Land Development Handbook 00_Land_FM_pi-xvi.indd 1 25/03/19 2:57 PM 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, 00_Land_FM_pi-xvi.indd 2 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. 25/03/19 2:57 PM 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 00_Land_FM_pi-xvi.indd 3 25/03/19 2:57 PM 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 MHID: 1-26-044076-1 The material in this eBook also appears in the print version of this title: ISBN: 978-1-26-044075-1, MHID: 1-26-044075-3. eBook conversion by codeMantra Version 1.0 All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. 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Neither McGraw-Hill Education nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill Education has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill Education and/ or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise. To the hardworking Dewberry employees who dedicate their talent, energy, and passion to building amazing places. 00_Land_FM_pi-xvi.indd 5 25/03/19 2:57 PM This page intentionally left blank 00_Land_FM_pi-xvi.indd 6 25/03/19 2:57 PM 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 00_Land_FM_pi-xvi.indd 7 25/03/19 2:57 PM This page intentionally left blank 00_Land_FM_pi-xvi.indd 8 25/03/19 2:57 PM 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 ix 00_Land_FM_pi-xvi.indd 9 25/03/19 2:57 PM This page intentionally left blank 00_Land_FM_pi-xvi.indd 10 25/03/19 2:57 PM 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 00_Land_FM_pi-xvi.indd 11 25/03/19 2:57 PM 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. 25/03/19 2:57 PM 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. 25/03/19 2:57 PM 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 25/03/19 2:57 PM 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 00_Land_FM_pi-xvi.indd 15 25/03/19 2:57 PM This page intentionally left blank 00_Land_FM_pi-xvi.indd 16 25/03/19 2:57 PM Part I Overview 01_Land_CH01_p001-016.indd 1 20/03/19 9:38 AM This page intentionally left blank 01_Land_CH01_p001-016.indd 2 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 3 20/03/19 9:38 AM 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. 01_Land_CH01_p001-016.indd 4 20/03/19 9:38 AM 1 ■ Overview of L and Development 5 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. 01_Land_CH01_p001-016.indd 5 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 6 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 7 ■ L and Development Design Process 7 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. 20/03/19 9:38 AM 8 C h a p t e r 1 ■ O verview of L and D evelopment 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. 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 9 ■ L and Development Design Process 9 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. 20/03/19 9:38 AM 10 C h a p t e r 1 ■ O verview of L and D evelopment 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, 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 11 ■ L and Development Design Process 11 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 20/03/19 9:38 AM 12 C h a p t e r 1 ■ O verview of L and D evelopment 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. 20/03/19 9:38 AM 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 01_Land_CH01_p001-016.indd 13 ■ 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. 20/03/19 9:38 AM 14 C h a p t e r 1 ■ O verview of L and D evelopment 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 20/03/19 9:38 AM 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 ■ 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. 20/03/19 9:38 AM This page intentionally left blank 01_Land_CH01_p001-016.indd 16 20/03/19 9:38 AM Part II Pre-Design 02_Land_CH02_p017-124.indd 17 23/03/19 11:56 AM This page intentionally left blank 02_Land_CH02_p017-124.indd 18 23/03/19 11:56 AM 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. 19 02_Land_CH02_p017-124.indd 19 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 20 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 21 ■ 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. 23/03/19 11:56 AM 22 C h a p t e r 2 ■ 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 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 23 ■ 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 23/03/19 11:56 AM 24 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.1 Figure 2.1B 02_Land_CH02_p017-124.indd 25 ■ Development Program, Site Selection, and Defining Property 25 Example of a plat. 23/03/19 11:56 AM 26 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 ■ 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. 23/03/19 11:56 AM 28 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 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 29 02_Land_CH02_p017-124.indd 29 23/03/19 11:56 AM 30 C h a p t e r 2 ■ D ue D iligence “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 23/03/19 11:56 AM 2.2 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 02_Land_CH02_p017-124.indd 31 ■ 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 23/03/19 11:56 AM 32 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 ■ 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 02_Land_CH02_p017-124.indd 33 23/03/19 11:56 AM 34 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 2.2 ■ 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 02_Land_CH02_p017-124.indd 35 Example of a land use plan. 23/03/19 11:56 AM 36 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 36 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. 