer width is
3 feet to accommodate utilities (like a fire hydrant) and traffic signs, or 6 feet or more so that landscaping can be accommodated. The minimum width of a sidewalk is often 5 feet,
which follows accessibility requirements of the Americans
with Disabilities Act (ADA)and is comfortable width for
pedestrian travel. In retail areas, the sidewalk may need to be
wider to accommodate concentrated volumes of pedestrian
traffic and provide enough clearance from site furnishings
such as benches, trash receptacles, and vegetation.
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Cross slopes for sidewalks and the adjacent utility or planting
areas are set at 2%, which is in compliance with ADA. The 2%
cross slope is directed to the street (curb and gutter) and provides a comfortable walking surface while promoting drainage.
Sidewalk surfaces are commonly concrete or asphalt but may
also be constructed of porous pavementeither open grid,1
gravel, or pervious concretesubject to the prevailing design
conditions including soils, climate, ownership/maintenance
responsibility, and accessibility criteria. Material selection for
sidewalks (and trails) is increasingly important especially for
projects utilizing a low-impact or sustainable design approach.
Shoulders, Side Slopes, and Ditches. In some low-density
residential areas and in most rural areas, shoulders, rather
than curbs, are normally used. Shoulders are 6 feet or wider
and typically gravel. Highways will often have wider shoulders and are generally paved with asphalt.
The cross slope of the shoulder is typically steeper than
the cross slope of the normal street section. Typical cross
slopes are two to three times greater than normal pavement cross slope, on the order of 4% to 6%. Where roads
are superelevated, shoulder cross slopes are dictated by
1
Open-grid pavement is generally considered to be 50% impervious or less with vegetation in the open areas.
the rollover between the outside edge of pavement and
the beginning of the shoulder, that is, the algebraic difference between the pavement cross slope and the shoulder
cross slope. For additional information on the treatment
of shoulder cross slopes in superelevated sections, see the
AASHTO Green Book.
Side slopes are used for connecting the road section to
natural ground. The slope gradient depends on the angle of
repose of the soil and the available right-of-way width. In
many cases, the slopes are constructed at a 3H:1V or 2H:1V
ratio. However, flatter slopes are generally desirable if they
can be provided within the available right-of-way. The type
of mowing equipment also factors into determining the
maximum side slope grades. Drainage ditches at the toe of
the slope control surface water from the pavement in excavated areas or through an adjacent property in embankment
areas. Figure 3.3G depicts a conceptual plan and section of a
roadway with an adjacent ditch.
Right-of-Way Widths. Right-of-way needs only be wide
enough to accommodate the pavement and other facilities
that are operated and maintained by the local DOT. The
right-of-way is important to show because it identifies if all
the road improvements are accommodated within an existing right-of-way or if land acquisition is required. The width
F i g u r e 3 . 3 G Plan and section of shoulders, side slopes, and ditches.
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of the right-of-way may be prescribed by the type of road,
such as a subdivision street with a 50-foot right-of-way, or
may be set just beyond the sidewalk (usually about 1 foot
beyond the sidewalk). A newer development may have a
consistent right-of-way width, but older road networks often
have variable width right-of-way, which can make new road
construction a challenge.
Profile Grade Line. One important element identified on
the typical section drawings is the profile grade line (PGL)
or theoretical grade line (TGL), an arbitrarily selected
reference point showing the proposed street elevations.
While the PGL appears as a line in the profile view, in the
typical section the PGL appears as a point (labeled as P in
Figure 3.3D). The PGL is selected for the convenience of
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design, stakeout, and construction. It can be the centerline
of the road, edge of pavement, top of curb, or any other
designated point as long as the horizontal and vertical relationship between the selected point and other elements of
the typical section are known. For a new roadway, the PGL
is usually set at the centerline of the road, whereas a road
widening project may set the PGL along the existing edge
of pavement. The PGL provide reference to the alignment
and profile geometry.
Example Road Sections. The roadway components are
used differently based on the project requirements. An example of a few different road sections for various road types
(urban, suburban, and rural) are shown in Figures 3.3H, 3.3I,
and 3.3J in both a conceptual and illustrative context.
A
B
F i g u r e 3 . 3 H Example of roadway components in an urban section, shown as (a) a two lane section with on-street parking and (b) a two lane with a
center turn lane section.
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A
B
F i g u r e 3 . 3 I Example of roadway components in a suburban section shown as (a) a four lane road with bike lanes and a median for turn lanes and
(b) a four lane section within a subdivision.
3.3.4. Intersection Types
Intersections are points where two or more streets join or
cross. The three basic categories of intersections in use are
at-grade, grade-separated, and interchange. At-grade intersections are used for joining local, collector, and many minor
arterial streets, whereas the latter two types are more generally used on high-speed highways or arterials. Except for
certain unusual situations, the number of legs at any one
intersection should not exceed four.
Intersections affect the overall performance of the intersecting roads. A measure of the operational performance
includes the speed of movement through the intersection.
The movement speed determines the capacity and thus
affects the suitability of a street to function as intended.
In other words, the LOS of a street is greatly affected by
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the LOS of the intersection. Design features that affect the
design of at-grade intersections include the following:
1. Adequate intersection sight distances
2. Angle of approach
3. Matching pavement grades on approaches
4. Width of the intersection
5. Radius of curvature of the curb returns
6. Need for auxiliary lanes (e.g., turn lanes) or
channelization
7. Intersection control
8. Need for traffic-calming measures
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A
B
Figure 3.3J
Example of roadway components in a rural section shown as (a) two lane road with shoulder and (b) a four lane section with roadside
ditches and a planted median.
Physical elements, traffic, human factors, and economics
encompass items for consideration in the design process of
at-grade intersections. For local streets in residential areas,
the major design considerations are adequate corner sight
distances, matching street grades, and minimum radius of the
curb returns. The complexities of intersection design increase
as the traffic, bicycle, and pedestrian volumes increase, and
the potential rate of conflicts (pedestrian-vehicle, pedestrianbicycle, vehicle-bicycle, and vehicle-vehicle) increases.
The three basic types of at-grade intersection are the “T”
intersection, the four-leg intersection, and the multileg intersection. Each type has numerous variations. The “T” intersection and four-leg intersection are the most prevalent in the
lower-category streets. In addition, the use of roundabouts,
which can accommodate all at-grade intersection configurations, is a common alternative to traditional intersections.
“T” Intersections. The “T” intersection, or three-leg
intersection, consists of three approaches, often with one
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approach that is not stop-controlled. The “T” intersection
has the least number of conflict points of any of the various
types of intersections and therefore provides a high level of
safety. Figure 3.3K illustrates some of the basic “T” intersection variations.
The intersection shown in Figure 3.3K(a) is the most common in residential land developments where minor or local
roads are prevalent and may be used when minor roads intersect important highways with an intersection angle less than
30° from normal. At locations with higher speeds and turning
volumes, which increase the potential for rear-end collisions
between through vehicles and turning vehicles, an additional
area of surfacing or flaring should be provided, as shown in
Figure 3.3K(b). In cases where left-turn volume from the
through road onto the minor road is sufficiently high but does
not require a separate left-turn lane, a bypass lane may be provided. This provides the space needed for through vehicles to
maneuver around the left-turning vehicles, which must slow
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Figure 3.3K
Types of “T” intersections. (From A Policy on Geometric Design of Highways and Streets,
2011, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with
permission.)
down before making their turns. Figure 3.3K(c) shows the
addition of auxiliary lanes on each side of the through highway approaching the intersection. This type of intersection is
suitable for locations where turn volumes onto the minor road
are high, from both directions of the through road. An intersection of this type will generally be signalized.
Four-Leg Intersections. The four-leg intersection may
or may not have an automatic right-of-way assignment.
Depending on the design volume of traffic at the intersection, control can be as simple as yield/stop at the lesser category street to signal control with auxiliary lanes and islands
to channel the turning movements. Variations of the latter
situation typically occur at intersections of major roads,
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such as collectors with minor arterials. Such intersections
frequently are necessary at the fringe of large development
projects in high-density areas. Figure 3.3L illustrates some
of the basic configurations and various levels of the four-leg
intersection.
The unchannelized intersection shown in Figure 3.3L(a)
is used mainly at locations where minor or local roads cross,
although it can also be used where a minor road crosses a
major arterial or collector road. In these cases, the turning
volumes are usually low and the roads intersect at an angle
that is not greater than 30° from normal.
When turning movements are more frequent, a flared
intersection with additional capacity for through and
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F i g u r e 3 . 3 L General types of four-leg intersections. (From A Policy on Geometric Design of Highways and Streets, 2004, by the American
Association of State Highway and Transportation Officials, Washington, D.C. Used with permission.)
turning movements, such as those in Figure 3.3L(b), can
be provided. The layout shown in Figure 3.3L(c) shows
a flared intersection with a marked pavement area that
divides traffic approaching the intersection and is suitable for a two-lane highway that is not a minor crossroad,
where speeds are high, intersections are infrequent, and
the left-turning movements from the highway may create
a conflict.
Figure 3.3L(d) shows a channelized four-leg intersection
with right-turning roadways in all four quadrants. This configuration is suitable in developments where turning volumes
are high. The channelized islands may provide for pedestrian refuge; however, this configuration may be criticized
for encouraging a free-flow condition for turning movements that challenges pedestrian safety. The configuration
presented in Figure 3.3L(b) and (c), where additional lanes
are provided for left-turning movements, can be adopted for
channelized intersections as well.
Figure 3.3L(e) demonstrates a simple intersection with
divisional islands on the crossroads. This configuration
accommodates a large range of traffic volumes.
The layout shown in Figure 3.3L(f) is suitable for a twolane highway that is not a minor crossroad and that carries
moderate volumes at high speeds or operates near capacity.
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Complete solutions and variations for various circumstances
are numerous.
Multileg Intersections. Multileg intersections (those
with five or more approaches) should be avoided wherever
practical in land development. At locations where multileg intersections are used, it may be satisfactory to have all
intersection legs intersect at a common paved area, where
volumes are light and stop control is used. At other than
minor intersections, traffic operational efficiency can often
be improved by reconfiguration that removes some conflicting movements from the major intersection. Such reconfigurations are accomplished by realigning one or more of the
intersecting legs and combining some of the traffic movements at adjacent subsidiary intersections, or in some cases
by converting one or more legs to one-way operation away
from the intersection. The distances between the newly
formed intersections should be such that they can operate
independently.
Angle of Approach. Right-angle intersections provide a
better view of traffic and better turning movements than
roads intersecting at acute angles. A recommended practice
is to limit the angle of approach of the intersecting road to
60° (relative to the through street). In those situations involving intersections of legs at acute angles, the engineer should
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F i g u r e 3 . 3 M Realignment of intersecting road to obtain 90°
configuration.
consider slight changes to provide a configuration closer to
the desired 90° alignment.
Figure 3.3M shows how an approach street with an acute
angle of approach can be modified to obtain a 90° intersection. On local roads, such an alignment is acceptable. On
higher category streets, this type of alignment should be used
cautiously. Although this type of alignment may improve visibility and turning movements, there are disadvantages to it.
Traffic control devices are obscured, and vehicles approaching the intersection through the short radius curve tend to
encroach into the opposite lane, creating a traffic hazard for
vehicles turning from the main road.
Within a relatively large project, adjusting lot lines and
street configurations can achieve proper alignment of intersections. For those projects that must access near or at an
existing intersection, local agencies may require extensive
modifications of the intersection to accommodate the additional volume of traffic. This can be costly and should be
identified early in the project planning.
Roundabouts and Traffic Circles. The last intersection
identified in this chapter is a roundabout and neighborhood
traffic circle, or mini-roundabout (Figure 3.3N). These two
roundabout configurations are shown in Figure 3.3N. The
primary difference between the roundabout and the neighborhood traffic circle is scale: roundabouts are designed for
somewhat higher traffic volumes and speeds than traffic
circles, and consequently require additional land to construct. However, both operate on the same principles: traffic within the roundabout or circle has the right-of-way,
with entering traffic controlled by yield signs. By forcing all
vehicles to travel counterclockwise around the central circle, these measures force vehicles to reduce speed through
the intersection. Roundabouts and traffic circles have been
found to be highly effective at reducing the number of
intersection accidents. In addition, accidents that do occur
tend to be less severe than those for signalized or stop-controlled intersections. This is primarily because the design of
roundabouts and traffic circles greatly reduces the possibility of head-on and right-angle collisions. The circulating
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nature of these designs limits most accidents that do occur
to sideswipe accidents.
One major concern regarding traffic circles is their
accessibility to larger vehicles. As the neighborhood traffic
circle is generally implemented within approximately the
same right-of-way as a standard intersection, the construction of the central circle can greatly inhibit the maneuverability of trucks or buses, which might not be able to
negotiate the tight radius around the circle. One solution
is to include a “truck apron” around the inner circle. This
is a paved outer ring with a mountable curb that allows
the larger vehicles to more easily negotiate the tight radius.
The mountable curb, which can easily be negotiated by
larger vehicles, helps to prevent smaller vehicles from taking advantage of the apron to travel through the intersection at a higher speed.
There are a number of resources available to assist in
the design of roundabouts. In the absence of local or state
F i g u r e 3 . 3 N Roundabout and mini-roundabout (neighborhood
traffic circle).
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guidelines, the reader is directed to FHWA (Federal Highway Administration) publication no. FHWA-RD-00-067,
Roundabouts: An Informational Guide.
3.3.5. Site Access and Intersection Design
A site’s transportation accessibility is one of the critical elements for the desirability of a site. A commercial development will seek high visibility and proximity to multiple
points of access from high traffic areas, while a rural residential development will seek a secluded site that discourages through traffic. The accessibility is important in the
context of vehicular traffic as well as pedestrian or bicycle
traffic. Most land development projects will only use atgrade intersections, such as a signalized intersection or
stop-controlled intersection. Grade-separated intersections with ramps and flyovers are typically only provided on
major highway systems.
Access Management. Traffic circulation and ease of site
access often determine a development’s consumer appeal. Retail
centers are often interested in corner sites, which allow for
increased visibility and more opportunities for access from arterial and collector roadways. In contrast, a suburban residential
development may be interested in discouraging traffic through
the neighborhood and might prefer a single point of access from
a minor collector roadway. Urban areas with a mix of uses benefit from multiple access points and internal road networks.
Figure 3.3O
03_Land_CH03_p125-304.indd 185
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Transportation Fundamentals 185
Access management is a reference to the controlling systems that are in place at the entrance into a site. The access
management of a site may govern the location of the site’s
driveway along the road, the need for turn lanes, or the
requirement for channelizing devices at the site entrance.
Channelizing devices may include a raised island at the
entrance or modifications to a highway median to prevent
specific turn movements.
The local DOT will often control the site access based
on the road classification, design speed, distance between
other intersections, and traffic characteristics of a site. The
DOT is concerned about preserving the mobility of the primary roadways to keep traffic moving. This mobility is often
accomplished by limiting the number of entrances and intersections. In contrast, a local jurisdiction and developer are
often interested in providing urban corridors that facilitate
access to development areas. In all cases, safety is a focus and
access management is often prescribed to limit the number
of potential for traffic accidents. Figure 3.3O depicts the
conflict points in relation to the regions of an intersection.
Compare the number of potential conflicts on the left, with
access points A and B and an unrestricted access condition,
to the condition on the right with controlled access from
point C. While ease of access is reduced, the potential for
conflicts and crashes is also reduced for the condition shown
on the right. This is especially critical when access points are
Areas of an intersection and conflict points.
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located in close proximity to all other movements that occur
near an interection.
Access locations are often controlled by the distance from
other intersections and access locations. Most jurisdictions
will set minimum distances between driveways, minor intersections, and major intersections. The length of turn lanes,
intersection queueing, acceleration, and deceleration areas
represent the functional area of an intersection and should be
kept clear of site access points (Figure 3.3O). As vehicles enter
and exit a site, the movements introduce conflict points and
increase the opportunities for accidents. The DOT will review
the type of access that can be provided for the site based on the
characteristics of the highway and site. If entrance spacing or
proximity to an intersection is a concern, it’s possible the DOT
will restrict turning movements in and out of the site.
It’s important to identify all access requirements during the early stages of design. The need for turn lanes, the
location of the site entrance, and the restrictions on turning movements will have a large impact on site design and
desirability. In some cases, the requirement for turn lanes
for a new site entrance may require land from adjacent sites,
which could be challenged by the adjacent property owners.
Intersection Spacing. The intersection control, highway
speed, roadway classification, and access type all influence
the spacing requirements of intersections and driveway
TA BL E 3 . 3 B
entrances. Signalized intersections should be spaced further apart than unsignalized intersections. Partial access
entrances (e.g., right in and right out only) can be located
closer together than full access points. The higher the speed
limit of the roadway, the larger the distance between the
intersections. For example, the Virginia DOT (VDOT)
requires 660 feet between driveways along a 30 mph road,
but 1,320 feet between driveways on a 50 mph roadway
(Urban Minor Arterial Classification). On local streets in
residential areas, the recommended minimum spacing is
generally less at 125 feet. On collector streets, spacing should
be increased to 250 feet. Table 3.3B provides some examples
of intersection spacing requirements based on speeds and
roadway classifications as indicated by VDOT.
In commercial areas, it is common for one site to have
numerous entrances from the main highway into the
office and retail establishment. The volume of traffic and
the numerous turning movements require these entrances
to be treated as intersections. Residential driveways, however, are not considered intersections because of the low
volume and few consecutive turning movements. Recommended spacing of minor commercial entrances is around
200 feet, depending on the traffic on the serving road and
the anticipated traffic through each entrance. Spacing may
be reduced if the turning movements into and out of the
Minimum Spacing Standards for Commercial Entrances, Intersections,
and Crossovers, VDOT Road Design Manual
Minimum Spacing Standards for Commercial Entrances, Intersections, and Crossovers
Centerline to Centerline Spacing in Feet
Highway
Functional
Classification
Urban
Minor
Arterial
Legal Speed
Limit (mph)
Signalized
Intersections/Crossovers
Unsignalized
Intersections/Crossovers &
Full Access Entrances
Partial Access
One or Two Way
Entrances
880
1,050
1,320
660
660
1,050
270
305
425
≤ 30 mph
35 to 45 mph
≥ 50 mph
Divided
Urban
Collector
≤ 30 mph
35 to 45 mph
≥ 50 mph
Rural
Minor
Arterial
≤ 30 mph
35 to 45 mph
≥ 50 mph
660
660
1,050
03_Land_CH03_p125-304.indd 186
≤ 30 mph
35 to 45 mph
≥ 50 mph
425
425
495
Divided
Undivided
440
440
660
200
305
425
1,050
1,320
1,760
Divided
Rural
Collector
Undivided
880
1,050
1,320
880
1,050
1,320
Undivided
570
570
645
155
250
360
270
360
495
Divided
Undivided
660
660
1,050
305
425
570
200
305
425
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entrances are limited to the direction of flow of traffic, in
other words, right-in and right-out movements.
Intersection spacing is always a matter for consideration
in maintaining safe streets and avoiding traffic queuing. Full
access conditions are often desired by new developments,
but uncontrolled access points introduce potential vehicle
and pedestrian conflicts. Many traffic projections use prescribed values for estimating the volume of traffic expected
at each entrance. These volumes, in conjunction with highway characteristics, are used to determine turn length and
queuing requirements. Local knowledge of traffic patterns or
attraction of the new development can further inform the
design recommendations.
Intersection Sight Distance. Another controlling factor
for intersection location and design is sight distance. Sight
distance is provided at intersections to let drivers identify the
presence of potentially conflicting vehicles and to be able to
stop or adjust their speed, as appropriate, to avoid intersection
collision. The driver of a vehicle approaching an intersection
should have an unobstructed view of the entire intersection,
including any traffic control devices, and sufficient lengths
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Transportation Fundamentals 187
along the highway to permit the driver to anticipate and avoid
potential collisions. To enhance traffic operations, intersection
sight distances that exceed stopping sight distances are desirable along the major road. Intersection sight distance for an atgrade intersection is subject to the type of traffic control at the
intersection or, where no traffic control devices are present, by
the rules of the road. At an intersection where no traffic control devices are present, a basic rule of the road would require
the vehicle on the left to yield to the vehicle on the right if they
arrive at approximately the same time.
Clear sight triangles are specified areas along intersection
approach legs and across their included corners that should
be kept clear of obstructions that might block a driver’s view
of potential conflicts. The dimensions of the legs of the sight
triangles depend on the design speeds of the intersecting
roadways and the type of traffic control used at the intersection. These dimensions are based on observed driver behavior and are documented in NCHRP Report 383, Intersection
Sight Distance. Two types of clear sight triangles are considered in intersection design: approach sight triangles and
departure sight triangles, as illustrated in Figure 3.3P.
(a)
(b)
F i g u r e 3 . 3 P Intersection sight triangles. (a ) Approach sight triangles (uncontrolled or yield controlled),
(b ) departure sight triangles (stop controlled). (Modified from A Policy on Geometric Design of Highways and
Streets, 2011, by the American Association of State Highway and Transportation Officials, Washington, D.C.
Used with permission.)
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Figure 3.3P(a) shows typical clear sight triangles to the
left and to the right for a vehicle approaching an uncontrolled or yield-controlled intersection. The vertex of the
sight triangle on a minor-road approach (or an uncontrolled
approach) represents the decision point for the minor road
driver, where braking to stop should begin if another vehicle
is present on an intersecting approach. The distance from
the major road, along the minor road, is illustrated by the
dimension a in Figure 3.3P(a). The geometry of a clear sight
triangle is such that when the driver of a vehicle without the
right-of-way sees a vehicle that has the right-of-way on an
intersecting approach, the driver of that potentially conflicting vehicle can also see the first vehicle. Being the distance b
away from the intersection, the driver of the vehicle with
the right-of-way should be able to slow, stop, or avoid other
vehicles, if it becomes necessary.
Figure 3.3P(b) illustrates departure sight triangles necessary for a stopped driver on a minor road approach to enter
or cross the major road.
For many land development projects, the most typical
type of traffic control is the stop control. The vehicle on
the minor street is required to completely stop, ensure that
movement through the intersection, either entering the
Figure 3.3Q
03_Land_CH03_p125-304.indd 188
intersecting road or crossing it, can be safely done without
colliding with oncoming traffic, and then proceed.
The ability to ensure proper sight distance will often govern whether a site entrance can be placed at a given location
along a roadway. Vertical or horizontal obstructions can
prohibit visibility and a site access point would therefore not
be allowed. Many jurisdictions will require a sight distance
easement to ensure the line of sight is kept clear of vegetation, signs, site furnishings, parked cars, or other obstructions. If the line of sight crosses an adjacent property, the
developer may be required to obtain a sight distance easement from the adjacent owner. A conceptual graphic of
intersection sight distance for two different site entrances
is depicted in Figure 3.3Q. For Site 1, the sight distance is
obstructed by a building and the angle of the sight lines are
undesirable. In Site 2, the road geometry and site location
allow the line of sight to exist within the roadway and avoids
horizontal obstructions.
The driver on the minor road must have an unobstructed
view for a sufficient distance to ensure that movement
through the intersection can be safely accomplished. This
sufficient sight distance depends on the speed of the oncoming vehicles and some nominal perception-reaction time
Sight distance for site analysis.
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3.3
representative of most drivers as well as the physical conditions of the intersection. For left turns, the number of lanes
in a roadway will govern the length of the sight distance. As
a vehicle makes a left turn they must cross all lanes of traffic, and a median, before reaching the receiving lanethis
condition increases the time of the turning movement that
requires more time. Additionally, steep grades at an intersection should be considered when calculating the timing
required to make a turning movementlarger vehicles may
require more time to traverse steep grades.
The three types of movements for the stopped vehicle, the
corresponding sight triangles, and the values of the related
sight distances are explained in detail in the AASHTO
Green Book. These three basic cases are briefly identified
here, while the detailed explanation of these and other cases
(intersection with no control, intersections with yield control
on the minor road, intersections with traffic signal control,
intersections with all-way stop control) can be found in the
AASHTO Green Book.
Crossing maneuver from the minor road: The vehicle
can proceed through the intersection, continuing
along the minor street (four-legged intersections
only). This requires that the vehicle clear traffic
approaching from either direction of the major street
and also vehicles turning left from the opposite direction on the minor street.
Left turn from the minor road: The stopped vehicle can
turn left onto the major street, which again requires
clearing traffic from both directions.
Right turn from the minor road: The stopped vehicle can
turn right, which requires clearing traffic coming from
the driver’s left.
The vertex (decision point) of the departure sight triangle
on the minor road and the corresponding assumed location of the driver’s eye should be 14.5 feet from the edge of
the major road traveled way (note the edge of traveled way
may be different from the edge of pavement). According to
AASHTO, this assumed vertical location of the driver’s eye is
3.5 feet above the roadway surface, and the object to be seen
is 3.5 feet above the surface of the intersecting road. In addition to AASHTO, the reader is referred to local highway design
criteria to establish the intersection sight distances. Without
the benefit of a specific method, a reasonable rule of thumb
applicable to local-local and local-collector intersections is to
allow 10 feet of distance for every 1 mph of design speed. For
example, an intersection where the design speed of the major
street is 35 mph requires a sight distance of 350 feet.
The preceding discussions on sight distance for atgrade intersections assume that grades on approach legs
are almost level. When the grade along the intersection
approach exceeds 3%, the leg of the clear sight triangle along
the approach should be modified by multiplying the calculated sight distance by the appropriate adjustment factor.
03_Land_CH03_p125-304.indd 189
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Transportation Fundamentals 189
Both sets of data are clearly provided in table format in
the AASHTO Green Book. The same publication also prescribes the height of eye and the height of object at 3.5 feet;
therefore, nothing in the line of sight, such as fences and
vegetation, should be higher than this value.
Curb Return Radius. The curb return is the arc of the curb
that joins two roadways. The ability of a vehicle to turn safely
and effectively depends on such factors as type of vehicle,
pavement width, curb return radius, speed through the turn,
longitudinal and transverse pavement slope, and skew angle
of the intersection. Ideally, as the vehicle makes a turning
movement, it remains within the limits of the designated
lane. On local streets, for which the design vehicle is predominantly small, and the design speeds and traffic volumes
are low, whether the vehicle stays within the confines of the
designated travel lane is of minor consequence. The radius of
curvature of the curb return affects the width of the entrance
of the street. Since pedestrian safety is paramount in residential areas and speeding is discouraged, short radius curb
returns are more desirable.
In commercial and industrial developments, pavement
widths and curb returns are matched to accommodate larger
vehicles and reduce (or eliminate) the need for the vehicle
to encroach into other lanes during the turning movement.
However, as the curb return radius increases, the entrance
width increases, and consequently lane designations through
the intersection are obscured. Additionally, pedestrians are
forced to traverse larger pavement areas, which is a safety
concern. The need for channelization by islands or special pavement markings to delineate the travel lanes then
becomes a consideration.
According to the ASCE/NAHB/ULI’s Residential Street
Design (3rd ed., 2001), the recommended range for curb
return radii is 10 to 15 feet at local-local street intersections,
15 to 20 feet at local-collector intersections, and 15 to 25 feet
at collector-collector intersections. However, most jurisdictions prescribe minimum radius of curvature for the curb
returns. The engineer should use discretion in setting these
radii. In some cases, the sidewalk adjacent to a curb return
may have a thick section (increased from 4 to 7 inches) to
accommodate the occasional overrun of a large vehicle. The
curvature is based on the largest type of vehicle that will
frequently use the street, pedestrian safety, and the cost for
maintenance of the right-of-way and the loss of usable land
for development.
In some cases the curb return radius does not need to
match the vehicle turning radiuswhen a road section
includes a bike lane or on-street parking the effective turning radius is larger than the curb return (Figure 3.3R). In
this case, the vehicle path should be simulated to verify
adequate clearance. Software can simulate vehicle turning
movements assist in checking these provisions. An example
of one template is shown in Figure 3.3S. Other templates
for the vehicles listed in Table 3.3C are provided in the
AASHTO manual. The minimum turning radii values for
20 different vehicle types are listed in Table 3.3C.
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F i g u r e 3 . 3 R Minimum turning path for WB-40 design vehicle. (From A Policy on
Geometric Design of Highways and Streets, 2011, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with permission.)
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Figure 3.3S
Transportation Fundamentals 191
Effective turning radius for streets with bike lanes.
Channelization. Wide street entrances at intersections,
such as those generated by large curb return radii or
skewed intersecting streets, create large areas of pavement
that confuse drivers and promote irregular vehicle movements. To mitigate these conditional hazards, painted
pavement markings or raised islands are installed to direct
the traffic flow (Figure 3.3T). Raised islands also serve as
pedestrian refuge points, separate opposing traffic, and
serve as attractive aesthetic focal areas for the development when landscaped. In the latter case, however, the
landscaping must not impede sight distances nor obstruct
the driver’s view of pedestrians.
The size, configuration, and method of channelization are very site specific. Factors to consider include
the volume of traffic, the type of intersection control, the
geometry of the intersecting streets, and, most important,
the requirements and standards of the approving public
agencies.
The islands must be large enough to be visible to drivers for each turning movement affected by the island.
03_Land_CH03_p125-304.indd 191
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AASHTO recommends the island area to be not less than
50 square feet as an absolute minimum and preferably
100 square feet in size. Triangular islands should be 12 feet
long on a side, not including the rounding at the corners.
Raised curb islands are set 2 to 4 feet from the edge of pavement of the through street on low-speed roads. The setback
increases with increasing design speed on the through street.
Curb Ramps. Curb ramps (or curb-cut ramps) are
required by ADA at most intersections and all crosswalk
locations. According to the ADA, curb-cut ramps shall
be a minimum 3 feet wide and the slope shall not exceed
8.33% (1V:12H). If located along a curb and gutter section, the gutter slope should be minimized to prevent a
large grade change across the gutter and into the rampa
gutter is usually set to 5% maximum adjacent to a curb. A
detectable warning surface such as truncated domes must
also be provided for 2 feet longitudinally and for the full
width of the ramp for individuals with vision impairments.
Most states and municipalities provide standards detailing
curb ramp dimensions and intersection locations. Issues
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TA BL E 3 . 3 C
Design
Vehicle
Type
Minimum Turning Radii of Design Vehicles
City
Passenger Single-Unit
Car
Truck
Symbol
P
SU
Intercity Bus
(Motor Coach)
BUS-40
BUS-45
Transit
Bus
Conventional Large2
School Bus School Bus Articulated Intermediate Intermediate
(65 pass.) (84 pass.)
Bus
Semitrailer Semitrailer
CITY-BUS
S-BUS36
S-BUS40
A-BUS
WB-40
WB-50
Minimum
Design
Turning
Radius (ft)
24
42
45
45
42.0
38.9
39.4
39.8
40
45
Centerline1
Turning
Radius
(CTR) (ft)
21
38
40.8
40.8
37.8
34.9
35.4
35.5
36
41
Minimum
Inside
Radius (ft)
14.4
28.3
27.6
25.5
24.5
23.8
25.4
21.3
19.3
17.0
Farm3
Tractor
w/One
Wagon
Design
Vehicle
Type
Symbol
Minimum
Design
Turning
Radius (ft)
Interstate
Semitrailer
*
WB-62
WB-65**
or WB-67
“Double
Bottom”
Turnpike
Double
Triple
Semitrailer/ Semitrailer/
Combination
trailers
trailer
Motor
Home
WB-67D
WB-100T
WB-109D*
MH
Car and
Camper
Trailer
Car and
Boat
Trailer
Motor
Home
and Boat
Trailer
P/T
P/B
MH/B
TR/W
45
45
45
45
60
40
33
24
50
18
Centerline1 41
Turning
Radius
(CTR) (ft)
41
41
41
56
36
30
21
46
14
14.9
25.9
17.4
35.1
10.5
Minimum
Inside
Radius (ft)
7.9
4.4
19.3
9.9
8.0
*
Design vehicle with 48-ft trailer as adopted in 1982 Surface Transportation Assistance Act (STAA).
