Urban Stormwater Retrofit: Design and Implementation of a Water
Urban Stormwater Retrofit: Design and Implementation of a Water
Quality Swale and Pocket Wetland for Friendship Park,
Winchester, VA
BSE 4126: Comprehensive Design Project
May 13, 2009
Purpose: The purpose of this report is to present the final design for a retrofit water quality swale and wetland at Friendship Park in Winchester, Virginia. This report includes an introduction to the problem and the site, a detailed literature review, an analysis of alternative designs, and final design specifications. The report also describes the economic analysis, and the maintenance and community outreach plans that were developed for the project.
Team Name:
Friendship Park
Group Members:
Dan Laird
Kelly Davis
Kerry Choi
Advisors:
Dr. W. Cully Hession (Associate Professor), Andrea Ludwig (PhD Candidate), Dr. David
Sample (Assistant Professor), Biological Systems Engineering, Virginia Tech
Client:
City of Winchester
Executive Summary:
Urban Stormwater Retrofit: Design and Implementation of a
Water Quality Swale and Pocket Wetland for Friendship Park, Winchester, VA
Opequon Creek is a tributary to the Potomac River in the Chesapeake Bay watershed. It has been listed on the Virginia’s Section 303(d) Total Maximum Daily Load (TMDL) Priority
List (VA DEQ, 2002) and undergone TMDLs analyses both bacteria and benthic impairments
(VA DEQ, 2006). A NFWF Targeted Watershed Grant for the Opequon Creek watershed was awarded to help develop a comprehensive project plan to lower nutrient loads within the watershed. As part of the project, several sites within the watershed have been identified as locations for best management practice (BMP) implementation, including ten constructed wetlands and six water quality swales. Friendship Park, in the City of Winchester, has been identified as a location for retrofitting of an existing urban stormwater management (SWM) facility to help reduce nutrients loads leaving the Friendship Park drainage area. The purpose of this project was to develop the retrofit plan for the park to improve sediment and nutrient retention.
The current Friendship Park SWM facility consists of a dry detention pond, with an area of 0.68 ha (1.68 ac), and a riprap water conveyance swale, with a length of 104 m (340 ft). The retrofit plan includes converting the current dry detention pond to a constructed wetland and converting the riprap swale to a water quality swale. To check that both the swale and the wetland will have the capacity to hold the desired stormwater, the program WinTR-55 was used.
From TR-55, we found that the storage volume is 2500 m 3 and 8800 m 3 for the 2-year and 10year storms, respectively. The peak stage for the 2-year and 10-year storms was 1.46 m and 2.85 m, respectively. The current dry detention pond was large enough to handle both storms and the design of the wetland ensured that this storage volume will not be decreased.
The conversion of the riprap channel to a water quality swale was based roughly on the
VA SWM handbook recommendations for a dry water quality swale. The water quality swale will encourage infiltration of stormwater runoff as it enters the swale. Infiltration will encourage nutrient uptake by the soil and then allow the stormwater to travel to the constructed wetland.
The design consists of leaving 0.5 m of the existing riprap in the channel, placing a geotextile liner over the riprap, and then placing 0.5 m of a permeable soil/sand mixture over the liner. The riprap is left in the channel as an alternative to placing an underdrain in the channel. The air voids between the riprap will act as an underdrain and allow the infiltrated stormwater to then travel to the wetland. The bottom width of the swale will be 2 m wide and the side slopes of the channel will be constructed at 3:1 slopes.
The conversion of the dry detention pond to a constructed wetland was based on a combination of the Meandering Flow wetland and the Cell wetland. The wetland will consist of 3 pools and a vegetated berm. The first pool will function as a sediment forebay and was designed to contain 0.25 cm (0.1 in) of runoff from the impervious area of the drainage area. It is built with a depth of 0.61 m (2 ft). Pool 2 was designed with a depth of 0.3 m (1 ft) and pool 3 with a depth of 0.35 m (0.8 ft). The surface areas at the top of pool 1, pool 2, and pool 3 were 210 m
2
,
139 m 2 , and 122 m 2 , respectively. The berm will be placed between pool 2 and pool 3 to increase the flow path of the wetland and to help increase the residence time of the stormwater in the wetland. The berm will be 15 m (49.2 ft) long and 0.9 m (3 ft) high. In between the pools, the flow will be from 5.1-10.2 cm (2-4 in). These areas of the wetland will act as shallow marshes.
The variation between pools and shallow marshes in the wetland helps to encourage nutrient uptake from the soil and wetland plants. The pools will be lined with a 45 mil EPMD plastic
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liner to address Karst concerns. To help reduce erosion within the pools, the sides will be lined with coconut fiber mats.
To function properly, the wetland and the swale must be planted with appropriate vegetation. Grasses and shrubs that can withstand moving water were selected for the swale.
Wetland plants that can survive inundation with water and moist conditions were selected for the shallow marsh areas. We selected trees and shrubs for the remainder of the site that are better suited for dryer conditions, but can handle occasional flooding.
The majority of the costs associated with the project will be covered under the Targeted
Watershed Grant. The total cost of the equipment and construction materials for the swale, wetland, and vegetation totaled $21,223, which was within our $30,000 budget. The majority of the cost was due to the soil matrix needed for the swale. Two things that were not included in the cost analysis were the trees and riprap for the berm. These materials are already available free of charge.
Maintenance of the wetland and swale is imperative for proper functioning. The most important maintenance occurs during the first three years, while vegetation is being established.
During this time, the vegetation must be kept alive and dead plants must be removed from the site and replanted with new plants. After the first three years, the wetland vegetation should be fully established. After this time, the site should still be periodically inspected for trash and debris, dead plants, and sedimentation build-up. Trash and debris must be kept clear of the flow path of the wetland. Pool 1 will need to be dredge to remove accumulated sediment roughly every five years.
The team has concluded that with proper implementation and maintenance, the proposed project will have a noticeable impact on nutrient levels of stormwater runoff leaving the
Friendship Park drainage area. The project area will also add aesthetic value to the surrounding community and provide an area for environmental education and recreation for local citizens.
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Table of Contents
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List of Figures
Figure 1. Existing stormwater structures Friendship Park and the Friendship Park drainage area.
Figure 8. Example constructed stormwater wetland design (from VA DCR, 1999). ................... 29
Figure 13. Layout of a water quality swale, with underdrain, checkdams, and forebay (from
Figure 14. Grassed swale showing the proper position of check dams (based on Yocum, 2007).41
Figure 15. Typical check dam configuration for the state of Virginia (from VA DCR, 1999). ... 42
Figure 16. Friendship park drainage area, with existing channel and dry pond highlighted. ....... 50
Figure 18. Existing riprap channel at Friendship Park that is to be retrofitted with a water quality swale. ............................................................................................................................................ 51
Figure 20. Plot of TSS results for stormwater collected during the October 25, 2008 storm at
Figure 19. Existing dry detention basin at Friendship Park that is to be converted to a wetland area. ............................................................................................................................................... 52
Figure 21. Hydrograph of the 2-yr and 10-yr, 24-hr design storms computed using WinTR-55 . 55
Figure 22. Team completing soil borings during second site visit. .............................................. 56
Figure 23. Exposed bedrock on the site is a primary constraint for this project........................... 59
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Figure 25. Meandering flow alternative design for the wetland at Friendship Park (from CWP,
Figure 32. Downstream view and side view of constructed berm (from VA DCR, 1992). ......... 72
Figure 34. Vegetation scheme for the “Nature Area” alternative. Wetland plants will be planted in the area shown in blue. Swale grasses will be planted in the area shown in yellow, and trees and shrubs will be planted in the area shown in green. ................................................................ 76
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List of Tables
Table 4. Results of TSS in stormwater runoff through the riprap channel during the October 25,
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I. Introduction
Title: Urban Stormwater Retrofit: Design and Implementation of a Water Quality Swale and Pocket Wetland for Friendship Park, Winchester, VA
Problem Statement
In recent years, environmental concern over nutrient loads to the Chesapeake Bay has been a pressing issue. As part of an ongoing project in the Opequon Creek watershed, a subwatershed of the Chesapeake Bay watershed, this group will design and implement a combination pocket wetland and water quality swale to improve the water quality leaving an urbanized drainage area upstream of Friendship Park in Winchester, VA. {looks like more of the solution than he problem – impaired & need some reductions}
Background Situation
The Opequon Creek watershed, part of the Upper Potomac watershed, is located within
Virginia and West Virginia and is approximately 880 km
2
(340 mi
2
). The Virginia portion of the creek spans 49.7 km (30.9 mi), and the West Virginia portion of the creek spans 54.9 km (34.1 mi). Opequon Creek receives both agricultural and urban nonpoint source (NPS) pollution from runoff and has been listed as impaired in Clean Water Act Section 303(d) (VA DEQ, 2002). At four monitoring stations within the watershed, total nitrogen and total phosphorous concentrations have exceeded U.S. EPA recommended levels in nearly every sample taken over the past five years (Heatwole et al., 2006). Nitrogen and phosphorus loads have been identified as the leading cause of the degradation of the watershed. From a 2004 report by the Potomac
Tributary Stakeholder Team, the Opequon Creek was found to have the highest levels of both phosphorus and nitrogen of any Potomac River sub-watershed within West Virginia (OCPT,
2008). Through the Targeted Watershed Grant Program, numerous agricultural and urban best management practices (BMPs) are being designed and implemented throughout the watershed
(Heatwole et al., 2006). As part of the this grant, this design will retrofit an existing dry detention pond with a pocket wetland and water quality swale at Friendship Park to help improve water quality in Abrams Creek, an impaired tributary of Opequon Creek. An aerial photograph of
Friendship Park, where the pocket wetland and water quality swale will be implemented is shown in Figure 1. A map of the Opequon Creek watershed is shown in Figure 2.