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 37 ■ 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 23/03/19 11:56 AM 38 C h a p t e r 2 Figure 2.2D 02_Land_CH02_p017-124.indd 38 ■ D ue D iligence Example of a sector plan. 23/03/19 11:56 AM 2.2 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. 02_Land_CH02_p017-124.indd 39 ■ 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. 23/03/19 11:56 AM 40 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 41 23/03/19 11:56 AM 42 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 23/03/19 11:56 AM 44 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.3 ■ 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 23/03/19 11:56 AM 46 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 02_Land_CH02_p017-124.indd 47 47 Figure 2.3B Zoning map showing zoning transition, modified from Fairfax County zoning maps. 23/03/19 11:56 AM 48 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 ■ 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. 23/03/19 11:56 AM 3128 600 CXd 50000 600 600 JE F F ER 1900 2800 3500 100 ME 1/4 section NOTE: This O entire lies within the CENTRAL 1900 RY 5401 CITY PLAN DISTRICT. S 5400 T. SW 2300 1100 RAIL UBL IC T MA JOR P 3130 1300 BIA 1400 CXd N 3400 OSd NC E 3200 A UD ITO R IUM 800 3300 600 700 NOTE: Zoning designations arePL subject A N to change; verify zoning prior to developmentDIS orTsales. R IC T Historic Landmark City Boundary Urban Service Area Boundary T HO OSdeg* 1900 3400 ST. 2800 2900 2700 2600 2500 2400 CXd Plan District Boundary HAW 1800 RN E 3000 3300 H Historic or Conservation District or N.R.M.P. Boundary BD S.W . LPR IN OPE OSdeg* ST. 1700 ST. SO U T CXd 1700 3200 3500 L UM 3000 UE 1800 OSdeg*s 3229 R10 (R10) 2000 2100 OSdeg*s 2200 OSdeg* OSdeg*s ZONING REVISED: Current Zoning 400 _ 90000 5400 OSdeg* L 100 ST. 1500 SEQ ST. 4100 AVE . H A VE. S.W CXd .M IL L ST. TG HIL 3100 700 1600 HA KET AVE . 6200 MA R 4000 S.W . 6T S.W . ST. 8000 W. MO N C LA Y 80000 7900 ST. ST. 1800 CE 8500 7500 7800 CXd CXd N 3100 S.W . 100 YAM 600 SPA MIL L SO 270 0 S.W . 3700 7700 400 300 1100 CO S.W . BR d 2600 ST. T ST. 200 OSd 2900 CXd DW AY OA 7600 OSdeg* 500 2000 RIC 300 100 N S.W . BC 2100 S.W . 1900 IST 200 3800 400 1600 2500 3900 IC D 700 600 90000 2400 80000 2200 OR 3900 500 ISO 900 S.W . K PAR ST. IST 800 ST. 6500 6600 MA D 700 S.W . 220 0 2300 3800 400 MA IN 2000 3500 LH 6300 200 3400 3700 6200 1700 180 0 2100 90000 S.W . ST. HIL 1600 E RI VER 6700 1000 3300 YAM 300 CXd 2400 4000 800 S.W . 700 N 4200 6800 1300 1200 1400 3400 4300 6900 7900 500 400 500 7800 3500 ETT 800 MO 2500 4500 4400 6100 OSdeg* ST. 900 T SAL 700 300 7700 8700 LAM CXd 2900 R OSdeg* TRIC 2500 2800 ST. AVE . 2400 2600 3300 6000 7000 S.W . 2700 3000 900 CXd 3200 4600 8600 BA 2700 3100 4700 7100 1000 WIL 6600 AVE . 6400 3200 NS T. 5800 5900 200 S.W . 2600 3100 ISO TAY LO ON 8500 DIS AD 7600 CXd ST. 3000 RIS T. TOR IC DW AY 3300 S.W .M HIL L 90000 1000 ER ITO 3700 3400 BR RXd 6500 CXd 3801 3500 OA OSd 6300 800 S.W . ST. BB S.W . 3600 6100 YAM 80000 MO R CXd NS 2800 NA 2200 3900 1200 7200 TO HIS 2300 A LD 5000 90000 ING 5100 AVE . 4300 S.W .W ASH 5200 2900 150 0 4200 4000 80000 2200 AVE . 6000 7300 3900 8600 5400 5500 2N D 2300 4100 5T H ST. 2400 2100 6T H 2500 8100 4400 S.W . 5900 K 1900 PAR 1600 8200 CT 3500 4400 8800 8700 8100 5300 7400 1300 2000 1800 N 100 AVE . 5500 OSd 1700 1500 MO 5400 AVE . 5700 ST. CXd 1200 1400 9T H R S.W . ! 200 1900 S.W . 8300 8500 8300 CXd 8900 7500 8400 S.W . 8400 7900 8000 9000 70000 S.W . 7700 7800 9100 1800 4800 5300 CXd 9300 9200 9600 Y. 8600 5600 TAY LO 9500 9400 8000 PKW CXd 8100 S.W . 100 8200 ST. 9700 AVE . ! 300 8500 AVE . ON 7700 3R D CXd RIS 7900 1S T 9500 3029 ST. S.W . 5100 5000 4900 8400 8800 8700 AVE . 9100 AVE . 9200 MO R 4800 4T H 5300 5200 D ue D iligence ■ 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 ■ 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 23/03/19 11:56 AM 52 C h a p t e r 2 ■ D ue D iligence 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). 23/03/19 11:56 AM 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 ■ 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. 23/03/19 11:56 AM 54 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.3 ■ Zoning 55 F i g u r e 2 . 3 H Lafayette, Louisiana, dimensional standards. 02_Land_CH02_p017-124.indd 55 23/03/19 11:56 AM 56 C h a p t e r 2 ■ D ue D iligence 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 ■ 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 23/03/19 11:56 AM 58 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 58 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. 23/03/19 11:56 AM 2.3 ■ 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. 