Design vehicle with 53-ft trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA).
1
The turning radius assumed by a designer when investigating possible turning paths and is set at the centerline of the front axle of a vehicle. If the minimum turning path is
assumed, the CTR approximately equals the minimum design turning radius minus one-half the front width of the vehicle.
2
School buses are manufactured from 42- to 84-passenger sizes. This corresponds to wheelbase lengths of 11.0 to 20.0 ft, respectively. For these different sizes, the minimum
design turning radii vary from 28.8 to 39.4 ft and the minimum inside radii vary from 14.0 to 25.4 ft.
3
Turning radius is for 150–200 hp tractor with one 18.5-ft-long wagon attached to hitch point. Front wheel drive is disengaged and without brakes being applied.
(From A Policy on Geometric Design of Highways and Streets, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.)
**
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Transportation Fundamentals 193
(a)
(a)
F i g u r e 3 . 3 T Detail of corner island. (a ) Details of corner island designs for turning road-
ways (urban location), (b ) details of corner island designs for turning roadways (rural cross section
on approach).
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(b)
F i g u r e 3 . 3 T (Continued )
for consideration in the placement and design of curb-cut
ramps include the following:
•• “T” intersections should not include a curb ramp
unless there is a painted crosswalk or traffic device
(stop sign or signal).
•• Low points should be avoided to prevent drainage
ponding.
•• Traffic islands provided for pedestrian refuge should
include curb ramps.
03_Land_CH03_p125-304.indd 194
•• No obstructions, such as parked cars, should be
within the path of a curb ramp.
•• Visibility of the pedestrians should be considered.
Typical curb-cut ramp types, and curb-cut ramp details with
recommended dimensions are shown in Figures 3.3U and
3.3V, respectively.
3.3.6. Design Criteria
While the design of streets should follow the standards and
guidance established by the well-known transportation
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F i g u r e 3 . 3 U Curb-cut ramp types. (Source: FHWA. Designing Sidewalks and Trails for Access,
Part I. Federal Highway Administration, U.S. Department of Transportation. Washington, D.C., 1999.)
F i g u r e 3 . 3 V Curb-cut ramp details. (From A Policy on Geometric Design of Highways and Streets, 2004, by the American Association of State Highway and Transportation Officials, Washington, DC. Used with permission.)
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agencies and organizations (e.g., FHWA or a DOT), additional considerations are provided for the design of streets
in a lower speed environment typically associated with
land development projects. Traffic calming measures and
bicycle facilities are a few concepts that designers need
to consider and introduce into their land development
street designs with the intent of creating a safer roadway
environment.
Most of the design guidance for high-speed, high-volume
highways has been established by organizations such as
AASHTO, Institute of Transportation Engineers (ITE), and
FHWA. Design guidance for low-speed roads is typically
established by local municipal or state agencies by adapting
the criteria from highway requirements. However, the popular AASHTO design manual, A Policy on Geometric Design
of Highways and Streets has incorporated discussions and
provided design guidance regarding local roads and streets
in both urban and rural settings. Many local agencies have
adopted the AASHTO manual entirely or with modifications
to reflect regional philosophies and conditions. AASHTO
guidance represents the consensus of state transportation
agencies, the FHWA, representatives of the American Public Works Association, the National Association of County
Engineers, and others. They provide a consistent basis for
street design with appropriate variations for climates, terrain, and economic factors.
The FHWA publishes the Manual on Uniform Traffic
Control Devices (MUTCD). This manual has been officially
adopted by most states and has become an important legal
document because it ensures that traffic control devices
such as signs, pavement markings, traffic signals, and other
devices would be uniformly and consistently applied. Local
jurisdictions, in most instances, are required by state law
to conform to the MUTCD. A local jurisdiction may have
a supplemental document for local traffic control requirements. Use of this standard applies to both the selections of
appropriate traffic control devices and for the design and
application of pavement markings and signs.
High-speed, high-volume highway design is mostly
governed by vehicle and driver characteristics. Local street
design is governed by the same criteria but also considers
operation and nonvehicular use. More emphasis is placed
on pedestrian safety, local service, land accessibility, and the
preference by residents for a pleasant environment.
The safety of pedestrians and bicyclists is a major consideration in the planning and design of roadways. This is
particularly important in urban environments where their
interaction with vehicular traffic is high. Both AASHTO
and ITE have published manuals that provide guidance for
the development of safe pedestrian and bicycle facilities. In
addition, the Americans with Disabilities Act Accessibility
Guidelines (ADAAG) must be followed during the design
process especially in public street applications.
In many localities, the actual construction and the cost
of construction of the street network of a development project is the responsibility of the developer. Once the streets
03_Land_CH03_p125-304.indd 196
are constructed, they are then accepted as part of the local
or state street network system and the costs of repair and
maintenance become the responsibility of public agencies.
Maintenance costs, in most cases, are funded through local
and state taxes. Higher-order roads on the other hand are
typically constructed with state and federal funds and maintained with state funds.
It is imperative for the streets to be designed and built
according to the governing criteria. Failure to do so often
results in additional design and construction costs in order
to have the street accepted into public roadway system. Nonstandard design may also cloud liability in accident cases.
Instead of public streets, some developers choose to provide
private streets. These streets are not turned over to the local
or state street network system. In these cases, the cost burden
for repair and maintenance will then be the responsibility of
the collective private owners.
The design capacity, that is, the traffic volume that can
be supported by a street, depends on factors such as width,
horizontal curvature, vertical curvature, longitudinal street
grades, superelevation, roadside obstructions, intersections, and traffic control measures. These criteria are usually specified as minimums for development streets and are
determined by the need for fire access and whether on-street
parking is permitted. For collectors and arterials, it is necessary to design the street for the traffic volumes it is expected
to maintain at some future date. These traffic volumes are
determined during the initial traffic studies conducted during the preliminary design phases. Traffic data, such as ADT
and peak-hour traffic volumes, are essential to establish the
geometric design features of the street.
Vehicle Characteristics. Roads are designed to accommodate the characteristics of the vehicles using it. There is
a wider range of vehicle types using arterial roads as compared to vehicle types using local streets. In addition, the
frequency of occurrence of specific vehicle types is higher
on arterial roads. For example, a large truck (e.g., moving
van) is only occasionally found on residential streets, but
large trucks are common on arterial streets. Therefore, for
economic reasons, streets higher in the functional hierarchy
must be geometrically and structurally designed to accommodate larger vehicles. In comparison, the design requirements for local streets may not have to be as stringent. For
example, lanes for local streets may not be as wide as for
arterials, curve radii may be shorter, and grades steeper.
Local requirements for pavement structure and operating
room for fire apparatus often provide more than adequate
design for other similar vehicles.
The largest vehicle type that is most likely to use the
roadway with regular frequency is selected as the design
vehicle. The physical and operating characteristics of the
design vehicle are then used to establish critical geometric
and structural design features of the road. Local streets, like
any roadway, must accommodate the predominant vehicles
using this type of functional street. The predominant vehicle
on local streets is the private passenger car, including small,
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medium, and large size cars, vans, and pickup trucks. In
high-density developments (e.g., mixed-used developments,
office parks), typically the travel way is not publicly owned
and maintained. Consequently, design criteria for minimum
widths and radius of curvature are even less than those prescribed for local public streets. However, the design engineer
must consider the use of these streets by emergency vehicles,
moving vans, trash trucks, school buses, and other large
vehicles. Typical dimensions and minimum turning paths
for a variety of these design vehicle types can be found in the
AASHTO Green Book.
Driver Characteristics. The characteristics of vehicles
and drivers using the roadway directly affect the geometric
design. Driver behavior and response to various situations
fluctuate based on conditions. Behavior and response for the
same situation and condition vary for the same driver at different times due to physical condition and mental attitudes.
Likewise, the performance of vehicles varies due to size,
weight, power, mechanical condition, and the driver. Road
design must account for some minimum standard of driver
behavior and vehicle performance.
Individuals vary, and therefore as an added measure of
safety designers must account for below-average driver
characteristics, with the presumption that a high majority
of drivers will have these minimum threshold characteristics. Driver’s fundamental characteristics include perception,
identification, emotion, and volition (PIEV).
Perception includes seeing and observing objects. Identification consists of comprehending an encountered object
and its surrounding elements. Emotion involves judgment
and the process of decision making, leading to responsive
actions. Volition or reaction is the driver’s will to react to
or execute the actions decided during the emotion process.
The total time elapsed through the PIEV process is referred
to as the PIEV time, or more commonly the perceptionreaction time.
PIEV time varies with the complexity of individual
situations. Factors such as the use of drugs and alcohol,
fatigue, and physical impairments cause increases in PIEV
time. Studies conducted in laboratory situations determined PIEV time as lying in a range from 0.5 to 7.0 seconds. Highway agencies typically use PIEV times of 2.5
seconds for stopping distance and 2.0 seconds for determination of adequate sight distance at intersections. The
latter is shortened because the driver anticipates a possible reaction at intersections. Similarly, familiarity with
common signs and traffic markings reduced PIEV time
because perception identification and emotion occur
almost instantaneously.
Pedestrians. Pedestrian safety is a concern on any roadway where a combination of vehicular traffic and people
occurs. Pedestrian density substantially increases in the
lower classifications of streets in both residential and commercial areas. In residential areas there is an added emphasis on child safety. Not to be overlooked are the safety
concerns of pedestrians in parking areas. Parking areas
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Transportation Fundamentals 197
are complex in that the driver, although proceeding slowly,
must contend with numerous vehicle and pedestrian movements occurring nearly simultaneously. Pedestrian movements along and randomly crossing travel ways, vehicles
entering and exiting parking spaces, as well as the movement and turns of the normal traffic, all contribute to parking area hazards.
Planning for the pedestrian is a high priority in the design
of roadways at the local level. Pedestrian safety provisions
included in roadway planning are sidewalks, crosswalks,
curb cuts, and ramps for the physically disabled. Another
such provision is special walkways for pedestrian freeway
crossings. Traffic control features aid both pedestrian traffic
and vehicular traffic.
Sidewalks should provide a continuous path to service
areas such as community centers, schools, parks, and shopping areas. Most ordinances require sidewalks on at least one
side of the street. In some residential areas, the ordinance
may require a paved trail in lieu of or in addition to the conventional sidewalk.
Conventional sidewalks typically are adjacent and parallel to streets and travel ways, whereas paved trails meander
along natural pedestrian circulation routes. Another provision is the need for landing and staging areas at bus stops.
Unfortunately, bus routes through new residential developments are rarely established prior to completion of the development. Providing landings and pedestrian circulation to
these locations, when possible, adds to the appeal and safety
of the land development project.
One of the most dangerous maneuvers for pedestrians is
to cross a wide street at an unsignalized intersection. One
option to increase pedestrian safety is to locate bulb-outs at
corners as shown in Figure 3.3W. This not only reduces the
pedestrian crossing distance but also creates an illusion of
a narrower street to drivers, who instinctively reduce their
speed. Additional traffic calming measures are discussed in
Chapter 5.4.
Bicyclists. Many existing roads were not designed to
accommodate an ever increasing number of bicyclists sharing the road. However, in areas of new construction, where
a significant number of bicyclists are expected, provisions
should be made by the designer.
Ideally bicycle routes should be located outside the paved
roadway section when sufficient right-of-way is available.
This additional space is not always available or may be occupied by sidewalks and utility strips. As a result, bicycle facilities can generally be designed as one or a combination of the
following types:
Type Ioff-street path: These bikeways should be used
on high-volume, high-speed roadways provided rightof-way space is available. They can be designed wide
enough to accommodate both bicyclists and pedestrians
(10 feet or more). Provisions such as low curbs or planting strips should be considered to separate these two
modes of transportation.
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Figure 3.3W
Bulb-outs (Source: FHWA. Designing Sidewalks and Trails for Access, Part I. Federal Highway Administration,
U.S. Department of Transportation. Washington, D.C., 1999).
Type IIbicycle lane: These bikeways are on-street
striped routes. They can be located adjacent to curb lines
or adjacent to parking lanes. In either case a narrow
painted buffer should be considered to separate the bicycle lane from the adjacent travel lane. These bikeways
can be installed where sufficient street width is available
or can be provided without restricting vehicular traffic.
Figure 3.3X shows a typical Type II bicycle lane with suggested pavement markings.
Type IIIbicycle route: The least desirable bikeway of the
three is the on-street signed route. They are to be used
where there is insufficient curb-to-curb width to accommodate the striped Type II bicycle lane and in some
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cases to provide continuity to other Type I and/or Type II
bikeways in the area.
When on-street bicycle lanes and/or off-street bicycle paths
are present in the land development, the design of the intersections in the development should be modified to reflect
the presence of these facilities.
The following measures can be introduced:
•• Special sight distance considerations
•• Wider roadways to accommodate on-street bicycle lanes
•• Special lane markings to channelize and separate
bicycles from right-turning vehicles
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Figure 3.3X
Transportation Fundamentals 199
Type II bicycle lane.
•• Provisions for left-turn bicycle movements
•• Special traffic signal designs (such as conveniently
located push buttons at actuated signals or even separate signal indications for bicyclists)
The subject of bicycle facilities is further explained
in the AASHTO Guide for the Development of Bicycle
Facilities.
3.3.7. Site Design
Traffic circulation and ease of access often determine a
land development’s consumer appeal. In purely residential
developments, the recommended practice is to discourage cut-through traffic, by limiting the number of access
points to the arterials or collectors or by designing the
internal streets in short, narrow, curvilinear, and discontinuous patterns. Although multiple access points might
encourage through traffic, they do provide for less congestion because of more dispersion of the traffic from within
the residential development. In mixed-use developments
and commercial/retail projects, the access and circulation pattern is more complex. Circulation and access are
important to facilitate the traffic flow through the commercial and retail sections. However, the commercial and
retail traffic should not be directed through residential
areas. Proper planning of the development necessitates
that the residential section be located apart from the business areas to reduce the high traffic volume’s impact. As
previously mentioned, through traffic can be discouraged
by street geometry and configuration, but this strategy is
most effective if the minor arterials have adequate capacity and provide a good LOS.
Internal Street Networks. Street circulation is site oriented
to a degree. Where and how the streets are oriented depend
on the interrelationships of various aspects such as lot layout,
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topography, noise levels, accident potential, and other factors. To accommodate access and circulation demands, street
networks have evolved into grid and/or curvilinear patterns
that fit certain situations and lend themselves to particular
solutions.
Many mixed-use and retail establishments are designed
to promote walking and biking, which can create conflicts
with motorized vehicles.
The safety of all modes of transportation should be considered with the design of a site. Pedestrian circulation should
be evaluated when providing sidewalks and street crossings should be prominent and accessible. The convenience
of street crossings and the safety of pedestrians should be
balancedinconvenient crossing locations may discourage
the use of formal crosswalks, while too many crossing locations introduce additional conflicts.
Grid Patterns. The grid pattern is a series of parallel streets
intersecting at right angles creating rectangular blocks. The
grid is best suited for use in high-density areas with widely
distributed traffic flows. Grid systems provide easily recognizable orientation for users. Their simplicity in providing
access is a definite advantage. The grid system simplifies
surveying and construction while also creating efficient and
uniform building sites.
Grid street systems dictate the topography by forcing
a specific grading design in order to obtain buildable lots.
Therefore, they are best suited for generally flat areas. Imposing a grid system on rolling terrain in most cases requires
extensive grading. Other drawbacks to this type of system
are the monotony of the layout and the lack of differentiation
of function between individual streets within the system.
The system also invites through traffic. These drawbacks
can be overcome with creative design and by variations from
the typical grid. Figure 3.3Y illustrates the simple grid pattern and modifications such as using secondary loop roads
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create a sense of neighborhood on a small scale by collecting
the lots into small groupings through the use of smaller elements of street patterns such as cul-de-sacs, loop courts, and
short streets (Figure 3.3Z). Such a pattern’s drawback is the
confusion created that may result when attempting to traverse the area. Extensive curvilinear patterns are not recommended for commercial and industrial developments.
Circulation and access may be considered from two different scales as they apply to the development’s proposed
public street system, and as they apply to a specific land
parcel. In both situations, circulation and access are constrained by the existing street network, adjacent property, and the intended type of land use for the proposed
development.
Frequently, providing for adequate circulation and access
requires improvements to existing infrastructure elements
beyond the limits of the site, and these are often financed by
the developer. Turn lanes added to the existing street or signalization of an intersection to improve capacity and safety
are examples of such required off-site improvements. Necessary considerations for circulation and access include the
following:
•• Will the intersection of the proposed street adversely
affect existing intersections?
•• Is there adequate horizontal and vertical sight distance at the new access point?
•• Will turn lanes be required?
•• Will any intersections require signal control?
F i g u r e 3 . 3 Y Grid pattern and variations. (J. DeChiara and
L. Koppelman: Time Saving Standards for Site Planning, 1984, McGraw-Hill.
Reproduced with permission of McGraw-Hill.)
to provide a more attractive environment while discouraging
nonlocal through traffic.
Curvilinear Patterns. Many medium to large projects
develop a curvilinear type of street pattern. Such a pattern
discourages “cut-through” traffic, optimizes land use, and
minimizes cut and fill grading operations during construction by taking advantage of the existing topography for streets
and, also, lot orientations and configurations. Such patterns
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Figure 3.3Z
Smaller elements of street patterns. (J. DeChiara and
L. Koppelman: Time Saving Standards for Site Planning, 1984, McGraw-Hill.
Reproduced with permission of McGraw-Hill.)
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•• Will any streets within the development be required
to terminate at a particular point on the boundary
to allow for access to adjacent properties for future
development?
Considerations for access to a particular parcel such as
entrances (driveways) into commercial/retail sites should
address:
•• Is the driveway length (i.e., throat length) adequate to
allow for vehicle queuing into and out of the parcel?
•• Is the driveway entrance too close to other driveway
entrances or to an intersection that causes conflicts
in turning movements?
•• If there is a median in the driveway, is it visible by the
vehicles turning into the entrance?
•• Are the profile and cross section of the driveway adequate and compatible with the roadway
conditions?
Circulation within the parcel should also be considered:
•• Is a loop road or perimeter road appropriate?
•• Does the parking area need to be partitioned or
somehow sectioned to facilitate traffic flow?
•• Are the landscaped islands adequately sized (e.g.,
radius and widths) and located for visibility and
circulation?
Cul-de-Sacs and Turnarounds. The cul-de-sac is a popular street type for use on local streets. Cul-de-sac streets are
open at one end and provide a turning area at the closed end.
Although the circular cul-de-sac and its variations may be
the most widely used, other types of turning areas include
“T” and “Y,” among others. Some of these are depicted in
Figure 3.3AA. The “T” and “Y” type require more inconvenient backing movements by the user but in low volume
situations conserve land and allow increased flexibility in the
land planning process. The type chosen for a particular situation depends on the primary type of vehicle being serviced
by the turning area, traffic volume, and, most importantly,
the standards accepted by the approving agency.
Because they are open only at one end, cul-de-sacs are limited to an ADT volume of about 200 VPD. If it is assumed that
a single-family residence generates 10 trips per day, then 20
residences will generate an ADT of 200 vehicles. Lot widths
range from about 65 feet for small lots to 100 feet for moderately sized lots. Based on these assumptions, the maximum
length of a cul-de-sac street is generally 650 to 1000 feet.
The recommended radius for an all-paved cul-de-sac is
45 feet. Most passenger vehicles require a minimum radius
of 30 feet to turn around. Small trucks and fire equipment
can turn around with one or two backing movements on
40-foot-radius cul-de-sacs. However, this presumes no vehicles are parked within the turnaround area.
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Transportation Fundamentals 201
Other variations or shapes of cul-de-sacs may be provided to permit vehicles to turn around by backing only
once. Several types [Figure 3.3AA(f–i)] may also be suitable
for alleys, which provide access to the side or rear of individual land parcels.
For practical reasons, cul-de-sacs should be relatively
level. Pavement grades across the cul-de-sac should ensure
adequate drainage over the pavement and yet allow a vehicle to access adjacent land. A vehicle should be able to turn
around without negotiating steep grades. Maximum pavement grades across the cul-de-sac of 3% to 4% are recommended. The street grade entering the cul-de-sac should be
between 3% and 5%. To ensure proper design, the edge of
pavement or top of curb around the perimeter of the cul-desac should be profiled. Spot elevations around the cul-de-sac
based on the resulting profile aid the contractor in construction. For more information on grading, see Chapter 3.4.
The lengths of the vertical curves and tangent grades
should be selected to provide a relatively flat area around
most of the perimeter of the cul-de-sac. This would allow
for smoother driveway connections. Site conditions and
drainage considerations are key factors in determining the
lengths, locations, and tangent grades of curves.
Parking. For most land development projects, the availability of vehicular parking is a critical characteristic of
the site. It’s often difficult to accommodate parking within
a siteeach parking space (including drive aisles) will
take up about 300 square feet of land area. As a comparison, a shopping center may require four parking spaces for
every 1000 square feet of shopping space (or one space per
250 square feet), this means the parking area will often
occupy more space than the actual building area. In an office
building, most employees likely work in a space smaller than
where they park their car.
There are generally three different parking classifications: surface parking, garage parking, and street parking. A
surface parking is at-grade and usually provided as a large
parking lot that can vary in geometry to fit the shape of the
site. A parking garage is more expensive (20 to 30 times) to
construct than surface parking spaces, but a parking garage
occupies less real estate and is therefore suitable for dense
development areas (Figure 3.3BB shows a surface parking lot
and a parking garage). A standalone parking garage (one not
attached to a building) usually has an efficient layout such as
a rectangle that maximizes the number of parking spaces. If
the parking garage is below a building, the shape conforms to
the building, which may not be an efficient layout but occupies less land area. Street parking is provided with spaces
parallel or angled to a roadway and is common in dense
development areas where parking lots or parking garages
are financially prohibitive. Within a development, such as a
town center, on-street parking can provide convenient parking adjacent to storefronts (Figure 3.3BB).
In addition to passenger vehicle parking, many jurisdictions require parking for delivery vehicles. The parking
spaces reserved for delivery spaces will generally be larger
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(a)
(b)
(c)
(f)
(d)
(g)
(e)
(h)
(i)
F i g u r e 3 . 3 AA Types of cul-de-sacs and dead-end streets. (From A Policy on Geometric Design of Highways
and Streets, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used with
permission.)
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Figure 3.3BB
Transportation Fundamentals 203
Example of surface parking and a parking garage.
(e.g., 15 × 25 feet), but the required number of spaces is significantly less with only a few throughout the development.
Retail tenants may have their own requirement for delivery
trucks while a school will likely establish their own requirements for parking buses. Bicycle parking may be required in
some regions and is usually separated into short term (bike
racks) and long term (internal bike storage rooms).
The common dimension of perpendicular parking space
is 9 feet wide by 18 feet deep, but this can vary by 1 or 2 feet
in each direction based on local requirements. For context of
scale, a small car is around 6 feet wide by 16 feet longthe
parking space dimension considers ingress and egress from
the vehicle. Some retail tenants prefer larger parking spaces
to accommodate shoppers that are loading goods. Office
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developments may prefer reduced parking areas because
parking is used primarily by an individual just twice a day.
Requirements. The required number of vehicle parking
spaces is usually dependent on the use of the site and size of
the development based on the local ordinance. Many parking requirements are based on the area of the building, the
number of employees, or the number of homes. Many retail
or office tenants will also have their own requirements for
parking, which may exceed the requirements of a local jurisdiction. A hospital may be interested in additional handicap
(ADA accessible) parking spaces or a grocer might request
more parking than the code requires. A lender or investor
may also have their own requirements for parking based on
how they evaluate a project’s marketability.
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In some cases, a developer may be interested in reducing
the amount of parking for a site to reduce construction costs
or encourage alternative modes of transportation. If the site
is located near a rail station, bus stop, or a bike trail network,
the site could function with less vehicle parking. In mixeduse development it’s likely that the peak demand for parking alternates between residential uses and office uses, which
allows parking areas to be shared.
When providing parking areas, it’s necessary to also provide parking in compliance with ADA. The minimum number of ADA spaces required is dependent on the number of
total parking spaces provided (Table 3.3D). The location of
ADA spaces should be as close to the primary entrance of the
building as possible. When providing ADA parking spaces
within a garage, the spaces can be split between levels but
should then be located near an elevator. If all ADA spaces are
provided within a parking garage it may be necessary to verify that the parking garage has adequate floor-to-floor clearance to accommodate a large ADA vanthis often requires
additional height over standard parking decks.
Many jurisdictions also have zoning requirements for
site parking. Landscaping is often required within parking
lots and is provided through parking islands (curbed areas
T A B L E 3 . 3 D Minimum Number of Accessible
Parking Spaces (From Americans with Disabilities
Act, 2010 by U.S. Department of Justice)
Total Number of
Parking Spaces
Provided in
Parking Facility
(per facility)
(Column A)
Minimum Number
of Accessible
Parking Spaces
(car and van)
Mininum Number
of Van-Accessible
Parking Spaces
(1 of six
accessible spaces)
1–25
1
1
26–50
2
1
51–75
3
1
76–100
4
1
101–150
5
1
151–200
6
1
201–300
7
2
301–400
8
2
401–500
9
2
500–1000
1001 and over
*
2% of total parking
provided in each lot
or structure
1/6 of Column A*
20 plus 1 for each
100 over 1000
1/6 of Column A*
one out of every 6 accessible spaces
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that occupy parking spaces to provide room for vegetation).
Parking visibility and setback is a common concern and there
are often requirements for screening of parking areas. These
requirements should be considered during early planning
stages as they can affect the available space for parking.
Layout Options. There are three configurations for passenger vehicle parking spaces: perpendicular, parallel, and
angled. The angled spaces can vary as 45° or 60° parking
spaces. Perpendicular spaces are the most common for parking lots and garages, whereas parallel parking spaces are
common for on-street parking. Angled parking can usually
allow for more spaces (compared to perpendicular), but the
angled spaces limit how parking is accessed through a parking lot and is better suited for lots that encourage directional
movement (e.g., a fast food restaurant).
Each jurisdiction will identify the geometric requirements
for parking spaces, which is usually identified in the zoning
ordinance or design standards manual. A perpendicular space
is usually 9 feet wide by 18 feet deep with a 24-foot-wide drive
aisle. Parallel spaces are narrower but longer at 8 feet wide
by 22 feet long with a drive aisle of 16 feet for one-way or 20
feet for two-way streets. Angled parking spaces require a larger
depth dimension than perpendicular, but the drive aisle can be
reduced because the angled parking space requires less space
to maneuver. If the parking drive aisles serve as primary routes
through the site, it’s important to consider requirements for
emergency vehicles, which often have minimum widths of 20
to 26 feet. Table 3.3E provides a sample of parking stall dimensions based on parking space alignment.
The optimum parking layout should consider the site use
while also maximizing the efficiency of the parking spaces.
Parking should be convenient to the users while also providing
a safe configuration for pedestrians. A layout should reduce
the number of dead ends for parking bays and should reduce
the number of drive aisle intersections. If queuing is expected
within the site (e.g., highway access, fast food drive through
lanes, loading areas, etc.), parking should be provided where it
doesn’t impede other prominent site movements.
Parking Planning Considerations. Parking generally
occupies large areas of a site and becomes an important measure of site’s appeal, but parking demand can easily diminish
because of other transportation improvements. New public
transportation, autonomous vehicles, and ride share services
can reduce the need for parking on a site. When designing a
site, it’s worth considering how to future-proof the site so that
parking areas could be redeveloped for higher value uses.
TA BL E 3 . 3 E
Parking
Angle
Parking Space Dimension Table
Width
Depth
Aisle Width
(one-way)
Aisle Width
(two-way)
45°
9
19
16
18
60°
9
20
17
19
90°
9
18
24
24
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Transportation Fundamentals 205
CARLISLE ROAD DIET
Location: Carlisle
Completion Date: 2011
Case Study: Carlisle’s downtown, located in Cumberland County, Pennsylvania, was plagued by excessive vehicle speeds
and long crosswalks. The downtown’s four-lane streets created an auto-dominance that was challenging for both pedestrians and bicyclists alike. The Borough desired to recreate the thriving, walkable downtown it once had. Dewberry was
selected to design traffic pattern changes that would calm traffic, enhance Carlisle’s small town feeling, increase safety,
reduce noise and air pollution, and promote walking and bicycling.
Consistent with these goals, Dewberry designed a road diet conversion of High and Hanover Streets from four to three
vehicular lanes with a dedicated bike lane in each direction. The bike lanes provide an added benefit of making parallel
parking easier and making entering and exiting one’s vehicle safer. Dedicated left-turn lanes were proposed at each signalized intersection in the downtown along High and Hanover Streets to improve traffic flow. In order to accommodate
the highly variable traffic volumes through Carlisle, significant upgrades to the existing uncoordinated traffic signal
system were recommended including an adaptive signal system, emergency vehicle preemption, and audible countdown
signals to better serve pedestrians.
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The Carlisle Road Diet project features the cutting edge of traffic signal technology. With the first full-scale implementation of the InSync Adaptive Signal System in Pennsylvania, traffic in downtown Carlisle will keep moving at a
steadier, calmer pace. An Encom wireless communication system keeps all 21 intersections synchronized. Pedestrians
benefit from leading pedestrian intervals and audible countdown signals, while Carlisle’s first responders are guided
through town via the new emergency vehicle preemption system. Curb extensions were also constructed to shorten the
pedestrian crossing distance and, in addition, truck mitigation signing was installed to reduce the number of trucks in
the downtown. On this project, pedestrians and bicyclists were the primary focus along with safety and traffic calming.
A significant challenge was the need to overcome initial public resistance to the road diet concept. Through many
public meetings, question and answer sessions, and finally the completion of construction, the Carlisle community has
accepted the changes and many now speak highly of the end results. One of the main reasons for the success of this
project was the cohesion and excellent working relationships among the owner, designer, and stakeholders. Everyone
had a strong sense of ownership in the project that was critical to keep the project moving forward and ensure that the
Borough of Carlisle’s needs and expectations were exceeded.
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Chapter 3.4
Grading Fundamentals
3.4.1. Introduction
Grading is configuring the surface of the land by removing or adding earthen material to shape the land to best
suit the development program. It is accomplished with
both large machines, such as bulldozers, hydraulic breakers, and dump trucks down to people, rakes and shovels,
and constitutes a major component of the function and
success of a land development project.
During site analysis it is important to examine the existing topography of a site. The preliminary information
gathered for the feasibility study and base map will usually include a topographic map of the site (as discussed
in Chapter 3.2). This is an important indication of development potential. It is important to know how to read a
topographic map and to understand the implications of the
existing topography.
This fundamental information will be crucial to be able to
interpret existing conditions and understanding the grading
process. In addition, it is important to know how these contours relate to drainage patterns and drainage divides. This
will be important to consider not only when grading, but
also the impacts for stormwater management, which is discussed in Chapter 3.5. Next, this chapter discusses an overview of grading and earthwork calculations. This process is
vital to apply during the site analysis to ensure that a piece of
property can be developed efficiently and successfully.
3.4.2. Topographic Maps
As introduced in Chapter 3.2, topographic maps display both
the horizontal positions of natural and man-made features
and boundary lines as well as elevation data (depicted as spot
elevations and contour lines). These maps can come from a
variety of sources, including field survey, aerial survey, remote
sensing, and geographic information system (GIS) databases.
Contour lines are a method for graphically depicting
three-dimensional information on two-dimensional media.