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Figure 1. Existing stormwater structures Friendship Park and the Friendship Park drainage area.
Figure 2. Map of the Opequon Creek Watershed from Middletown, VA to the Potomac River with red star showing Friendship Park project focus area (from Heatwole et al., 2006).
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Targeted Watershed Grant
The Opequon Creek watershed, located in Virginia and West Virginia, has been identified as a watershed with high levels of nutrient loading that contributes to the degradation of water quality in the Chesapeake Bay. Through The Targeted Watershed Grant, a comprehensive project has been developed to lower the nutrient loads leaving the watershed. The
Opequon Creek watershed has been shown to have significant nutrient loads from agriculture and urban nonpoint sources (NPS) (Heatwole et al., 2006). The project aims to reduce NPS nutrient loads from the Opequon Creek watershed by at least 48,988 kg (108,000 lb) of nitrogen
(N) and at least 6,123 kg (13,500 lb) of phosphorus (P) per year on average through the implementation of ten constructed wetlands and six water quality swales (Heatwole et al., 2006).
Connection to Contemporary Issues
In today’s increasingly urban and suburban world, the loss of open space to development has resulted in a variety of hydrologic problems (Randolph, 2004). The introduction of impermeable land cover causes higher stormwater runoff rates and volumes that can cause flooding, erode stream banks, and increase nutrient loading in surface water (Randolph, 2004).
The design and installation of BMPs can help control the negative effects of urbanization on water quality. By increasing detention time, allowing for infiltration, and reducing peak flow, it is possible to reduce nutrient and sediment loading. The methods used in this project for designing multiple, small-scale BMPs in series may be applied in other urban or suburban areas where space is limited and expensive.
Objectives
Survey Friendship Park drainage swale and potential pocket wetland location
Create a hydrologic model of the drainage basin
Estimate the flow into the Friendship Park from the drainage basin
Design a small, innovative wetland that may be used in urban environments
Design a water quality swale to install upstream of the wetland area
Develop a design for both BMPs to include CAD plans and construction documents
Complete an economic analysis to determine cost for installation
Include and inform stakeholders (local land owners, city engineers and planners)
Educate the community about wetlands using pamphlets
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Deliverables
Design an innovative wetland and water quality swale for stormwater control and nutrient reduction in Friendship Park, Winchester, VA
Develop design, vegetation, and maintenance plans
Present design options to the community through presentations at City of Winchester
Natural Resources Advisory Board meetings
Develop pamphlets to provide for public education
Work Plan
All team members had equal responsibility in the division of labor and the timely completion of the assignments. The team attempted to follow the dates of the timeline and Gantt chart, but reevaluation of the timeline occurred weekly. During the weekly meetings that occurred after the weekly meeting with advisors, each group member was assigned specific tasks for the next meeting that were vital for the overall design completion. Since this design was projected to be installed by April 2009, it was important that necessary goals and milestones were met and tasks were completed in a timely manner. While setbacks occurred that forced made it impossible to install the design at the site by the originally scheduled date, the team was able to create a final design that can be implemented in the future.
In the first semester of the project, the team accomplished several tasks that were necessary for the final design development. A site survey and visual assessment was completed in October 2008, and an AutoCAD surface was created based on the survey data. This AutoCAD surface is shown in Appendix A. This information was extremely useful during the creation of the final design. In addition to the site survey and AutoCAD work, the team completed
Geographic Information System (GIS) work, an extensive literature review, a work plan, the determination and ratings of alternative designs, and hydrologic analysis using TR-55. Lastly, the team has also prepared and presented the technical presentation outlining the progression of the design.
The team finished the hydrologic analysis and final design in the final semester of the project. The team made a public presentation in January and April for the Natural Resources
Advisory Board, whose approval is the first step in the implementation of the project. Specific vegetation was chosen for the wetland and swale based on the depth zones and location within the wetland. Along with the design creation, the team compiled costs of materials, estimates of
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maintenance costs, and a maintenance plan for the area. In addition, a community outreach plan was developed. Now, the next step is design approval and implementation which will be undertaken by Dr. Hession in the coming year.
There were many obstacles that slowed the progress of the project and influenced the final design. After the preliminary investigation of the site, bedrock was found at the surface of the existing dry pond. Since Karst {define?} is prominent in the area, certain provisions had to be made in the design to prevent structural problems related to Karst. While the team and advisors had originally decided that special considerations were not necessary for the site since it was already a stormwater structure, Winchester stakeholders and DEQ contacts informed us that liners were necessary for the pools. This is a relatively simple solution, but it took weeks to get in touch with our contacts to get an answer which was valuable time that was lost. The team encountered other obstacles that prevented the further progression of the design which resulted in deviation from the original timeline. The orthographic photographs of the drainage area were not properly geo-referenced in the ArcGIS model, and the team was missing data layers for roads and storm drains. This limited the ability to accurately estimate drainage area and determine the impervious surface area earlier in the project. In addition, the missing storm drain layer was required to understand what portion of the runoff within the drainage area was actually being conveyed to the site. Since no progress was made on this issue after using multiple on campus resources, the City of Winchester was contacted and the layers were made available. Using the data layers from the City and assumptions based on current standards, the impervious drainage area was calculated, and the team completed hydrologic analysis using TR-55. In addition, GIS was used to determine the watershed drainage area and impervious surface area, the proposed wetland area, and the swale length. However, the initial geo-referencing problem added to the delays in project implementation.
The two most important components of the work plan include the project milestones and project tasks. The project milestones are the important checkpoints in the project, while the project tasks are individual assignments used to achieve each milestone. The milestones and tasks are shown below. A Gantt chart is shown in Appendix B.
11/14 Task: Measure TSS results:
Kelly and Kerry
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11/20 Milestone: Finish Final Lit Review
Task: Swale: Dan
Task: Wetland: Kelly
Task: Economic Research: Kerry
11/18 Milestone: Review and Evaluate Alternative Designs
11/ 20 Task: Determine Alternative Designs: All members
11/18 Task: Determine selection criteria: All
11/18 Task: Discuss with advisors at team meeting: All
11/20 Task: Create Decision Matrix: Kelly
12/2 Milestone: Give Technical Presentation
12/1 Task: Create Technical Presentation: All members
12/15 Milestone: Complete Final Report
1/12 Milestone: Winchester Meeting
2/20 Milestone: Formulation of Final Design
11/15 Task: GIS work: Kerry
1/12 Task: Complete Hydrologic Analysis: All
2/6 Task: Soil Subsurface Survey: All
2/20 Task: CAD work: Dan
2/17 Task: Sediment Forebay Design: Kelly
2/20 Task: Perform research on materials: All
2/20: Task: Select materials: All
3/ 6 Milestone: Perform economic analysis for wetland and swale design
3/2 Task: Final CAD Design: Dan
3/ 3 Task: Determine Materials Cost
Task: Wetland: Kelly
Task: Swale: Dan
3/3Task: Determine Construction Cost: Kerry
3/23 Milestone: Midterm Presentation
3/23 Task: Midterm review of swale: Dan
3/23 Task: Midterm review of wetland
CAD: Dan
Vegetation: Kerry
Vegetation Scheme: Kelly
Pools: Kelly
4/23-24 Milestone: Final Presentations
4/23 Task: Final review of swale
4/23 Task: Final review of wetland
4/23 Task: Community Outreach: Kelly
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4/23 Task: CAD output: Dan
4/23 Task: Economics: Kerry
5/13 Milestone: Final Report
Project Timeline
The project timeline was adhered to as strictly as possible. However, due to the obstacles previously discussed in the work plan, all of the projected dates could not be met and the final project implementation at the site did not occur. The original projected date Gantt chart is shown in Appendix B. The Gantt chart with the actual date of work is shown in Appendix C. The following timeline shows the projected dates in black, and the actual finish date in parentheses.
September 16, 2008 First meeting with advisors
September 16
September 23
Preliminary research and grant review
Scope of work review with advisors
September 25
October 9
October 10
October 15
October 16
October 23
Scope of work due
Submit revised scope of work and cover page
Project notebook due
Review of surveying procedure and equipment
Site visit and survey
Submit revised cover page and scope of work with resources in ASABE format
Create site model from survey information October 24
October 31
November 7
November 13
Determine appropriate BMPs for the site
Complete all necessary research on the chosen BMPs and materials
Submit project notebook
Prepare safety, regulatory, and environmental considerations
Determine design criteria and alternatives
November 20
Create decision matrix and work plan
Complete literature review
November 28
Project notebook due
Evaluate alternative designs
December 1 Select final design
December 2 Technical presentation in class
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December 9
December 15
Project notebook due
Create PowerPoint presentation for public meeting
December 16 Final report I due
January 12, 2009 Present wetland plan to Natural Resources Advisory Board in Winchester
January 12-31
February 2
February 6
February 16
Modify design plan based on NRAB feedback
Project notebook due
Site visit to determine depth to bedrock
Midterm progress report due
February 17
February 19
February 20
February 23
Meeting with Denton Yoder for AutoCAD help
AutoCAD design work
Complete hydrologic analysis
Project notebook due
March 2 Revised work plan
March 3 (May 1) Final AutoCAD design
March 6 (May 1) Finalize all costs
March 6 (April 23) Second NRAB meeting
March 10 (April 23) Complete construction documents and AutoCAD design
March 23 Mid-term presentation
March 23 (April 20) Finalize vegetation
March 30 Project notebook due
April 5 (April 23) Finalize community outreach plan
April 10-15 (-) Complete installation of BMPs
April 20
April 23 (-)
Final report draft due
Final stakeholder meeting and presentation of wetland
April 24
May 1
Poster Presentation
Finalize maintenance plan
May 4
May 13
Final project notebook due
Final report due
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II. Literature Review
TMDL Plan
In order to meet provisions of the Clean Water Act, Section 303(d), Virginia has completed water sampling and pollution studies to determine the health of its water bodies
(USEPA, 2008). From these studies, many sections of Opequon Creek were found to be impaired water bodies. Abrams Creek, the tributary to Opequon that accepts stormwater from the
Friendship Park drainage area, has been listed as impaired due to bacterial and benthic impairment (VA DEQ, 2006). In 2004, the Opequon water quality assessment showed that
Abrams Creek had a violation rate of 22%, and 17% of collected fecal coliform sample concentrations exceeded 1,000 cfu/100 mL (VA DEQ, 2006).