23/03/19 11:56 AM 60 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 60 Rear Lane Parkway Low Impact Road Rural Road Military Roads Other (specific street section) Beaufort, South Carolina, street regulating plan. 23/03/19 11:56 AM 2.3 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 23/03/19 11:56 AM 62 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 62 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. 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 63 ■ Zoning 63 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 23/03/19 11:56 AM 64 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 65 ■ 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 23/03/19 11:56 AM 66 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 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 67 02_Land_CH02_p017-124.indd 67 23/03/19 11:56 AM 68 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 68 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 23/03/19 11:56 AM 2.4 ■ Subdivision Ordinance, Review Process, Building Codes, and Development Costs 69 (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 02_Land_CH02_p017-124.indd 69 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 23/03/19 11:56 AM 70 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 70 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. 23/03/19 11:56 AM 2.4 ■ Subdivision Ordinance, Review Process, Building Codes, and Development Costs 71 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 02_Land_CH02_p017-124.indd 71 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. 23/03/19 11:56 AM 72 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 72 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 23/03/19 11:56 AM 2.4 ■ Subdivision Ordinance, Review Process, Building Codes, and Development Costs 73 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 02_Land_CH02_p017-124.indd 73 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 23/03/19 11:56 AM 74 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 74 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. 23/03/19 11:56 AM 2.4 ■ Subdivision Ordinance, Review Process, Building Codes, and Development Costs 75 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 02_Land_CH02_p017-124.indd 75 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 23/03/19 11:56 AM 76 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 76 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 23/03/19 11:56 AM 2.4 ■ Subdivision Ordinance, Review Process, Building Codes, and Development Costs 77 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. 02_Land_CH02_p017-124.indd 77 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 23/03/19 11:56 AM 78 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 78 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. 23/03/19 11:56 AM 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 79 02_Land_CH02_p017-124.indd 79 23/03/19 11:56 AM 80 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 80 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. 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 81 ■ 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. 23/03/19 11:56 AM 82 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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. ■ 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. 02_Land_CH02_p017-124.indd 83 23/03/19 11:56 AM 84 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 2.5 Figure 2.5B 02_Land_CH02_p017-124.indd 85 ■ Environmental, Geotechnical, and Historical Considerations 85 Sample flood insurance rate map. 23/03/19 11:56 AM 86 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 87 ■ 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 23/03/19 11:56 AM 88 C h a p t e r 2 Figure 2.5C ■ D ue D iligence 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. 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 89 ■ 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. 23/03/19 11:56 AM 90 C h a p t e r 2 ■ D ue D iligence 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.) 02_Land_CH02_p017-124.indd 90 23/03/19 11:56 AM 2.5 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. 02_Land_CH02_p017-124.indd 91 ■ Environmental, Geotechnical, and Historical Considerations 91 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. 23/03/19 11:56 AM 92 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 92 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 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 93 ■ Environmental, Geotechnical, and Historical Considerations 93 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. 23/03/19 11:56 AM 94 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 94 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. 23/03/19 11:56 AM 2.5 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 02_Land_CH02_p017-124.indd 95 ■ Environmental, Geotechnical, and Historical Considerations 95 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 23/03/19 11:56 AM 96 C h a p t e r 2 ■ D ue D iligence •• 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 02_Land_CH02_p017-124.indd 96 23/03/19 11:56 AM 2.5 ■ 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. 