A contour is a graphic line connecting points of equal elevation and is formed by the intersection of a horizontal plane
with the ground surface. Contour lines provide a legible
description of a site’s topographic condition. A topographic
map depicts the contours of a site, which provides a reference to the ground shape. A topographic survey of a site is
prepared by collecting elevation points throughout the site
and contour lines are interpolated from these points—in this
way, a contour map is not a perfect depiction of all details for
a site but a visual reference (Figure 3.4A).
The vertical distance between successive contour lines
is the contour interval. Most topographic maps, especially
those associated with a land development project, have a
constant contour interval. For instance, a 2-foot contour map
means that the plan shows contour lines that indicate a 2-foot
change in elevation. At every 10-foot change in elevation, the
contour line may have a bold or different style to represent
a major interval as compared to the minor 2-foot contour
intervals. The contour interval provides a reference to the
resolution of the data collected, where smaller intervals (such
as 1 foot) mean that more information has been collected
(refer to mapping accuracy information in Chapter 3.2).
When a topographic survey is requested, the contour intervals will set the accuracy requirements. When a contour map
is referenced into a design document, the contour intervals
should not deviate from how the map was prepared—any
additional interpolation that adds contour lines would imply
accuracy that’s not available.
State and local agencies, through ordinance and design
standards as described in Chapter 2, often require specific
contour intervals for drawings submitted for review. While
large-scale topographic maps, with 5- to 10-foot contour
intervals, are suitable for feasibility studies, smaller scale
maps, with 1- and 2-foot contour intervals, are used for final
design and detailed studies. As discussed in Chapter 3.2,
contour maps that are developed from a jurisdiction’s GIS
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F i g u r e 3 . 4 A A spot elevation map without contours and with contours.
data will likely have a larger contour interval as compared to
project survey.
Characteristics of Contour Lines. The depiction of contour lines should follow established rules that are used when
interpreting the topographic information. The following are
the main characteristics of contour lines:
1. All contour lines eventually close on themselves if
traced in their entirety: Any apparent break in a
contour line is due to the limitations of the map.
Contour lines that extend beyond the limits of the
subject area terminate at the map edge.
2.
Contour lines cannot cross or overlap: This would
imply that two elevations occur at a single point.
The exception is a vertical face, where contour lines
will overlap on a two-dimensional plane, or an overhang where contour lines will cross to represent different planes.
3.
Spacing of contours indicates the general steepness
of the ground: Closely spaced contours indicate
steep slopes. As the ground slope becomes flatter,
the distance between the contours increases. Contour lines that are spaced further apart represent a
mild slope.
4. A contour line cannot split, nor can several lines join
to form one line.
In general, irregularly spaced contour lines designate
rough, rugged landforms; while parallel, equally spaced contour lines indicate a smooth, uniform slope. On a relatively
large scale, the natural ground line is considered smooth and
continuous. Relatively few ground features show sharp jagged
03_Land_CH03_p125-304.indd 208
or abrupt changes in ground relief. This smoothness is carried
over to the concept of contour lines.
Contour lines indicate distinct elevations through a
site but only provide information at set contour intervals.
The actual ground between contour lines may deviate as
much as the contour interval space—the area between
10-foot contour intervals could change nearly 10 feet
from the assumed smoothed ground line, as schematically shown in Figure 3.4B. When determining an elevation between contour lines, the ground is assumed
straight unless additional information (such as a spot
elevation) is provided.
Contour Line Patterns for Natural Surfaces. Many natural ground features can be identified based on the patterns
of contour lines. The identification of these natural surfaces
can identify potential areas of concern (depressions without
drainage) or suitable building locations.
Hills and Depressions. A series of contours that close on
themselves within the mapped area indicate either a localized hill or localized depression. Figure 3.4C(a) shows a hill
and Figure 3.4C(b) shows a depression. In Figure 3.4C(a)
the elevations shown on the contour lines increase up to
the summit. Conversely, in Figure 3.4C(b) the elevations
decrease toward the bottom of the depression.
Valleys and Ridges. Valleys and ridges are indicated by
contour lines configured in V-shapes. To distinguish the
ridge from a valley, notice the direction of the apex of the V.
On ridges, the V points down ridge (i.e., downhill), while the
V points upstream (i.e., uphill) in valleys.
Overhangs. Technically, contour lines never cross. However, in the case of an overhanging cliff or cave, where contour lines may appear to cross, the contour lines represent
different planes.
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Figure 3.4B
Grading Fundamentals 209
Irregularities in natural ground.
Contour Line Patterns for Constructed Surfaces. For an
existing constructed surface, such as a bridge, wall, building, or channel, the depiction of a contour line will appear
irregular. The contour line will appear to break at the constructed surface as it then begins to follow along the face of
the surface.
Culverts or Bridges. At the end of a culvert or bridge there
are, for all practical purposes, two land surfaces to consider.
One is at the level of the invert of the structure (at the bottom)
and the other is the “at grade” (above the structure). Typically, the ground will slope up and away from a culvert opening and contours will be parallel and rather close together. A
bridge will have a much more complex facial configuration
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with possible wingwalls, piers, earth floor, etc. At the face of
a bridge the contours may appear to cross as two separate
ground surfaces are being presented. Figure 3.4D illustrates
this concept.
Retaining Walls. Although it is physically impossible
for several contours to join and form a single contour line,
retaining and exterior building walls can appear in plan view
to do just this. For a vertical slope (wall) the space between
the contour lines disappears. Consider the face of a wall as
a series of contour lines stacked one on top of another as
shown in Figure 3.4E.
In tracing the contour line around a wall, the contour line
intersects the wall at the contact elevation, that is, where the
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(a)
(b)
Figure 3.4C
Contour patterns: hill, ridges, depressions, valleys.
F i g u r e 3 . 4 D Contour patterns for culverts and bridges.
ground meets the wall, and then continues along the face of
the wall. The contour line leaves the wall where the wall intersects the ground at that elevation. Many earthen retaining
walls do not have a vertical face and instead have a steep slope,
or batter, along the face of the wall. The batter of the wall
should be accurately considered in a grading plan because the
wall height will govern the projection of the wall face. A batter
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of 1-inch horizontal for every 8-inch vertical is common for
gravity retaining walls.
Exterior Walls of Buildings. Exterior walls of buildings
serve as earth retaining walls, as well as structural support.
Similar to the conventional retaining wall, the contour
line enters the wall where the ground surface intersects
the wall at the prescribed elevation, shown as points A,
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F i g u r e 3 . 4 E Schematic and plan view of retaining wall.
B, and C in Figure 3.4F. The contour line then follows the
exposed face of the building wall until it reaches the point
where the ground surface is the same elevation; shown as
points A′, B′, and C′ in the plan view of Figure 3.4F. Since
a contour line is continuous, if it enters the wall, it must
exit the wall.
Conveyance Channels. Four frequently used conveyance
channel sections are trapezoidal, V-ditch, rectangular, and
semicircular. In the plan view of Figure 3.4G, each of the
channels is 2 feet deep. The depth of the channel is evident
by comparing the elevation at the top of bank, with the elevation at the bottom of the channel. On each type of channel
section, point A, the top of bank, has an elevation of 6 feet.
A line drawn perpendicular to the flow line, through the
top of bank, intersects the flow line at elevation 4 feet, representing the elevation of the channel invert. Hence, the
depth is 2 feet. The V-ditch section shows the V-apex pointing upstream—a quick indication of the direction of flow.
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Direction of flow in the other channel sections is evident
from inspection of the contour elevations.
Another indication of the depth of the channel is how far
upstream the contour line runs. The spacing of the contour
lines along the sides of the channel is an indication of the
steepness of the bank. Figure 3.4H shows a 2- × 4-foot-deep
V-ditch with the same longitudinal slopes and the same top
widths, w. The contour lines extend farther upstream for the
4-foot-deep channel as compared to the 2-foot-deep channel. Because the top widths are the same, the 4-foot-deep
channel has steeper side slopes, evidenced by the contour
line spacing on the sides of the channel.
Streets. Two types of streets commonly used in development projects are the crown street with curb and gutter, and
the crown street with shoulder and ditch. Figure 3.4I shows
the contour line pattern for a curb and gutter street section with longitudinal slope S. For a normal crown section
street, the elevation decreases on a line perpendicular to the
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Figure 3.4F
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Schematic and plan view of exterior building.
centerline due to the cross slope of the pavement. From the
prescribed elevation on the centerline (point A) the contour
line follows a straight line path that leads uphill until it meets
a point at the edge of gutter (point B) equal in elevation to
the centerline elevation.
The break in the contour at the edge of the gutter (point B)
results if the cross slope in the gutter pan differs from the cross
slope of the street. From the edge of the gutter, the contour line
continues uphill to the point on the flow line with the same
elevation (point C). The contour line then follows the face of
the curb downhill to the point on the top of the curb with the
same elevation (point D). The contour line intersects the outside edge of the sidewalk at the prescribed elevation. Typically,
sidewalks are sloped toward the street. This is apparent from
the contour line’s downhill direction (points D to E) across the
sidewalk. A similar trace of the contour line is shown in the
shoulder/ditch type of street of Figure 3.4J.
Typically, surface drainage on the pavement flows toward
the curb and gutter (or ditch).
Berms and Ponds. Just as natural hills and depressions display contours that close in on themselves, berms and ponds
also have concentric contours. However, man-made features
of this nature are typically less irregular, and the contours
tend to be evenly spaced and/or parallel.
Spot Elevations. Often, contour lines alone cannot provide
sufficient grading information to detail the existing ground
conditions. As a result, the level of precision needed to construct the proposed features detailed on land development
plans is not afforded by contours alone. Therefore, spot elevations are used to identify specific elevations at precise locations.
F i g u r e 3 . 4 G Contour line pattern for conveyance channels.
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Figure 3.4J
F i g u r e 3 . 4 H Example of 2- × 4-foot-deep channels.
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Contour line pattern for shoulder.
A spot elevation is usually indicated in the plan view
by a “+” or “×” symbol with the elevation written next to
it. Spot elevations identify discontinuous or abrupt grade
breaks in the ground surface, where straight-line interpolation between contours does not give the intended elevation.
Therefore, spot elevations are used when the uncertainties
associated with scaling distances and interpolating between
contours cannot be tolerated.
Typically, spot elevations are used for
•• Precise information regarding utility structures
(such as storm inlets)
•• Changes in slope that occur between contour
intervals
•• Identification of high and low points in the grading
scheme
•• Description of retaining walls, that is, top and bottom of wall elevations
•• Elevations at building entrances and corners
F i g u r e 3 . 4 I Contour line pattern for curb and gutter.
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The house-grading plan of Figure 3.4K illustrates the liberal use of spot elevations. Abbreviations are written next to
the spot elevation when the elevation pertains to a specific
feature, for instance top of curb (TC), TC = 105.5. Selected
abbreviations are given in Table 3.4A.
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Figure 3.4K
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House grading plan.
3.4.3. Drainage Patterns
A good way to visualize how a site is graded, and to better
understand the contour lines, is to consider the drainage of
the site. Water flows downhill following a path that is perpendicular to the contour lines. By considering a single drop of
water falling onto the site, the drainage pattern can be identified by tracing the path of the water downhill. This will show
TA BL E 3 . 4 A
Spot Elevation Nomenclature
Abbreviation
Meaning
TW/BW
Top/bottom of wall
TC/BC
Top/bottom of curb
FF El.
Finished floor elevation
BF El.
Basement floor
elevation
HP/LP
High/low point
Inv. El.
Invert elevation
MH El.
Manhole elevation
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which direction the water flows and the low point can then be
identified. By picking multiple points, it will be easier to see
how the entire site drains and understand the existing grades
of the site. In addition, the path of a single drop of water can be
traced uphill to find the high point on the site.
Drainage Areas. Drainage areas, or catchment basins, are
the delineated boundary of drainage in a site. At a large scale,
for instance when considering a river system, a drainage area
may be referred to as a watershed. All water captured within
the drainage area will flow to the outfall at the low point of
the boundary. Drainage divides are the lines separating different drainage areas.
A drainage area can be used to calculate the area of the site
draining to a single storm drain or may show a larger portion of the site that drains to a pond. See Figure 3.4L to see
how multiple drainage areas can be depicted on a topographic
map. The drainage divide is the line between each drainage
area and is shown as the (dashed line with arrow).
Figures 3.4M and 3.4N describe the process to draw a
drainage area.
Drainage areas are drawn by first identifying the low point
of a site, as can be seen in step 1 of Figure 3.4M. From there,
the divide is traced uphill perpendicular to the contour lines,
as can be seen in step 2. When two lines are drawn in opposite directions from the low point, they will trace back to the
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F i g u r e 3 . 4 L Drainage divides on a topographic map.
same high point, and that will form the drainage area, as can
be seen in step 3.
Ridges will usually form the border on the upland side of
the divide. As depicted in the graphics, the drainage divides
are not defined by the property line. The drainage divide for
a site may include off-site drainage. A common error is to
assume that a drainage divide terminates where the contour
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Grading Fundamentals 215
map terminates, which is often at the limits of the site. It’s
necessary to reference off-site contour information to accurately depict the entire drainage area.
On a large site, there may be several drainage areas, as shown
in Figure 3.4N. One side of a ridge may flow in one direction,
while the other side will flow in the opposite, as can be seen in
case B. Separate valleys within a site will each have their own
drainage areas, as can be seen in case C. Multiple valleys that
are connected at one low point, however, will have one outfall
and thus one drainage area, as can be seen in case D.
The starting point of the drainage area does not necessarily have to be at the low point of the site. A specific starting point can be used to calculate different areas of drainage
within a site. Therefore, as a site is developed, multiple drainage areas will need to be drawn to be able to identify how
water is captured across the site. This will be utilized for
stormwater management facilities design, storm inlet design,
and with erosion and sediment control measures, which is
discussed in Chapters 3.5, 3.6, 5.5, 5.6, and 5.7.
It is important to identify drainage areas during the site
analysis to know the existing conditions and drainage patterns of the site. Generally, existing drainage patterns should
be maintained as the site is developed and graded. This will
minimize earthwork efforts, as well as work to preserve existing downstream conditions. The low point on a site will be
critical to know when considering stormwater management
strategies, which is discussed in Chapter 3.5.
F i g u r e 3 . 4 M Process to draw a drainage area.
B
A
C
F i g u r e 3 . 4 N Multiple drainage areas on one site.
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Environmental Impact. The drainage of a site is directly
related to the water concerns that are discussed in Chapter 2.5.
During storm events, water quality and quantity must be considered as it drains across a site. Naturally, on undeveloped land,
this water permeates through the surface into the ground and
slowly reaches downstream waterways. On developed and more
urban sites, however, impervious surfaces, like buildings and
concrete, decreases the amount of permeable surfaces, which
increases the storm runoff. This runoff is often too much for
the downstream waterways to accommodate—the runoff is also
moving quicker and can be heavily polluted. More information
about water quality and quantity, as well as stormwater management strategies to manage runoff, are provided in Chapter 3.5.
During construction, though, specific strategies are required to
manage this runoff (as identified in Chapter 5.7).
Erosion and Sediment Control. For any land development
project, it is critical to develop an erosion and sediment
control plan to manage storm runoff during construction.
Construction activities remove the vegetative cover and
expose the soil underneath to the forces of water from storm
events. Earthwork during construction can change the natural drainage patterns of the site as well as negatively affect
slope stability amongst other concerns. Runoff can then
erode the soil on the property and pollute downstream sites
with excess sediment conveyance. The drainage across a site
is not limited to the property lines. The consequences of the
construction activities could then affect neighboring parcels,
which must be avoided and/or mitigated.
The erosion and sediment control plan prevents erosion and
the offsite migration of sediments. As a condition of the Clean
Water Act (as described in Chapter 2.5), most states require
the preparation of a Stormwater Pollution Prevention Plan
(SWPPP) as part of every construction project that will involve
land disturbing activities. An erosion and sediment control plan
is required by the SWPPP. A general erosion and sediment control program must ensure compliance with local code regulations, which will detail the specific requirements of the plan.
To control runoff and sediment on a construction site,
typical elements of erosion and sediment control plans
include stabilized construction entrances; temporary sediment barriers such as silt fence, stormwater inlet filters,
temporary sediment ponds, runoff diversions, check dams,
dust control measures, temporary stockpile, and soil stabilization; and permanent measures such as permanent vegetative cover, conduit outlet protection, slope stabilization,
or armoring. Many “nonstructural” design elements should
also be incorporated into these plans including minimizing
site disturbance during construction, minimizing soil compaction in vegetated areas to reduce runoff, and protecting
environmentally sensitive areas, such as wetlands, by establishing adequate buffers from construction activities.
More information about developing an erosion and sediment control plan will be discussed in Chapters 4.4 and 5.7.
3.4.4. Slope and Grade
After understanding the drainage across the site and environmental impacts that must be accounted for, it is important to
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further analyze the topography of the site. The topography of
the site should be considered early in the design phase as grading will likely influence the site design. This section introduces
the terminology of slope and grade, which helps communicate
the existing topographic conditions. This terminology is used
to define standard slopes and grades for site conditions.
Grade and slope are often used interchangeably in defining incline or steepness of terrain. Slope may reference a
numeric ratio whereas grade may reference a percentage, but
this is not a standard format across the industry.
Along a linear path, like a roadway, the terminology of longitudinal grade and cross slope is used to define directions of
grade. Longitudinal grade refers to the grade along the length
of the path. Cross slope is the grade cut perpendicular to the
path. There are often different requirements for longitudinal
and cross slopes, as well as a maximum slope in any direction.
Ground Slope. Ground slope is the rate of change in elevation, with respect to the horizontal distance, commonly
expressed as a ratio. The ratio is provided in a format of
horizontal:vertical distance. A slope of 4:1 indicates that
for every 4 feet of horizontal distance the vertical elevation
changes by 1 foot. This is usually spoken as “4 to 1” when
written as a ratio. Traditional nomenclature uses an elevation
change of 1 foot, so it’s not common to see a 5:2 slope, which
would instead be written as 2.5:1.
The ratio format is best used to define steep slopes (e.g., 5:1,
3:1, 1:1), whereas mild slopes are often described with a percentage or decimal. If the elevation drops 1 foot across 4 feet,
the percentage is calculated as rise/run, or 1/4 = 0.25 (25%).
Slope is usually written as a positive value with a directional
arrow in plan view. When shown in a profile for a road where
there is defined direction of travel the slope may be written as
negative for downhill and positive for uphill. Because there is
no industry standard, even mild slopes may be written as a
ratio of a different format (vertical:horizontal). A road cross
slope is often referenced as 1:48, or 2.08%.
Table 3.4B provides a list of common grades in both percent and ratio format.
TA BL E 3 . 4 B
Percent (%)
Common Grades in Percent and
Ratio Format
Ratio (H:V)
2
5
10
20
5:1
25
4:1
33
3:1
50
2:1
100
1:1
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Note that these values (especially as percent format)
should not be confused with geometric angles. Slope is
rarely referred to with an angle in design but may be used
when referencing geotechnical soil stability. The geometric conversation is determined by the ArcTan of the decimal format of the percentage slope (for a 25% slope, the
ArcTan(0.25) is 14°).
The slope within a site can vary dramatically and should
be considered when evaluating site features. Mild slopes
(2% to 7%) can provide ideal locations for buildings with
minimal earthwork requirements. Steep slopes (2:1 or 1:1)
may suggest the need for walls.
Given two points in a site (A and B) at a given distance
apart, with elevations ElA and ElB, the average ground slope,
Savg, between the two points is
S avg =
El B − El A ∆v
=
x B − x A ∆x
(3.4A)
By calculating the slope between two points, it’s also possible to identify the approximate elevation between those points
through interpolation. For instance, Figure 3.4O shows points
A and B that are 100 feet apart with a drop in elevation of
20 feet (20% slope). Point C is between those points, 40 feet
from point A. The elevation of point C is calculated as
40 × 0.20 = 8, such that point C is 8 feet lower in elevation
than point A. Therefore, point C is at an elevation of 92.
The grade can be easily calculated between contours based
on the contour interval. For instance, if the contours are
shown at a 2-foot interval and the distance between mapped
contours is 20 feet the slope is 10%. This graphic relationship
between the distance of contours and the steepness of the site
can allow for a quick review of the site to identify suitable
building locations and areas of concern (Figure 3.4P).
Figure 3.4O
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Grading Fundamentals 217
Standard Slopes and Grades. When establishing the proposed grades the site engineer should consider all national
and local requirements. Some jurisdictions will place limitations on how steep the ground can be graded based on soil
conditions and maintenance (2:1 and 3:1 are common limitations based on soil characteristics and maintenance). Local
jurisdictional requirements are usually found in the Development Standards Manual of the local subdivision ordinance, as discussed in Chapter 2.4. A retail tenant or home
builder may also have grading requirements that should be
followed, which may set maximum and minimum grades
throughout the site. In addition, the Americans with Disabilities Act (ADA) sets requirements for maximum grades
of parking and accessible routes for persons with disabilities.
The use of the site should also be considered when developing a grading plan to provide grades appropriate for the
proposed use. For example, in a retail development milder
slopes would be recommended to promote walkability
throughout the site.
When designing a grading plan, it’s important to consider
errors in measurement and construction. The measured grade
on paper should not depict the exact minimum or maximum
grades. Any minor construction (or survey) error would result
in a violation of the grading requirements, and most construction specifications allow for a tolerance of constructed elevations. This is especially important to consider when designing
accessible routes that have requirements for maximum grades.
Parking (including ADA). Many jurisdictions will set requirements for grades within a parking lot. A 5% maximum grade is
a common limit because it considers the challenge associated
with opening a car door on a hill side. For parking along a ramp
within a parking garage structure, the 5% is also a standard
limit. While 5% may be a common limit for a parking lot or
garage, on-street parking will follow the grade of the street.
Slope calculation for points A, B, and C.
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F i g u r e 3 . 4 P Example of mild slope areas and steep slope areas.
For a parking space that is marked to meet ADA parking requirements, the grade should be less than 2% in any
direction—a longitudinal slope of 2% and a cross slope of 2%
violates the maximum grade because it is 2.8% in the diagonal
direction. Some retail tenants will set requirements for minimum and maximum grades of a parking lot as they consider
shoppers with carts—a typical requirement is 1% minimum
(for drainage) and 3% maximum throughout the parking lot.
Roads. A road will typically have a cross slope of 2.08%
(1:48) unless it is super-elevated or a jurisdiction permits
lesser grades. Longitudinal slopes of a road will vary dramatically by geography. In areas with more mild terrain, such as
Texas, the department of transportation (DOT) has a minimum established grade of 0.25%. The AASHTO Green Book
recommends 0.5% as a standard minimum grade, but many
private developments plan for a minimum grade of 1% to
promote constructability and drainage. Maximum grades are
typically restricted by a jurisdiction and should consider the
roadway classification, speed, and the number of large vehicles
traveling along the roadway. For land development projects,
it’s common to establish a maximum road grade near 5% to
allow for a parallel accessible sidewalk route, but in mountainous terrain it’s not uncommon to see grades above 10%.
All road grades and changes in grade are restricted by vertical
geometry requirements, as discussed in Chapters 3.3 and 5.3.
When establishing the longitudinal grades of roads, it is
important to consider the adjacent building frontage. For example, a series of retail shops or townhomes along a road will often
seek a constant floor elevation. The constant elevation across
the building reduces breaks or steps in the building but can create challenges with how steep the grade between building and
road becomes. Mild slopes or localized high and low points can
provide a nearly flat condition across the building face.
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When evaluating driveway grades, it’s important to consider the grade change between a primary road cross slope
and the driveway grade. As noted in the AASHTO Guide
for Geometric Design of Driveways, the maximum desirable
grade difference is near 8% before a vertical curve is recommended. This guidance considers the effect of abrupt grade
changes on vehicles, which may scrape along the roadway
if the transition is large. Considering a 2% road cross slope
(down), the maximum desired driveway slope (up) should
begin at no more than 6% (for an 8% difference). A similar
design guide should be used when considering site ramps
for loading or parking garages as well as transitions from a
driveway into a garage home.
One other grade consideration for roads, or curbed
parking areas, is to provide a small shelf with a mild grade
behind the curb when possible (Figure 3.4Q). The shelf
should be graded near 2% or 3% for about 2 feet before
steeper slopes are proposed. This shelf provides a level
surface for curb inlets, site lighting, hydrants, and other
site furnishings. Additionally, the shelf provides structural
support for the back of curb. Conversely, if there is a steep
slope toward the roadway or parking area there should be
enough width to transition to the road grades and provide
a swale, which prevents stormwater from running into the
street.
Sidewalks and Trails. A sidewalk and trail will typically
have a requirement for a 2% cross slope, with drainage
toward the street. Sidewalks and trails that are located adjacent to a street will likely follow the same vertical geometry
of the roadway. If a road grade is steeper than 5%, it may be
required that the sidewalk alignment deviates from the road
to provide a route that is less than 5%—although this is not
always practical and a ramp may be required.
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3.4
Figure 3.4Q
Grading Fundamentals 219
Example of a shelf behind a curb line.
For accessible routes, the ADA requirements will govern
the allowable grades for sidewalks and trails. The maximum
grade along an accessible route is 5% (1:20) before it is considered a ramp. A ramp may be as steep as 8.3% (1:12) if the
total rise is limited to 2.5 feet, a handrail is provided, and a
5-foot landing area (2% maximum slope) is provided. The
maximum length of a ramp is 30 feet to achieve a total rise of
2.5 feet. The effective or average grade of a route with ramps
and landings, when considering the landings, is near 7%.
Curb ramps, the ramp between the street and a sidewalk,
are also subject to ADA requirements. The 8.3% rule is still
applicable as a maximum grade for the ramp but a handrail
is not required. When a curb ramp is provided with a gutter
section the gutter should not have a grade steeper than 5%
(1:20), which often requires minor deviations from the standard gutter slope. When designing a curb ramp the longitudinal grade of the route should be considered. If the sidewalk
has a longitudinal slope the length of the curb ramp may need
to be increased to account for the change in relative elevation
between the start and end of the ramp. When a curb ramp is
proposed along a sidewalk route with a steep grade, the ramp
length may be impossible to accommodate. Some jurisdictions
set maximum lengths of a ramp. Figure 3.4R depicts two different scenarios of curb ramps: one on a conceptual flat street
and the other on a street with a longitudinal grade of 4%.
When considering the grade of a sidewalk or trail it is
necessary to consider the effect of curved alignments. The
inside of a curve has a shorter distance and should therefore
be considered when evaluating the grade of the sidewalk.
Building Perimeter. The floor elevation of a building is
referred to as the finished floor elevation. The grading around
a building perimeter should be evaluated to ensure positive
drainage away from the building. Additionally, if the elevation
around the building varies along the perimeter the variations
should be coordinated with the architect to ensure compatibility with building features, such as windows and facade
treatments. Within a retail strip or a row of townhomes the
builder may request mild slopes to prevent steps between
shops or homes—vertical differences between attached units
can add cost and restrict how a retail space can be used.
Many home builders will have criteria for grades adjacent to a building. For instance, within the first 10 feet of
the home it’s common to see a grade of 5% and beyond that
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distance the minimum permissible grade may be 3% within
the yard. A retail tenant may have requirements for 0.5% to
1% grades within a given distance of the main entrance to
account for shopping carts. Steep grades adjacent to a building entrance may also create issues with doors opening or
accessibility. Some tenants may also require mild grades near
loading docks to provide a near-level condition for loading
and unloading.
Open Space. Grading within an open space is typically less
restrictive than hardscape areas and building perimeters. As
always, drainage should be considered when establishing
grades. In lawn areas and sports fields, the minimum grade is
often 2% to 3%. Large open space areas should be evaluated to
ensure drainage is accommodated with yard inlets or defined
channels to prevent ponding—lawn areas are often prone to
differential settlement which causes localized depressions.
Steep Slopes. Topography of the site, especially steep
slopes, will influence the developable area of the site and
increase construction costs associated with earthwork. A
steep slope is usually referenced as anything more than 3:1
(33% grade), but the specific threshold will vary based on
soil conditions. Steep slopes are challenging for pedestrians
to traverse and are well beyond grades acceptable for vehicle
travel. After a certain threshold the soil requires reinforcement to prevent failure of the slope. If the site has a large area
of steep slopes in the existing conditions, it is likely that the
developable area will be less than the total site area, and that
walls may be required to develop the site.
For example, a project site has a total land area of 10
acres, but an initial evaluation of the site topography identifies that large areas of the site have slopes steeper than
15%. This analysis is important because the site will be constrained as the internal roads and building pads are established at mild slopes (5% or less). The construction of walls
could expand the developable area for the site, but there are
several conditions that should be documented during the
site analysis:
•• Retaining walls will add significant cost to the design,
construction, and maintenance.
•• Most wall systems require reinforcement behind the
wall (70% to 100% of the wall height is common)
which limits what can be constructed.
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F i g u r e 3 . 4 R Example of a curb ramps on various street grades.
•• If a wall is located near the edge of the property, the constructability and the reinforcement systems may require
the wall to be offset into the site (or permission and
easements may be required from the adjacent parcel).
Steep slopes within a site will also require special maintenance considerations. Vegetation on the steep slope should
not require significant maintenance, but a vegetative cover is
often recommended to enhance slope stability.
3.4.5. Preliminary Grading and Earthwork
The development of final grading plan is typically reserved
until later phases in the design, but a preliminary analysis
of site grading can influence the site layout. Steep slopes
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may limit the buildable area or identify the need for walls.
Significant elevation changes may require long access roads
to prevent excessive road grades. At this stage it’s important
to carefully analyze the site and begin considering how the
final topography will be established. The grading design
work begins in concept and schematic design phases (refer
to Chapter 4 for more information). The preliminary grading effort is usually limited to the following actions:
Determine critical points: The critical topographic points
within a site include access locations, desirable building
locations, and drainage outfall points. The elevation at the
road entrances, existing elevation around the property,
and internal access will quickly shape the site topography.
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3.4
Identify steep slopes: Steep slopes throughout the site are a
sign that significant grading will be required. Most developed areas, such as sidewalks, parking areas, and buildings
will have mild (or flat) slopes. The steep slopes will need
to be regraded to accommodate proposed development.
Calculate potential walls: The potential for walls should
be identified early as the infrastructure can add significant cost. It’s important to understand which wall system
can be used for the project because the supporting infrastructure and construction may require the walls to be
setback from the property line.
Consider geotechnical conditions: While a geotechnical
report may not be available in early stages of design, it’s
possible that some information has been collected. The
presence of undesirable material, such as subsurface
rock, should be considered when identifying potential
area of cut in the site.
Aim for balance: There’s often an economic benefit to
balancing the site earthwork, meaning that if the site
needs significant grade the amount of cut will be similar
to the amount of fill required.
Avoid drainage issues: Drainage issues can arise if large
depressions are created in the site (sags, sinks, or bowls)
with no way for water to escape. Even within storm
drains this scenario should be avoided because drainage
systems can be clogged or overwhelmed.
Understand limits: Most projects will keep all grading work
within the project parcel as permission from adjacent owners is often challenging. The grading limits should consider
the parcel line as an area of the site that can’t be changed,
or permission to work off-site should be sought and established early in the project. Within the property there may
also be limitations on grading because of local regulations—
see Chapters 2.3 and 2.4 for more information.
Keep aesthetics and accessibility in mind: While considering all else, the form and function of the site should always
be considered. A developer may prefer a more attractive
grading scheme even if it’s not the most economical. Additionally, comfort and accessibility can enhance the site.
Grading Fundamentals. A good design integrates the natural landforms of the site with the proposed program to create
an aesthetically pleasing, yet functional, and cost-effective
site plan. Because a grading scheme must consider function
and utility, as well as aesthetics, it is both a science and an art.