As a result of its impaired water status, Virginia developed a total maximum daily load
(TMDL) and TMDL implementation plan for pollutants in Abrams Creek (VA DEQ, 2006). The implementation plan contains actions on stormwater runoff and development guidelines to reduce pollutants to meet standard levels set by the Clean Water Act and ordinances to enforce them (VA DEQ, 2006). For the Abrams Creek watershed, these actions include replacing 44 failing septic systems, establishing 11.7 ha (29 ac) of riparian zones, treating 668.5 ha (1,652 ac) with bioretention/infiltration areas, and enhancing erosion and sediment control practices (VA
DEQ, 2006). The installation of a pocket wetland retrofit to treat impervious runoff at the
Friendship Park Site will contribute to the success of initiatives of the Opequon Creek TMDL plan, specifically targeting the impairment of Abrams Creek by increasing stormwater infiltration and treatment.
Watershed Background
Winchester Demographics
The city of Winchester is a ,2420 ha (5,971 ac) independent city located within Fredrick
County, VA (US Census Bureau 2008). The city of Winchester has had a 7.1% increase in population from 2000 to 2006. As of 2006, the total population of the city was 25,265 (US
Census Bureau, 2008), and the primary land use of the city was residential. In 1990, 60% of the developed land of the City of Winchester was either residential or streets and alleys. The poulation density of the city was 10.1 persons/ha and the housing density was 4.4 houses/ha (US
Census Bureau 2008). A residential community consisting of medium density single family
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homes constructed around 1950 surrounds Friendship Park (Planning Commission, 1999). This area drains into Oppequon Creek which is a tributary to the Chesapeake Bay.
To the west of Friendship Park is a growing regional commercial center (Planning
Commission 1999). The major employeers of the area are wholesale and retail trades (26%), and the government (9%) as of 1988. Growth is expected to continue in the retail and service industires as Winchester becomes a regional center (Planning Commission, 1999). By 2000,
4,157 of the 24,041 city resident workers were employed in retail trade (US Census Bureau,
2008).
The majority of city residents hold jobs within the city (54%). Seventy percent of those who do commute to the city for work do so from the surrounding county (Planning Commission,
1999). The mean travel time for all workers living within the city is on 20.1 min (US Census,
Bureau 2008). Only 4.3% of city residents are unemployed as of September 2008 (Bureau of
Labor Statistics, 2008).
The median household income of city residents in 1990 was 84% of the state average, confirming that the majority of city residents are middle-income families. As of 2000, the median household income of city residents was $34,335 (US Census Bureau, 2008).
There are 4 elementary schools, 1 middle school, and 1 high school located within the city (Planning Commission, 1999), and one of these schools is adjacent to Friendship Park. The city has a total enrolled school population of 5,762 for the year 2000 (US Census Bureau, 2008).
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Figure 3. Map of the northeast area of the City of Winchester as of 2002, showing major land use categories and Friendship Park identified with the red circle (from Planning Commission, 1999).
The Winchester MS4
The State of Virginia has designated the City of Winchester as an “urbanized area” (City of Winchester, 2006). Because of this classification, the city is required to obtain a National
Pollutant Discharge Elimination System (NPDES) Phase II stormwater permit. To fulfill this permit, the city has a municipal separate storm sewer system (MS4) and sanitary sewer system.
Stormwater enters the city’s storm sewer system and is discharged directly into receiving surface waters. As of March 2003, the City of Winchester is covered by a Virginia Pollutant Discharge
Elimination System (VPDES) Permit for municipalities with separate storm sewer systems.
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States and supports more than
3,600 species of plants, animals, and fish (MDDNR, 2008). The Bay’s watershed is 165,760 km
2
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(64,000 mi²), approximately 320 km (200 mi) long, and contains over 100,000 streams and rivers
(CBP, 2008). States within the watershed include Delaware, Maryland, New York, Pennsylvania,
Virginia, and West Virginia, as well as the District of Columbia (MDDNR, 2008).
Since the first English settlement in Jamestown, VA was established in 1607, the population of the Chesapeake Bay Region has boomed and is residence to over 16.6 million people today (CPB, 2008). As the colonists grew in numbers and began to colonize the region, forests and land were cleared for homes, commercial, recreational and agricultural use. Without much knowledge of the consequences, waste was dumped into the bay and the first signs of environmental degradation began to show in the 1700’s (CPB, 2008).
Several different sources deliver loads of nutrients and sediment to the Bay. Three major sources are responsible for excess nutrients that reach the Bay: 1) specific, identifiable pipes from wastewater treatment plants; 2) runoff of the land from agriculture, farm animal waste, and urban areas; and 3) air pollution from vehicles, industries, and other sources (CBP, 2008). Other natural sources include soil, plant material, and wildlife waste.
The nutrients of most concern are nitrogen and phosphorous. Although these and other nutrients are needed by aquatic plants and animals to grow and live, excess levels are harmful.
During the summer season, the excess nutrients can cause algae to grow. The algae deprive the water of oxygen and block sunlight to the benthic vegetation. As the vegetation dies, organisms lose food, shelter, and the oxygen needed to survive (CBP, 2008).
In severe cases where there are higher concentrations of nitrogen and phosphorous in the water, eutrophication occurs. Eutrophication causes large algal blooms to form and creates dead zones where there is not enough oxygen to support a healthy ecosystem (CBF, 2008). This causes fish kills and diminishes aesthetic qualities with its strong smell and unnatural look, both reducing the values of the Chesapeake Bay and its tributaries (Sierra Club, 2008). Other organisms affected negatively include crabs, oysters, clams, water fowl, reptiles, amphibians, and plants in tidal and non-tidal wetlands (CBF, 2008).
Sediments that get transported into the Bay have adverse effects as well and result from the same mechanisms that deliver nutrients. Sediments come from erosion of the land, stream banks, and shorelines and from re-suspension. Although these are natural processes, the land cleared for development and agriculture use in the region has accelerated erosion to dangerous levels and 8.5 million MT (18.7 billion lbs) enter the Bay each year (CBP, 2008). As the primary
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culprit of habitat degradation, sediments can carry nutrients and chemical contaminants, block sunlight to underwater grasses, smother bottom-dwelling species, and can accumulate in ports and channels to the point where they are impassable (CBP, 2008). Due to the excess non-point source pollution from nitrogen, phosphorous, and sediments, the Chesapeake Bay and its tributaries have been placed on the Clean Water Act’s list of impaired waters (CBF, 2008).
Karst Terrain
Karst terrain is prevalent in the Ridge and Valley region of Virginia (CSN, 2008). Karst terrain has large spatial variability. Subsurface conditions and the risk of sinkholes can change within the matter of yards across a given site. Karst terrain also increases the interaction between surface and groundwater. This interaction leads to an increased risk of groundwater contamination from pollutants in surface runoff (CSN, 2008). Because of this increased interaction, infiltration of stormwater runoff is generally not desired. In Karst terrain, large centralized stormwater practices are not idea because of the increased risk of sinkholes and infiltration. Instead, several small and shallow practices are desired. In areas of Karst terrain, designers should reduce the risk of a SWM practice that can become a Stormwater Hotspot.
Stormwater Hotspots are those SWM practices that are known to produce higher concentrations of stormwater pollutants (CSN, 2008).
In Karst terrain, special site investigations should be conducted. The investigation should include the location of any existing sinkholes. The investigation should also include analysis of existing geological and topographic maps, aerial maps, and a site visit (CSN, 2008). The purpose of the investigation is to identify areas of subsurface voids, cavities, fractures, and discontinuities. It should also include bedrock characteristics and soil characteristics.
Stormwater designs options are limited in Karst terrain. In general, the design should minimize site disturbance and changes to the soil profile. The stormwater practice should be designed to disperse flows over the broadest area possible. The design should avoid ponding, concentration, and soil saturation when possible. Stormwater practices and their suitability in
Karst regions are shown in Table 1. Stormwater practices often need to be closed and lined to prevent groundwater interaction. Closed practices refer to practices that are closed from groundwater. Closed practices should posses an impermeable filter fabric or liner and include an underdrain. For stormwater practices in Karst regions, the minimum depth of the filter bed can be reduced to 18 inches if specific site conditions are problematic (CSN, 2008).
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Table 1. Suitability of stormwater practices in Karst regions (CSN, 2008).