02_Land_CH02_p017-124.indd 97 23/03/19 11:56 AM 98 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 98 23/03/19 11:56 AM 2.5 ■ 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 02_Land_CH02_p017-124.indd 99 23/03/19 11:56 AM 100 C h a p t e r 2 ■ D ue D iligence 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, 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 101 ■ Environmental, Geotechnical, and Historical Considerations 101 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. 23/03/19 11:56 AM 102 C h a p t e r 2 ■ D ue D iligence 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 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 103 ■ 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- 23/03/19 11:56 AM 104 C h a p t e r 2 ■ D ue D iligence 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). 02_Land_CH02_p017-124.indd 104 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 23/03/19 11:56 AM 2.5 Figure 2.5I ■ 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 23/03/19 11:56 AM 106 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.5 ■ 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. 23/03/19 11:56 AM 108 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.5 •• 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 ■ 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. 02_Land_CH02_p017-124.indd 109 23/03/19 11:56 AM 110 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 110 23/03/19 11:56 AM 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. 02_Land_CH02_p017-124.indd 111 ■ Environmental, Geotechnical, and Historical Considerations 111 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. 23/03/19 11:56 AM 112 C h a p t e r 2 ■ D ue D iligence 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. 23/03/19 11:56 AM 2.5 Figure 2.5P 02_Land_CH02_p017-124.indd 113 ■ Environmental, Geotechnical, and Historical Considerations 113 Boring log. 23/03/19 11:56 AM 114 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 114 23/03/19 11:56 AM 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 02_Land_CH02_p017-124.indd 115 ■ 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.) 23/03/19 11:56 AM 116 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 116 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 23/03/19 11:56 AM 2.5 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 02_Land_CH02_p017-124.indd 117 ■ Environmental, Geotechnical, and Historical Considerations 117 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 23/03/19 11:56 AM 118 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 118 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) 23/03/19 11:56 AM 2.5 •• 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. 02_Land_CH02_p017-124.indd 119 ■ Environmental, Geotechnical, and Historical Considerations 119 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. 23/03/19 11:56 AM 120 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 120 23/03/19 11:57 AM 2.5 ■ Environmental, Geotechnical, and Historical Considerations 121 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. 02_Land_CH02_p017-124.indd 121 23/03/19 11:57 AM 122 C h a p t e r 2 ■ D ue D iligence 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. 02_Land_CH02_p017-124.indd 122 23/03/19 11:57 AM 2.5 ■ Environmental, Geotechnical, and Historical Considerations 123 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. 02_Land_CH02_p017-124.indd 123 23/03/19 11:57 AM 124 C h a p t e r 2 ■ D ue D iligence 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 02_Land_CH02_p017-124.indd 124 23/03/19 11:57 AM 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. 125 03_Land_CH03_p125-304.indd 125 25/03/19 5:09 PM 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 126 03_Land_CH03_p125-304.indd 126 25/03/19 5:09 PM 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. 03_Land_CH03_p125-304.indd 127 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 127 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 25/03/19 5:09 PM 128 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 128 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 25/03/19 5:09 PM 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. 03_Land_CH03_p125-304.indd 129 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 129 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 25/03/19 5:09 PM 130 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 130 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 25/03/19 5:09 PM 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? 03_Land_CH03_p125-304.indd 131 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 131 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: 25/03/19 5:09 PM 132 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 132 Landlocked parcel, due to property lines, that could necessitate extension of public infrastructure by the developer. 25/03/19 5:09 PM 3.1 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. 03_Land_CH03_p125-304.indd 133 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 133 •• 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. 