The grading of a site serves three basic purposes:
1. Grading re-forms the land for the intended use: The
relative elevations and gradients of streets, buildings, parking areas, and pedestrian/vehicle accesses
must be mutually compatible if they are to function
as a system. Similarly, they must be compatible with
the surrounding existing terrain. Incompatibility
with the existing terrain, which leads to excessive
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earthwork, the use of retaining walls, and drainage
problems, increases construction costs.
2. Grading establishes new drainage patterns. To be
cost effective, the grading design should allow for
the efficient collection, conveyance, and control of
stormwater runoff. Proper grading prevents flooding
structures, soggy yards, foundation damage, erosion,
and muddy stream waters.
3.
Grading defines the character and aesthetics of the
site. Site design is the foundation upon which many
other elements of development depend. Proper grading should be cost effective to the developer, appealing to the user, and responsive to the opportunities
and constraints offered by the site. In this way, it
enhances property value and contributes to the success of a land development project.
Site grading is one of the most important steps of the
land development design process. It determines the extent of
the clearing limits of the project and the potential for costly
infrastructure, such as walls.
Prior to initiating work on the grading plan the design
engineer should review soils mapping and available geotechnical reports to understand and evaluate existing site
and soils conditions. These investigations should include
a review of information regarding problem soils, groundwater levels, rock formations, suitability of on-site soil
for reuse, and other conditions that may affect the grading design. In areas of poor soils it’s possible that material
would need to be removed and replaced, which can add
cost. Similarly, digging in areas with significant rock formations will add cost and should be minimized when possible. Refer to Chapter 2.5 for additional information on
soil classifications.
A grading plan is established through refinement of different schemes over the course of several trials. During the
first few trials, grades are adjusted to accommodate site
constraints, earthwork, different building designs, and the
preferences of the developer. A grading plan should be established to meet the following criteria:
1.
Drainage: Ensure adequate drainage within the site
2.
Earthwork: Balance the site earthwork, to minimize
import and export of soil material
3.
Access: Provide accessibility throughout the main
site routes, for pedestrians and vehicles
4.
Aesthetics: Create an aesthetically pleasing landform
Experience and creativity both play a role in developing
the grading scheme. As the designer begins to work through
the process, relationships between proposed improvements
and existing conditions begin to coalesce. It’s possible to have
a technically correct solution that isn’t necessarily the most
desirable solution. Important relationships begin to dictate
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patterns, such as existing elevations at site entry points compared to ground elevations at proposed building sites.
When possible, it’s economically and environmentally
beneficial to minimize the amount of native soil disturbed
when developing a site. Or in the case of redevelopment,
existing impervious areas should be reduced, and native
plant material would be reintroduced to the site. The site
design does not always require clearing and grading of the
entire site, and the grading plan should limit the amount of
disturbed land when possible.
Drainage. Perhaps the most important aspect of grading
is to ensure proper drainage of the site. If water is trapped,
flows toward undesirable locations, or causes erosion, the
grading has failed in its most basic requirement. The design
engineer must consider runoff within the site as well as runoff that flows onto the site from off-site areas. The drainage
study sets the basic parameters for the grading design and
often influences the site layout. The following is a list of goals
for a site grading plan from a drainage perspective:
•• Collect and convey stormwater runoff and direct it to
adequate outfall points.
•• Convey runoff away from buildings to protect them
from damage.
•• Establish overland relief to prevent localized flooding.
Drainage is conveyed either overland or subsurface.
Overland flow in its most benign form is called sheet flow,
where little or no concentration of water exists as it moves
across uniform, fairly level areas. Sheet flow is an ideal way
to convey water because it helps absorption and is nonerosive. However, after about 200 feet (or less for steep slopes)
the sheet flow becomes shallow concentrated flow. Shallow
concentrated flow should be directed to adequate conveyance systems prior to causing erosion. The designer’s job is
to artfully manipulate this transition from sheet flow to shallow concentrated flow using sound engineering principles
without cluttering the site with drainage system components.
Within most site there’s a condition that will lead to localized depressions or sump conditions. These locations will
need to be drained through inlets and closed conduit systems. But these areas should be graded with consideration
for overland relief—if the inlets and pipe systems are overwhelmed the localized flooding should not affect buildings.
As discussed in more detail with Chapters 3.5 and 3.6, most
storm drain systems are designed only for 10 year storms,
which means they may be surcharged with larger storms.
During the surcharged condition the site should be graded
to allow the overflow to be carried away from the buildings.
Adequate drainage through a site is also important prior
to site stabilization. Construction delays could also occur:
Very flat slopes are difficult to achieve in the field, even
given the precision of modern construction equipment. Flat
areas also remain wet for a longer period of time, which may
cause construction delays following rain. Yet, site constraints
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often require slopes less than those recommended, thus, they
are not entirely uncommon. The use of flat slopes should be
carefully considered, and avoided if possible.
Earthwork. The term “earthwork” refers to the manual
movement of soil. Earth is taken from one location and moved
to another in order to form the land as desired. In general,
moving earth from one location on a site to another can be
expensive. It is even more costly to a project when earth must
be removed from or brought onto a site. Therefore, it is very
important to have a balanced site where there will not be a
large amount of excess soil or, vice versa, soil demand.
The term “cut” refers to an area where soil is removed, while
the term “fill” refers to the area where soil is added. Cut and
fill areas are indicated on the grading plan by comparing the
existing contour lines to the proposed contour lines. Where the
proposed contour elevation is higher than the existing contour
elevation, the area is a fill. Conversely, a cut area is one in which
the proposed elevation is lower than the existing elevation.
Figure 3.4S shows a grading plan of a building with cut and
fill areas. As an example of determining the depth of cut from
comparison of contour lines, consider point A, where the existing 106 contour line intersects the proposed 100 contour line.
The section view shows the 6 feet depth of cut at this point.
The left side of the building is a cut area and the right
side is a fill area. The plan view shows a line around the cut
Figure 3.4S
Plan and section view showing cut and fill areas.
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3.4
and fill areas known as the zero cut/fill line. This line connects the points where no fill or cut occurs and separates
the cut areas from the fill areas. Additionally, notice that the
line also follows the points where the proposed contour lines
connect to the existing contour lines around the perimeter of
the graded area. Since the basement floor elevation has been
established at elevation 99.0 feet, the zero line follows the
existing 99-foot contour line through the center of the building. Although this grading plan shows only one cut and one
fill area, other projects may have several areas of both types.
The zero cut/fill lines are helpful for determining earthwork
quantities, which is discussed later in the chapter.
Consideration of cut and fill quantities is very important
when developing a grading plan. For many reasons, a “balance” of cut and fill is usually desirable. Balance is achieved
when the quantity of cut is roughly equal to the amount of
fill. For example, in order to create a flat area on a hillside,
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it is most effective to cut into the hillside and use the excess
soil as fill on the lower portion of the site.
One of the most compelling reasons to achieve a balance
of earthwork is its effect on project cost. Moving soil around
a site is less costly than either importing fill to the site or
hauling excess cut from the site. Generally, importing fill
onto the site costs more than hauling excess material away.
Balancing the earthwork helps keep costs under control and
often will help the finished site appear more in harmony with
its surroundings.
The relationship between cut and fill is simple in concept,
but other factors must be considered which complicate the
equation. These factors include
Soil conditions: It cannot be assumed that onsite material
may be used as fill, especially in areas where a load, such
as a building or wall, is to be placed. The designer must
F i g u r e 3 . 4 T Grading to improve aesthetics. (Courtesy of Maurice Nelischer, ed., Handbook of Landscape Architectural Construction, Vol. 1, 2nd ed. Washington, D.C.: Landscape Architecture Foundation, 1985.)
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verify the engineering characteristics of on-site soils
rather than assume they may be used as fill material.
Undercut: The cut/fill analysis should consider that the
proposed grade condition might not represent the total
amount of cut or fill. Undercut occurs if the soils are poor
and need to be excavated and replaced with good material.
Topsoil: In undeveloped areas of a site the existing natural grade (turf or forested conditions) can have nearly a
foot of topsoil. This material is required to be removed
(cut) but can be only be reused in proposed turf and
forested area.
Pavement and building slabs: Pavement and building slabs
have an associated thickness of material below grade, which
reduces required adjustments to the cut and fill calculations. Areas of the site with proposed pavement or building
slabs will decrease the amount of fill required in a fill condition and increase the amount of cut in a cut condition.
Shrink and swell: Soil, especially expansive clays, will
shrink and swell based on changing moisture conditions. The shrink and swell will modify the volumetric
calculations. Similarly, when reporting the amount of cut
for other soils the contractor should understand that in
situ volumes of material will be smaller than the volume
when stockpiled or transported in an uncompacted state.
Borrow across projects: A site developer (or the contractor) may have several concurrent projects with different
cut and fill needs. It may be desirable to intentionally
produce a need for fill on one project to dispose of
excess fill from another nearby project.
Additional details on necessary adjustments to earthwork
are described in Chapter 5.4. In all cases it’s important to consider the daylight limits for the site are proposed. The daylight
limit represents the point at which the proposed grade is equal
to the existing grade and generally represents the boundary of
the site or the limits of work. If a significant cut or fill condition exists near the limits of work, it’s possible that a retaining
wall will be required to prevent earthwork off-site.
Access. The site’s access grading for both pedestrians and
vehicles should be designed based on the proposed use of
the development. A residential site will have different access
requirements than an industrial site because the volume of
traffic and variety of vehicles. Large trucks are impeded by
steep grades or abrupt grade transitions, whereas passenger vehicles can traverse steep terrain without a significant
impact on vehicle behavior.
Grading can also create intentional vertical barriers
that limit access. A berm may be used to limit pedestrian access or a retaining wall may establish a separation between a roadway and an adjacent green space.
When possible, all pedestrian routes should meet ADA
accessibility requirements—existing steep roadways may
prohibit adjacent pedestrian routes and alternate routes
should be developed.
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Aesthetics. Drainage and earthwork considerations form
the basis of functionally sound grading; however, form
should follow closely to function. A site must appeal to
a user’s aesthetic sense in order to be successful. If a commercial development is set below the road elevation, the
visibility (and marketability) of the site is poor. If a home is
set too high above the road elevation, it may appear unwelcoming and require an uncomfortably steep driveway. When
designed properly, grading can transform a flat, featureless
site into a visually pleasing series of rolling landforms that
established a visual interest in the site and enhances the consumer’s experience.
Figure 3.4T illustrates how grading can be used to enhance
aesthetics. The grading itself can become the feature, as in
the creation of landforms where none exists [Figure 3.4T(a)].
Grading can also be used to influence what we see, by hiding
a visually undesirable element, as shown in Figure 3.4T(b),
or opening up a view, shown in Figure 3.4T(c). The landforms created for aesthetic reasons can simultaneously serve
functional roles. The landform may aid in the balancing of
earthwork by providing an on-site location to dispose of
excess soil or be used to direct wind away from buildings or
outdoor use areas.
However, using the grading to enhance the site aesthetically is not achieved by simply following formulas and rules,
it must be coupled with creativity and a thorough knowledge
of the site. Because most of these grading devices have an
impact on project costs, the aesthetic gain must be compared
to the extra construction expense. Sometimes the designer
can justify the expense, but must be prepared with lower cost
solutions if necessary.
Impacts of Grading and Earthwork. It is important to
consider the impacts of grading and earthwork during early
design stages before the final grading is commenced. Earthwork is often a major cost of a land development project and
often requires a significant amount of construction time.
When possible, a permit for clearing and rough grading may
allow for earthwork to occur while other site elements are
being final designed.
During construction it’s possible that undesirable conditions arise as rock is discovered, or unsuitable material is
identified—the grading plan should consider the site geotechnical reports to limit costly earthwork conditions. For
example, if the geotechnical report identifies shallow rock
conditions the grading plan should reduce the amount of cut
to prevent the need for blasting.
By removing vegetation from the site during the earthwork efforts of construction, the runoff from storm events
increases. Vegetation is naturally permeable and allows for
rainwater to percolate into the ground. Compacted dirt across
a construction site does not allow for drainage into the soil
and will instead sheet flow runoff across a site. This can damage the site and existing waterways. To avoid harming the
environment, this runoff is mitigated during construction
through appropriate erosion and sediment control measures,
which will be discussed in more detail in Chapter 5.7.
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Chapter 3.5
Stormwater Fundamentals
3.5.1. Introduction
The transformation of land results in a change in the hydrologic and hydraulic characteristics of the watershed during
construction and after the development is complete. The
natural environment allows for permeation of stormwater
runoff into the ground. Land development, however, has historically increased the amount of impervious area. The built
environment of structures, asphalt, and concrete decreases
the amount of natural permeable surfaces.
This development has resulted in an increase in the postdevelopment flow rates, the runoff volumes, and frequency of
flooding as well as the degradation of surface water quality. To
convey the increased runoff: systems consisting of curb and
gutter, storm pipes, and channels are typically developed to
safely convey the runoff through the developed basin. These
man-made conveyance systems directly increase flow velocity that decreases the basin time of concentration, resulting
in higher peak flow rates. This increase can be dramatic—
even moderate development conditions can increase the peak
discharge from two to five times higher than pre-developed
conditions. Perhaps more importantly, the volume of runoff
can be increased by several orders of magnitude.
Figure 3.5A depicts a hydrograph of the same size site before
and after development. In the figure, each line represents the
discharge rate of stormwater from the site, with the peak flow
occurring about midway through the storm. The volume of
runoff is illustrated in Figure 3.5A by the area under each curve.
Along with the increase in water quantity, urbanization
results in an increase in nonpoint source (NPS) pollutants.
Trace metals from galvanized downspouts, flashing and roofing materials, and other pollutants are washed into natural
channels. Other pollutants such as tire particulate, hydrocarbon products from pavement and fuels, and mechanical part
flakes can all end up in the surrounding streams, lakes, and
groundwater. The damaging effects of the many pollutants
might not be immediately apparent. It may be years before fish
and wildlife are affected by land use changes or before these
effects become noticed. Damaging effects from other pollutants such as phosphorus and nitrogen promote algae growth as
they are washed into natural water systems. If left unchecked,
eutrophication and water quality degradation are imminent.
Stormwater management (SWM) is the mechanism for
controlling stormwater runoff for the purposes of reducing
downstream erosion and flooding, and mitigating the negative effects resulting from urbanization. Many localities have
ordinances which require specific action to mitigate such
potential damage. Within the broad scope of the Clean Water
Act (as defined in Chapter 2.5) local authorities are compelled to implement measures such as sediment and erosion
control, stormwater quantity, and quality controls to diminish the negative effects of land use changes. Best Management Practices (BMPs) are those techniques used to control
NPS pollution. Local jurisdictions may also have additional
SWM performance requirements, commonly groundwater recharge, which can greatly affect the overall site SWM
strategy. While traditional SWM techniques centered on the
detention of stormwater runoff, current techniques look to not
only decrease the peak runoff rate but also the total volume of
runoff. To manage runoff rate and volume, these techniques
utilize infiltration or collect rainwater for reuse to replicate the
pre-development hydrologic conditions of the site.
The modern approach to SWM often involves a combination of facilities and technologies to address the various performance requirements in a given jurisdiction and
accommodate the unique characteristics of a site. This chapter discusses the basic concepts for hydraulic analysis, design
techniques for sizing and locating various SWM facilities,
and various types of BMPs that can be incorporated into the
land development program.
This text focuses on two of the most common methods of
hydrology; rational method and Natural Resources Conservation Service (NRCS) methodology. The rational method
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F i g u r e 3 . 5 A Hydraulic and hydrologic effects of urbanization.
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is best used for very small drainage areas and provides a
good estimate of peak runoff rate based on a few simple variables. The NRCS method, in reference to Technical Release
55 (TR-55), is a more robust methodology which uses synthetic storm models over a given duration (usually a 24-hour
storm) to evaluate the quantity of runoff as well as the rate.
Both methods are generally accepted and the differences
between each will be discussed in depth.
Storm Model Units. When evaluating stormwater management, it’s necessary to understand the source of input values
and the available precision of the calculated values. Hydrology and hydraulics analysis uses modeled storm events based
on historical rainfall data—but an actual rainfall event will not
match the modeled event. The values used for the storm models are determined by statistical methods based on the observed
events. When reporting the results and preparing a design, it’s
important to consider the available precision and the appropriate significant figures in documentation. In most cases, a rainfall model is built on a value of inches measured to the tenth
of an inch (usually three significant figures). As an example,
the calculated channel flow based on a rainfall intensity of 7.03
inches per hour and calculated as 145.17569 cfs (cubic feet per
second) should be reported as 145 cfs—anything more suggests
a measured accuracy that is not actually available.
An additional concept to consider is the unit of measurement for calculating runoff: cubic feet per second (or cfs).
This unit of measurement is an industry standard, but it may
be easier to visualize the equivalent volume for gallons. One
cubic foot is equal to about 7 and half gallons (7.48052).
For reference, a 24-inch-diameter concrete pipe at a slope of
2% can convey about 32 cfs or 240 gallons per second.
The volume of runoff is usually evaluated based on a given
rain event and measured as inches of runoff for an area to determine the cubic feet of runoff (for larger systems it’s also common to see a measurement of acre-feet for a unit of volume). In
the example shown earlier in Figure 3.5A, the volume of runoff
from a 10-acre site during a 10-year rain event was calculated at
172,000 cubic feet—this converts to 1,287,000 gallons, or almost
enough water to fill two Olympic sized swimming pools (each
Olympic pool is about 660,000 gallons).
When considering the velocity of stormwater, the common
unit of measurement is feet per second (ft/s). For a common
reference, each foot per second is equivalent to about 0.7 mile
per hour. For most pipe (or man-made) conveyance systems
the maximum recommended velocity is 10 to 20 ft/s (7 to 14
mph), depending on the material. For natural channel systems,
the maximum velocity is based on the land cover conditions
but is often less than 3 ft/s to prevent erosion of the channel.
By considering the units of measurement, it’s easy to
appreciate the magnitude of stormwater management systems. Large storm events can have disastrous consequences
to a community—and even small events can lead to continuous degradation of the natural environment.
3.5.2. Hydrologic Analysis
Site layout, grading, and drainage are interrelated: stormwater
management systems are integral parts of a site and require
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early planning and design to ensure sufficient space is allotted and an adequate outfall is available. Therefore, consideration of stormwater is important in the planning stages of a
project. To develop a successful stormwater management program (one that meets applicable jurisdictional guidelines and
adheres to client/developer expectations and project goals),
the engineer must understand the site hydrologic conditions
and options for stormwater management systems.
Hydrology focuses on the stormwater runoff from a site
(hydraulics focuses on conveyance, which is identified in
Chapter 3.6). The hydrologic conditions of a site are affected
by the characteristics of the site’s topography, soils, land cover
condition, and rainfall pattern. These characteristics will influence the rate, volume, and quality of runoff from a site. A site
is evaluated in a before and after (pre and post) development
condition, which documents how stormwater management
systems provide necessary treatment. This comparison determines the applicable stormwater management requirements
and reveals feasible strategies for managing site runoff.
Like all components of the design process, the hydrologic
analysis is often iterative, especially when it comes to assessing
the post-development condition. Hydrologic analysis performed
as part of the preliminary engineering effort is often updated,
checked, and rechecked as the site layout and grading scheme are
refined. A larger building, additional parking spaces, or additional
roadways will change the stormwater management requirements.
The preliminary engineering hydrologic analysis (usually
performed during concept and schematic design) should be
performed at a level of detail sufficient to confirm applicable
requirements and ensure that adequate facilities can be provided to meet those requirements. This typically means that
the size and location of the stormwater management systems
are estimated based on the proposed land cover condition.
Instead of calculating the exact square footage of buildings
the impervious area of a site can be estimate as a percentage—
estimate examples are provided later in this chapter.
In the early stages of design, the location of the outfall should
be identified to verify that the downstream system is adequate
for the post-development condition. A site may have multiple
outfall points, which all must be evaluated. The adequacy of an
outfall is determined by the geometric condition and the capacity. The geometric condition of the outfall should be at an attainable elevation such that stormwater can gravity-drain from the
site. The capacity of the downstream system should accommodate the post-development flow rate without causing flooding
or erosion. If the downstream system is not adequate it will
require additional SWM systems to control the site discharge,
improvements to the outfall system, or both.
The importance and validity of the preliminary plan is
established by ordinance and, in some jurisdictions, reaffirmed through proffers or development conditions often
related to specific infrastructure components such as stormwater management facilities. As such, it is important for the
preliminary engineering hydrologic analysis and related
design efforts to be accurate but conservative.
The following discussion presents a primer on the popular methodologies for hydrologic analysis. It is important to
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understand that certain sites may warrant detailed investigation
and at times, alternate methodologies may be required either
due to site conditions or jurisdictional design standards. The
site engineer should be aware of the jurisdictional requirements
prior to initiating the hydrologic study. The techniques, applications, and limitations of both the rational and NRCS methods
are identified in this chapter. Before introducing the hydrology
methods, it’s important to understand the components that are
common to both methodologies: rainfall and runoff.
Additional information and detailed computations and
examples are presented in Chapter 5.5—this chapter introduces the concepts and fundamentals.
Rainfall and Runoff. Every rainfall event is unique. Temporal and spatial precipitation vary seasonally as well as within
a storm event due to the prevailing climatic conditions at
the time of the storm. Just as every rainfall event is unique,
the resulting runoff from a storm event is also unique. The
runoff condition of the site will change as the site transitions
from pre-development to post-development.
The site characteristics such as the amount of vegetation,
land use, soil classification, topography, and other factors affect
the rate and quantity of runoff. When designing the individual
components of a storm drain system such as a road inlet, the
effects of the temporal and spatial distribution of runoff have
little impact. However, the temporal effects of runoff must
be considered when designing larger components, such as
stormwater management facilities or major conveyance systems. With larger drainage areas, the conveyance time could
take hours, which impacts how the peak runoff rate is calculated based on the corresponding temporal distribution of the
storm (refer to time of concentration content in this chapter).
Hydrologic data is historic by nature—rainfall data is collected through observation of a storm event. When calculating
runoff conditions for a storm event, the rainfall condition is
based on a theoretical storm model that’s derived from historic
events. The storm model represents the practical method for
analyzing the runoff condition of a given storm and the documented results should consider the source of inputs.
Exceedance Probability and Recurrence Interval. Generally, hydrologic events are predicted by stating their exceedance probability or recurrence interval. The exceedance
probability represents the likelihood that an event of specified
magnitude will be exceeded in each time period. Typically, the
time period is 1 year for most hydrologic events. Similarly, the
return period represents the average length of time that will
pass between events having the same magnitude. For example,
a 100-year frequency return period for a rainfall event means
that on the average, there is a 1% chance (1/100) that this rainfall event will be exceeded in any year. A 10-year frequency
return period rainfall event would on average have a 10%
chance (1/10) of being exceeded in any year, and so on.
Specifically, the recurrence interval is
Tr =
1
× 100
P
(3.5A)
where Tr is the recurrence interval and P is the probability
in percent.
03_Land_CH03_p125-304.indd 228
The concept of exceedance probability and recurrence
interval is often misinterpreted. A 100-year storm does not
mean that it will only occur once every 100 years. Likewise,
if a particular event occurs today, then it will not occur for
the next Tr years is not the proper interpretation of the recurrence interval. The recurrence interval represents the statistical average number of years between similar events given a
relatively long period of record.
Occasionally, it is necessary to determine the probability of a specific event being exceeded within a specific time.
The probability P of an event having a given return period Tr
occurring at least once in N successive years is given as

1
P = 1 − 1 − 
T

R 
N
(3.5B)
From this equation the probability of a 100-year storm
occurring at least once in 100 years is 63%. A distinction exists
between the probability of an event occurring at least once and
exactly once in a given time period. Another form of the risk
equation determines the probability of an event to occur a precise number of times in a given period. In this equation,
P=
 1
( N !)  
Tr 
I

1
1 − 
 Tr 
I !( N − I )!
N −I
(3.5C)
Here, I is the exact number of times the event with Tr occurs
in N successive years. From this equation, there’s almost a
19% chance of a 100-year storm occurring twice in 100 years.
A jurisdiction will often use the exceedance probability to prescribe the design storm for inlets, channels, and
stormwater management systems. A pipe system may be
required to demonstrate capacity for the 10 year storm,
while a pond should be checked to make sure the system
does not overflow during a 100-year storm event. It is good
practice to understand how larger storms would perform on
a site as well, even if the system is not designed to accommodate them.
3.5.3. Design Storms
A design storm is the defined result of a statistically estimated rainfall-runoff event used in the design of hydraulic
systems. Depending on the hydrologic technique selected,
the design storm can be inferred from point precipitation
depths (rainfall data), fabricated (synthetic) hydrographs,
or isohyetal maps using predetermined spatial storm patterns. It is important to note that the design storm is not
an actual storm of record. Rather, it is a fabricated storm
compiled from average characteristics of previous storm
events, and for convenience and standardization, most
review agencies dictate the design storm(s) for use in the
design process.
Every storm produces different peak discharges of runoff,
has different times to the peak discharge and consequently
different volumes of runoff. Therefore, a specific design
storm is often characterized by the following items:
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3.5
•• Duration—the length of time of the storm event (hours)
•• Depth—precipitation for the duration of the storm
event (inches)
•• Type—reference to NRCS distribution patterns (I, IA,
II, III)
•• Frequency—design storm based on exceedance probability (years)
•• Intensity—precipitation depth divided by duration
(inches/hour)
Each of these items by themselves will not provide enough
information to compute the hydrologic conditions from a
design storm. Duration is usually defined as a 24-hour storm
event, which matches NRCS synthetic rainfall distribution
models. The depth is the precipitation depth throughout the
entire storm duration. It is important to recognize that the
precipitation depth (and volume) is not necessarily equal
to the runoff depth (and volume). Runoff is the amount of
excess precipitation, that is, the amount of rainfall after all
abstractions, including infiltration, evapotranspiration, and
depression storage, have been accounted for.
The storm type, as defined by NRCS TR-55, identifies a
synthetic storm model that is assigned to a geographic region.
This is important to consider because the storm intensity will
vary over the duration. Even with the same 24-hour duration
and same precipitation depth the peak intensity of a storm
will be different in southern California than it is in Colorado.
Storm frequency is geographic specific, meaning that a 10-year
storm in Illinois does not have the same precipitation depth as
Florida (about 4 inches as opposed to about 10 inches).
The storm intensity references the peak intensity, which is
used to determine peak flow rate and evaluate channel capacity.
The peak intensity occurs near the middle of the 24-hour storm
(depending on the storm type) over several minutes. The peak
intensity of a storm is based on the depth of precipitation as well
as the duration and type of storm. A 24-hour storm duration
with a precipitation depth of 10 inches does not imply 0.4 inch/
hour (10/24) of a peak intensity because the intensity of rainfall
varies throughout the duration of the storm event.
The appropriate intensity used from a design storm is
determined based on the site characteristics. Intensity is a
function of the time of concentration for the site. A shorter
time of concentration will result in a higher intensity whereas
a larger time of concentration results in a lower intensity.
Table 3.5A provides general guidelines for recurrence
interval storms for selected hydraulic systems typical of
many local, state, and federal requirements. The duration is
specified by the local public agencies.
Time of Concentration. The time of concentration is the
time for water to flow from the most hydraulically remote
point of the drainage area to the outlet point. Recognize that
this does not imply the most distant point in terms of length.
Rather, it is considered as the longest flow time from some
point in the drainage area to the outlet point. For example, the
point most distant could be drained by storm sewers, which
would decrease the travel time to the outlet point, while an
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T A B L E 3 . 5 A Guidelines for Design Storms for
Various Hydraulic Systems
Hydraulic System
Design Recurrence Interval
Minor storm drain system
2- to 25-year
Major storm drain system
10- to 50-year
Road culverts crossing minor
streams
10- to 50-year
Road culverts crossing major
streams
25- to 100-year
Small on-site
detention/retention ponds
2-, 10-, 25-, 100-year
Large on-site or regional pond
100-year to PMF*
Floodplains on minor streams
10-year to 100-year
Floodplains on major streams
100-year+
*
Probable maximum flood.
area closer to the outlet point could travel over dense flat terrain, thus slowing it down. The hydraulically most distant
point would be the area travel through natural terrain (if it
takes longer than the storm system conveyance). When runoff
from the most hydraulically remote point reaches the outlet,
the entire catchment area is then contributing to the discharge.
The time of concentration is the sum of two components:
(1) the overland flow time (or inlet time) and (2) the channel
(or conveyance) time (Figure 3.5B). Overland flow is typically thought of as a flowing thin layer without any significant
depth, before it concentrates in defined swales and channels.
This could also be referred to as inlet time, since overland
flow is basically confined to a short stretch often draining to a
catchment, such as a street inlet. Channel time is that part of
the flow time when the runoff proceeds as concentrated flow
in perhaps irregular but well-defined channels. Often, flow
time is determined by computing the velocity of the stormwater within the channel over the length of the channel.
Figure 3.5B
Example of a flow path.
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The time of concentration varies according to hydraulic
characteristics of the watershed and the storm event itself.
Generally, the time of concentration used in analysis is never
less than 5 minutes—even for the smallest catchment and
nearly impervious ground surface. The time of concentration
for a developed project will often range from 5 to 15 minutes
where there are large impervious areas and pipe conveyance systems. When analyzing a natural area, the time of
concentration could be 15 minutes just for the first 100
TA BL E 3 . 5 B
feet of sheet flow on a mild slope. Large areas (hundreds
of acres) could easily have a time of concentration that
exceeds an hour.
There are numerous empirical methods to determine the
inlet time of concentration. The method selected depends
on the information available and the preferences dictated by
local review agencies and the site engineer. Some of the various methods are listed in Table 3.5B. Example computations
are provided in Chapter 5.5.
Summary of Time of Concentration Formulas
Method and Date
Formula for tc (min)
Remarks
Kirpich (1940)
tc = 0.0078L0.77S −0.385
L = length of channel/ditch from
headwater to outlet (ft)
S = average watershed slope (ft/ft)
Developed from SCS data for seven rural basins
in Tennessee with well-defined channel and steep
slopes (3–10%); for overland flow on concrete or
asphalt surfaces multiply tc by 0.4; for concrete
channels multiply by 0.2; no adjustments for overland flow on bare soil or flow in roadside ditches.
California Culverts Practice
(1942)

L3 
t c = 60 11.9 

H
L = length of longest watercourse (mi)
H = elevation difference between divide
and outlet (ft)
Essentially the Kirpich formula; developed from
small mountainous basins in California (U.S.
Bureau of Reclamation, pp. 67–71, 1973).
41.025(0.007 i + c ) L0.33
S 0.33i 0.67
i = rainfall intensity (in/hr)
c = retardance coefficient
L = length of flow path (ft)
S = slope of flow path (ft/ft)
Developed in laboratory experiments by Bureau of
Public Roads for overland flow on roadway and turf
surfaces; values of the retardance coefficient range
from 0.0070 for very smooth pavement to 0.012 for
concrete pavement to 0.06 for dense turf; solution
requires iteration; product i times L should be ≤500.
L0.5
S 0.33
C = rational method runoff coefficient
L = length of overland flow (ft)
S = surface slope (%)
Developed from airfield drainage data assembled
by the Corps of Engineers; method is intended for
use on airfield drainage problems, but has been
used frequently for overland flow in urban basins.
0.94 L0.6 n 0.6
i 0.4S 0.3
L = length of overland flow (ft)
n = Manning roughness coefficient
i = rainfall intensity (in/hr)
S = average overland slope (ft/ft)
Overland flow equation developed from kinematic
wave analysis surface runoff from developed
surfaces; method requires iteration since
both i (rainfall intensity) and tc are unknown;
superposition of intensity-duration-frequency curve
gives direct graphical solution to tc .