Stormwater Practice Suitability in Karst Regions Design and Implementation Notes
Bioretention (closed) Preferred Lined with underdrains
Urban Bioretention
Rain Tank/Cistern
Rooftop Disconnection
Green Roofs
Preferred
Preferred
Preferred
Preferred
4.6 meter (15 feet) foundation setback
Dry Swale
Filtering Practices
Filter Strips
Grass Channel
Preferred
Preferred
Adequate
Adequate
Lined with underdrains
Water-tight or lined with underdrains
Conservation filters
Compost amendments
Soil Compost Amendment
Small Scale Infiltration
Micro-bioretention
Permeable Pavers
Constructed Wetlands
Wet Ponds
Dry Ponds
Wet Swale
Large Scale Infiltration
Adequate
Adequate
Adequate
Adequate
Adequate
Discouraged
Discouraged
Prohibited
Prohibited
Not at stormwater hotspots
Closed systems
Use liner or linear cells
Liner required
Liner required
Pollutant Removal Mechanisms
Nitrogen (N) and phosphorus (P) are both nutrients needed by plants and animals to live and grow. These nutrients occur naturally and can come from soil, plant material, wild animal waste, and the atmosphere (CBP, 2008). Due to anthropogenic effects, the excessive levels of nutrients found today no longer sustain life but limit it instead. The excessive levels create algal blooms that block sunlight and rob the water of oxygen (CBP, 2008). In order to improve water quality, the mechanisms of how these nutrients are removed from water need to be known.
Phosphorus
Sedimentation is the main physical process in phosphorus removal (CWP, 2008).
Sedimentation is the settling of suspended particles to the bottom of the wetland. Because incoming particles from the influent have phosphorus adsorbed to them, most phosphorous in runoff can be removed by sedimentation (Kadlec and Knight, 1996). To obtain high rates of sedimentation, the constructed wetland should be designed for low flow velocity, rooted wetland vegetation, and long residence time (CWP, 2008).
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Another important mechanism of phosphorus removal is the adsorption of soluble phosphorus (Kadlec and Knight, 1996). Adsorption is the adherence of the soluble phosphorus to sediments, vegetation, or ditritus material within the wetland (CWP, 2008). Some characteristics of the wetland to improve adsorption include low flow velocity, long residence time, complex microtopography, and dense emergent vegetation (CWP, 2008).
Nitrification and Denitrification
The surfaces of constructed wetlands provide encouraging conditions for active microbial growth and processes that are important in water quality (CWP, 2008). Via nitrification and denitrification, the removal of nitrogen (N) from water is possible. First, under aerobic conditions, bacteria transform ammonium (NH
4
+
) to nitrate (NO
3
-
) through the nitrification process (CWP, 2008). Then, under anaerobic conditions, bacteria convert NO
3
-
into nitrogen gas through denitrification, effectively releasing nitrogen into the atmosphere (Groffman and
Hanson, 1996). Denitrification requires carbon sources, supplied by vegetation, and long water retention to function properly and efficiently (CWP, 2008). Longer periods of inundation also provide higher dentrification rates. This is supported by a study done by Groffman and Hanson
(1996) in Rhode Island where they found that poorly drained soils achieved higher denitrification rates than well-drained soils.
Constructed Wetlands
Background
The idea of using constructed wetlands to retain runoff and remove nutrients from stormwater was first developed in the mid-1980’s and gained popularity throughout the 1990’s
(CWP, 2008). Constructed wetlands mimic natural wetlands to mitigate the water quality and quantity impacts of development (Metropolitan Council, 2001).
With the increased urbanization that has occurred in Winchester since installation of the dry pond at Friendship Park, the area receives a much greater stormwater runoff and pollutant input. While a dry pond is currently in place at the site, it was proposed to retrofit the area with a stormwater wetland to alleviate the impact of nutrients and other pollutants carried in the runoff.
There are four main types of stormwater wetlands: shallow marsh, extended detention
(ED), pond/wetland system, and pocket wetland (CWP, 2008). A shallow marsh is a large, shallow wetland that can achieve moderate pollutant removal rates, but requires a drainage area
~ 24 ~
greater than 10.1 ha (25 ac) (CWP, 2008). An ED wetland uses a shallow marsh with vertical storage for stormwater runoff to achieve moderate pollutant removal rates (CWP, 2008). This type of wetland often experiences low plant diversity as a result of the highly fluctuating waterlevels caused by the vertical storage. The pond/wetland system is designed with a large, deep wet pond cell to decrease incoming water velocity and cause sedimentation and with a shallow marsh area to reach a high level of pollutant removal (CWP, 2008). Pocket stormwater wetlands are most appropriate for small sites because they can be designed for areas of less than 4.1 ha (10 ac)
(CWP, 2008). However, there are many issues that arise with pocket wetlands. Since they are small, they usually lack base flow and must be supported by stormwater runoff (CWP, 2008).
This makes them subject to highly fluctuating water-levels often in excess of 20.32 cm (8 in) above normal water elevation, which has a negative effect on both vegetation and pollutant removal rates (CWP, 2008). The wetland design should include measures to minimize water level fluctuation to provide for gradual increases and decreases in water level and ease the stress on vegetation (VA DCR, 2004). While some sources suggest that pocket wetlands no longer be used because they lack species diversity and habitat value (CWP, 2008), other sources propose that pocket wetlands are a cost effective way to improve water quality and provide habitat for a wide variety of organisms, including fish, insects, and birds (Metropolitan Council, 2001).
Since Winchester has become increasingly urbanized over the past twenty years, a stormwater pocket wetland in place of the dry detention pond at Friendship Park has the potential of attenuating nutrients and providing an atmosphere for passive recreation. According to recent literature, retrofitting dry ponds with wetlands is now an accepted practice because many dry ponds become unintentional wetlands from lack of maintenance and changes in hydrology
(CWP, 2008).
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Figure 4. A dry pond that was naturally converted to a wetland (from CWP, 2008).
The typical constructed wetland is created with a meandering flow as shown in Figure 8.
This lengthens the flow path and the time for water to reach the outlet to allow for more infiltration and biological interactions to decrease nutrient loads. According the Virginia
Stormwater Management (SWM) Handbook (VA DCR, 1999), there are four depth zones required for a constructed wetland: 1) the deep pool; 2) the low marsh; 3) the high marsh; and 4) the semi-wet area. The deep pool depth zone should cover approximately 10% of the wetland area, the low marsh depth zone should cover approximately 40% of the area, and the high marsh depth zone should cover 50% of the wetland area. The low marsh depth can range between 15.24 cm (6 in) to 20.32 cm (8 in), and the high marsh depth should range from 0 to 15.24 cm (6 in)
(VA DCR, 1999).
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Figure 5. Depth zones in a constructed wetland (from VA DCR, 1999).
Another type of constructed wetland is an island system. The island wetland design alternative is unique by utilizing an island located in the middle of the wetland, while also using a sediment forebay. The forebay allows sediments to settle out and the presence of the island alters the flow path, not allowing water to flow straight to the outlet, both increasing detention time. The island will extend above the normal water surface and can support a variety of emergent vegetation in the high marsh area, while also supporting plant species that prefer drier conditions in the higher elevated parts of the island (CWP, 2008). An example of this design is shown in Figure 6.
Figure 6. Island alternative wetland design (from CWP, 2008).
A third type of wetland that has recently been proposed by the Center for Watershed
Protection (CWP, 2008), is the use of a wetland cell system. This system creates micro-
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topography within the wetland with pools of varied depth zones to encourage nutrient removal while limiting water level fluctuations and allowing for continuous flow of water. According to the Center for Watershed Protection, each cell within the system should 25% of the area of the total system, and the total system should have a length to width ration of at least 3:1. The cells should alternate between deep and shallow pools, with the deepest pool being 45.72 – 60.96 cm
(18 – 24 in) and the shallowest pool being 5.08 – 10.16 cm (2 – 4 in). An example of wetland cells is shown in Figure 7.
Figure 7. Wetland cells in conjunction with a wet pond (from CWP, 2008).
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Figure 8. Example constructed stormwater wetland design (from VA DCR, 1999).
While there are limitations of using wetlands for stormwater management, there are design methods that can be implemented to help deal with these limitations. Most designed wetlands currently in place are not considered forested wetlands because they only include emergent vegetation (CWP, 2008). However, forested wetlands with trees and shrubs have been shown to maximize pollutant removal and provide increased species diversity. In addition, uniform topography within the wetland can allow for one or two species to dominate vegetation, and water-level fluctuations greater than 20.3 cm (8 in) can cause a decline in species variation
(CWP, 2008). By reducing water-level fluctuations with the design of multi-cell ponds and ensuring that the topography of the designed wetland is properly maintained, there is an increased chance of sustained species variation. Other BMPs, such as forebays and micropools, can be used along with wetlands to improve their function (Davis, 2004).
Wetland Pollutant Removal
The physical, biological, and chemical interactions that occur in wetlands can be extremely effective in the removal of water pollutants and nutrients, and there are several mechanisms for pollutant removal within a wetland ecosystem. Constructed wetlands are designed to maximize the removal of pollutants and nutrients through plant uptake, retention of
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stormwater runoff, settling, and adsorption (Metropolitan Council, 2001). In addition, chemical precipitation of pollutants, water filtration by wetland plants, and microbial transformation can occur in constructed wetlands under certain conditions (CWP, 2008). Studies show significant, though widely ranging, pollutant removal rates for constructed wetlands. Removal rates are highly variable for several reasons. They can vary throughout a storm water event because removed pollutants are subject to re-suspension in a pocket wetland (CWP, 2008). Removal rates can vary throughout the year as a result of the decrease in biological activity during winter months (VA DCR, 1999). Constructed wetland removal rates also fluctuate over time as the wetland and its vegetation matures (VA DCR, 1999). Expected removal efficiencies are difficult to predict due to site specific soils, hydrology, climate, and design (VA DCR, 1999). There is also uncertainty associated with the biological cycling processes of phosphorous in the wetland environment which add to the complexity of determining expected removal rates (VA DCR,
1999).
Figure 9. Pollutant removal pathways that exist within a functioning constructed stormwater wetland (from
CWP, 2008).