25/03/19 5:09 PM 134 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 134 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. 25/03/19 5:09 PM 3.1 •• 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 03_Land_CH03_p125-304.indd 135 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 135 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 25/03/19 5:09 PM 136 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 136 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 25/03/19 5:09 PM 3.1 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. 03_Land_CH03_p125-304.indd 137 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 137 •• 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. 25/03/19 5:09 PM 138 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 138 25/03/19 5:09 PM 3.1 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 139 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 03_Land_CH03_p125-304.indd 139 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. 25/03/19 5:09 PM 140 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 140 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 25/03/19 5:09 PM 03_Land_CH03_p125-304.indd 141 141 F i g u r e 3 . 1 D Example of a legend. 25/03/19 5:09 PM 142 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 142 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. 25/03/19 5:09 PM 3.1 ■ Feasibility Study, Site Inspection and Plan Sheet Comprehension 143 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 03_Land_CH03_p125-304.indd 143 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. 25/03/19 5:09 PM 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 144 03_Land_CH03_p125-304.indd 144 25/03/19 5:09 PM 3.2 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. ■ Base Map and Site Diagram 145 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. 03_Land_CH03_p125-304.indd 145 25/03/19 5:09 PM 146 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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) 03_Land_CH03_p125-304.indd 146 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 25/03/19 5:09 PM 3.2 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. 03_Land_CH03_p125-304.indd 147 ■ Base Map and Site Diagram 147 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 25/03/19 5:09 PM 148 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 148 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 25/03/19 5:09 PM 3.2 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 03_Land_CH03_p125-304.indd 149 ■ Base Map and Site Diagram 149 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 25/03/19 5:09 PM 150 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 150 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.” 25/03/19 5:09 PM 3.2 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. 03_Land_CH03_p125-304.indd 151 ■ Base Map and Site Diagram 151 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 25/03/19 5:09 PM 152 C h a p t e r 3 Figure 3.2B ■ S ite A nalysis and E ngineering F undamentals 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). 03_Land_CH03_p125-304.indd 152 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 25/03/19 5:09 PM 3.2 Figure 3.2C ■ Base Map and Site Diagram 153 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. 03_Land_CH03_p125-304.indd 153 25/03/19 5:09 PM 154 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 25/03/19 5:09 PM 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 03_Land_CH03_p125-304.indd 155 ■ Base Map and Site Diagram 155 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. 25/03/19 5:09 PM 156 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 156 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 25/03/19 5:09 PM 3.2 (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). 03_Land_CH03_p125-304.indd 157 ■ Base Map and Site Diagram 157 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 25/03/19 5:09 PM 158 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals F i g u r e 3 . 2 I Codes and abbreviations used for data collectors. 03_Land_CH03_p125-304.indd 158 25/03/19 5:09 PM 3.2 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. 03_Land_CH03_p125-304.indd 159 ■ Base Map and Site Diagram 159 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, 25/03/19 5:09 PM 160 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 160 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 25/03/19 5:09 PM 3.2 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 03_Land_CH03_p125-304.indd 161 ■ Base Map and Site Diagram 161 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. 25/03/19 5:09 PM 162 03_Land_CH03_p125-304.indd 162 Example of a base map. Figure 3.2J 25/03/19 5:09 PM 03_Land_CH03_p125-304.indd 163 Site diagram: area classifications. 163 Figure 3.2K 25/03/19 5:09 PM 164 03_Land_CH03_p125-304.indd 164 F i g u r e 3 . 2 L Site diagram: zoning. 25/03/19 5:09 PM 03_Land_CH03_p125-304.indd 165 165 F i g u r e 3 . 2 M Site diagram: resource protection areas. 25/03/19 5:09 PM 166 03_Land_CH03_p125-304.indd 166 F i g u r e 3 . 