Izzard (1946)
0.385
tc =
Federal Aviation
Administration (1970)
t c = 1.8(1.1 − C )
Kinematic wave formulas
Morgali and Linsley (1965)
Aron and Erborge (1973)
tc =
SCS average velocity charts
(1975, 1986)
tc =
Overland flow charts in Figure 3-1 of TR 55 show
1 L
≤
average velocity as function of watercourse slope
60 V
and surface cover.
L = length of flow path (ft)
V = average velocity in feet per second from
Figure 3.1 of TR 55 for various surfaces
Source: Kibler, David F., ed. Urban Stormwater Hydrology Monograph 7. Copyright 1982 by the American Geophysical Union, Washington, DC.
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3.5
Often, large catchments will require the consideration
of several flow paths in determining which represents the
time of concentration. The flow path with the longest travel
time is typically selected for design, but this condition is best
suited for homogeneous drainage areas (i.e., consistent land
use and topography), which increase linearly with length.
Both the shape of the drainage basin and its homogeneity
affect the peak discharge at various points within the catchment. For those situations where catchment area and length
are not linearly related or when the catchment has widely
varied land use (e.g., a large grass area flowing to a parking lot), selecting the flow path with the longest tc does not
always produce the peak discharge at the specified location.
The time of concentration is important in hydrology, for
both the rational method and the NRCS method. Both hydrologic methods use the variable when determining runoff conditions based on the storm model. Professional judgement
should be used when selecting an appropriate time of concentration, and an early evaluation can provide insight into
stormwater management requirements.
Hydrographs. Runoff from a watershed is graphically shown
by a hydrograph, which is a plot of the discharge as a function
of time. In the most simplistic case a hydrograph has a rising
limb, which reflects rainfall characteristics, a crest segment,
and a recession curve that reflects watershed characteristics.
A hydrograph may be classified as a natural hydrograph, one
which is derived from observed data from a stream flow gage
or it may be classified as a synthetic hydrograph, one which is
derived from presumed characteristics related to the rainfall
Figure 3.5C
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Stormwater Fundamentals 231
and the watershed. The area under a runoff hydrograph represents the volume of runoff from the watershed.
The following parameters define the timing aspects of the
hydrograph (Figure 3.5C):
Time to peak (tp)—the time from beginning of runoff to
the peak.
Lag time (tL)—the time from center of mass of rainfall
excess to the peak rate of runoff.
Time of concentration (tc)—the time of equilibrium
of the watershed. On a hydrograph, tc is the time from
the end of excessive rainfall to the inflection point on the
recession limb.
Time base (TB)—total duration of the direct runoff
hydrograph.
As might be deduced from the hydrograph sketch and the
above definitions, the hydrograph shape is affected by
•• The intensity, duration, and distribution (both temporally and spatially) of rainfall
•• The size and shape of the watershed
•• Factors which influence the time of concentration
(land slope, channel length, and land cover/use)
A short time of concentration results in a higher peak discharge rate and a shorter time to peak while a long time of concentration results in a lower peak discharge rate and longer time
Elements of a hydrograph.
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F i g u r e 3 . 5 D Effect of basin shape and tc on hydrograph shape. (Modified for color and style,
courtesy of Martin P. Wanielista, Hydrology and Water Quality Control, New York: John Wiley & Sons.
Reprinted with permission of John Wiley & Sons, Inc.)
to peak (assuming the land cover conditions and drainage area
are constant). These concepts are illustrated in Figure 3.5D.
Generally, land development activities will decrease the
time of concentration and increase the imperviousness for
a basin. These changes decrease the time to peak discharge
as well as increase the peak discharge and the total runoff
volume; it is these increases which must be mitigated with
stormwater management methods. A low-impact development (LID) or sustainable design approach to stormwater
management seeks to replicate the pre-developed hydrologic
conditions and reduce these inherent increases related to site
development. An example of an LID is a bioretention system
(rain garden) or infiltration trench, whereas a traditional
method is an underground detention pipe. By employing a
03_Land_CH03_p125-304.indd 232
variety of SWM techniques or other site features that maintain
or even elongate the time of concentration or improves the
land cover condition, the structural SWM facilities required
can be minimized.
Unit Hydrographs. A unit hydrograph is defined as a runoff hydrograph generated from a unit depth of rainfall excess
occurring at a constant rate over a specified duration of time,
uniformly distributed over the watershed. The area under a
unit hydrograph represents one unit of runoff depth over the
entire watershed. Each unit hydrograph has a specific duration,
that is, time base, which represents the duration of the rainfall
excess. Therefore, a D-hour unit hydrograph is defined as the
hydrograph, which results from a storm with a constant rainfall excess of 1 inch over a duration of D hours (Figure 3.5E).
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Stormwater Fundamentals 233
F i g u r e 3 . 5 E Direct runoff from a unit hydrograph.
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F i g u r e 3 . 5 E (Continued )
The physical features of a watershed vary little from storm
to storm. Therefore, in unit hydrograph theory, a storm event
of equal duration but different intensity produces a direct runoff hydrograph with an equal base length and similar shape as
that of a unit hydrograph. If the ordinates of the direct runoff hydrograph are proportional to runoff volume, multiplying the ordinates of a unit hydrograph by the rainfall intensity
generates a hydrograph corresponding to that intensity.
For more complex unit hydrographs, it is necessary to
generate direct runoff hydrographs through multiplicationtranslation-addition procedures (convolution) to obtain
the direct runoff hydrograph. This can only be done if the
assumptions of linearity are valid, that is, the time base
remains constant regardless of the runoff depth. Figure 3.5E
shows how to obtain a direct runoff hydrograph using multiplication-translation-addition procedure.
3.5.4. Rational Method Hydrology
The rational method is best utilized for determining peak
flows for small drainage areas with homogenous land
cover conditions. Most localities have maximum restrictions on the applicability of the rational method, ranging
03_Land_CH03_p125-304.indd 234
from 20 to 200 acres. Even at 20 acres, the rational method
may not be appropriate if there is large variation in land
cover (nonhomogeneous). Other jurisdictions also place
time of concentration restrictions on the use of rational
method, limiting the maximum time of concentration to
60 minutes.
For small urban drainage areas, common in minor
storm drainage design, it is assumed that short-duration
high-intensity storms are the cause of flooding. For such
short-duration storms and small drainage areas, the rainfall
intensity is often assumed constant. The peak runoff rate
then occurs when the entire drainage area is contributing
to the runoff. If a storm of constant intensity begins instantaneously, the rate of runoff for the catchment steadily
increases until the entire drainage area is contributing to
the discharge at the outlet point. From then on, the drainage area is in equilibrium. All precipitation is converted to
runoff and the peak runoff remains uniform for the duration of the constant intensity rainfall.
Peak runoff from the rational method is given by
QP = CiA
(3.5D)
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3.5
where Qp is the peak discharge in cubic feet per second
(cfs), A is the drainage area in acres, C is a runoff coefficient
characteristic of the ground surface (0 < C < 1), and i is the
average rainfall intensity (inches/hour). The precision of the
peak discharge depends on the estimated values of C and i.
The average rainfall intensity is a function of the time of concentration of the drainage area. The units in this equation are
not consistent (cfs, inches/hour, acre) but the conversion of
inches/hour and acres is nearly one (1.0083), and common
practice is to ignore the conversion factor.
Runoff Coefficient. In Equation (3.5D), the product iA can
be considered as the inflow to the catchment while also representing the maximum possible runoff rate. The ratio of peak
discharge, Qp, to inflow, iA, is the runoff coefficient, C. This
coefficient can be considered as a lump-sum parameter that
accounts for abstractions (losses before runoff begins—mainly
interception, infiltration, and surface storage), antecedent runoff conditions (index of the runoff potential of the soil before
a storm event) and other variables affecting the runoff rate.
Table 3.5C identifies the ASCE’s (American Society of Civil
Engineers) version of the runoff coefficient and the standards
used in Austin, Texas, are shown as an example in Table 3.5D.
Note, the coefficient, C, is also a function of the recurrence interval of the storm. The reason for this function is an
attempt to approximate soil saturation conditions. For larger
storm events, it is understood that the soil has already been
saturated to such an extent that it no longer has the infiltration characteristics associated with everyday conditions.
Therefore, since the soil is saturated, the rainfall will produce
more runoff; the greater the saturation of the soil, the higher
TA BL E 3 . 5 C
Description of Area
Business
Downtown
Neighborhood
Residential
Single-family
Multiunits, detached
Multiunits, attached
Residential, suburban
Apartment
Industrial
Light
Heavy
Parks, cemeteries
Railroad yard
Unimproved
■
Stormwater Fundamentals 235
the C coefficient and hence, the greater the runoff. The NRCS
method, discussed in this chapter, has a similar mechanism for
adjusting a runoff factor based on a limit of initial abstraction.
Other localities account for the change in C coefficient versus recurrence interval by using a correction factor. For example, using the correction factor, Equation (3.5D) becomes
QP = C f CiA
(3.5E)
where the correction factor Cf varies by recurrence interval.
Comparing the City of Austin example in Table 3.5D, the C
coefficient would remain the same for all storms; however,
the Cf factor would change for storms greater than the 2-year
event. Cf would equal 1.066, 1.107, 1.178, 1.233, 1.301, and
1.370 for the 5-, 10-, 25-, 50-, 100-, and 500-year events,
respectively.
Use of hydrologic soil groups is more common in NRCS
hydrology; however, Table 3.5E is useful in that it correlates
the C coefficient to hydrologic soil groups and slope ranges
with various types of land use.
Whenever a single catchment area consists of several
areas with different C coefficients a weighted coefficient is
computed. The weighted coefficient is found by
m
Cw =
∑C A
i
i
i =1
(3.5F)
Ar
where Cw is the weighted C coefficient, Ai is the area of the
subarea with Ci coefficient, and AT is the total area of the
catchment.
Runoff Coefficients, C, Recurrence Interval ≤ 10 Years*
Runoff Coefficients
Character of Surface
Runoff Coefficients
0.70–0.95
0.50–0.70
Pavement
Asphalt or concrete
Brick
0.70–0.95
0.70–0.85
0.30–0.50
0.40–0.60
0.60–0.75
0.25–0.40
0.50–0.70
Roofs
Lawns, sandy soil
Flat, 2%
Average, 2–7%
Steep, 7% or more
Lawns, heavy soil
0.50–0.80
0.60–0.90
0.10–0.25
0.20–0.35
0.10–0.30
Flat, 2%
Average, 2–7%
Steep, 7% or more
0.05–0.10
0.10–0.15
0.15–0.20
0.13–0.17
0.18–0.22
0.25–0.35
Source: From “Design and Construction of Sanitary and Storm Sewers,” ASCE Manual of Practice No. 37, revised by D. Earl Jones, Jr., 1970.
*
For 25- to 100-year recurrence intervals, multiply coefficient by 1.1 and 1.25, respectively, and the product cannot exceed 1.0.
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TA BL E 3 . 5 D
Runoff Coefficients for Use in the Rational Method
Return Period (Years)
Character of Surface
2
5
Developed
Asphaltic
0.73
0.77
Concrete/roof
0.75
0.80
Grass areas (lawns, parks, etc.)
Poor condition (grass cover < 50% of the area)
Flat, 0–2%
0.32
0.34
Average, 2–7%
0.37
0.40
Steep, over 7%
0.40
0.43
Fair condition (grass cover on 50–75% of the area)
Flat, 0–2%
0.25
0.28
Average, 2–7%
0.33
0.36
Steep, over 7%
0.37
0.40
Good condition (grass cover > 75% of the area)
Flat, 0–2%
0.21
0.23
Average, 2–7%
0.29
0.32
Steep, over 7%
0.34
0.37
Undeveloped
Cultivated land
Flat, 0–2%
Average, 2–7%
Steep, over 7%
Pasture/range
Flat, 0–2%
Average, 2–7%
Steep, over 7%
Forest/woodlands
Flat, 0–2%
Average, 2–7%
Steep, over 7%
10
25
50
100
500
0.81
0.83
0.86
0.88
0.90
0.92
0.95
0.97
1.00
1.00
0.37
0.43
0.45
0.40
0.46
0.49
0.44
0.49
0.52
0.47
0.53
0.55
0.58
0.61
0.62
0.30
0.38
0.42
0.34
0.42
0.46
0.37
0.45
0.49
0.41
0.49
0.53
0.53
0.58
0.60
0.25
0.35
0.40
0.29
0.39
0.44
0.32
0.42
0.47
0.36
0.46
0.51
0.49
0.56
0.58
0.31
0.35
0.39
0.34
0.38
0.42
0.36
0.41
0.44
0.40
0.44
0.48
0.43
0.48
0.51
0.47
0.51
0.54
0.57
0.60
0.61
0.25
0.33
0.37
0.28
0.36
0.40
0.30
0.38
0.42
0.34
0.42
0.46
0.37
0.45
0.49
0.41
0.49
0.53
0.53
0.58
0.60
0.22
0.31
0.35
0.25
0.34
0.39
0.28
0.36
0.41
0.31
0.40
0.45
0.35
0.43
0.48
0.39
0.47
0.52
0.48
0.56
0.58
Note: The values in the table are the standards used by the City of Austin, Texas. Used with permission.
3.5.5. Intensity-Duration-Frequency Curves
The hydrologic procedure selected to establish the rainfall-­
runoff relationship determines what type of data is required
to generate the design storm. Simple types of computational procedures, such as the rational method, require
basic intensity-duration-frequency (IDF) curves, whereas
more sophisticated hydrologic approaches require hyetographs (time variation of precipitation) or hydrographs
(time variation of runoff) as input. Data specific to the
model selected is available from various public agencies.
The engineer should always check with the jurisdiction to
determine the applicable rainfall values to use for design
03_Land_CH03_p125-304.indd 236
purposes—the design storm values can vary between an
adjacent city or county.
IDF curves present hydrologic data for use as design storm
information. These curves show precipitation intensity on
the ordinate (y axis), duration along the abscissa (x axis), and
a series of curves representing individual storm frequencies.
The IDF curves are developed through statistical analysis of
long time series rainfall data. They graphically represent the
probability that a certain average rainfall intensity will occur
given a duration. Note: This is quite different from the misconception that they represent an actual duration or actual
time history of rainfall. A single IDF curve represents data
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Runoff Coefficients for the Rational Formula by Hydrologic Soil Group and Slope Range
A
B
C
D
Land Use
0–2%
2–6%
6% +
0–2%
2–6%
6% +
0–2%
2–6%
6% +
0–2%
2–6%
6% +
Cultivated land
0.08*
0.14†
0.13
0.18
0.16
0.22
0.11
0.16
0.15
0.21
0.21
0.28
0.14
0.20
0.19
0.25
0.26
0.34
0.18
0.24
0.23
0.29
0.31
0.41
Pasture
0.12
0.15
0.20
0.25
0.30
0.37
0.18
0.23
0.28
0.34
0.37
0.45
0.24
0.30
0.34
0.42
0.44
0.52
0.30
0.37
0.40
0.50
0.50
0.62
Meadow
0.10
0.14
0.16
0.22
0.25
0.30
0.14
0.20
0.22
0.28
0.30
0.37
0.20
0.26
0.28
0.35
0.36
0.44
0.24
0.30
0.30
0.40
0.40
0.50
Forest
0.05
0.08
0.08
0.11
0.11
0.14
0.08
0.10
0.11
0.14
0.14
0.18
0.10
0.12
0.13
0.16
0.16
0.20
0.12
0.15
0.16
0.20
0.20
0.25
Residential lot
Size 1⁄8 acre
0.25
0.33
0.28
0.37
0.31
0.40
0.27
0.35
0.30
0.39
0.35
0.44
0.30
0.38
0.33
0.42
0.38
0.49
0.33
0.41
0.36
0.45
0.42
0.54
Lot size ¼ acre
0.22
0.30
0.26
0.34
0.29
0.37
0.24
0.33
0.29
0.37
0.33
0.42
0.27
0.36
0.31
0.40
0.36
0.47
0.30
0.38
0.34
0.42
0.40
0.52
Lot size 1⁄3 acre
0.19
0.28
0.23
0.32
0.26
0.35
0.22
0.30
0.26
0.35
0.30
0.39
0.25
0.33
0.29
0.38
0.34
0.45
0.28
0.36
0.32
0.40
0.39
0.50
Lot size ½ acre
0.16
0.25
0.20
0.29
0.24
0.32
0.19
0.28
0.23
0.32
0.28
0.36
0.22
0.31
0.27
0.35
0.32
0.42
0.26
0.34
0.30
0.38
0.37
0.46
Lot size 1 acre
0.14
0.22
0.19
0.26
0.22
0.29
0.17
0.24
0.21
0.23
0.26
0.34
0.20
0.28
0.25
0.32
0.31
0.40
0.24
0.31
0.29
0.35
0.35
0.46
Industrial
0.67
0.85
0.68
0.85
0.68
0.86
0.68
0.85
0.68
0.86
0.69
0.86
0.68
0.86
0.69
0.86
0.69
0.87
0.69
0.86
0.69
0.86
0.70
0.88
Commercial
0.71
0.88
0.71
0.88
0.72
0.89
0.71
0.89
0.72
0.89
0.72
0.89
0.72
0.89
0.72
0.89
0.72
0.90
0.72
0.89
0.72
0.89
0.72
0.90
Streets
0.70
0.76
0.71
0.77
0.72
0.79
0.71
0.80
0.72
0.82
0.74
0.84
0.72
0.84
0.73
0.85
0.76
0.89
0.73
0.89
0.75
0.91
0.78
0.95
Open space
0.05
0.11
0.10
0.16
0.14
0.20
0.08
0.14
0.13
0.19
0.19
0.26
0.12
0.18
0.17
0.23
0.24
0.32
0.16
0.22
0.21
0.27
0.28
0.39
Parking
0.85
0.95
0.86
0.96
0.87
0.97
0.85
0.95
0.86
0.96
0.87
0.97
0.85
0.95
0.86
0.96
0.87
0.97
0.85
0.95
0.86
0.96
0.87
0.97
Source: Kibler, D.F., et al., 1982. Recommended Hydrologic Procedures for Computing Urban Runoff in Pennsylvania. Commonwealth of Pa. Harrisburg Pa.: Dept. of Environmental Resources.
*
Runoff coefficients for storm recurrence intervals less than 25 years.
†
Runoff coefficients for storm recurrence intervals of 25 years or more.
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from several different storms. The IDF is fabricated from
extracting rainfall depths from selected time segments of
longer storms. Procedures for constructing IDF curves are
discussed in McPherson (1978). These curves are mainly
used in conjunction with the rational method for determining peak runoff. See Figure 3.5F for a typical IDF curve.
IDF curves are available through many local agencies
such as the state highway departments and the NRCS. In
rare cases where IDF curves cannot be obtained, they can
be developed from current and applicable U.S. Weather
Service Maps or from frequency analysis using local rainfall
information.
Rational Method Limitations. The rational method provides
adequate results for computing peak discharges if it is used
properly, with an understanding of the underlying assumptions and limitations. Even with proper understanding of the
rational method, as the catchment increases in size, the results
Figure 3.5F
03_Land_CH03_p125-304.indd 238
become suspect due to the assumption of a steady uniform rain
over a catchment area. Additionally, the inherent uncertainties in the C coefficient are magnified as the catchment area
increases and homogeneity is less likely. There are different
applications for determining the upper limit of the catchment
size that can effectively utilize the rational method. Values of
20 acres, to 200 acres, and up to 1 square mile (640 acres) have
been proposed. Certainly for the relatively small catchments
(less than 20 acres) encountered in minor storm drain design
the rational method should be satisfactory for use.
The key element in using the rational method is proper
determination of the time of concentration. Due to the
hyperbolic shape of the IDF curves, a small error in tc (i.e.,
rainfall duration) causes large discrepancies on the intensity. If the estimated tc is less than the actual tc, the rainfall
intensity will be too high, resulting in a high Q. Another
important consideration, when performing the hydrologic
Intensity-duration-frequency curves.
25/03/19 5:10 PM
3.5
analysis, is the dynamics of the land use in the catchment.
For projects within a catchment undergoing development,
the runoff coefficient should represent the catchment as it
might ultimately appear, rather than current conditions.
To summarize, the basic assumptions in the rational
method are
•• Small drainage basins are appropriate, usually 20 acres
or less, and should have a homogenous land cover
condition.
•• Rainfall intensity is uniform and constant over the
catchment and the duration of this rainfall intensity
is at least as long as the time of concentration of the
catchment.
•• Peak rate of runoff occurs when constant rainfall
intensity falls on a catchment for as long, or longer
than the time of concentration.
•• Runoff coefficient is the same for each rainfall intensity and for all return intervals (or adjusted by event).
This is an assumption inherent in the original proposal by Kuichling in 1877. Runoff coefficients are
typically higher for the less frequent storms because
of the reduction effect of the rainfall abstractions.
Runoff coefficients are also increased for the higher
intensity rainfalls for the same reason.
•• Frequency of the peak discharge is the same as that of
the rainfall intensity for the given time of concentration. Although this may not be necessarily true due
to variations in surface conditions.
3.5.6. NRCS Methodology
For relatively small catchments the rational method can be
used to determine peak runoff discharge. However, designers
often prefer to use more sophisticated rainfall-runoff models. Although a more sophisticated model does not necessarily provide greater accuracy, there is greater flexibility for
calibrating the model to local observations. One such hydrologic model, developed by the NRCS, is widely accepted
and well documented. Underlying fundamentals of this
method are found in the National Engineering Handbook,
Chapter 4, Hydrology (NEH-4) and the computer program
documented in Technical Release 20 (TR-20) and Technical
Release 55 (TR-55) Urban Hydrology for Small Watersheds.
These documents are available from the Government Printing Office, Washington, D.C. and online.
The NRCS method is commonly used with computer
software because there are numerous steps in modeling
a storm event and computing peak runoff (as compared
to the rational method with relatively simple equations).
By modeling the entirety of the storm event, the NRCS
method can be used to determine volume of runoff or evaluating multiple sites with different land cover conditions.
The model allows for designing stormwater management
03_Land_CH03_p125-304.indd 239
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Stormwater Fundamentals 239
systems in addition to determining peak flow (for sizing
conveyance systems).
Many jurisdictions require NRCS methodology for the
design of larger SWM facilities, where downstream safety is
a major concern in the event of dam failure. Local agencies
usually require a hydrologic analysis of large storm events
such as the probable maximum precipitation (PMP), the
probable maximum flood (PMF), or a percentage of the
PMF. The PMF is the flood discharge, which may be expected
from the most severe combination of critical meteorologic
and hydrologic conditions that are reasonably possible in the
region. The PMP is, theoretically, the greatest depth of precipitation for a given duration that is physically possible over
a given size storm area at a particular geographic location at
a certain time of the year.
Runoff Curve Numbers. The curve number is analogous
to the runoff coefficient used in the rational method. It converts the mass rainfall to runoff and is based on such factors
as the hydrologic soil group (HSG), cover type, hydrologic
conditions, and antecedent moisture conditions. In part,
the curve number (CN) for a particular soil depends on the
HSG classification. Soils are divided into four hydrologic
soil groups, A, B, C, and D, according to their minimum
infiltration rate. Soils classified in hydrologic group A generally have high infiltration rates (sand), whereas the HSG
D has the lowest infiltration rates (clay). The cover type
describes the surface of the catchment, such as type and
denseness of vegetation and impervious or semi-impervious
pavements. It is determined from field reconnaissance,
aerial photographs, specialized photography (infrared,
etc.), and land use maps. Hydrologic condition (poor, fair,
or good) is a measure of the effects of the cover type on
infiltration and runoff.
Table 3.5F presents CN values for several types of soils
and cover types. It should be noted that these CN values are
based upon an average antecedent runoff condition. The
NRCS publishes soil surveys for the majority of localities
in the United States and contains soil classification information. The surveys are found in many different formats,
including databases and GIS layers. The NRCS is constantly
updating information available electronically; refer to the
United State Department of Agriculture (USDA) website for
the latest information.
Once the hydrologic soil group and the cover type and
antecedent runoff condition have been determined, a
weighted CN can be found by determining the areal coverage of each set of conditions and consulting an NRCS curve
number table. Figure 3.5G shows the HSG groups overlain on
a land use map. A soil’s map is used to identify the soil series,
which is then converted to a hydrologic soil group. This map
was created by referencing the HSG map onto the land use
map. The worksheet shown in Figure 3.5H is used to tabulate
the data and determine the composite CN.
The NRCS curve number is used to determine the depth
of runoff (using basic equations) and can be used within a
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TA BL E 3 . 5 F ( A )
Runoff Curve Numbers for Urban Areas1
CURVE NUMBERS FOR HYDROLOGIC SOIL GROUP
COVER DESCRIPTION
Cover Type and Hydrologic Condition
Average %
Impervious Area2
Fully developed urban areas (vegetation established)
Open space (lawns, parks, golf courses, cemeteries, etc.)3
Poor condition (grass cover < 50%)
Fair condition (grass cover 50–75%)
Good condition (grass cover > 75%)
Impervious areas:
Paved parking lots, roofs, driveways, etc. (excluding right-of-way)
Streets and roads:
Paved: curbs and storm sewers (excluding right-of-way)
Paved: open ditches (including right-of-way)
Gravel (including right-of-way)
Dirt (including right-of-way)
Western desert urban areas:
Natural desert landscaping (pervious areas only)4
Artificial desert landscaping (impervious weed barrier, desert
shrub with 1- to 2-in sand or gravel mulch and basin borders)
A
B
C
D
68
49
39
79
69
61
86
79
74
89
84
80
98
98
98
98
83
83
76
72
89
89
85
82
92
92
89
87
93
93
91
89
63
96
77
96
85
96
88
96
Urban districts:
Commercial and business
Industrial
85
72
89
81
92
88
94
91
95
93
Residential districts by average lot size:
1⁄ acre or less (town houses)
8
¼ acre
1⁄ acre
3
½ acre
1 acre
2 acres
65
38
30
25
20
12
77
61
57
54
51
46
85
75
72
70
68
65
90
83
81
80
79
77
92
87
86
85
84
82
77
86
91
94
Developing urban areas
Newly graded areas (pervious areas only, no vegetation)5
Idle lands.
1
Average runoff condition and Ia = 0.2S. For range in humid regions, use Table 3.5F(b).
The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition.
3
CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open-space cover type.
4
Composite CNs for natural desert landscaping should be computed based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CNs are
assumed equivalent to desert shrub in poor hydrologic condition.
5
Comparable CNs to use for the design of temporary measures during grading and construction should be computed based on the degree of development (impervious area
percentage) and the CNs for the newly graded pervious area.
Source: USDA, TR-55.
2
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Stormwater Fundamentals 241
Runoff Curve Numbers for Urban Areas (Continued )
CURVE NUMBERS FOR
HYDROLOGIC SOIL GROUP
COVER DESCRIPTION
Hydrologic
Condition1
A2
B
C
D
Poor
Fair
Good
68
49
39
79
69
61
86
79
74
89
84
80
Meadow-continuous grass, protected from grazing and generally
mowed for hay
—
30
58
71
78
Brush-brush-weed-grass mixture with brush the major element2
Poor
Fair
Good
48
35
303
67
56
48
77
70
65
83
77
73
Woods-grass combination (orchard or tree farm)4
Poor
Fair
Good
57
43
32
73
65
58
82
76
72
85
82
79
Woods5
Poor
Fair
Good
45
36
303
66
60
55
77
73
70
83
79
77
—
59
74
82
86
Cover Type
Pasture, grassland, or range-continuous forage for grazing1
Farmsteads-buildings, lanes, driveways, and surrounding lots
1
Poor: <50% ground cover or heavily grazed with no mulch.
Fair: 50–75% ground cover and not heavily grazed.
Good: >75% ground cover and lightly or only occasionally grazed.
2
Poor: <50% ground cover.
Fair: 50–75% ground cover.
Good: >75% ground cover.
3
Actual curve number is less than 30: use CN = 30 for runoff computations.
4
CNs shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CNs for
woods and pasture.
5
Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.
Fair: Woods are grazed but not burned, and some forest litter covers the soil.
Good: Woods are protected from grazing, and litter and brush adequately over the soil.
Source: USDA, TR-55.
computer model to determine the peak runoff for the site
(traditional tabular methods are also an option). Examples of
using curve numbers are provided in Chapter 5.5.
Rainfall Models. The NRCS has developed four synthetic
rainfall distributions, which are indicative of the rainfall
intensities inherent to geographic regions of the United
States. These four standard rainfall distributions, labeled
type I, IA, II, and III, have been developed from numerous publications. Since most rainfall data is reported on a
24-hour basis, the NRCS used 24 hours as the duration for
these distributions. The location of the peak rainfall intensity (early, center, or late peaking) in each storm is intended
03_Land_CH03_p125-304.indd 241
to mimic the location of the peak intensity for the particular region of the United States. For example, peak intensities for type I and IA storms occur around 8 hours, similar
to the storms in the far western part of the United States.
Type II and III storms have peak intensities occurring
around the midpoint of the duration. Specific geographical
areas are shown in Figure 3.5I.
Computer Models for NRCS Method. Computer software
applications have made hydrologic modeling easier and
quicker. The designer should have a strong understanding
of the hydrologic processes as well as knowledge of the fundamental limitations of the programs. The reliability of the
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F i g u r e 3 . 5 G Hydrologic soils designation on topographic mapping.
output from any computer program is only as good as the
input data. The designer should be familiar with the inherent
assumptions associated with the model as well as the numerical techniques used to simulate the model.
A number of computer models are available, well documented and supported, and powerful in terms of their
ability to perform hydrology and manipulate hydrographs.
These models may be used to generate hydrographs from
either synthetic or historical design storms, combine
hydrographs together, and perform storm drain design and
channel and pond routings. The type of computer model
03_Land_CH03_p125-304.indd 242
selected depends on the user, available data, and possibly any preferences by the reviewing agencies. The NRCS
WinTR-20 is a popular application, but there are numerous
options available.
3.5.7. Stormwater Management
To mitigate the effects of development, stormwater management systems are usually required for new projects.
These systems provide a storage volume that mitigates peak
flow conditions to reduce the rate of flow (and sometimes
the volume) from a storm event during post-development
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Stormwater Fundamentals 243
F i g u r e 3 . 5 G (Continued )
conditions. Designing stormwater management facilities should be considered during the concept design
to determine the required site area for the system. This
preliminary engineering effort will help the designer to
understand the characteristics of the site as they prepare
initial plans. In order to size the facilities, it is important
to determine the specific performance requirements for
the facility. As mentioned previously, these may include
water quality or pollution removal, groundwater recharge,
and quantity control (detention/retention) requirements
for the proposed development. These requirements vary
greatly from region to region through the United States
and even at the state and local levels. Although the exact
configurations and grading of such facilities are subject to
change during final engineering, it is important to ensure
that adequate provisions have been made to accommodate
and locate these facilities to achieve the required stormwater management objectives.
Stormwater management facilities are commonly grouped
into several facility types which are selected based on site
considerations, performance requirements, aesthetics, and
other factors (cost). These facilities usually fall into one of
the following types:
03_Land_CH03_p125-304.indd 243
•• Detention basin (i.e., dry pond for the temporary
impoundment of surface water runoff)
•• Retention basin (i.e., wet pond that maintains a permanent pool of water with additional storage volume
above the permanent pool for detaining runoff)
•• Infiltration facilities
•• Reuse systems (i.e., cisterns for irrigation or building
graywater)
•• Structural facilities (i.e., underground manufactured
system)
Exact design parameters for each of these facility types
may vary depending on local requirements, and often combinations of these generalized facility types are used together
to achieve performance goals. For instance, a stormwater
management facility may employ infiltration of runoff as
a means to provide groundwater recharge and water quality functions, while providing controlled outflow for larger
storms. Similarly, a wet pond may be used for water quality
treatment in conjunction with extended detention to reduce
peak runoff rates. If runoff volume control is required, the
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F i g u r e 3 . 5 H Runoff curve number worksheet. (Source: TR-55.)
post-development stormwater management program will
have to include permanent storage facilities (retention),
infiltration facilities, stormwater harvesting/reuse facilities
(typically structural), or some combination of the three in
order to minimize the total volume of runoff leaving the site.