Suspended solids removal rates in constructed wetlands can range from 45 – 95% (CWP,
2008). According to one study (Harter and Mitsch, 2003), sedimentation rates in a constructed
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marsh wetland averaged about 4.9 cm/yr or 36 kg/m
2
/yr and ranged from 1.8 to 9.2 cm/yr. The study found that sedimentation rates were much higher for deep, open pools rather than shallow vegetated areas. In addition, the study concluded that by slowing water velocity, wetlands create a depositional environment for sediment which can also carry sorbed nutrients (Harter and
Mitsch, 2003). Total phosphorous removal rates range from 15 – 75% (CWP, 2008). The
Virginia Stormwater Manual (VA DCR, 1999) suggests that the targeted removal efficiency over the long term is approximately 30% when the wetland is designed to carry twice the water quality volume and a sediment forebay is present. Phosphorous removal can be accelerated and increased by incorporation of woody vegetation that serves as a phosphorous sink (CWP, 2008).
Wetland conditions also promote growth of microbial populations that extract soluble carbon and nutrients and potentially reduce BOD and fecal Coliform levels (Metropolitan
Council, 2001). Bacteria removal rates from constructed wetlands range from 40-85%, and nitrogen removal rates rate from 0-55% (CWP, 2008). Nitrogen removal rates can be as low as
0% during periods of very low inflow nitrogen concentrations. When this is the case, it is possible for internal nitrogen processes within a wetland to create more nitrogen than was entering the wetland from runoff, resulting in no net nitrogen removal (Kadlec and Knight,
1996).
Nitrogen removal rates can be increased by dense emergent vegetation growth (CWP,
2008), and by the addition of organic matter (Burchess et al., 2007). While adding organic matter can accelerate the aging of the wetland, it can also significantly increase biomass growth and denitrification. The recent study showed that by increasing organic matter from 50 g/kg to 110 g/kg, the NO
3
-
wetland treatment efficiency was significantly increased (Burchess et al., 2007).
Wetland Maintenance
Unlike natural wetlands, constructed wetlands are not self maintaining and require routine inspection and maintenance (Metropolitan Council, 2001; CWP, 2008). Often, constructed wetlands are poorly maintained, so the pollutant removal capacity degrades, forebays and deep pools fill with sediment, and invasive species may overtake native plants. To prevent this, a long term vegetative management plan should be developed, and a maintenance agreement should be established (VA DCR, 1999). With proper access to the constructed wetland, maintenance practices are relatively easy and inexpensive. While maintenance is required throughout the lifespan of a constructed wetland, the first three years of the maintenance plan are critical to ensure that vegetative diversity and density is established (CWP, 2008).
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During this time, the constructed wetland should be inspected at least twice a year (VA DCR,
1999) during both the growing and non-growing season and detailed observations should be recorded. Observations of interest include type, dominance, and distribution of plants within the wetland, as well as evidence of invasive species (Metropolitan Council, 2001). Sediment accumulation, stability of designed depth zones, and amount of standing water should also be evaluated (Metropolitan Council, 2001). Vegetative management practices include care of existing desirable vegetation (watering, pruning, and thinning), supplemental plantings, and the removal of invasive species or undesirable plants (CWP, 2008). According to the Virginia
Stormwater Manual (VA DCR, 1999), it is likely that reinforcement planting will be required at the beginning of the second growing season. Harvesting before winter dieback is unnecessary for wetland plants because most of the nutrients are contained within the roots (VA DCR, 1999).
Because sedimentation can reduce the water-quality volume of a constructed wetland, dredging will need to be employed from time to time. If a small forebay or micropool is present, it is to be dredged every other year or as needed (Metropolitan Council, 2001). If present, larger forebays and micropools are to be dredged on a five- or ten-year cycle, depending on the rate of sedimentation (Metropolitan Council, 2001). According to the Virginia Stormwater Manual (VA
DCR, 1999), sediment must be removed from the forebay every three to five years or after 15.2-
30.5 cm (6-12 in) of sediment accumulates.
Other maintenance practices are also required and must be performed as needed, including: clearing debris after storm events; removing trash from trash racks and outlet structures; and mowing grasses (CWP, 2008). These activities are not difficult and do not require a high of expertise, so they can be completed by citizen volunteers. Mowing should occur biannually for the access road and embankments (VA DCR, 1999).
Vegetation
Plants native to their region and tolerant to inundated soils are significant in constructed wetlands. Because they are adapted to the climate, soils, and pests of the region, native plants will grow faster and will need less maintenance when compared to non-native plant species
(Wetland Watch, 2008; VA DCR, 2008). Along with contributing to greater aesthetic values, different wetland plants perform different yet significant processes required for a healthy wetland
(CWP, 2007).
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Dense vegetation is very important in nutrient removal (CWP, 2007). The vegetation improves water quality by providing surfaces for pollutants to adsorb to; providing a substrate where microbes can transform nutrients; creating aerobic conditions by pumping oxygen from their leaves down to their root zones; and providing carbon sources for denitrification (CWP,
2007).
Different vegetation must be considered for different areas and depths of the constructed wetland. For deep zones of the wetland that are inundated constantly, such as fore-bays and micro-pools, submerged aquatic vegetation is to be used (VA DCR, 1999). Providing habitats and food to diverse communities of aquatic organisms, submerged aquatic vegetation also supply invaluable functions to wetlands: 1) provide oxygen in the water; 2) filter and trap sediments; 3) absorb nutrients and toxic metals; and 4) increase productivity levels (FWS, 2008).
A thick and dense distribution of submerged aquatic vegetation is desirable to best provide the benefits of ecological function. Either nursery-grown or wild stock may be used.
Collection of these plants should occur between mid-April and late June preferably be planted within 24 hours, as the plants lose vigor quickly (MRC, 2000). Instead of planting them throughout the deep zones, three test plots of 2 m by 2 m in size to see if the species selected will become established and succeed (MRC, 2000). Once proven successful, re-planting projects on larger scales may be planned to achieve dense vegetation. Proper monitoring and assessment are essential to ensure the establishment of the submerged aquatic vegetation. Naturally occurring species in the Chesapeake Bay region include Eurasian watermilfoil, sago pondweed, redhead grass, horned pondweed, wild celery, common elodea, water stargrass, coontail, and southern naiad (Orth and Moore, 1979).
Shallow marsh areas of consistent inundation in the wetland are locations for emergent wetland plants (VA DCR, 1999). Like submerged aquatic vegetation, emergent wetland plants provide many of the same benefits of habitat, food, oxygen, nutrient uptake, and increasing productivity levels (CBP, 2008). However, because these plants are closer and grow above the water level of the wetland, they provide extra values. Benefits include reducing flow velocity, preventing re-suspension of sediments, provide habitat for predatory insects, keeping a check on mosquitoes, and improve aesthetics (VA DCR, 1999).
At minimum, five to seven emergent wetland species should be included in the constructed wetland and planted during the growing season (VA DCR, 1999; Metro Council,
~ 33 ~
2001). Due to the difficult germination of emergent plant seeds and the chance for them to get transported by moving water (Grelsson and Nilsson, 1991), nurseries that provide these species already grown in small containers or plugs are better suited for quicker establishment. Like submerged aquatic vegetation, nursery-grown or wild stock plants may be used for re-vegetation.
With saturated soils, digging holes large enough for the plugs with your hands is possible. The plants should be planted 46 cm (18 in) apart (Hoag, 2000). Although these plants have long planting windows, they generally establish better when planted during spring and spread faster
(Hoag, 2000). Sedges, spikerushes, bulrushes, and rushes are common emergent plants found in natural wetlands (Hoag, 2000).
Trees and shrubs are important and innovative aspects of constructed wetland vegetation that will be planted in high marshes where wet and dry periods will occur. When compared to submerged vegetation and emergent plant species, the deeper and wider root systems of trees and shrubs are more efficient at removing nutrients and pollutants from water and promote infiltration, both improving water quality (CWP, 2007; FWS, 2008). The more efficient pollutant removal is also improved by the greater transpiration rates of these plants. Nutrient and pollutant uptake most often occurs from water through plant roots. The greater transpiration rates translate into more water being absorbed by the plants, carrying with it the nutrient and pollutants that degrade water quality. Table 2 compares the different transpiration rates between emergent plants and trees. The superior transpiration rates of trees indicate that they hold much more nutrient and pollutant removal capabilities.
Table 2. Transpiration Rates of Various Plant Species (CWP, 2007).
Plant Name
Common Reed
Great Bulrush
Sedge
Cottonwood
Hybrid Poplar
Cottonwood
Weeping Willow
Plant Type
Emergent
Emergent
Emergent
Tree (2 years old)
Tree (5 years old)
Full mature tree
Full mature tree
Transpiration Rate
1.17 cm/day
2.18 cm/day
4.83 cm/day
7.57-14.20 L/day/tree
76-150 L/day/tree
190-1320 L/day/tree
760-3030 L/day/tree
Like the other wetland plants, trees and shrubs can provide habitats for wildlife as well.
These plants will provide safe shelter for birds and supply nesting and feeding opportunities
(Kusler, 2006). They will also supply more habitats for amphibians and reptiles (Kusler, 2006).
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Trees and shrubs also discourage people and nuisance animals, such as geese, from entering the constructed wetland (VA DCR, 1999).