2 N Site diagram: soils map. 25/03/19 5:09 PM 03_Land_CH03_p125-304.indd 167 Site diagram: steep slopes. Figure 3.2O 167 25/03/19 5:09 PM 168 03_Land_CH03_p125-304.indd 168 F i g u r e 3 . 2 P Site diagram: drainage patterns. 25/03/19 5:09 PM 03_Land_CH03_p125-304.indd 169 Site diagram: utilities. 169 Figure 3.2Q 25/03/19 5:09 PM 170 03_Land_CH03_p125-304.indd 170 F i g u r e 3 . 2 R Site diagram composite map. 25/03/19 5:09 PM 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 layoutgrids 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 171 03_Land_CH03_p125-304.indd 171 25/03/19 5:09 PM 172 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 172 Principal Arterial. A principal arterial is often referred to as a freewayroads 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 25/03/19 5:09 PM 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 ■ Transportation Fundamentals 173 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 agencytypically 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 agencythese 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. 03_Land_CH03_p125-304.indd 173 25/03/19 5:09 PM 174 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 174 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. 25/03/19 5:09 PM 3.3 ■ Transportation Fundamentals 175 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. 03_Land_CH03_p125-304.indd 175 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 25/03/19 5:09 PM 176 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 176 25/03/19 5:10 PM 3.3 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 03_Land_CH03_p125-304.indd 177 ■ 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. 25/03/19 5:10 PM 178 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 pavementeither open grid,1 gravel, or pervious concretesubject 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. 03_Land_CH03_p125-304.indd 178 25/03/19 5:10 PM 3.3 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 ■ Transportation Fundamentals 179 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. 03_Land_CH03_p125-304.indd 179 25/03/19 5:10 PM 180 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 180 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 25/03/19 5:10 PM 3.3 ■ Transportation Fundamentals 181 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 03_Land_CH03_p125-304.indd 181 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 25/03/19 5:10 PM 182 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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, 03_Land_CH03_p125-304.indd 182 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 25/03/19 5:10 PM 3.3 ■ Transportation Fundamentals 183 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. 03_Land_CH03_p125-304.indd 183 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 25/03/19 5:10 PM 184 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 184 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). 25/03/19 5:10 PM 3.3 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 ■ 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. 25/03/19 5:10 PM 186 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:10 PM 3.3 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 ■ 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.) 03_Land_CH03_p125-304.indd 187 25/03/19 5:10 PM 188 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 25/03/19 5:10 PM 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 lanethis 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 movementlarger 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 ■ 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 radiuswhen 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. 25/03/19 5:10 PM 190 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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.) 03_Land_CH03_p125-304.indd 190 25/03/19 5:10 PM 3.3 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 ■ 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 rampa 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 25/03/19 5:10 PM 192 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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.) ** 03_Land_CH03_p125-304.indd 192 25/03/19 5:10 PM 3.3 ■ 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). 03_Land_CH03_p125-304.indd 193 25/03/19 5:10 PM 194 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals (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 25/03/19 5:10 PM 3.3 ■ Transportation Fundamentals 195 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.) 03_Land_CH03_p125-304.indd 195 25/03/19 5:10 PM 196 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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, 25/03/19 5:10 PM 3.3 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 03_Land_CH03_p125-304.indd 197 ■ 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 Ioff-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. 