The designer must evaluate and determine the most appropriate facility or combination of facilities to achieve the
applicable performance requirements.
Locating SWM Facilities. The location of SWM facilities
should be integrated with the site design, either to minimize
the impact on the development project or to enhance the
development in terms of function and aesthetics. It’s important
to understand the scale, location, and strategy of stormwater
management systems during early design phases. If possible, a
03_Land_CH03_p125-304.indd 244
SWM facility should be located so that runoff will drain into it
naturally, without requiring additional engineering measures
such as storm sewers or channels to artificially force drainage
divides. The most economical design for any SWM facility is
one that requires the least earthwork and structural components. Further, facilities should be located with respect to outfalls in order to maintain natural (existing) drainage divides as
well as protect and improve the condition of outfalls through
adequate capacity analysis and erosion reduction.
When designed properly, stormwater management systems can also enhance the site aesthetics. For large sites, a wet
pond can act as a prominent feature of the site (Figure 3.5J).
In smaller sites, bioretention systems (rain gardens) can provide stormwater management benefits while also adding to
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Stormwater Fundamentals 245
F i g u r e 3 . 5 I Approximate geographic boundaries for NRCS rainfall distributions. (Source: USDA, TR-55.)
the aesthetic character of the site. The shape of these systems
may also benefit from a natural formation, as opposed to an
engineered geometric shape.
Estimating the Volume of Storage Required. When initially
sizing an SWM facility, the required storage volume to meet
detention requirements is unknown. An initial estimate of the
required storage volume may be made based on the inflow
hydrograph and the required outflow rate. The amount of storage required for a given design storm is equal to the representative volume between the inflow and outflow hydrographs.
To obtain a first estimate of the storage required, the outflow hydrograph can be approximated by drawing a straight
line from the beginning of substantial runoff on the inflow
hydrograph to the point on the receding limb corresponding
to the allowable peak outflow rate. Alternatively, TR-55 provides a dimensionless graph relating the ratio of storage volume to runoff volume to the ratio of peak outflow discharge
Figure 3.5J
03_Land_CH03_p125-304.indd 245
Example of a stormwater management facility.
to peak inflow discharge for the four types of synthetic
storms (Figure 3.5K).
Once an initial estimate of the required volume has been
made and the location determined, a preliminary grading
plan (for earthen or surface facilities) or volumetric design
(for structural or infiltration based facilities) can be performed. This usually occurs during concept and schematic
design phases. In preparing the grading plan, the objective
is to obtain the preliminary storage volume while keeping in
mind such things as minimizing earthwork, nominal height
requirements of the embankment, depth and clear height
(confined space) limitations for safety, sediment storage,
aquatic vegetation, depth to groundwater table, cost and aesthetic considerations.
Many stormwater management system design software
packages can perform preliminary sizing estimates for
detention basins, utilizing the above methods. These can
be helpful in streamlining the evaluation of various basin
designs. The graph published by NRCS in TR-55 also provides a volume estimate that can be used with an anticipated
depth to determine a size requirement. For instance, if the
estimated storage volume is computed as 10,000 cf and the
outfall elevation allows for a system of 5 feet deep, a land
area of about 2000 sf would be required. When estimating
the size of a system, it’s important how the size is impacted by
grading, freeboard requirements (additional system depth),
dam height limitations, or pretreatment requirements. For
instance, once the grading and freeboard parameters are
considered the estimated land area should be larger than the
simple computation of volume divided by depth.
3.5.8. Stormwater Quality Control
The Clean Water Act (as introduced in Chapter 2.5) regulates the restoration and maintenance of the chemical, physical, and biological integrity of the Waters of the
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Approximate detention basin routing for synthetic rainfall distribution. (Source: USDA, TR-55.)
United States. When first enacted, the Clean Water Act
was primarily aimed at point-source discharges. However,
as point-source discharges decreased, the awareness of
the detrimental effects from non-point-source discharges
increased. NPS discharges are, as the name implies, pollutant discharges emanating from a dispersed area. Figure 3.5L
illustrates some of the NPSs. One of the difficulties with
standardizing and implementing specific controls for NPSs
of pollution is that many of the sources are transient with
respect to time.
In 1978, the EPA provided funding and guidance to a
5-year study called the Nationwide Urban Runoff Program
(NURP). NURP studied the runoff from commercial and
residential areas across the United States. These studies concluded that the effects of urban runoff on receiving water
quality are highly site specific. They depend on the type, size,
and hydrology of the water body, the characteristics of the
runoff quantity and quality, the designated beneficial use of
the receiving water, and the concentration levels of the specific pollutants that affect that use. Certain types of water
bodies are more vulnerable to NPS pollution than others.
For example, lakes, reservoirs, and estuaries, which have long
residence times, may be subject to accelerated eutrophication because pollutants and sediment may be retained, leading to nutrient buildup.
For jurisdictions that have a municipal separate storm
sewer system (MS4s) stormwater is ultimately discharged
03_Land_CH03_p125-304.indd 246
directly to the natural waters without being processed
through a treatment plant (as is seen with wastewater). The
EPA regulations focus on stormwater quality control for a
constructed site as well as management during construction
activities (often referred to as erosion and sediment control,
described in detail in Chapter 5.7).
NPS Pollutants. NPS pollutants include sediments, oxygen
demand, bacteria, nutrients, metals, and other toxic chemicals. In identifying the source and the type of pollutant, the
engineer is better able to recommend a suitable best management practice (BMP) and its appropriate placement in context of the site and outfall. The national distribution of the
following described selected primary types of NPS pollutants
is shown in Figure 3.5M.
Sediment is one of the largest contributors to NPS pollution by volume. Too much sediment in the receiving waters
may cause fish gills to clog; reduce the size of spawning areas;
block sunlight, preventing plant growth; reduce organism
species and numbers because of food-chain perturbations;
and reduce aesthetic values. Construction sites and agricultural areas are primary generators of sediment. Sediment
buildup in existing ponds (including manmade stormwater
management system) will reduce the volume of the systems
and decrease the effectiveness.
Oxygen demand, free oxygen, or dissolved oxygen (DO)
is necessary in water to maintain aquatic life. DO is an
indicator of the health of lakes and streams. If the oxygen
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F i g u r e 3 . 5 L Nonpoint pollutant sources. (Adapted from Martin P. Wanielista, Stormwater Management, New York: John Wiley & Sons. Reprinted
with permission of John Wiley & Sons, Inc.)
demanding bacteria exceed the oxygen replenishing algae,
the DO will be depleted. DO is consumed by microorganisms as they decompose organic matter. A low-DO can lead
to fish death and to a reduction in aesthetic values.
Common bacteria are coliform, fecal coliform, and specific pathogens, such as Shigella, Salmonella, and Clostridium.
Bacteria, in addition to causing a low DO, may be a health
hazard. Generally, surface waters are tested for coliform bacteria. While coliform bacteria are not pathogenic, their presence is an indicator that more pathogenic organisms may
03_Land_CH03_p125-304.indd 247
be present. Sources include animal droppings, garbage, and
sanitary wastewaters.
Nutrients are chemicals that stimulate the growth of
algae and water plants, which in excess can contribute to
the degradation of lake and stream water quality. Micronutrients are nutrients that are needed in very small quantities.
Nitrogen and phosphorus are the most common micronutrients. Water quality problems that result from excess
nutrients include algal scums, water discoloration, odors,
toxic releases, and overgrowth of plants. Sources include
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F i g u r e 3 . 5 M Primary types of NPS pollution in lakes in the United States.
(Courtesy of Engineers & Surveyors Institute Northern Virginia.)
gardens, lawns, golf courses, and other areas that are frequently fertilized.
Metals are widely varied and may be present in stormwater. An ingestion of excess metals may cause health problems
and birth defects; this is a primary concern in waters used
for recreational purposes and in terms of fish and seafood
cultivation. The most common metals in urban runoff are
lead (Pb), copper (Cu), and zinc (Zn). Sources include flashing, gutters, downspouts, brake linings from vehicles, paints,
catalytic converters, and tires.
Other toxic chemicals found in surface waters may include
phenols and creosols (wood preservatives), pesticides and
herbicides, oils and greases, petroleum products, and many
other manufactured chemicals.
Because of laws such as the Clean Water Act, designers are often required to reduce the amount of pollutants
leaving a development area to some percentage of predevelopment levels. This generally involves quantifying
the pre- and post-development pollutant loads from some
key pollutants (frequently nutrients). The method for calculating pollutant loads is identified in Chapter 5.5. The
engineer then attempts to reduce the post-development
loads by implementing some form of best management
practice (BMP).
Best Management Practices. Best management practices
(BMPs) are policies, practices, procedures, and structures
implemented to mitigate the adverse impacts to surface water
quality resulting from development (they may also provide a
benefit to volume or water quantity). There are many different BMPs that can be used within a site and the appropriate
03_Land_CH03_p125-304.indd 248
BMP will be based on site characteristics and the jurisdiction’s pollutant removal goals. In terms of land development
engineering, either structural or nonstructural BMPs may be
utilized. Structural BMPs include such controls as extended
detention ponds, dry ponds, infiltration trenches, shallow
marshes/wetlands, porous pavements, and water quality
inlets. Nonstructural BMPs include street cleaning, fertilizer
application control, and certain vegetative practices such as
grass swales and filter strips.
The primary mechanisms for pollutant removal in BMP
facilities are
1. Settling of pollutants
2. Filtering pollutants
3.
Infiltration of soluble nutrients through the soil
profile and biological and chemical stabilization
of nutrients
A list of common systems is included in Appendix Chapter 7.2.
The selection for and effectiveness of a BMP facility
depend on numerous site conditions such as climate, watershed size, soil permeability, ground slope, subsurface conditions (e.g., bedrock and groundwater), and land use to name
a few (Figure 3.5N). Figures 3.5O through 3.5Q recommend
various restrictions to specific BMP facilities.
Another factor influencing the design and selection
of BMPs is the governing criteria for efficiency. Local criteria prescribe the allowable post-development pollutant
loading. Typical policies range from “no net gains,” that is,
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F i g u r e 3 . 5 N BMP selection criteria.
249
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F i g u r e 3 . 5 O Watershed area restrictions for BMPs. (Courtesy of Thomas R. Schueler, Controlling Urban Runoff:
A Practical Manual for Planning and Designing Urban BMPs, Washington, D.C.: Metropolitan Washington Council of
Governments, 1987.)
F i g u r e 3 . 5 P Soil permeability restrictions for BMPs. (Courtesy of Thomas R. Schueler, Controlling Urban Runoff: A
Practical Manual for Planning and Designing Urban BMPs, Washington, D.C.: Metropolitan Washington Council of Governments,
1987.)
post-development pollutant loading must be less than or
equal to pre-development pollutant loads, to reducing the
post-development pollutant loads to a specific percentage of the pre-developed conditions. Other criteria may set
the allowable loadings on a site-by-site basis, which may be
contingent on location within a water or sewer district. The
03_Land_CH03_p125-304.indd 250
pollutant removal efficiencies shown in Figure 3.5R might
be considered for ideal conditions. These values are to be used
as a guide since some studies have shown substantial differences from these efficiencies. Of the types of BMP measures
listed in Figure 3.5R the most effective types of facilities,
according to a study done in Maryland (John, 1982), are the
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Figure 3.5Q
Other common restrictions on BMPs. (Courtesy of Thomas R. Schueler, Controlling Urban Runoff: A Practical Manual for Planning
and Designing Urban BMPs, Washington, D.C.: Metropolitan Washington Council of Governments, 1987.)
wet ponds, extended detention ponds, artificial marshes, and
infiltration trenches (Table 3.5G).
Settling. For a BMP to use settlement practices the stormwater is detained in a basin and slowly drained over 24 to
48 hours, which provides time for the pollutants to settle.
Settling column experiments have been performed to estimate the removal of pollutants from urban runoff. Suspended sediment removal frequently forms the basis of BMP
designs. It should be noted that the efficiency of removal of
pollutants through settlement is related to
1. The particle size of the pollutants (which affects the
settling velocity)
2. The velocity of flow through the storage area
3. The depth and total storage volume available (related
to the hydraulic residence time, or the length of time
during which settling may occur)
Examples of BMPs that use settling practices are wet ponds
and dry ponds, but some filtering systems also rely on settling practices in addition to filtration.
Filtering. Filtering mechanisms allow runoff water to
pass through an engineered media, such as sand, gravel,
or a proprietary manufactured filter. Unlike a practice that
retains runoff for to allow settling, a filter system may not
reduce the discharge rate of stormwater runoff. Many of the
filtering systems require extensive maintenance to replace
the filters after a certain number of years. These systems are
commonly designed as subsurface systems (as opposed to
03_Land_CH03_p125-304.indd 251
most settling systems), which increases cost but decreases
the land area occupied by the BMP. Underground systems
can usually be installed below parking areas or private road
networks.
Examples of filtering systems are sand filters, tree box soil
media filters, porous pavers, and bioretention systems.
Runoff Reduction. Runoff reduction systems provide
quality control by removing the runoff volume from entering
the downstream waterway. Runoff reduction can be achieved
through infiltration, where runoff is collected and allowed to
seep back into the soil—this condition relies on soil testing to
verify the soils can infiltrate the stormwater at an acceptable
rate. Stormwater can also be reused within the development.
Water reuse is often implemented for irrigation systems or
nonpotable uses (washing vehicles, lavatory water, etc.).
Many vegetative BMP systems have inherent runoff reduction benefits because of evapotranspiration and vegetative
uptake to support plant growth.
Examples of runoff reduction include infiltration
trenches, water reuse cisterns (irrigation or building reuse),
and vegetated BMPs.
REFERENCES
John Galli, Analysis of Urban BMP Performance and Longevity in
Prince George’s County, Maryland, Washington, D.C.: Metropolitan Washington Council of Governments, 1982.
McPherson, M.B. 1978. The Design Storm Concept. Urban Runoff
Control Planning Miscellaneous Report Series. U.S. Environmental Protection Agency. Washington, DC: U.S. Government
Printing Office.
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F i g u r e 3 . 5 R Comparative pollutant removal of urban BMP designs. (Courtesy of John Galli, Analysis of Urban BMP Performance
and Longevity in Prince George’s County, Maryland, Washington, D.C.: Metropolitan Washington Council of Governments, 1982.)
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TA BL E 3 . 5 G
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Stormwater Fundamentals 253
Summary: General Attributes of BMP Systems Field Surveyed
I. BENEFITS
Factor
Porous
Grass
Infiltration Pavement and Infiltration Filters/ ED Dry Wet Artificial Pocket
Oil/Grit
Dry
Trenches
Dry Wells
Basins Swales Ponds Ponds Marshes Wetlands Separators Ponds
Downstream channel
protection
●
○
○
○
●
●
●
●
○
○
Removal of particulate
pollutants
●
○
●
○
●
●
○
●
○
○
Removal of soluble
pollutants
●
○
○
○
○
●
●
○
○
○
Aquatic/wildlife habitat
creation
○
○
○
○
○
●
●
○
○
○
Wetland creation
○
○
●
○
○
●
●
●
○
○
Thermal impact
protection
●
●
●
○
○
●
○
○
○
○
II. DISADVANTAGES
Soils limitations
●
●
●
●
●
●
●
●
○
●
Maintenance
requirements
●
●
●
●
●
●
●
●
●
●
Space consumption
●
○
●
●
●
●
●
●
○
●
Public safety hazards
○
○
●
○
●
●
●
●
○
●
Functional life/reliability*
○
○
○
●
○
●
●
○
○
○
(Source: Courtesy of Galli, John. 1982. Analysis of Urban BMP Performance and Longevity in Prince George’s County, Maryland. Washington, D.C.: Metropolitan Washington
Council of Governments, 777 No. Capital St. NE, Suite 300, Washington, DC 20002-4226, 202/962-3256).
○ Low
● Moderate
● High
*
General ability to provide water quality/quantity control benefits for 5 years or more without regular sediment/trash removal.
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OAKWOOD BEACH FLOOD RESILIENCY STUDY WITH NATURE BASED INFRASTRUCTURE
Location: Staten Island, NY
Client: New York State Department of Environmental Conservation (NYSDEC) through the New York State Office of
General Services (NYSOGS)
Completion Date: April 4, 2014 (Design)
Case Study: After Superstorm Sandy impacted the Oakwood Beach area, the New York State Department of Environmental Conservation (NYSDEC) had the opportunity to infuse natural infrastructure solutions into USACE’s hard
infrastructure coastal flood protection system. The low-lying portion of Oakwood Beach on the south shore of Staten
Island, New York is subject to flooding from coastal storm surge and heavy rainfall.
For this high-visibility project, which was mentioned in the NYS Governor’s State of the State address, Dewberry
worked with stakeholders to prepare a Feasibility Study and Conceptual Design Plan for a combination of natural
and gray infrastructure to provide storage for a 100-year rainfall event and reduce damages from the 500-year coastal
storm event. This comprehensive conceptual design plan would provide environmental/recreational benefits and
help to improve flood resiliency by protecting 1,843 houses along with DEP’s critical infrastructure within Oakwood
Beach community.
Dewberry’s study evaluated stormwater Best Management Practices (BMPs) and freshwater and tidal wetland restoration opportunities in concert with storm damage reduction strategies. Coastal hydrodynamic and wave modeling
assessed tidal wetland restoration alternatives. Hydromorphic assessment and hydrology and hydraulic analyses were
also performed. Alternatives were based on availability of parcels, wetland restoration benefits, ability to manage stormwater runoff, mitigation of storm surge, and resiliency offered from projected sea level rise.
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Chapter 3.6
Utility Fundamentals
3.6.1. Introduction
Most urban land development projects will be served by
public utility systems, whereas smaller rural sites will have
private on-site systems. The site engineer is typically focused
on the design of the storm, sanitary, and sewer utility systems
for a site. Other utilities, such as power, gas, and communication, are often coordinated with the utility provider but may
be designed by the site engineer. Large facilities, such as a
university campus or hospital, may have a central plant with
steam, chilled water, and hot water systems. There is generally one public utility provider for a region’s water, sewer,
and power, but there may be more than one provider for
communication.
The availability and existing capacity of utilities, such as
sanitary and water, can impact the development potential for
a site. If there is no public water or sewer available near a new
residential community the development will need large lots
to accommodate septic systems and well systems. In some
cases, a public utility may be available, but the capacity would
not support the desired development—in this case the developer would need to consider upgrading the system (which
could be prohibitively expensive) or they may be required to
decrease the desired density.
Each utility system has individual design, construction,
maintenance, and operation requirements. Each utility provider and jurisdiction will have a set of requirements, and
often a set of unique details, for the utility systems. In dense
development projects, one of the challenges is to accommodate the utility systems within a confined space. Water and
sanitary systems are generally smaller diameter pipes (less
than 10 inches is large enough to serve most developments).
Storm drainage systems usually have a 15-inch minimum
pipe size, and can quickly grow to 60 inches—very large
systems may require larger pipes (96 inches is a common
maximum) or box culverts, which range from 4 × 4 feet up
to much larger sizes.
In addition to the physical size of each utility, there are
separation requirements (both horizontal and vertical). A
public utility may also require an easement, which can vary
from 10 to 25 feet in width along the center of the utility.
For major utility systems, such as power transmission, the
easements can be over 100 feet in width. These easements
are required to protect the utility system and accommodate
maintenance activity. The easements often include restrictions on development. Many easements must be clear of
vegetation, buildings, or other site furnishings. These conditions are especially challenging as utility systems compete
with other site requirements like landscaping and screening.
The required separation and easement requirement should
be determined early in the design phase.
Utility design should be carefully coordinated with building requirements. The connection between a building and
the site is critical, and each discipline will likely have different design requirements. An estimate of utility requirements
is important in the early phases of design so that the capacity
can be verified. If capacity does not exist in the utility systems, there may be a need for costly upgrades and in some
cases, it may not be possible to modify the system.
This chapter is separated into four parts: (A) Storm
Drainage, (B) Sanitary Sewer, (C) Water Distribution, and
(D) Dry Utilities. These utilities are often the focus of site
engineering design work, but all other utilities should be
considered and carefully coordinated. This chapter introduces the terminology, materials, and design strategies of
utility systems. Design examples and detailed equations are
provided in Chapter 5.6.
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PART A—STORM DRAINAGE
3.6.2. Introduction
After the planning of the site layout and initial hydrologic
analysis of the site, the horizontal placement (and later, the
sizing) of the storm drainage system is performed. Conveyance systems may vary from nonstructural systems
such as grass channels to structural systems comprised of
pipes, culverts, and inlets. The selection of conveyance systems may vary due to development conditions, low-impact
development (LID) goals, available site area, topography,
soils, as well as local jurisdictional requirements. The
implementation of LID techniques is often encouraged
for storm conveyance systems as additional measures of
stormwater management. The design team must evaluate
and consider these objectives and incorporate these various
design elements as appropriate. In some cases, the other site
infrastructure may govern the type of conveyance system
used. For instance, a road with curb and gutter will require
inlets and pipes, while a road with just a paved shoulder
would require a roadside ditch.
Once the type of conveyance and collection system has
been established, these systems are incorporated into the site
plan as the grading and drainage plan is developed. But this
may require iterations of design to determine the best storm
network layout. Preliminary grading leads to the placement
of stormwater management facilities, which leads to the
design of the conveyance system. But the ultimate routing
of the conveyance system may alter the stormwater management facility locations, which would then change the grading
of the site. The site design and buildings can change, especially during the site analysis and preliminary engineering
phase of a project, which may require edits to the storm system. Figure 3.6A depicts the final design for a storm drainage system for a residential community. The design includes
inlets, pipes, channels, and stormwater management systems.
Preliminary sizing of the conveyance drainage system should be performed on each component to provide
adequate conveyance of stormwater runoff to proposed
stormwater management facilities or points of discharge
(outfalls). It is important to verify the design storm and sizing requirements for these systems with local authorities
having jurisdiction. A common design storm for conveyance systems is 10- to 15-year storm events, but the location of the system can require conveyance for larger design
storms (such as 25- or 50-year storms adjacent to a highway
crossing).
The designer must also account for safe runoff conveyance either via overland flow, via the collection system, or
via other secondary means of conveyance for storm events
exceeding the required design storm for the collection system. This is necessary to ensure that drainage water is conveyed to appropriate stormwater management systems even
as the pipes and channels are overwhelmed.
F i g u r e 3 . 6 A Example of a storm drainage system.
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Utility Fundamentals 257
Profiles of the storm sewer system are typically part of
the final design; however, select profiling should occur during preliminary engineering to check vertical conflicts with
outfall systems and other utilities. Additionally, flow capacities of proposed and existing systems should be checked. It
is important to ensure that proposed developments on a site
do not negatively affect surrounding properties and existing
drainage systems. Since the adequacy of the drainage outfalls
is critical, as-built information of existing storm systems and
field run cross sections of existing drainage channels (natural
or man-made) should be obtained.
Drainage systems are often divided into two categories:
minor and major. The minor system, which consists of
swales, small ditches, gutters, small pipes and the other various types of inlets and catch basins, collect and convey runoff to a discharge area or impoundment. Components in the
minor system are sized to carry runoff generated by the more
frequent, short-duration, storm events. The major drainage
system includes natural streams, channels, ponds, lakes,
retention and detention facilities, large pipes, and culverts.
Design criteria for the major system are based on significant
amounts of rainfall produced by the less frequent, long duration storms and are further governed by the hydraulic concepts related to bridges and large conveyance structures.
This part of this chapter will build from Chapter 3.5 by
introducing the storm system that conveys water into the
stormwater management facility. This chapter begins by
describing the parts of a storm sewer system, including the
materials and preliminary sizes. Then the design and location of the system within a property will be discussed, to
allow for the optimal design of a site. A holistic understanding of stormwater will enable the success of a project.
3.6.3. Storm System Materials
A storm drainage system is generally comprised of concrete
structures (manholes and inlets) and pipes of a variety of materials. The layout of structured systems (pipes, inlets, manholes,
endwalls) generally follows a layout in which straight pipe segments form connections between storm structures. At each
storm structure the pipe can change horizontal direction or
vertical direction (pipe slope). The layout of a storm network
can often look like a constellation pattern as the infrastructure
meanders around the site. There is a large catalog of storm
structures and the details usually vary by jurisdiction.
Storm Junctions. Manholes are typically precast circular
concrete barrel sections, in 3 to 4 feet lengths, that stack on
top of each other (Figure 3.6B). The elevation of the top of
the manhole is adjusted to meet grade with spacer rings. The
top is covered with an iron ring fitted with an iron cover.
Manholes are used to change horizontal and/or vertical
direction of pipes, while also acting as a junction, allowing
the convergence of several incoming pipes. Many types of
inlets use the manhole barrel sections but have a precast
throat or grate that fits to the top.
The diameter of the manhole depends on the size of the
pipes connecting to it. Standard manhole diameters are
03_Land_CH03_p125-304.indd 257
Figure 3.6B
Precast unit assembly diagram.
approximately 4 feet but can increase to 8 feet or more. When
several pipes converge at a manhole the size and angle of the
pipes should be evaluated to ensure that manhole can accommodate the connections. For large systems, concrete junction
boxes may be required to accommodate pipe connections.
In Figure 3.6C(a), the size and angles of the pipes are such
that they can be connected to the manhole without interfering with each other. Compare this to Figure 3.6C(b), where
the pipe sizes and angles cannot fit into the manhole at the
same elevations (or nearly same elevations). In the second
case the elevations of the pipe should be staggered enough to
provide for the necessary clearance. In each case the thickness
of the pipe walls should be considered when determining the
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Figure 3.6C
and
E ngineering F undamentals
Pipes connecting at manhole at same elevation.
necessary manhole size. Wall thickness and other details are
often available from manufacturers or in details from local
jurisdictions.
Similar to the preceding discussion, where several pipe
connections to the manhole must be checked for proper
fit, the skew angle of the pipe connection at a rectangular
structure must be checked for fit. Figure 3.6D(a) shows
an incoming 36-inch-diameter pipe properly fitted to a
3 × 5-foot (outside dimensions) structure. In Figure 3.6D(b),
the pipes are skewed and do not properly connect to the
structure. In this example, the center of the structure is
aligned with the centerline of the pipe and the skew angle
causes part of the incoming pipe to overlap onto the 3-foot
side of the structure.
The angle of the inflow and outflow pipe should be more
than 90° to prevent a U-turn condition of the flow, which
reduces hydraulic efficiency. At the base of a structure, inlet
shaping is usually provided to promote flow through the
system and reduce energy losses (Figure 3.6E). Inlet shaping is the practice of creating a channel from the inflow
pipes to the outflow pipe. The inlet shaping is most effective when pipe inverts are similar for the inflow and outflow pipes.
Storm Collection Structures. There is a large catalog of
storm structures and the details vary by jurisdiction. In most
cases, storm collection structures will include inlets and endwalls (or headwalls, depending on whether they receive or
discharge stormwater). An endwall or headwall (Figure 3.6F)
is located where a pipe system connects to an open channel,
and the structure retains the adjacent earth.
Storm inlets are generally classified into four categories,
as identified by Federal Highway Administration (FHWA) in
the HEC-22 Urban Drainage Design Manual (Figure 3.6G):
a. Grate
F i g u r e 3 . 6 D Skew angle for pipes connecting to rectangular
structure.
03_Land_CH03_p125-304.indd 258
b.
Curb opening
c.
Combination (grate and curb opening)
d.
Slotted drain
Many jurisdictions, including the state department of
transportation (DOT), will have a prescribed catalog of
inlet types. Some jurisdictions limit the use of certain inlets,
such as grate inlets, if they consider them prone to clogging.
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F i g u r e 3 . 6 E Example of inlet shaping, modified from Virginia Department of Transportation Road and Bridge
Standards.
Figure 3.6H depicts various inlet types located along a roadway. Inlets may be located within the roadway, parking lots,
hardscape, or outside of paved areas based on the grading
and hydraulic requirements. Smaller variations of these inlet
types are typically used for plaza, landscape, or yard drainage [usually high-density polyethylene (HDPE) or polyvinyl
chloride (PVC)]. Inlets can be precast or cast in place and
Figure 3.6F
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Example of a headwall.
will generally have a manhole or other vertical structure
below the inlet top to allow for pipe connections.
Pipe Materials. Pipe systems (closed channel conveyance)
are common in suburban and urban development, and typically seen in conjunction with curb and gutter streets that
have inlets. Pipes can be manufactured from many different
materials but the most common are reinforced concrete pipe
(RCP) and HDPE. Other plastic pipe systems may be used
for smaller yard drains or building connections. Corrugated
metal pipe (CMP) is often used for driveway culverts. Each
material will often have a variety of specifications for installation, trenches, and connections. For example, a storm pipe
that connects to a wet pond and is constantly submerged will
have a different joint specification than a pipe used just for
conveyance.
It is important to determine acceptable pipe materials
selection based on locality specifications and site constraints.
pH levels and resistivity of the soils, high or low fill areas, corrosive environmental conditions, and tidal areas will be some
of the site constraints. Others may be the preference of contractor, the proximity to manufacturing plants, and the cost of the
different pipe materials. The engineer must be careful to consider the pipe material when designing a storm sewer system.
The various pipes available on the market can have an enormous range of Manning’s “n” values (from as little as 0.007 to
as high as 0.033). Since the n value plays an important role in
pipe capacity and hydraulic grade line calculations, the selection of the pipe material could dramatically affect the design.
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F i g u r e 3 . 6 G Example of storm drain inlets (FHWA, HEC-22, 3rd edition, Urban Drainage Design
Manual).
F i g u r e 3 . 6 H Example of inlet shapes.
An acceptable n value for concrete pipe is usually 0.013, and
HDPE may be 0.011 to 0.12. Additional information on ‘n’ values and Manning’s equation is provided in Chapter 5.6.
Some localities will provide guidance as to the types of
materials preferred. Many other jurisdictions will allow the
contractor to choose the pipe material. When the contractor
is given the option of pipe materials, it will either be up to the
contractor or the engineer to determine whether the material selected will still meet the design requirements of the
construction documents. If the design documents use HDPE
with a lower n value (and greater capacity) but the contractor chooses RCP (higher n value), the design may need to be
03_Land_CH03_p125-304.indd 260
modified. Therefore, it is important that the engineer understand the requirements of the local industry before determining pipe materials to be used for design and analysis.
Pipes are generally circular and will range from small
diameter plastic pipes (3 to 10 inches) for building and
landscape drains with larger diameter options for RCP and
HDPE, as well as other common materials. The pipe size
option usually starts around 12-inch diameter and increases
in 3-inch increments to 36-inch diameter before increasing at
6-inch increments. As with all pipe materials, the wall width
should be considered in the design of the pipe network—
the diameter references the interior dimension but the wall
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Utility Fundamentals 261
Concrete Pipe Size, Wall Thickness and Weight
Internal Pipe Diameter
(in)
Wall Thickness
(B Type, in)
12
15
18
24
30
36
42
48
54
60
72
2
2¼
2½
3
3½
4
4½
5
5½
6
7
Weight
(lb/ft)
(lb/ft)
93
127
168
264
384
524
686
867
1068
1295
1811
F i g u r e 3 . 6 I Dimension and weights of pipes.
thickness can add as much add 4 to 18 inches to the outside
diameter for RCP. Figure 3.6I provides a list of common concrete storm pipe sizes, wall thickness, and weight.