With careful planning and design, the vigor and life of trees and shrubs will not be negatively affected. The placement of trees and shrubs on top of the embankments is prohibited because of their deeper root systems: they can damage the integrity of the embankment, causing its failure, and, without enough depth to grow, will lose vigor and lead to premature mortality
(VA DCR, 1999). Trees and shrubs already grown, but not fully grown, in nurseries will help them become established faster and better, similar to submerged aquatic vegetation and emergent plants. Planting holes should be three times wider and deeper than the root ball and five times deeper and wider for container-grown stock (VA DCR, 1999). The planting holes should also be located in areas where proper depth will not interfere with root growth and spaced so root zones will not interfere with each other or block sufficient sun light (VA DCR, 1999). Recommended by the Center for Watershed Protection (2007), trees most often used for phytoremediation include willow, poplar, and mulberry trees because of their deep root systems, fast growth, high tolerance to moisture, and ability to control migration of pollutants by consuming large amounts of water.
In regard to the species that should be chosen, selections should be based on depth tolerance, price, commercial availability, and beneficial wildlife contributions (DoW, 2008).
According to the Metropolitan Council of Minnesota (2001), more priority should be given to plant species that have proven to be successfully established. With the success of the
Ceadermeade pocket wetland nearby in Winchester, special emphasis will be put on the plants established there. The plants used there were Fox Sedge, Soft Rush, Blue Flag, Black-eyed
Susan, Silky Dogwood, Square Stemmed Monkey Flower, Partridge Pea, Wool Grass,
Elderberry, Blue Vervain, Blunt Broom Sage, American Cranberry, Green Bulrush, and
Common Sneezeweed. Tables from the Virginia Department of Conservation and Recreation
(1999) are available to assist in the selection of different native submergent, emergent, trees and shrubs found in Virginia that can be implemented in constructed wetlands.
Swales
Grassed Bioswales are BMPs used to treat stormwater runoff. The state of Virginia refers to these swales as either Grassed Swales or Water Quality Swales. Two main distinctions are made between the types of swales. Grassed swales are used primarily to carry stormwater
~ 35 ~
volumes; water quality swales are used primarily to treat the required water quality volume in addition to carrying stormwater volumes. The Virginia Stormwater Management Handbook definition of a grassed swale is…
“an earthen conveyance system which is broad and shallow with erosion resistant grasses and check dams, engineered to remove pollutants from stormwater runoff by filtration through grass and infiltration into the soil.”(VA DCR 1999)
Grassed swales and water quality swales are intended for drainage areas with a relatively high impervious cover. According to the state of Virginia, the intended impervious cover of the drainage area for a grassed swale is 16%-21% and the intended impervious cover for a water quality swale is 16%-37%. When used within the intended percent impervious cover for a drainage area, the State of Virginia has established expected removal rates of phosphorus for both grassed swales and water quality swales. Virginia’s expected phosphorous removal rates are shown in Table 3. As shown, the target phosphorous removal efficiency for a water quality swale is 20% more than for a grassed swale. A typical grass swale used for stormwater runoff is shown in Figure 10.
Table 3. Pollutant removal efficiency for stormwater swales (VA DCR, 1999).
Water Quality BMP
Target Phosphorus
Removal Efficiency
Impervious
Cover
Grassed Swale
Water Quality Swale
15%
35%
16-21%
16-37%
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Figure 10. A typical grass swale with riprap check dams (from VA DCR, 1999).
Swales are advantageous over other more conventional types of stormwater conveyance.
They have relatively lower capital costs, they provide a reduction in peak flow, and they promote some infiltration and pollutant removal (EPA 1999). Swales are not a practical SWM solution in areas of very flat or very steep grades and they become ineffective and erodible in high flow volumes or velocities (EPA 1999). Grassed swales are also not effective in areas of wet or poorly draining soils. Existing soil conditions are less influential on water quality swales due to the engineered media underlying them (VA DCR 1999).
Grassed Swales
Grassed swales, or channels, are open channels that provide limited water quality treatment. They are able to reduce flow velocities and can increase filtration capacity (CWP
2007). They cannot however provide the same degree of pollutant removal as water quality swales. A typical grassed swale configuration is shown in Figure 11.
Maintenance of grassed swales is fairly straightforward. The must be mowed and the vegetated cover must be kept dense and living. Grass should ideally be kept at a height of 0.15 m
(6 in). The hydraulic properties of the swale must also be maintained (VA DCR 1999).
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Figure 11. Typical configuration of a grassed swale (from VA DCR, 1999).
Water Quality Swales
Water quality swales are referred to by several terms: bio-swales, dry-swales, wet-swales, or water quality swales (VA DCR 1999; CWP 2007; Grove Unknown). A typical water quality swale configuration is shown in Figure 12. An overall layout design of a water quality swale is shown in Figure 13.
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Figure 12. Typical configuration of a water quality swale (from VA DCR, 1999).
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Figure 13. Layout of a water quality swale, with underdrain, checkdams, and forebay (from CWP, 2007).
Wet-swales are essentially linear wetlands within a stormwater conveyance swale. A wetswale incorporates a shallow permanent pool within the swale to provide water quality treatment
(CWP, 2007). The wet-swale provides a higher level of nutrient removal, but is not an appropriate approach for residential or commercial settings due to the shallow standing water.
The shallow pool created by the wet-swale provides a high risk of mosquito breeding (US EPA,
2006).
Dry-swales are a linear soil filter system (CWP, 2007). They are more like a bioretention trench, and are what the state of Virginia refers to as water quality swales. For water quality swales to work properly, a sand/soil combination media must be engineered to underlay the swale (CRD, 2008). The engineered media is necessary to increase permeability and increase infiltration capacity of the swale (Yocum Unknown). The state of Virginia suggests that this media be comprised of 50% sand, 20% leaf mulch, and 30% top soil (VA DCR 1999). In addition, water quality swales with compost added to bed material have been shown to experience faster growth, thicker coverage, and higher removal efficiency (Yocum Unknown).
Water quality swales may have an underdrain underlying the engineered media (CWP
2007) . This underdrain is usually a perforated pipe, surrounded by gravel, stretching from the inlet to the outlet of the swale. The purpose of the underdrain is to convey treated stormwater runoff downstream of the water quality swale (CWP 2007).
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Many swales make use of check dams to slow water down and create ponding behind the check dams to increase nutrient and sediment removal. A typical swale with check dams is shown in Figure 14. Check dams are riprap dams placed along the swale to increase the water quality treatment of the swale. The check dams should be spaced far enough apart so that the proper water quality volume is provided behind the dams (VA DCR 1999). A typical check dam configuration from Virginia is shown in Figure 15.
Water quality swales must be properly maintained to ensure adequate water quality treatment. The vegetation within the swale must be kept dense and living. At the start of each growing season, dead vegetation should be removed and reestablished. Vegetation should be maintained at a height of 15 cm (6 in). Accumulated sediment must be removed periodically.
Figure 14. Grassed swale showing the proper position of check dams (based on Yocum, 2007).
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Figure 15. Typical check dam configuration for the state of Virginia (from VA DCR, 1999).
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Vegetation
Swales must have a dense, established cover in order to work properly. Appropriate vegetation must provide a dense cover and strong root structure and stand upright in strong water flows. The vegetation should also be able to tolerate periodic flooding and drought. It cannot be dormant during any rainy season (Yocum Unknown). The state of Virginia suggests: Kentucky-
31 tall fescue, reed canary grass, redtop, and rough-stalked blue grass (VA DCR 1999).
Safety
Water Safety
Since Friendship Park is located in a residential area within walking distance of a school and recreational fields, it is important to ensure that the BMP design will not endanger local children and residents. During high-flow periods, the swale may receive a large amount of water and high peak flows that will result in high flow velocities that may be unsafe for small children.
Keeping this in mind, the swale can be designed with check dams to reduce the flow velocity
(VA DCR, 1999).
Deep pools within the constructed wetland present drowning concerns that must be addressed in the design. By designing the wetland with a wide aquatic shelf of low and high marsh zones surrounding the deep pools, children can be kept away from the deep pool. In addition, dense vegetation can be planted along the perimeter of the deep pools and the sediment forebay to discourage unauthorized access. Local codes may also require fencing of deep pools.
However, the Friendship Park wetland should become an area of passive recreation for the residents, so fencing people out of the entire wetland is not ideal. Therefore, warning signage and other alternatives to fencing will be proposed to the community and the town engineer.
Flooding may also be a safety concern for local residents, especially for those who own property adjacent to the site. To ensure that flooding does not occur after the installation of the
Friendship Park BMPs, hydrologic analysis will be completed based on both the 2-yr and 10-yr,
24-hr storms to determine the required storm water runoff volume. In addition, the team will not decrease the existing storage volume at the site, only increase the capacity if necessary and change the flow path of the water.
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Vector Control
Mosquitoes can be a major pathway of disease transmission, or vector, in the world (VA
DCR, 2005). Mosquitoes are capable of carrying and transmitting many diseases, including West
Nile Virus, which has been found in Virginia. Since mosquitoes thrive in standing water that occurs often in stormwater BMPs, it is important to carefully design the wetland and swale system to eliminate and control mosquito habitat. This can be done by minimizing shallow depths and designing for continuous circulation within the wetland. The grassed swales should be designed to ensure that no standing water occurs for more than 24 hours to eliminate mosquito habitat (Yocum, 2007). In addition, VA DCR suggests implementing steep side slopes in the wetland to allow for more water to circulate within the system (VA DCR, 2005).
Structural Considerations
Structural safety considerations must be addressed when designing any stormwater management system. The main structural concern for a constructed wetland or grass swale is the failure of created slopes. In the wetland, side slopes will be designed considering safety and machinery restrictions and made no steeper than 3:1 (VA DCR, 1999). The cross slopes of the swale should be built at a maximum horizontal to vertical ration of 4:1 so that maintenance can occur safely (ASABE, 2006).