25/03/19 5:10 PM 198 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 IIbicycle 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 IIIbicycle 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 03_Land_CH03_p125-304.indd 198 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 25/03/19 5:10 PM 3.3 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, 03_Land_CH03_p125-304.indd 199 ■ 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 balancedinconvenient 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 25/03/19 5:10 PM 200 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 200 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.) 25/03/19 5:10 PM 3.3 •• 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. 03_Land_CH03_p125-304.indd 201 ■ 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 siteeach 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 25/03/19 5:10 PM 202 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals (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.) 03_Land_CH03_p125-304.indd 202 25/03/19 5:10 PM 3.3 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 longthe 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 03_Land_CH03_p125-304.indd 203 ■ 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. 25/03/19 5:10 PM 204 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 vanthis 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 03_Land_CH03_p125-304.indd 204 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 25/03/19 5:10 PM 3.3 ■ 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. 03_Land_CH03_p125-304.indd 205 25/03/19 5:10 PM 206 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 206 25/03/19 5:10 PM 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 207 03_Land_CH03_p125-304.indd 207 25/03/19 5:10 PM 208 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 25/03/19 5:10 PM 3.4 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 03_Land_CH03_p125-304.indd 209 ■ 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 25/03/19 5:10 PM 210 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals (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 03_Land_CH03_p125-304.indd 210 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, 25/03/19 5:10 PM 3.4 ■ Grading Fundamentals 211 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. 03_Land_CH03_p125-304.indd 211 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 25/03/19 5:10 PM 212 C h a p t e r 3 Figure 3.4F ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 212 25/03/19 5:10 PM 3.4 Figure 3.4J F i g u r e 3 . 4 H Example of 2- × 4-foot-deep channels. ■ Grading Fundamentals 213 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. 03_Land_CH03_p125-304.indd 213 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. 25/03/19 5:10 PM 214 C h a p t e r 3 ■ S ite A nalysis Figure 3.4K and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 214 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 25/03/19 5:10 PM 3.4 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 ■ 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. 03_Land_CH03_p125-304.indd 215 25/03/19 5:10 PM 216 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 216 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 25/03/19 5:10 PM 3.4 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 03_Land_CH03_p125-304.indd 217 ■ 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. 25/03/19 5:10 PM 218 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 218 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. 25/03/19 5:10 PM 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 03_Land_CH03_p125-304.indd 219 ■ 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. 25/03/19 5:10 PM 220 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 220 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. 25/03/19 5:10 PM 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 03_Land_CH03_p125-304.indd 221 ■ Grading Fundamentals 221 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 25/03/19 5:10 PM 222 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 222 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. 25/03/19 5:10 PM 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, ■ Grading Fundamentals 223 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.) 03_Land_CH03_p125-304.indd 223 25/03/19 5:10 PM 224 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 224 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. 25/03/19 5:10 PM 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 225 03_Land_CH03_p125-304.indd 225 25/03/19 5:10 PM 226 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals F i g u r e 3 . 5 A Hydraulic and hydrologic effects of urbanization. 03_Land_CH03_p125-304.indd 226 25/03/19 5:10 PM 3.5 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 03_Land_CH03_p125-304.indd 227 ■ Stormwater Fundamentals 227 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 25/03/19 5:10 PM 228 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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: 25/03/19 5:10 PM 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 03_Land_CH03_p125-304.indd 229 ■ Stormwater Fundamentals 229 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. 