Alternative pipe size and geometry may provide solutions
to difficult design situations, such as a utility crossing with
other systems. A reduction of a pipe size will have a significant
impact on available capacity. Storm pipes are also available in
elliptical shapes, which can reduce the total height or width
of a pipe while maintaining capacity. Rectangular shapes (box
culverts) are often used for large capacity systems and conveyance below a road crossing. Pipes, whether circular, box, elliptical, or arch, can also be used for underground stormwater
detention systems when surface systems (such as ponds) are
not feasible.
Open Channel Systems. Open channel conveyance systems provide an alternative to pipe systems. Open channels
may be used for stormwater quality (vegetated channels can
remove pollutants) or may be used to reduce construction
costs of a conveyance system (usually at the expense of land
area). Unlike pipes, open channels can change horizontal
or vertical direction without the need for a storm structure.
Channels often parallel a highway or rural subdivision roads
but can be used for conveyance in other applications as well.
For man-made channels, the geometry is usually trapezoidal or triangular, with side slopes typically ranging from
4:1 to 1:1 based on design criteria and channel material.
Channel geometry can also be irregular, when considering
natural channels, or can be established as parabolic or rectangular. Determining the capacity of a channel can be challenging because the shape, slope, and contributing drainage
area can vary along the channel alignment. Evaluating the
capacity at different locations (minimum slope, maximum
slope, complete drainage area) is often necessary to ensure
the channel is adequate.
03_Land_CH03_p125-304.indd 261
Channel material can vary between natural linings, such as
vegetation, or manufactured lining such as concrete, riprap,
or geotextile fabrics. The appropriate lining is determined
by an iterative process of selecting a material, determining
the velocity and shear stress (based on Mannings “n” of the
material), evaluating whether the velocity and shear stress
is permissible, and choosing a different material as needed.
For smaller conveyance systems, vegetated lining is generally
acceptable until a velocity of 3 to 6 feet/second based on the
vegetation, slope, and underlying soil conditions.
Culverts. A culvert is a relatively short length of conduit,
typically less than 250 feet long, used to transport water
through (or under) an embankment. A culvert, which acts
as an enclosed channel through the embankment, serves as
a continuation of the open channel. However, flow through
culverts depends on entrance geometry and depth of flow
at the downstream end. Consequently, flow computations
for culverts are more complex than the open channel flow
analysis associated with pipes and ditches. Culverts through
roadway and railway embankments are designed to pass the
design discharge without overtopping the embankment or
causing extensive ponding, or inundation at the upstream
end. Local requirements may allow nominal depths over the
embankments for lesser frequency storm (greater recurrence
interval) events.
Major components of a culvert design include specifying the materials—the barrel, end treatments such as headwalls, endwalls, and wingwalls, outlet protection, and inlet
improvements such as debris control structures as well as
determining the environmental permitting requirements.
Except for the barrel, these components are used as the specific situation warrants.
Barrels. Barrels are available in various sizes, shapes, and
materials. Figure 3.6J shows the commonly used culvert
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Figure 3.6J
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Common shapes of culverts.
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3.6
shapes as well as applications of the various shapes. Selection of shape depends on construction limitations, embankment height, environmental issues, hydraulic performance,
and cost. The most commonly used culvert materials are
corrugated steel, corrugated aluminum, and precast or cast
in place concrete. Factors such as corrosion, abrasion, and
structural strength determine the selection of material. In
cases where the culvert is located in a highly visible area, the
selection of shape and material may be based on aesthetics,
as well as the functional aspects.
End Treatments. Headwalls and wingwalls are examples of
end treatments. They protect the embankment from erosion,
serve as retaining walls to stabilize the bank and add weight
to counter any buoyancy effects. Ideally, the centerline of the
culvert should follow the alignment and grade of the natural channel. In many cases this cannot be done, and skewing
headwalls and wingwalls help transition the natural stream
Figure 3.6K
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Utility Fundamentals 263
alignment to the culvert alignment. Figure 3.6K shows four
types of end treatments.
Debris barriers are sometimes constructed on the
upstream end to prevent material from entering and clogging the culvert. The barriers are placed far enough away
from the entrance so that accumulated debris does not clog
the entrance.
At the inlet and outlet ends of the culvert, endwalls and
wingwalls serve as retaining walls and erosion protection for
the embankment and help to inhibit piping along the outside surface of the culvert. Downstream wingwalls provide
a smooth transition between the culvert and the natural
stream banks.
All culverts that cross underneath roadway embankments
inherently create potential stability concerns for the embankments. The same can be said of storm sewer pipe installations. This is a function of the construction of the culvert
Four common end inlet treatments.
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system. Compaction around a pipe, whether it is circular or
rectangular, is more difficult than standard compaction of fill
slopes. The contractor must take care in compacting around
culvert systems, usually requiring several “lifts” (e.g., 6 to
12 inches layers of soil) to compact the surrounding fill material to specification. Consideration should always be given,
by the designer and contractor, that storm runoff will seep
into roadway embankments, or groundwater tables will rise
during flood events. If the compaction is inadequate, water
can run along the voids in the soil of the “uncompacted” portion of the culvert. When water continues to flow along a
pipe or culvert, the “uncompacted” material slowly washes
away from the surrounding pipe, and failure of the embankment may occur. This is one of the most common causes of
embankment failures at culvert installations and roadway
pavement failures at storm sewer installations. This type of
failure is referred to as piping failure.
Culverts and storm sewers are typically constructed with a
stone base to provide support and flexibility. For most installations, this stone base is porous and allows any water inside
the embankment to travel downstream, along the path of the
culvert. Typically, this aggregate base aids in reducing piping failures, in that it concentrates the water flowing through
the embankment. However, should there be excessive ponding of water behind an embankment, the standard bedding
provided at culvert installations will not be adequate to pass
the seepage flow safely along the conduit, and piping failures
may occur. The designer should consult with a geotechnical
engineer to determine appropriate countermeasures, such as
concrete cradles, use of impervious materials, or embankment draining devices such as toe-drains to minimize the
potential for piping failures. Permitting requirements for
natural channels may require the countersinking the culvert
3 to 12 inches depending on the local, state, and federal laws.
The depth of countersinking may be dictated by the type of
ground material. Countersinking the culvert will provide a
natural channel bottom through the culvert for aquatic habitat. The culvert analysis must consider the loss of flow area in
the culvert, a modified Manning’s “n” value for the channel
bottom, and model the effective opening.
3.6.4. Storm System Layout
The storm system design, like most design processes, is iterative. The results from one iteration provide the details necessary for further refinement of the design in the next iteration.
Each iteration attempts to optimize the design to provide adequate drainage control, without an excessive number of inlets
or length of pipe. The design of the minor system involves:
•• Outfall determination—identify all outfall points
for site
•• Inlet location—based on preliminary grading and
drainage divides
•• Conveyance system layout—connecting inlets, buildings, manholes, and SWM systems
•• Preliminary analysis—checking hydrologic and
hydraulic assumptions for inlet locations and pipe/
channel sizes
•• Verify design—determine if the system follows local
criteria and standards, and any impacts to other
components of the project
F i g u r e 3 . 6 L Example of a road with location of inlets.
03_Land_CH03_p125-304.indd 264
Requisite information to begin design of the storm drain
system generally includes on-site and off-site topographic
maps, current and projected land use maps, soil identification maps, floodplain delineation maps, floodplain reports,
existing site plans of surrounding property, stream and channel outfall information, and the plans of the proposed conditions of the project.
Location of Inlets. On curb and gutter roads and parking
lots, inlets are placed at all low points, such as sag vertical
curves (low points within the road). In sag locations, where
there is a higher propensity for clogging of inlets, the spread of
ponded water presents a traffic hazard. Flanking inlets should
be considered, if they are not required by code, in the sump
area of a sag vertical curve. The addition of flanking inlets,
which limit the spread and ponding at the low point, provides
relief when the sump inlet is clogged or overwhelmed.
On normal crown street sections the locations of inlets
are usually similar for each side of the road. If the road is
sloped to one side (both lanes drain to one curb), the location of inlets will often be limited to the low side of the road;
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3.6
however, if there is a large area adjacent to the roadway additional inlets may be placed on the upper side of the road to
capture water prior to spilling across the roadway. Refer to
Figure 3.6L for examples of inlet locations shown on the
cross section of a road. This is important when considering
how snowmelt may drain across a roadway if not captured
with an inlet.
Additional inlets adjacent to the roadway (beyond the
curb and gutter) may also provide relief from drainage across
the roadway.
To properly locate the drainage inlet on superelevated
roadways, the design engineer should check the elevations
of the outer edge of pavement along the superelevation transition segment to determine the location of any “false low
points.”1 Most urban streets have curbed or curb and gutter sections. In those situations, where the urban street is
required to have superelevation through horizontal curves,
certain conditions create the situation where a low point
occurs and is not in an obvious sump. To attain full superelevation or to go from full superelevation to normal crown
(or reverse superelevation in the case of compound curves)
requires a length of transition. If the rate of superelevation
is greater than the longitudinal rate of grade change, a low
point in the gutter area results. The engineer should be aware
of the conditions that present this predicament and locate
the low point and inlet location.
Inlet placement in an intersection requires detailed
analysis of the intersection street’s profiles and cross sections. Intersections have the potential for creating unusual
drainage patterns and collection networks. The design of an
intersection involves matching the cross slope of the intersecting street with the longitudinal profile of the through
street. Sometimes the crowns meet in the intersection but
other times the crown is only carried through on the major
through street. This usually depends on the classification of
the streets in the intersection. Two major roads will see the
crowns meet in the intersection but, for example, an arterial
and a local street will usually only have the crown of the arterial carried through the intersection. Drainage patterns can
be complicated by deviations from the cross slopes shown on
the typical section, which may be relaxed through the intersection to fit the pavement from the intersecting street to the
pavement of the through street.
Curb returns (the horizontal curve portion of the curb
used to join the curb of the two intersecting streets) and the
cross sections of the intersecting streets coupled with any
horizontal and vertical curves can create sump areas in inconspicuous and unfavorable locations. Additionally, the profile
of the curb return itself may not follow a known mathematical function, for example, a parabolic curve, and typically
does not exactly coincide with the street profile. Typically,
inlets are placed along the curb and gutter of the downhill
street before the curb return to capture water before entering
1
The term false low point is used to indicate a low point location difference from the
low point location given on the PGL.
03_Land_CH03_p125-304.indd 265
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Utility Fundamentals 265
the intersection. Detailed grading may be required along the
curb return to force water back to the inlet, or through the
curb return to the next street.
The designer should also consider the location of any
handicap ramps or pedestrian crossings prior to placing
inlets at the end of the curb returns. Depending on the longitudinal street slope and the gutter flow, an inlet just upstream
of this location may also be necessary. Locating inlets at the
ends of curb returns also reduces flows around the curb
returns, which may be a consideration if the longitudinal
gradient of either street is very steep, or if the area is a high
pedestrian traffic area.
After inlets have been placed at the required locations in
sumps and at intersections, the location of all remaining inlets
is dictated by the limitations on spread of flow into the street
(on-grade inlets, as opposed to sump inlets). The location of
on-grade inlets is dictated by the allowable spread of gutter
flow. Most jurisdictions will prescribe a maximum spread for
gutter flow (often half a vehicular travel lane). The spread is
calculated using Manning’s equation for channel flow where
the curb, gutter, and road define the channel. A steeper longitudinal road section will provide a higher capacity (less
spread) than a mild slope—this means mild road slopes will
require more frequent inlets to control the spread. Refer to
Chapter 5.6 for formulas and calculations of spread.
In parking areas, the grading plan determines the drainage pattern and inlet location. In commercial building sites,
pedestrian traffic is an important consideration for grading and drainage. Sump areas should be avoided in pedestrian travel ways such as crosswalks or near entrances to the
building. Additionally, sump areas should be avoided in areas
where passengers discharge from vehicles such as bus stops
or walkways. Although there is typically no limitation for the
spread of flow into privately maintained travel lanes and parking areas, good judgment is needed to avoid people parking
in puddles when stepping out of their vehicles and to limit the
spread in cold regions, since the formation of ice sheets is a
concern. Handicap ramps and parking spaces also need to be
considered when locating drainage inlets and sump areas.
In most moderate density developments, manholes and
inlet structures will be spaced less than several hundred feet
apart by necessity. Most localities will set design criteria
limiting the maximum distances between manholes, which
often are a function of the pipe size. Table 3.6A is the recommended spacing provided by AASHTO. This distance is usually determined by maintenance and accessibility concerns,
verify acceptable distances per locality.
Location of Yard Inlets. Ideally, the drainage system
should accommodate the site or roadway plan, and not be
the driving factor behind other design components. For
example, single-family detached projects favor drainage
patterns toward the rear and side of the houses. Swales and
inconspicuous ditches can convey the water across the rear
of the lots with minimal intrusion and little impact to the
overall site plan. Large grate inlets and concrete channels are
not desirable conditions for a residential yard.
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T A B L E 3 . 6 A Recommended Structure
(Manhole and Inlet) Spacing
Pipe Diameter
Distance (ft)
12–24
350
27–36
400
42–54
500
≥60
1000
The rate of flow being conveyed across a lot is limited to
nonerosive velocities. Maintenance and ownership of swales
and ditches is a factor in drainage design in suburban developments. The desirability of a lot is reduced if a drainage
easement runs through the middle of a rear yard. Drainage
easements may be necessary to prevent homeowners from
inadvertently impeding drainage patterns when improving
their yards. In most cases, the drainage system should be
kept within the public right-of-way.
Open spaces, including parks and plazas, may require
special attention to analyze the drainage patterns and
determine the best conveyance system. Sometimes these
open spaces can drain into the curb and gutter along a
street. But large parks may have too much flow for a street
inlet to handle. Yard inlets may then be required to capture
this flow, and swales can be utilized to route the drainage
into a yard inlet. These swales can be innocuous and blend
in to the landscaping of a park. Low points could also be a
concern. Yard inlets may again be required in low points,
but small plazas may have smaller drains to strategically
capture water.
Pipe Design. After inlet locations are established, the pipe
conveyance system is determined. The pipe network typically
converges to the outfall point. Occasionally, due to site conditions, the network is separated into several systems, each
discharging at different outfall points or connecting to
another storm sewer network. Outfall points can be natural channels of adequate capacity, retention/detention
areas, lakes, and rivers. The size, slope, and depth of the
pipe in the network are controlled by the elevation of the
outfall point. This is more of a problem in flat terrain,
such as Texas, where systems may also include inverted
siphons to maintain an outfall location. In flat terrain,
the engineer may have design problems trying to match
outfall elevations; whereas in steep terrain, high velocity
and high energy losses may create a potential problem. In
general, the slope and size of the pipe is kept to the minimum required to carry the design flows at near full capacity. Many jurisdictions have a requirement for a minimum
allowable pipe size and slope (commonly set to 15-inch
diameter, with the exception of building and landscape
drains that may be smaller).
03_Land_CH03_p125-304.indd 266
The relationship between pipe size, slope, and conveyance
capacity can be seen in Table 3.6B.
Common practice is to set the pipe at minimum depth
(i.e., minimum cover or minimum inlet depth) or at the shallowest depth possible (to ease in constructability) and select
the pipe size and slope that convey the design discharge at
near full capacity. The pipe network is typically designed
to prevent pressure flow (the conveyed flow is below the
pipe capacity). In pressure conditions, the pipe network is
surcharged for storms that generate runoff greater than the
design storm.
Discharge velocity in a pipe should be kept within a
nominal range to prevent scour and eliminate sediment
buildup. Sediment transported by very high velocities
causes abrasion damage to the pipe. Low velocities cause
sediment to settle out and reduce the pipe capacity. A recommended velocity range is 2 to 12 ft/s, but some materials may warrant velocities up to 20 ft/s. Reducing velocity
within a steep system may require mild pipe slopes with
drops between manhole structures (the drops would then
require steel plates at the base of the manhole, or other protective measures).
TA BL E 3 .6 B
Pipe Size, Slope, and Conveyance
Capacity
CONCRETE PIPE INTERNAL DIAMATER (INCHES)
Pipe Slope (%)
266 C h a p t e r 3
15
18
24
0.5
5
7
16
1
6
10
1.5
8
2
30
36
48
60
29
47
102
184
23
41
67
144
260
13
28
50
82
176
319
9
15
32
58
94
203
368
2.5
10
17
36
65
105
227
412
3
11
18
39
71
115
249
451
3.5
12
20
42
77
125
269
487
4
13
21
45
82
133
287
521
4.5
14
22
48
87
141
305
552
5
14
23
51
92
149
321
582
5.5
15
25
53
96
156
337
610
6
16
26
55
100
163
352
638
6.5
16
27
58
105
170
366
664
7
17
28
60
108
176
380
689
Pipe capacity (cubic feet per second)
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F i g u r e 3 . 6 M Typical underdrain.
Because storm sewer flow is driven by gravity, any conflicts with other proposed nongravity dependent utilities are
usually resolved by redirecting those utilities. Attempts are
made to locate the proposed storm system around any existing gravity or nongravity dependent utilities. It can be costly
to redirect existing utilities and only in the unavoidable case
is this done.
Underdrain Design. The inlet and storm sewer design
has focused on surface runoff. However, there are many
instances where water collects underneath a pavement section, either in locations of cut/fill transitions, or just by infiltration. Water will usually collect in the subgrade portion of
the pavement, especially in areas where aggregate or stone
is used in the design. This collection of water underneath
the pavement can be extremely problematic, leading to loss
of stability and strength of the pavement structure, fatigue
based on freeze/thaw conditions, or loss of cohesion and
bearing pressures of the surrounding soil.
Given certain situations, this excess water underneath the
pavement will require that a secondary drainage system be
03_Land_CH03_p125-304.indd 267
constructed to reduce the water collected underneath roadways. This is usually done by placing a combination of small
plastic pipes (sometimes perforated or wrapped in geotextile
fabric) along the aggregates section of the pavement box and
discharging the water either into a ditch or a structure (i.e.,
inlet or manhole) associated with the roadway drainage system. These devices are called underdrains.
In most urban situations, underdrains are placed along
the edge of the pavement, below the curb and gutter section. Typical roadway design requires that the pavement be
sloped to the edge to facilitate drainage. The same applies
to pavement design as well, such that all layers of the pavement box (top surface course, base course, subgrade, etc.)
drain to the outside. Curb and gutter sections typically are
constructed on stone or aggregate bases, meaning that water
can flow easily along the corridor. Underdrains placed longitudinally below the curb line, within the aggregate, can
capture the majority of water trapped beneath the pavement
and discharge it directly into the roadway drainage system.
Figure 3.6M shows a typical underdrain.
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GWINNETT COUNTY STORMWATER SYSTEM ASSESSMENT PROGRAM (MULTIPLE
WATERSHED PROJECTS)
Location: Gwinnett County, GA
Client: Gwinnett County Department of Water Resources
Completion Date: December 2012 thru March 2015; Multiple Projects
Case Study: Gwinnett County Department of Water Resources (GCDWR), in Georgia, launched a countywide program
to assess its stormwater pipe infrastructure system for each of its major watersheds. GCDWR spends approximately
$15 million annually replacing and rehabilitating failed pipe infrastructure. The primary driver for this is to address
aging infrastructure—approximately 67 percent of the 1400 miles of county-maintained stormwater pipe is corrugated
steel pipe (CSP) and is rapidly approaching the end of its useful life. In most cases the existing pipe’s capacity level of
service (LOS) or upgrade needed is unknown.
GCDWR recognized the next step to enhance its active stormwater infrastructure asset management was to conduct a
comprehensive system assessment to identify the capacity LOS of the county-maintained piped drainage system in order
to better plan system rehabilitation and replacement projects. Dewberry was selected to lead a pilot study in the Level
Creek watershed that included over 24 miles of stormwater pipe infrastructure and 7 miles of open channels. To date
under this program, Dewberry has completed twelve comprehensive watershed studies, which include approximately
900 miles of pipe and 1000 miles of streams and open channels. Tasks include
• E
nhanced stormwater inventory geodatabase to include pipe inverts, structure depths, and rehabilitation/replacement
quantities
• Assess construction work areas required for open trench pipe replacements to identify easement needs and conflicts
• Established asset values properly accounting for appurtenant items
• W
atershed-wide dynamic rainfall-runoff modeling on an ESRI ArcGIS platform (PCSWMM) for all countymaintained stormwater pipe systems and appurtenant natural channels to assign pipe LOS
• Scenario-based pipe rehabilitation and replacement modeling to achieve a desired LOS
• Development of Capital Improvement Project cost estimation decision support and planning tool
The aggregate result of these comprehensive system-wide analysis tasks is an asset management decision support tool. This
tool allows daily operational decisions involving stormwater pipe rehabilitation and replacement to be made based on a
comprehensive understanding of the existing capacity LOS, needed upgrades to meet the desired LOS, and associated costs.
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3.6
PART B—SANITARY SEWER
3.6.5. Introduction
Proper management of wastewater is one of the most important factors for ensuring the general health of a community
and its surface water quality. The design of any sewer system always involves several parties. Input from all impacted
parties should be organized early in the design. The design
engineer must be familiar with the local, state and federal
laws and regulations as they apply to the proposed design.
Also, it is important to determine if the downstream segments of the sewer system and the treatment facilities that
will receive the flow have adequate capacity. If not, a treatment plant and disposal of the treated effluent must be a
part of the planning phase.
Generally, there are two common types of wastewater collection and treatment methods implemented on land development projects: individual subsurface disposal systems
(septic systems) or public collection and treatment systems.
A less common third type of system is a private community
system that may utilize subsurface disposal or small, often
packaged treatment plants. Ordinarily, the decision as to
which type of system will be utilized is made during the feasibility stage. At this time it is also necessary to make a determination as to whether sufficient capacity is available when
using public collection and treatment systems.
During the preliminary engineering phase the design of
the collection system begins with a projection of anticipated
sewage flow. These calculations may be based on regulatory
requirements for projected flows, historical data obtained
from similar size and type of facilities, or other accepted
reference sources. See Chapter 5.6 for more information
regarding quantity of sewage. It is important for the design
engineer to recognize that the system must be designed
according to anticipated capacity requirements as well as
regulatory requirements, either of which may end up governing the design. It is preferable for sanitary sewer collection systems to be gravity systems to the maximum extent
possible, to eliminate the increased cost and maintenance of
pumping facilities. It is also important to evaluate and assess
the potential need for future expansion of the new system
and make appropriate accommodations if this is anticipated
to occur. If the collection system will be tied into an existing system, as-built information on the downstream system
should be evaluated as part of the design.
Profiles of the sanitary sewer are normally part of final
design; however, select profiling should occur during preliminary engineering to locate vertical conflicts with other utilities, and determine whether there is adequate cover over pipes
such as at stream crossings and in roadways. Many jurisdictions will require minimum clearances, cover, separation, and
occasionally encasement of sanitary collection systems from
other utilities and in areas such as stream crossings.
If a pump station is required to convey sanitary flow, a
detailed analysis of the various options for placement of the
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station and the route of the gravity collection piping as well as
the force main should be performed in the feasibility stage. In
the preliminary engineering phase, the pumping station will
need to be sized based on anticipated flows, allowable residence time, cycle length, and required wet well depth. Careful
consideration must be given to construction feasibility and
cost associated with each element of the pumping station.
In the feasibility phase, downstream treatment plants
should be contacted to confirm that they have capacity for
the proposed flows from the development. If public sanitary
sewer is not available to the project, alternative methods for
wastewater treatment will need to be explored in the preliminary engineering phase. As an example, if septic systems will
be used for sewage treatment, they should be preliminarily
sized and configured. In order to do this, percolation tests
in the area of the disposal fields will have to be performed
to determine the rate at which the soil can absorb the effluent. The tests (including number and location) need to be
coordinated with the geotechnical engineer and must be performed per the applicable regulatory requirements.
3.6.6. Sanitary Sewer Materials
A sanitary sewer system is generally comprised of concrete
structures (manholes and inlets) and pipes of a variety of
materials. Similar to storm networks, the layout of generally
follows a constellation pattern, where structures are used for
pipe junctions and to change the horizontal or vertical direction of a pipe system. The engineer has several types of pipe
and structure material options available for use in sewer construction. It is important that the proper selection be made
for the specific installation conditions.
Manholes. Manholes are a required appurtenance to any
sanitary sewer system. They provide access to the sewer for
inspection and maintenance. Manholes should be placed
at every change in grade or horizontal alignment, at every
change in sewer size, at every sewer intersection, and at locations between long spans of pipe (jurisdictions often require
manhole access within 400 feet).
Only precast reinforced concrete manholes should be
used except where special field conditions make it necessary
to field construct the manhole with brick or cast-in-place
concrete. Figure 3.6N shows two typical type manholes. The
standard manhole includes a base that is generally cast as a
part of the first barrel section. The base shown is an extended
base, which spreads the manhole weight over a larger area.
Additional barrel sections are used as required to bring the
manhole to where the eccentric cone is added. A manhole
frame and cover is placed at the surface. The manhole supplier should adjust the height of the sections to comply with
installation requirements. The precast supplier tailors each
manhole for the specific site. Rubber boots are available
that can be cast into the manhole at the locations where the
sewers will pass through the manhole wall. A stainless steel
clamp is used to secure the sewer to the boot making a watertight connection. The inside of the connection is grouted
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F i g u r e 3 . 6 N Typical precast manhole design. (Source: Handbook of Steel Drainage and Highway Construction Products, reprinted with permission
from McGraw-Hill.)
along with the placement of the bench (only shrink proof
grout is used). Corrosion resistant steps are placed inside the
manhole on a 12 or 16 inches vertical spacing. The exterior
of the manhole is waterproofed using bituminous mastic on
the surface and in the joints.
Figure 3.6O shows two types of drop manholes. A drop
manhole is used to reduce slopes of incoming sewers or
to permit connecting a sewer entering the manhole at a
higher elevation than the main sewer. Generally, if the difference in elevation is less than 5 feet and groundwater, or
obstacles are not a problem, the slope of the higher sewer
is increased to lower the sewer to where the drop manhole
will not be needed.
Doghouse manholes are typically used when a new gravity sewer will intersect an existing gravity sewer and an
existing manhole is not present. The doghouse manhole
is located at the intersection of the two lines and includes
the construction of the manhole base in the field through
forms and cast-in-place concrete around the existing gravity sewer line, see Figure 3.6P. Additional barrel sections
03_Land_CH03_p125-304.indd 270
and a cone section are added to the constructed base section until ground elevation is reached. Once the manhole is
constructed, the top is cut out of the existing gravity sewer
pipe to allow the sewage from the new line to enter the pipe,
joining the collection and conveyance system.
An epoxy coating or interior liner may also be used with
manhole construction if, in the opinion of the engineer,
highly corrosive situations may be present that would lead
to the early deterioration of the concrete manhole. Typically, these conditions could exist at manholes that receive
sewage discharge from a sanitary forcemain. There is a
wide variety of coatings and linings, which include paintedon epoxies through precast high-density polyethylene
linings that are formed with the original construction of
the concrete barrel sections. These products can also be
used in the rehabilitation of existing manholes that have
not reached their useful life expectancy or are cost prohibitive to replace.
The bench (similar to inlet shaping for storm systems)
is constructed to provide a smooth section through the
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Figure 3.6O
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Typical drop manhole design.
F i g u r e 3 . 6 P Typical doghouse manhole base.
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manhole so as to reduce energy losses, to prevent the accumulation of solids in the manhole and to provide a place for
the maintenance person to stand when working in the sewer.
The bench should extend to at least the springline (horizontal reference line located at the mid-point of the pipe) of
the sewer. For sewers larger than 10 inches in diameter, the
bench should extend to ⅔ of the sewer diameter. The specifications should require that the base section be supplied with
an extended base, that a boot be cast in the section for all
sewer connections and that the channels and bench be cast
as a part of the section. All sewers entering a manhole should
be provided a smooth channel into the main channel. When
there is a change in sewer diameter at a manhole, the invert
depths of the sewers should be placed at the same elevation.
This will prevent any upstream surcharging at full flow. The
change in flow direction at a manhole should not exceed
90°. Where a greater change in direction is required, use two
manholes with a segment of sewer between them.
In the past it has been common practice to allow for a
0.1-foot drop in sewer invert through a manhole where
there is no change in pipe diameter. This is no longer necessary (but might still be required by a local jurisdiction). A
well-constructed channel will not require this drop. Many
manhole suppliers now provide the base section with the
channel and bench cast in place when the manhole was
formed. The channel invert must be smooth, having the
same shape as the sewer. In all instances, the penetrations
(openings in manhole wall for sewers) should be cast in the
manhole or core drilled where an additional penetration
is needed. A penetration made with a jackhammer should
never be permitted.
A short section of sewer is used at the manhole to provide
a joint in the sewer not more than 3 feet from the manhole.
This allows some flexibility for any difference in settlement
between the manhole and the sewer. A pipe joint provides a
connection from the pipe (usually plastic) to the manhole
(concrete). Manufactured rubber compression joints should
be used in sewer construction unless there are unusual conditions. The joint should be free of debris and the rubber gasket lubricated prior to installing the pipe.
The manhole frame and cover should be of cast iron. A
typical frame and cover is shown in Figure 3.6Q. A standard
frame and cover as well as a watertight/locking frame and
cover are also shown in the figure. The engineer’s library
should include a catalog on standard manhole castings.
The weight of the frame and cover must be selected to carry
the expected loading. For example, a traffic bearing frame
and cover is required when the manhole is located in a
street, whereas a lighter weight one will be suitable for offstreet locations. Locking frame and covers are available for
use where required. Waterproof frame and covers should
be used at locations where the area is subject to flooding.
Ventilation to the sewer is provided through the manhole
cover. When waterproof frame and covers are used, alternate ventilation should be provided at least every 1000 feet
of sewer.
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Many utilities have a specific design on the manhole covers, such as the name of the utility. These utilities stock the
covers and sell them to the contractor.
Building Spurs. When manholes are placed in the street,
the building spur (also referred to as building lateral) should
be installed from the sanitary sewer to a minimum of 1 foot
inside the property line. A separate spur should be connected
for each lot or building site. Where sidewalks are to be constructed, the building spur should be constructed to 5 feet
beyond the back of the sidewalk. It is important that each
building spur be shown on the sewer plan with the station
of the connection being confirmed as a part of the as-built
drawings. SDR-35 or heavier pipe should be used for building
spurs. The spur should enter the main sewer through a manufactured wye or tee. An approved saddle may be used when
connecting to existing sewers. Some localities require that the
connecting wye or ell be of ductile iron because mechanical
rodding equipment will bore through a PVC connection if
care is not used. Building spurs should be laid to a grade of at
least 0.5% slope, with a minimum slope of 1% being provided
where possible.
A wye may be installed and the extended line capped at
the surface at the lot line for access in the future as needed.
Where the main sewer is excessively deep the spur should be
brought to a reasonable depth prior to reaching the lot line,
but the spur must be kept deep enough to serve the building. Figure 3.6R shows typical connections for building spurs
(service connections).
Pipe Materials. Since all pipe material used in sewer construction is essentially smooth wall, there will be little difference in the pipe roughness and, hence, in the Mannings “n”
value of n to be used in a design for different pipe material.
The selection of the type of pipe to be used in a design and
included in the contract specifications as being acceptable
for use in construction should be based on the long-term
serviceability of the pipe material. Serviceability factors
should include expected useful life, resistance to problems
with infiltration, ease of installation, resistance to corrosion
and erosion, and maintenance requirements. Initial cost
should not be the only determining factor.