In addition, it is important to ensure that existing underground utilities are mapped and will not be affected by the design. Most utility companies will not allow their lines to be disturbed or submerged by a permanent pool of water (VA DCR, 1999). Therefore, costs of moving underground utilities will be estimated, and the design will have to be created around existing utilities if moving them is unfeasible.
Regulations
Federal
There are a number of federal, state, and local regulations and laws that must be considered for the design of constructed wetlands. The Clean Water Act sets the basic structure for regulating discharges of pollutants into the waters of the United States (US EPA, 2008).
Because of this act and the Bay’s degraded state, the Bay and its tributaries have been listed in the Clean Water Act’s impaired waters. Under Title III, water quality standards are risk-based requirements that are set by the states. The water quality standards (US EPA, 2008) set site-
~ 44 ~
specific allowable pollutant loads for individual bodies of water and depend on the use of the water body (recreational, habitat protection, water supply, and agricultural use) and water quality criteria (nutrient concentrations and water quality volume). Because the proposed wetland is part of a tributary to the Chesapeake Bay, the water quality of the effluent will be considered under this act. Proper nutrient and sediment removal mechanisms will have to be established within the wetland to meet the water quality standards set by the state of Virginia.
The Federal Endangered Species Act (US EPA, 2008) may be considered as well. This act provides the program for the conservation of threatened endangered plants and animals and their habitats. If any endangered specie(s) is present within the immediate area of the wetland and grass swale, the design of either may be altered to provide a habitat or preserve the habitat of the endangered specie(s).
Along with federal laws, there are a number of state laws that must be abided by.
The Chesapeake Bay Agreement, created in 1983, is committed to removing the Bay and its tributaries off of the Clean Water Act’s list of impaired waters (CBF, 2008). It was signed by
Pennsylvania, Maryland, Virginia, the District of Columbia, and the Environmental Protection
Agency. This agreement established the Chesapeake Executive Council as the chief policymaking authority in the Bay region to achieve and maintain the water quality necessary to support aquatic living resources of the Bay and its tributaries (MD DNR, 2008). For the removal from the impaired waters list, best management practices (BMP’s) have been implemented to improve water quality. For the proposed wetland, it must meet the water quality requirements set by the Chief Executive Council (MD DNR, 2008) by reducing sediment and nutrient loads.
State and Local
The main standard for constructed wetlands and swales in Virginia is the Virginia
Department of Conservation and Recreation Stormwater Management Handbook (VA DCR,
1999). This Handbook covers Virginia regulations for designs of earthen embankments, principal spillways, emergency spillways, and multiple best management practices related to stormwater.
The use of this handbook is mandated in Chapter 9: Water Protection Ordinance in the Code of the City of Winchester (City of Winchester, 2006). These standards are also in accordance with
§10.1-560 et seq. of the Code of Virginia which mandates Virginia Erosion and Sediment
Control Law. In addition, 4VAC50-60-30, Virginia Stormwater Management Program (VSMP)
Permit Regulations also apply to every land disturbing activity regulated under §10.1-603.8 of
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the Code of Virginia. Before the BMP is implemented at the site, an Erosion and Sediment
Control Plan must be completed for any land disturbing activity that an erosion impact area as determined by the local authority (City of Winchester, 2006). This plan must include temporary and permanent erosion and sediment controls as defined by the Virginia Erosion and Sediment
Control Handbook. In addition, a Land Disturbance Application Package must be completed prior to construction (City of Winchester, 2006). The Winchester Water Protection Ordinance also mandates that the 10-yr post-development runoff rate not exceed the 10-yr pre-development runoff rate, and the 2-yr post-development runoff rate and velocity may not exceed the 2-yr predevelopment runoff rate and velocity (City of Winchester, 2006).
Virginia Stormwater Management Regulation Updates
The state of Virginia is in the process of updating the statewide SWM regulations. The new regulations are expected to be adopted and in place by fall 2009 (Wetland Studies, 2008).
The new regulations specify updated water quality and water quantity requirements. The new water quality requirement for redevelopment is to reduce phosphorous to 20% below predevelopment amounts. For new developments, the water quality requirement will be to have a maximum of 0.28 lbs/ac/yr of phosphorous (Wetland Studies, 2008). Projects will be able to satisfy water quantity requirements through two options: the channel protection option and the flooding protection option. To satisfy the channel protection option, the system must be able to carry 2-yr post development peak flows without causing erosion. To satisfy the flooding protection option, the system must be able to hold the 10-yr post development peak flow within the system without causing flooding (Wetland Studies, 2008).
Wetlands
From the Virginia Stormwater Management Handbook (VA DCR, 1999), the minimum watershed drainage area for a constructed wetland is 4 ha (10 ac), but in reality 6 to 8 ha (15 to
20 ac) or the presence of base flow is needed to maintain wetland hydrology. The surface area of the constructed wetland must be designed to be at least 20% of the contributing area (VA DCR,
1999).
The constructed wetland must be located 6.1 m (20 ft) from property lines or structures,
30.5 m (100 ft) from septic drainfields, and 15.2 m (50 ft) from slopes greater than 10% (VA
DCR, 1999).
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According to the stormwater manual, “permeable soils are not suited for constructed stormwater wetlands (VA DCR, 1999).” Final design and acceptance must be based on subsurface analysis and permeability tests of the soil strata to verify the soils ability to hold water. A liner is required if the soil infiltration rate is too great. Although other types of liner may be used, a clay liner should have a minimum thickness of 30.2 cm (12 in), and a layer of compact topsoil 15.2 to 30.2 cm (6 to 12 in) deep should be placed over the liner. If a clay liner is used, it must meet the requirements of Table 3.09-3 (VA DCR, 1999).
Chapter 3 minimum standards for the sediment forebay, earthen embankments, principle and emergency spillways, and landscaping all apply to the creation of a constructed wetland.
Outlet structures should be designed to pass the design storm. However, the principle spillway, emergency spillway, and earthen embankments are already in place, so modifications will not be necessary for this structure. All engineering calculations will be confirmed with the VA DCR
Stormwater Management Handbook and will be discussed in further detail in the design section of this report (VA DCR, 1999).
Grassed Swales
All grassed swales and water quality swales constructed in the state of Virginia should conform to chapter 3.13 of the Virginia Stormwater Management Handbook. Although the preferable side slope is 4:1, the maximum allowable side slope cannot exceed 3:1.The bottom width of the swale must be no less than 0.6 m (2 ft) and no greater than 1.8 m (6 ft). The flow velocity of the for the 2-year and 10-year storms is a maximum of 1.2 m/s (4 ft/sec) and 2.1 m/s
(7 ft/s), respectively. The longitudinal slope along the swale must be no less than 0.75 % and no greater than 5%. All swales should pass the peak flows from the 10-year storm with a minimum freeboard of 0.15 m (6 in) (VA DCR, 1999).
For water quality swales, the flow depth cannot exceed 0.10 m (4 in) and the flow velocity of the water quality volume must be no greater than 0.46 m/s (1.5 ft/sec). The engineered soil required for water quality swales consists of 50% sand, 20% leaf mulch, and
30% top soil. The depth of the engineered soil must be 0.76 m (30 inches) deep and be accompanied by a perforated pipe and gravel underdrain system. The under drain must consist of
AASHTO #7, ASTM M-43, or VDOT No. 3 Open-graded Course Aggregate; AASHTO M-278,
10.16 mm (4 in) rigid schedule 40, perforations of 9.5 mm (3/8 in) diameter at 15.2 cm (6 in) centers, 4 holes per row PVC pipe; and filter fabric (VA DCR, 1999).
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If check dams are used, they must be a maximum of 0.46 m (18 in), and must be anchored into the swale wall at a minimum of 0.61 m (2 ft). The maximum ponding depth behind check dams is 0.46 m (18 in). The check dam must also have an overflow notch to allow the 2year and 10-year storms to pass freely without damaging the abutments. Class A1 riprap should be used for the construction of check dams (VA DCR, 1999).
Environmental Considerations
Protected Lands
Constructed wetlands are usually implemented in low lying areas with favorable hydrological conditions to achieve adequate base flow and storage (VA DCR, 1999). Often these areas contain preexisting wetland characteristics and sensitive ecosystems, so they may already contain wetlands, streams, or wild life habitats that are protected by law. To ensure that environmental considerations are met, local wetland maps will be reviewed to determine the site’s protected status before development plans are created (VA DCR, 1999).
Erosion Control
Land disturbance is inevitable during the creation of a wetland or swale. Land disturbance can result in bare soils that are susceptible to erosion which raises the amount of total suspended solids and can cause an increase in sediment bound phosphorous levels (CWP, 2008).
Therefore, proper erosion control measures will be taken including installation and proper maintenance of silt fences and hay bales during development. Since the City of Winchester requires an erosion control plan for land disturbance this will be undertaken during the permitting stage.
Grassed swales must maintain a dense cover to function properly. To maintain a proper cover and to reduce the potential for erosion, erosion-resistant vegetation must be well established. Erosion control matting should also be used to stabilize the soil while an established cover is forming (VA DCR, 1999).
Vegetation
Native plant species are important for thriving wetland systems (VA DCR, 2008).
Invasive species can overtake natural species and devastate the wetland ecosystem once introduced (CWP, 2008). Therefore, only native species will be used in the landscape design. In
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addition, the areas with trees will be over planted to shade out invasive vegetation, and a long term vegetative maintenance program will be established (CWP, 2008).
Wildlife
Stormwater wetlands can provide valuable habitat for wildlife such as birds, amphibians, dragon flies, and aquatic organisms in an urban setting (Davis, 2004), and various techniques can be employed to encourage wildlife within the wetland. Mudflats, large deep pools surrounded by riparian buffers, and nesting boxes can be employed to support shore birds and waterfowl
(Metropolitan Council, 2001). By understanding the needs of desired species, the appropriate vegetation and design measures can be chosen that meet the needs of each species (CWP, 2008).