25/03/19 5:10 PM 230 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 230 25/03/19 5:10 PM 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 03_Land_CH03_p125-304.indd 231 ■ 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. 25/03/19 5:10 PM 232 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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). 25/03/19 5:10 PM 3.5 ■ Stormwater Fundamentals 233 F i g u r e 3 . 5 E Direct runoff from a unit hydrograph. 03_Land_CH03_p125-304.indd 233 25/03/19 5:10 PM 234 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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) 25/03/19 5:10 PM 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. 03_Land_CH03_p125-304.indd 235 25/03/19 5:10 PM 236 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:10 PM 3.5 TA BL E 3 . 5 E ■ Stormwater Fundamentals 237 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. 03_Land_CH03_p125-304.indd 237 25/03/19 5:10 PM 238 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 ■ 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 25/03/19 5:10 PM 240 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 03_Land_CH03_p125-304.indd 240 25/03/19 5:10 PM 3.5 TA BL E 3 . 5 F ( B) ■ 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 25/03/19 5:10 PM 242 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:10 PM 3.5 ■ 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 25/03/19 5:10 PM 244 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:10 PM 3.5 ■ 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 25/03/19 5:10 PM 246 C h a p t e r 3 ■ S ite A nalysis Figure 3.5K and E ngineering F undamentals 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 25/03/19 5:10 PM 3.5 ■ Stormwater Fundamentals 247 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 25/03/19 5:10 PM 248 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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, 25/03/19 5:10 PM 03_Land_CH03_p125-304.indd 249 F i g u r e 3 . 5 N BMP selection criteria. 249 25/03/19 5:10 PM 250 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:11 PM 3.5 ■ Stormwater Fundamentals 251 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. 25/03/19 5:11 PM 252 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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.) 03_Land_CH03_p125-304.indd 252 25/03/19 5:11 PM 3.5 TA BL E 3 . 5 G ■ 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. 03_Land_CH03_p125-304.indd 253 25/03/19 5:11 PM 254 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 254 25/03/19 5:11 PM 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. 255 03_Land_CH03_p125-304.indd 255 25/03/19 5:11 PM 256 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 256 25/03/19 5:11 PM 3.6 ■ 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 25/03/19 5:11 PM 258 C h a p t e r 3 ■ S ite A nalysis 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. 25/03/19 5:11 PM 3.6 ■ Utility Fundamentals 259 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 03_Land_CH03_p125-304.indd 259 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. 25/03/19 5:11 PM 260 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:11 PM 3.6 ■ 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 25/03/19 5:11 PM 262 C h a p t e r 3 ■ Figure 3.6J 03_Land_CH03_p125-304.indd 262 S ite A nalysis and E ngineering F undamentals Common shapes of culverts. 25/03/19 5:11 PM 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 03_Land_CH03_p125-304.indd 263 ■ 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. 25/03/19 5:11 PM 264 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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; 25/03/19 5:11 PM 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 ■ 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. 25/03/19 5:11 PM ■ S ite A nalysis and E ngineering F undamentals 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) 25/03/19 5:11 PM 3.6 ■ Utility Fundamentals 267 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. 25/03/19 5:11 PM 268 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 268 25/03/19 5:11 PM 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 03_Land_CH03_p125-304.indd 269 ■ Utility Fundamentals 269 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 25/03/19 5:11 PM 270 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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 25/03/19 5:11 PM 3.6 Figure 3.6O ■ Utility Fundamentals 271 Typical drop manhole design. F i g u r e 3 . 6 P Typical doghouse manhole base. 03_Land_CH03_p125-304.indd 271 25/03/19 5:11 PM 272 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 03_Land_CH03_p125-304.indd 272 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 25/03/19 5:11 PM 3.6 Figure 3.6Q 03_Land_CH03_p125-304.indd 273 ■ Utility Fundamentals 273 Typical manhole frame and covers. 25/03/19 5:11 PM 274 C h a p t e r 3 ■ S ite A nalysis and E ngineering F undamentals 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. 25/03/19 5:11 PM 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 03_Land_CH03_p125-304.indd 275 ■ Utility Fundamentals 275 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 h