Pipe materials are classified as being either flexible or
rigid. Pipe manufactured from materials such as concrete
and vitrified clay are classified as rigid wall pipe. A deflection
of the wall of a rigid wall pipe will result in pipe failure. The
most common type of flexible pipe is PVC. Ductile iron and
high-density polyethylene are also flexible pipe materials.
Rigid pipe is classified based on bursting strength, and for
pressure pipe the internal pressure rating is also included.
The bursting strength is established by standard tests and is
provided by the manufacturer in pounds of external vertical force per foot of pipe. It is listed as the three-edge bearing strength that reflects the type of standard test procedure.
Pipe is designed to withstand both the internal pressure and
the external loading simultaneously. The manufactured wall
thickness is adjusted to provide the required strength. The
engineer should have library catalog data available for the
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Figure 3.6Q
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Typical manhole frame and covers.
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F i g u r e 3 . 6 R Service connections showing building spurs or laterals.
types of pipe material being considered for incorporation
into a design. The strength of rigid pressure pipe will be
shown by pipe class such as class 150, 200, and so on.
Gravity flow sewer pipe is designed to withstand external
pressure loadings. Rigid wall pipe is manufactured in several wall strengths as may be required for design conditions.
For example, pipe classes are designated as 1500, 2400, 3300,
4000, and 5000. The class strengths represent the design
crushing strength in pounds per linear foot of pipe.
Flexible wall pipe is classified by the ratio of the external diameter to the wall thickness. This is expressed as
dimension ratio (DR) or standard dimension ratio (SDR).
Since limited deflection of the pipe can be tolerated, the
strength is expressed in terms of the force required to cause
a defined deflection. Pressure flexible pipe is designated by
pressure class. Flexible sewer pipe, such as PVC, is designated by the standard dimension ratio as noted for flexible
pressure pipe. The most often used sewer flexible pipe is
SDR-35. Heavier-wall-thickness pipe, such as DR-21, are
available for use where required to meet laying conditions.
Table 3.6C provides a list of common SDR-35 sizes, wall
thickness, weight, and laying length.
Sanitary sewer pipes are circular, typically with a minimum size of 8-inch diameter for sanitary mains. Smaller
diameters are used for building connections. The size of a
03_Land_CH03_p125-304.indd 274
sanitary main can increase to 10, 12, 15 inches or larger (with
carrying size increments).
3.6.7. Sanitary Sewer Layout
The sanitary sewer networks are deigned to allow connections from each site building to an existing public sanitary
sewer system (or septic system). Sanitary sewer lines are
often deeper than other utilities because the elevation at
a building connection will be deep (especially if there is a
T A B L E 3 . 6 C Characteristics of the Most
Commonly Used Sizes of SDR-35 Pipe
Nominal
Size Laying
Length (in)
External
Diameter
(in)
Wall
Thickness
(in)
Weight
(lb/ft)
8
8.4
0.24
4.42
20
10
10.5
0.300
6.93
20
12
12.5
0.36
9.91
20
15
15.3
0.43
14.90
Laying
Length
12.5
Reprinted with permission from American Society for Testing and Materials. ASTM
Standards in Building Codes, Vol. 4.
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3.6
basement without a pump). The system should be designed
to minimize the number of manholes, where possible, and
monitor the depth to prevent excessively deep manholes (20
feet or more).
Gravity Sewers. Sewer depth is established by the elevation of the buildings to be served. The sewer should be sufficiently deep to permit gravity drainage from all buildings
if possible. For residential subdivisions, the sewer depth
is based on the building construction. If the homes have
basements, the sewer should be at sufficient depth to gravity sewer the basement. Below the basement, an additional
2 feet of depth is needed to allow for the basement foundation, as the house sewer should be located below the
wall foundation and not through the foundation (pumping would be required if the outfall is not below the basement foundation). The building spur (sometimes referred
to as the house lateral or house sewer) should be placed on
a minimum slope of ¼ in per foot (0.0208 ft/ft). Plumbing
codes allow a slope of ⅛ in per foot (0.0104 ft/ft) for sewers
under conditions where the greater slope cannot be conveniently provided.
The design engineer should follow applicable rules in
selecting sewer slope. The slope for each segment of sewer
running between adjacent manholes is selected based on
design conditions. The overall design objective is to design
the collection system that can be constructed most economically while maintaining good engineering practice. This generally means keeping the sewer as shallow as possible while
complying with the following criteria:
1. Minimum sewer slope—the slope required to provide a velocity of 2.25 feet per second (fps), or the
slope required to carry the required flow, whichever
is greatest. Most jurisdictions will prescribe a minimum slope for public sewers, which can be 0.5%
or less (allowable slope often decreases as pipe size
increases).
2. Minimum cover
a. S ewers located in streets should have at least
6 feet of cover. If less than 6 feet of cover is provided, special bedding may be needed because
of the superimposed load from traffic.
b. Sewers located within off-street easements
should be placed below the applicable freeze
depth or at least to a depth of 3 feet.
c. Sewer segments serving a building lateral should
be at sufficient depth to provide gravity drainage
from the building served.
The requisite cover, size, and slope conditions may vary
between jurisdictions and should be determined prior to
design.
Pressure Sewers. A pressure sewer collection system is
used as an alternative to conventional gravity sewer systems.
The technology may be the only feasible means of sewering
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some areas. Pressure sewers are particularly applicable for
sewering less populated areas and developments or communities located in hilly or rocky terrain. Conventional gravity
sewers may need to be deep and costly to construct in areas
where the topography is undulating. Also, a high water table
may make the construction of gravity sewers economically
unfeasible. Pressure sewers also have merit other than lower
construction cost. Reliable pumping equipment is available. The technology requires no modification to the house
plumbing and therefore the use of pressure systems causes
little inconvenience to the homeowner. However, a gravity
system is often the desired solution because there is no reliance on moving parts or power.
The two major types of pressure sewer systems are the
grinder pump (GP) system and the septic tank effluent
pressure (STEP) system. Figure 3.6S shows the basic elements of both systems. A typical grinder pump is shown
in Figure 3.6T. The GP system consists of a grinder pump
that receives the flow from a dwelling or other facility and
pumps the flow into a pressure force main. The GP system
may also be installed to replace an existing septic tank system as shown in Figure 3.6S. The STEP system follows a
conventional septic tank where the flow is pumped into a
pressure force main.
Pumping equipment for the GP technology is well developed. The pump shown in Figure 3.6T is available in several sizes and capacities to serve uses from a single family
dwelling to a small commercial or industrial flow. The unit
is supplied in a pre-piped and wired fiberglass enclosure
that can be installed in a basement, crawl space, or below
ground level in a lawn. The units are available with one or
two pumps, for reliability. The unit is installed, and the
4-inch building sewer connected. Pumps are available for
pumping against discharge heads of more than 100 feet.
Pressure systems are designed similar to force mains, except
a determination of the number of pumps operating on the
main at any one time and the resulting flow. GP suppliers
have developed this type statistical data and it can be found
in the respective catalog.
The normal wet well size for a single-family dwelling is
60 gallons (pump enclosure provides wet well volume). This
constitutes some wastewater storage availability during periods of power outages. The units are wired to permit placing
a high water alarm at an appropriate location in the home,
usually in the kitchen, so that the homeowner is made aware
that the pump is not operating and that the wet well is full.
The pumping units are designed for easy removal of the
entire unit in case of pump failure. The pumps should give
10 or more years of service under normal conditions. It is
desirable for the community with a GP system to have service provided by a utility or by a private plumbing company
that will keep spare pumps in stock for rapid installation. The
nonfunctioning pump is then taken to the shop for repair or
replacement and kept in stock for the next replacement need.
If rapid service is not available, the homeowner should install
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Flow diagram for pressure pumping systems.
the duplex unit so that a backup pump is always available,
allowing time to have the nonfunctioning unit repaired.
The septic tank effluent pressure system (STEP) is a means
of eliminating the need for onsite treatment and disposal,
such as the soil absorption field. In past years, septic tanks
absorption fields have been installed at locations that are no
longer environmentally acceptable. Criteria for siting absorption fields have improved as more knowledge on soil percolation and potential for groundwater pollution has become
available. The STEP systems are being installed to eliminate
failing absorption fields and as a means of providing central sewerage service to both existing and new communities.
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The septic tank located ahead of the pump eliminates most
of the grease and solids in the flow to be pumped. The force
main design is similar to the GP system except for the type of
pump required. The homeowner must continue to maintain
a septic tank as a part of the STEP design.
Septic Sewer Systems. If there is not public sanitary
sewer service for a project, which may occur on rural residential sites, a septic system can be proposed. A septic system is installed on the lot. The system includes a septic tank
that holds wastewater and a drain field. The system relies on
soil and groundwater conditions, specifically for the drain
field component.
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F i g u r e 3 . 6 T Typical grinder pump complete with housing. (Reprinted with permission from Environmental One Corp. 1994, Grinder Environment
One product catalog, Schenectady, NY.)
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GOOSE CREEK INTERCEPTOR PIPELINE REPLACEMENT/REHABILITATION
Location: Boulder, CO
Client: City of Boulder
Completion Date: September 2016 through March 2017
Case Study: The City of Boulder, Colorado, operates and maintains an expansive wastewater collection and transmission system that services the city. Based on work performed as part of the City’s 2016 Wastewater Collection System
Master Plan update, a few areas within the city’s wastewater collection and transmission system were identified as requiring upgrades, either due to pipe capacity limitations, pipe condition, or both. One of these areas identified by the city
includes a portion of the Goose Creek Interceptor pipeline.
The city contracted with Dewberry to provide engineering services to perform preliminary design tasks for replacement
and/or rehabilitation of the interceptor segment. These tasks included analyzing the pipeline hydraulic conditions of the
interceptor segment, reviewing third party CCTV footage to categorize the current condition of the interceptor segment,
and developing and evaluating replacement or rehabilitation alternatives to correct both existing hydraulic/capacity
issues and the existing pipe condition issues.
Dewberry collected survey data on the manholes and pipe inverts of the interceptor, and in combination with existing GIS information, constructed and analyzed a hydraulic model at various flow rates to confirm all areas of hydraulic concern were identified. Dewberry reviewed the existing CCTV footage to identify and categorize the physical
condition of all pipe segments. With the hydraulic model results and physical condition assessment complete, Dewberry then used existing Lidar elevation data and existing GIS mapping data to develop design alternatives utilizing a combination of replacement and rehabilitation methodologies to provide the necessary improvements for the
interceptor segment.
The design replacement and rehabilitation alternatives were evaluated using economic (quantitative) and noneconomic
(qualitative) criteria to define a preferred approach for rehabilitation and/or replacement. The findings of the evaluation were documented in an alternatives analysis report, which was delivered to the city for their use in future project
planning.
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3.6
PART C—WATER DISTRIBUTION
3.6.8. Introduction
Water supply is ordinarily facilitated by either connection
of the development to a public water distribution system or
installation of an on-site groundwater supply well. In most
cases, it is preferable to provide a connection to public water
supply if it is available to the site or can be extended to serve
the site at a reasonable cost. The decision as to which type
of system will be utilized to serve the development is made
during the feasibility stage along with a determination as
to whether sufficient capacity for fire and domestic flow is
available from the existing water system.
A water supply system includes the water supply, treatment facilities, pumping facilities, transmission lines, and the
local distribution network. The distribution network consists
of pipes, fittings, valves, and other appurtenances designed to
convey potable water at adequate pressures and discharges.
The water utility company is responsible for the water quality and operation of the distribution system. A water utility
company can either be a public entity that, like many other
public bureaus, exists for the health, safety and welfare of the
public, or it may be a privately owned utility providing water
for profit. In some of the larger metropolitan and suburban
areas, the water utility company heavily controls the design of
a water distribution system. Personnel within the water company perform most of the design, analysis, and layout. In this
case, the site designer of the land development project only
performs the lesser water works design aspects and incorporates the analysis, along with the water company’s information,
into the construction drawing set. In smaller locales, the analysis and design may be performed entirely by the private consultant working for the developer. Typically, the water supply
company, to ensure conformance with local standards, reviews
the private consultant’s design. Regardless of whether the water
supply company is public or private, the design must conform
to the state health department’s criteria, as well as any other
controlling public agencies. Design parameters and regulations
for water works are available from state board of health and
the local city/county health departments. Additional information on water quality, as well as material and construction standards, are available from the Environmental Protection Agency
(EPA), the American Water Works Association (AWWA), and
the American National Standards Institute (ANSI).
The overall waterworks system consists of the following
elements:
The water source, usually a lake, river, or aquifer, which
serves as the municipality’s main supply. Larger municipalities may have more than one source. Lakes and reservoirs are located in outlying areas to take advantage
of runoff from large catchments and where there is less
pollution. Along rivers, water is usually extracted on the
upstream side of the population centers. The minimum
supply volume at the source must coincide with the present and projected demand, which it is to serve.
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A treatment facility to purify the water for safe use. Since
pumping clean water is less expensive than pumping
sediment-laden water, the location of the filtration plant
should be near the source. The treatment facility treats
and disinfects the water to meet the water quality standards set by government regulations. The law mandates
that the water supply company provide quality potable
water and consumers expect and rely on the water company to do so.
Transmission lines convey the water leading from the
source to the treatment facility, as well as from the
treatment facility to the distribution network. For
moderate to large population areas, transmission lines
can be 6 feet in diameter (and larger) and operate at
pressures in excess of 200 psi. They serve as the link
between the source, the treatment facility, and the distribution network. Therefore, transmission lines have
limited branches and service taps.
Pumping facilities provide necessary energy to move
the potable water to consumers. Pumping facilities are
needed to convey the water through transmission lines
to the distribution system. These pumping facilities
can be simple, such as the well pumps in small systems,
or they can be complex, high-capacity pumping stations needed for large municipalities. Pumping stations within the distribution network, called booster
stations, are used to maintain required minimum
pressures.
Intermediate storage facilities (e.g., water tanks) are
located near the distribution network. Intermediate storage facilities stabilize the line pressures, serve as reserve
for peak demand periods, and storage for fire flow
requirements. All domestic water line systems operate
under pressure. In times of high demand, the pressure
in the network is decreased, requiring the water supply
to be supplemented from the storage tanks along with
the water from the treatment plant. During low-demand
periods, when supply pressure in the distribution lines is
high, the storage tanks refill. Discounting fire flow storage, water storage volumes fluctuate 40 to 70% on a daily
basis.
Distribution lines are used to link the intermediate
storage facilities and the feeder lines that connect the
residential, commercial, and industrial service areas.
Typically, distribution networks are laid out as interconnecting loops. The looped (grid) layout allows for
bidirectional flow of water. Since water travels toward
the area with the lowest pressure, which is often the area
of highest demand, looping delivers twice the volume of
water for fire flows and other heavy demands. Additionally, grid layout allows for isolation of small areas during
repairs. A branching type of configuration consists of a
main feeder line with single-dead-end branches to service areas. In comparison to the grid system the branch
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system is not as economical for several reasons: (1) closing down a branch for maintenance disrupts service to a
larger area, (2) poor water quality resulting from stagnation and sedimentation in the branch ends requires periodic flushing at the ends. Although dead ends cannot be
avoided, they should be minimized. Fire hydrants, flushing hydrants, or blow-off valves at the dead ends allow
for such periodic flushing.
Appurtenances such as fire hydrants, valves, auxiliary
pumps, and fittings such as wyes, elbows, crosses, and
tees augment the operation of the distribution system.
The arrangement of the appurtenances and fittings is
specific to the local area they serve. Valves allow the
system to isolate small service areas when repairs are
needed. Other fittings are used to change flow direction
and provide economic flexibility in sizing pipes.
electric, pneumatic, or hydraulic. Whereas automatic valves
are more commonly used at the treatment facilities or pump
stations, most valves used in the distribution pipe network
are manual.
In addition to their function, valves are also described by
their numerous characteristics such as size, material, fitting
end, pressure rating, and actuator. The five most common
types of valves used in water lines along with their specific
characteristics, provide an extensive list for specifying a particular valve. These are identified as follows.
A ball valve (Figure 3.6U) has a slotted spherical or section
of a spherical closure element that rotates within the casing.
The valve is opened when the opening is parallel to the flow
direction. A ¼-turn of the opening closes the valve. Typically,
ball valves are used for on-off service and throttling the flow.
The design of the entire system requires an evaluation of
numerous factors. Location and size of main water supplies
depend on general characteristics of the region such as climate,
hydrology, geology, and topography. Design of treatment facilities, transmission and distribution lines depends on existing
water quality at the source, existing and projected population
and its spatial distribution. Additionally, as the municipality
grows, the increasing population and expanding development
inevitably influences hydrologic and topographic factors as
well as others, which in turn affect water demand.
3.6.9. Water Supply Materials
Typically, pipes used in waterworks systems are classified
according to its design working pressures. The four most
common classes of working pressures are 100, 150, 200, and
250 psi. Other parameters for identifying pipe strength are
the bursting strength rating, which identifies the strength
against internal forces caused by hydrostatic and hydrodynamic pressures, such as pressure surges and water hammer,
and the crushing strength, which deals with the external
forces related to soil, vehicles, and impact loadings.
Essential to a distribution system are such components as
valves, fire hydrants, meters, thrust restrains, joints, and fittings. The location and size of these components impacts the
system’s hydraulic efficiency and cost-effectiveness as well as
the health, safety, and welfare of the public.
Valves. In a waterworks system, valves are used in controlling flow directions, regulating flow rates, pressure control,
isolating flows, and transient wave suppression. Typically faucets, bibs, stoppers, and plugs are types of valves used in the
water supply lines within a building, while gate valves, butterfly
valves, and check valves are those typically used in water distribution lines.
A valve is completely closed when some type of operating
mechanism, such as a wheel, forces the gasket or plug tightly
against the fixed seat. The operating mechanism, that is, actuator, for a valve is either manual or automatic. Manual actuators include wheels and levers; automatic actuators can be
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(a)
(b)
F i g u r e 3 . 6 U Ball valve. (Courtesy of Flowerve Corp.)
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A plug valve (Figure 3.6V) is similar to the ball valve in
that a cone or cylinder attached to a shaft, with a rectangular
slot or circular orifice, opens and closes the flow by rotating
the slot parallel to the direction of flow. Eccentric plug valves
use a seat that consists of only part of the cone or cylinder to
(a)
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Utility Fundamentals 281
close the valve. The shaft rotates the seat over the pipe opening to regulate the rate of flow. Typically, plug valves are used
for control and isolation purposes. Smaller plug valves are
used on service connections and are referred to as service or
corporation cocks. Plug valves have low head losses and typically cost more than gate, globe, and butterfly valves.
Gate valves, a type of the broader classification of
slide valves, are the most frequently used when trying to isolate flows in the pipe network (Figure 3.6W). The gate valve
(shutoff valve) is used to completely stop the flow through
the pipeline. They operate by raising or lowering a plate
or disc into the flow path. Note, a shutoff valve’s intended
use is to operate in the full open or full closed positions
only, and not as a throttling valve (partially opened state).
Considerable wear and tear on the mechanism occurs in the
partially opened position and the head loss, at this position,
is very high. Typically, gate valves are placed in valve boxes
that extend to the ground surface to permit access. The valve is
operated with an extension wrench that reaches down through
the box to turn the operating nut on top of the valve stem.
As shown in Figure 3.6X, a butterfly valve, also a type of
rotary valve, consists of a thin disk that rotates about a thin
shaft. When the face of the disc is parallel to the flow directions the valve is fully opened. Butterfly valves can be used
for either shutoff or throttling purposes. Although these
types of valves have relatively low head loss, they present
problems when cleaning, since the disk is in the flow stream.
The globe valve, which is another type of slide valve, consists of a flexible disk attached to a screw-operated stem. A
schematic diagram of a globe valve is shown in Figure 3.6Y.
Raising and lowering the disc onto a horizontal seat controls
the flow. Use of globe valves is most common in smaller
domestic water lines; a familiar globe valve is the household
faucet. High-pressure losses are typical of most globe valves.
A summary of the important characteristics for these particular valves appears in Table 3.6D. In addition to the basic
valves just described, there are other specialty valves used in
waterworks systems:
Check Valves are pressure-activated valves. Typical
water main systems are bidirectional in that water flows
according to the pressure differential in the pipes. Check
valves are inserted to restrict flows to one direction. For
example, a check valve on the discharge end of a pump
or treatment plant prevents the water from reversing
directions and flooding out the facility when the facility shuts down operation. Check valves are also used to
control flows at the boundary of different water supply
districts and service areas of different pressures.
(b)
F i g u r e 3 . 6 V (a) Schematic diagram of a plug valve, (b ) eccentric
plug. (Reprinted with permission of DeZurik, Sartell, Minn.)
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Pressure reducing valves (PRV) are automatic valves that
protect against excess pressure in the water lines. Pressure reducing valves reduce pressure by controlling the
flow and intentionally causing high head losses. Large
pressure-reducing valves are operated by a smaller pilot
valve. A wheel on the pilot valve sets the internal spring
tension, which in turn sets the activating pressure.
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(a)
Figure 3.6W
(a)
Figure 3.6X
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(b)
Gate valve. (a ) Schematic diagram, (b ) resilient seat gate valve. (Reprinted with permission of Mueller Co., Decatur, Illinois.)
(b)
(a ) Butterfly valve, (b ) AWWA butterfly valve. (Reprinted with permission of DeZurik, Sartell, Minn.)
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F i g u r e 3 . 6 Y Schematic diagram of a globe valve.
TA BL E 3 .6 D
Valve Type
Common Size Particularly
Range (in.)
Adapted to
Valve Summary
Main Advantages
Main Disadvantages
Gate
½–24
Isolation services in
distribution grids
Low cost in small sizes
Low friction
Good service life
Ease of installation
High cost in large sizes
Large sizes are quite heavy
Poor for throttling; should not be used
where frequent operation is necessary
Butterfly
3 & up
Isolation and automatic
control
Low cost in larger sizes for
normal service pressures
Some types have very short
lengths
Ease of operation
Higher friction loss than gate valve
Difficult to open in lines where differential pressures exist
May cause problems when relining pipe
Leaks because of seat damage
Globe
½–24
Isolation in smaller sizes
Flow control in larger
sizes
Pressure control
Simple construction
Dependable, can be used for
throttling
Good for pressure control
Sediment or material unlikely
to prevent complete closing
High friction loss
Very heavy and expensive in large sizes
Ball and plug
½–3
½–12
Isolation and throttling
Dependable
Very low friction loss
Slow shut-off characteristic
minimizes closing surges
Ease of operation
Resistant to erosion
Long life
High cost in large sizes
Very heavy
(Reprinted from American Water Distribution Operator Training Handbook, by permission. Copyright © 1976, American Water Works Association.)
283
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Figure 3.6Z
and
E ngineering F undamentals
Two-inch blow-off valve detail.
distribution systems change elevations as the pipe
follows the terrain. In effect, the changes in elevation
change the pressure in the system. Consequently, the
air dissociates from the water and collects at the higher
points in the pipeline. Since the return of air back into
the water does not occur as readily as it leaves the water,
air pockets, which add 10 to 15% more resistance to the
flow, are formed. Additionally, these pockets can lead
to air lock of the system, which can completely stop the
flow. Typically, air relief valves are installed at the local
peaks in the pipe system. As shown in Figure 3.6AA,
a peak is any section of pipe that slopes up toward the
hydraulic grade line or runs parallel to it. Air release
valves should also be considered in sections where there
is an increase in the downward slope or decrease in the
upward slope2 of the pipe.
Larger PRVs are placed on line between service areas
of different elevations. Smaller PRV (less than 2 inches)
valves are used to protect residential plumbing from
excessive pressures in the main line. The valve acts as the
transition between high-pressure zones and low-pressure
zones. For example, if line pressures are greater than
70 psi, a pressure-regulating valve installed on a domestic service line reduces the service line to pressures so
that the pressure will not damage household fixtures.
Altitude valves are automatically activated according to
the pressure differential on either side of the seat. This
type of valve is usually used to control the flow to and
from an elevated storage tank. The valve opens, allowing
the tank to refill, when the pressure in the distribution
line is greater than the static pressure of the water in the
tank. In times of high demand, when the pressure in
the main decreases, the valve opens to allow the water
in the storage tank to equalize the distribution system.
Blow-off valves and drain valves are used to dewater lines
for repairs or remove accumulated sediment. These
valves are most commonly used at the end of branches
and at low points of the water main (Figure 3.6Z).
Air release valves—water at standard conditions contains
about 2% to 3% dissolved air by volume. The amount
of dissolved air in a water line system is governed by
the turbulence before it enters the system and the pressure and temperature within the system. Pressurized
03_Land_CH03_p125-304.indd 284
Hydraulic surge (water hammer) can occur when water
flowing in a long pipeline suddenly stops. Surge can actually
cause a water column separation at high points, or peaks, on
the main. This, at first, exerts a tremendous negative pressure (or vacuum) on the main that can cause it to collapse.
If the system does not collapse, the vacuum will be followed
immediately by a pressure spike, which occurs as the separated water columns reunite, that also can rupture the pipe.
2
Golden Anderson Automatic Valve Reference Manual, Golden Anderson Industries,
Inc. Pa.
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3.6
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Utility Fundamentals 285
F i g u r e 3 . 6 AA (a ) Typical bubble behavior, (b ) suggested air release valve locations. (Reprinted with permission of GA Industries, Inc., Cranberry
Township, PA.)
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To prevent this, vacuum relief, or combination air release
and vacuum relief valves, are used at points on water mains
where analysis indicates water column separation might
occur. The vacuum relief valve lets air into the pipeline during hydraulic surge to both relieve the vacuum and provide
a volume of air that cushions the pressure spike caused by
the two water columns coming together again. Note, hydraulic surge analysis is a specialized field and must be done by
qualified personnel.
Fire Hydrants. A fire hydrant consists of the above ground
barrel that extends below grade to the water supply line. The
supply line connecting the barrel to the water main is a minimum 6-inch-diameter pipe. Typically, a gate valve located
in an underground valve box allows for shut-off and repair.
Maximum recommended friction losses in the hydrant are
2.5 psi, and 5 psi in the water supply line, from the main to
the hydrant, for flows of 600 gpm. Hydrants have various
nozzle arrangements, but the most common hydrant has two
2.5-inch hose connections and one 4.5-inch pumper connection. The bend directly below the barrel connects the barrel
to the supply line. A thrust block prevents movement from
lateral forces and a concrete block underneath the bend prevents excessive settling. After use of the hydrant, the barrel is
drained by way of a drain near the base of the barrel. A gravel
bed surrounds the thrust block or bend to allow for draining
of the barrel.
Meters. Meters are commonly used to measure water for
setting cost, collecting payments from customers, and determining fair distribution of water delivery costs. Other uses
for meters include flow measurements to and from treatment
plants and reservoirs, blending of water and chemicals, and
measuring water sold to other jurisdictions.
Meters used to measure usage for residential dwellings
and commercial use are located in areas easily accessible to
the water utility company. They should not interfere with
public safety nor act as a hazardous obstacle. Most meters are
located below ground surface levels for safety and to reduce
damage due to weather and tampering. Occasionally meters
will be located inside the building with a remote recorder
mounted on the outside.
Meters are available in the same sizes as pipe diameters.
Usually the meter is sized one diameter size less than the
pipe for reasons associated with accuracy, head loss, and
cost. Unlike pipe, where the cost differential between successive pipe sizes may be on the order of 10 to 25%, the cost
differential between successive meter sizes can be significantly more. Downsizing the meter also down sizes succeeding appurtenances and fixtures thereby reducing costs. The
increase in accuracy in using the smaller size meter is counterbalanced by an increase in the head loss. Usually the cost
savings and the accuracy offset the drawback incurred from
the increased head loss.
3.6.10. Thrust Restraint
Two basic forces associated with flowing water under pressure are the hydrostatic forces and hydrodynamic forces.
03_Land_CH03_p125-304.indd 286
This pressure force acts perpendicular to a surface. Thus, for
a pipe with full flow the pressure acts radially outward and
along the longitudinal axis of the pipe.
Hydrodynamic forces are the result of changes in momentum of the moving fluid. Any change in direction or magnitude of flow velocity results in a change in momentum of
fluid. Fittings such as bends, tees, and wyes change the direction of flow; nozzles, valves, and reducers change the crosssectional area of the flow path. Each of these fixtures has
force acting on them due to hydrostatics and hydrodynamics.
On horizontal bends, the placement of the thrust block is
against the outer edge of the bend. On vertical down bends,
similar placement would put the thrust block on top of
the pipe. In situations where thrust blocks cannot be used,
another common method for thrust resistance is the use of
restrained joints. A restrained joint is a specially designed
joint that provides longitudinal restraint. The thrust force
at the component is transferred to the surrounding soil
through frictional resistance and bearing from a predetermined length of pipe.
3.6.11. Pipe Materials
Standard pipe diameters used in distribution systems range
from 6 through 20 inches in 2-inch increments. Then, beginning with 24 inches, diameters increase in 6-inch increments. For residential plumbing pipe, diameters begin at ½
inch and increase in ¼-inch increments. Since manufacturers and pipe material dictate the available pipe diameters,
diameters other than those mentioned here might be available. The supplier should be contacted to verify what pipe
diameters are available and if they are in stock.
Although ductile iron pipe (DIP) is the most popular cast
iron pipe, other common pipe materials for water distribution lines include grey cast iron, steel, plastic, and polyethylene. Note that this same type of piping is used for water
service lines larger than 2 inches. Copper or plastic tubing is
commonly used for service lines smaller than 2 inches.
Plastic pipe—distribution lines for small water systems are normally constructed of PVC pipe because of its
ease of construction and economy. Plastic pipe used for
drinking applications should be certified by an acceptable testing laboratory (such as the National Sanitation
Foundation) as not producing objectionable taste, and as
being nontoxic. Standards for PVC pipe (up to 12 inches)
for underground service are ASTM D1785, ASTM D2241,
and ANSI/AWWA C900. ASTM D1785 pipe is available in
schedule 40, 80, and 120 thickness. ASTM D2241 pipe has
pressure ratings up to 250 psi, while AWWA C900 pipe is
available in pressure ratings up to 200 psi. AWWA C900
pipe is manufactured to ductile-iron pipe outside dimensions, and both ASTM D2241 and ASTM D1785 are manufactured to steel pipe outside dimensions. Joining these
pipes requires a transition gasket used with standard ductile-iron fittings.
Ductile-iron pipe—is standardized in ANSI/AWWA
C150/A21.50-96 and is available in sizes from 3 to 64 inches,
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3.6
in various pressure classes. Push-on or mechanical joints
are used on DIP for underground service. For water service,
DIP is normally lined with cement-mortar. Soft water will
leach the cement-mortar lining if not properly conditioned.
Depending on soil conditions, DIP is often installed in polyethylene wrapping to aid in corrosion protection.
Steel pipe—steel pipe is normally used for large diameter installations beyond the sizes of which DIP is available,
and is not often used for small water systems. However, it is
available in various sizes and thickness classes. It is covered
under AWWA C200 standard. Note buried steel pipe is more
susceptible to corrosion than DIP, and must be adequately
protected. It is usually installed underground with a tapewrap coating as a minimum, and the pipe interior is normally specified to be provided with a cement mortar lining
for drinking water service.
Polyethylene pipe (PE)—PE is becoming popular for small
water service, mainly because it is economical to purchase
and install. PE is available in sizes larger than 4-inches and
the standard for PE is AWWA C906.
High-density polyethylene pipe (HDPE)—HDPE is popular
in directional drilling applications. The pipe is fused together
with a machine that provides a “jointless” connection. The
pipe is flexible and can be installed around a radius without
the use of joints. The pipe is covered under ASTM D1248,
ASTM 3350, AWW