Pests
Pests, such as mosquitoes, can be an area of great concern for local residents. Stormwater management practices such as constructed wetlands and swales can increase the mosquito breeding habitat. However, proper site design measures can be taken to control mosquito propagation. Creating deeper pools for mosquito predators and connecting shallow pools with the general flow path can eliminate mosquito problems. Grassed swales and wetlands should be designed so that positive drainage occurs over the entire length since standing water is a concern for mosquito habitat. To prevent the breeding of mosquitoes, water should not stand in the grassed swale or constructed wetlands for any period longer than 24 hours (Yocum, Unknown).
In addition, mosquito habitat can be eliminated by controlling and eliminating dense, monoculture stands such as cattails.
Geese may also be considered a nuisance that cause problems for constructed wetlands because they contribute to higher e. coli and bacteria levels in outflow. A riparian buffer that is greater than 7.6 m (25 ft) can limit the year round habitation of geese in the constructed wetland
(Metropolitan Council, 2001).
III. Site Characteristics
Friendship Park is located at 623 North Pleasant Valley Road Winchester, VA. The soils of Friendship Park are Frederick series in hydrologic soil group ‘B’ and are well drained (USDA,
1987). However, the residential development has caused significant compaction to the soils, and the well-drained top soil has been removed from the site. The current dry detention pond at the site is 0.68 ha (1.68 ac), and the length of the riprap channel is 104 m (340 ft).
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GIS Analysis
Using ArcGIS (ESRI, 2008), 0.61 m (2 ft) contours were utilized to delineate the drainage area of the dry detention basin, and estimated to be 34.3 ha (84.6 ac). The parcels layer and streets layer were clipped to the drainage area to find the percentage of imperviousness in the drainage area. The total area of building footprint was 4.4 ha (10.9 ac). The area of the roads was to 3.8 ha (9.4 ac). Without the sidewalks or driveways layers, assumptions were made to determine these areas. The Virginia Department of Transportation (2005) requires sidewalks to have widths of 1.5 m (4 ft). Assuming that every road has at least one side with a sidewalk, the total length of the sidewalks was estimated at 4350 m (14,271 ft), resulting in an area of 0.65 ha
(1.61 ac). Five randomly picked houses were chosen and their driveways measured, giving an average area of 43.5 m
2
(468 ft
2
). With 198 homes in the drainage area, the total area of the driveways was 0.86 ha (2.1 ac). With these area calculated, the total area of imperviousness was
10.4 ha (25.8 ac), and the percentage of imperviousness equaled 30.4%. The Friendship Park site, the current riprap channel, and its drainage area are shown in Figure 16.
Figure 16. Friendship park drainage area, with existing channel and dry pond highlighted.
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Initial Site Visit
An initial site visit of Friendship Park was conducted on October 17, 2008. During the site visit, an inspection of the site revealed numerous outcroppings of visible bedrock. A survey of the site was also conducted, and each bedrock outcropping was included during the site survey. The survey points were imported into AutoCAD and a surface representing the riprap channel and the dry detention pond was created. The AutoCAD surface drawing can be seen in
Appendix A. The drawing created in AutoCAD shows 0.2 m and 1.0 m contours. Each bedrock outcropping that was visible is represented by a blue circle on the drawing.
Figure 17. Design team conducting a site survey during a visit to Friendship Park.
During this visit, the team completed a visual assessment of the site. Existing riprap channel and dry detention basin are shown in Figures 18 and 19, respectively.
Figure 18. Existing riprap channel at Friendship Park that is to be retrofitted with a water quality swale.
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Figure 19. Existing dry detention basin at Friendship Park that is to be converted to a wetland area.
Initial TSS Results
Samples were taken of stormwater runoff in the existing riprap channel during a storm event on October 25, 2008. The team analyzed the samples for total suspended solids (TSS). In addition, duplicate samples were sent away for nutrient analysis, but the nutrient results have not yet been received. The TSS results provide a base pre-retrofit TSS concentration of stormwater runoff to compare with post-retrofit stormwater runoff TSS concentrations. The results of the
TSS analysis are shown in Table 4. A plot of the TSS concentrations over time for the storm event is shown in Figure 20. The results show that the highest TSS concentration of 35.29 mg/L occurred early in the sample collection and decreased to 3.20 mg/L by the end of the runoff event. This data can be compared to data collected from the site after project implementation.
Figure 20. Plot of TSS results for stormwater collected during the October 25, 2008 storm at Friendship Park.
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Table 4. Results of TSS in stormwater runoff through the riprap channel during the October 25, 2008 storm.
Date
Collected
Bottle ID Volume filtered (L)
Initial filter mass (g)
Final filter mass (g)
Suspended solids mass (mg)
TSS
(mg/L)
10/25/2008 FP-SW-01
10/25/2008 FP-SW-02
10/25/2008 FP-SW-03
10/25/2008 FP-SW-04
10/25/2008 FP-SW-05
10/25/2008 FP-SW-06
10/25/2008 FP-SW-07
10/25/2008 FP-SW-08
10/25/2008 FP-SW-09
10/25/2008 FP-SW-10
0.289
0.274
0.276
0.292
0.292
0.200
0.295
0.280
0.175
0.250
0.1276
0.1279
0.1267
0.1273
0.127
0.1274
0.1302
0.1319
0.1302
0.1278
0.1378
0.1333
0.1295
0.1301
0.1305
0.1286
0.1338
0.1333
0.1314
0.1286
10.2
5.4
2.8
2.8
3.5
1.2
3.6
1.4
1.2
0.8
35.29
19.71
10.14
9.59
11.99
6.00
12.20
5.00
6.86
3.20
Hydrologic Analysis
The next step in the design process was the completion of hydrologic analysis that will be prepared using the modeling software WinTR-55. This program requires multiple data inputs and uses the graphical discharge method to determine the peak flow that results from a 24-hour design storm over a drainage area (Randolph, 2004). The data inputs were found using the available GIS layers, provided by the City of Winchester’s GIS Division, and the site assessments. WinTR-55 requires land use details, areas of imperviousness, drainage areas, and slope to determine peak runoff rates and volumes of the given design storms (Ward and Trimble,
2004).
The first step was to calculate the watershed curve number and runoff volume (Randolph,
2004). The curve number is a measure of the impact of various land use, cover, and soils in the watershed on infiltration and runoff. The curve number for the watershed was computed by assuming the soils were in hydrologic soil group ‘C’ due to the urbanization, lack of topsoil, and soil compaction in the area. With the details of impervious area given in the GIS Analysis section, the WinTR-55 model determined an area weighted average curve number equal to 80.
The next step in the TR-55 process was calculating the total time of concentration (T c
), or the time for runoff to travel from the farthest point in the watershed to the outlet (Randolph,
2004) at Friendship Park. The time of concentration is dependent on sheet flow, shallow concentrated flow, and open channel flow (Randolph, 2004). Sheet flow is overland flow that is dependent on frictional resistance that is modeled using Manning’s roughness coefficient, n
(Ward and Trimble, 2004). The time of concentration of sheet flow is also based on the land
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slope and flow length. Because an extremely uniform surface would be required, the length of sheet flow was limited to 30.5 m (100 ft) in this analysis (NRCS, 2002). The Manning’s roughness coefficient was set by the model and equaled 0.24 for dense grass. The slope was calculated by taking the differences between elevations and dividing that difference by the length of the flow, giving a slope of 0.0148. This same method was used to determine the slopes for the rest of the flows.
Following the topography of the area, the sheet flow becomes shallow concentrated flow once it meets the street. The time of shallow concentrated flow is also dependent on Manning’s roughness coefficient, flow length, and land slope. Two different shallow concentrated flows, paved and unpaved, were used in this analysis. The lengths of flow were 473.5 m (1,553 ft) for the paved flow and 121.8 m (400 ft) for unpaved flow. Manning’s roughness coefficient was set by the model as ‘paved’ and ‘unpaved’ for each shallow concentrated flow. The slopes for the paved and unpaved portions were 0.0127 and 0.0076, respectively.
Shallow concentrated flow becomes open channel flow in the drainage area once the runoff drains into the riprap channel. Like the last two types of flow, open channel flow requires length of channel, channel slope, and Manning’s roughness coefficient, but also needs crosssectional flow area and the wetted perimeter. The length of the channel was 243.6 m (799 ft) and the slope equaled 0.0252. Because the channel is filled with riprap, the Manning’s roughness coefficient was determined to be 0.03 (ODOT, 2005). Using the AutoCAD surface developed, two different cross sections of the riprap channel were used to find an average cross-sectional area and wetted perimeter, which were 4.2 m
2
(45.2 ft
2
) and 6.7 m (22 ft), respectively. The total time of concentration is the sum of the individual time of concentrations of each individual type of flow (Randolph, 2004) and equaled to 0.56 hours.
Because of the outlet present at the site, further inputs were required in the structure data of the model. Inputs that were needed included two pond surface areas, pipe diameter, and the height of the pipe to the spillway. The pond surface area is the surface area of the water at different stages. Two of these surface areas were necessary to model the basin as a trapezoid rather than a rectangle and to obtain more accurate results (NRCS, 2002). The first pond surface area was at the spillway crest and equaled 0.0043 ha (0.011 ac). The second pond surface area was the area 1.4 m (4.59 ft) above the spillway crest and equaled 0.27 ha (0.67 ac). The diameter of the pipe equaled 0.46 m (1.5 ft). The third parameter refers to the distance between the pipe
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and the emergency spillway, which was set to 2.7 m (8.86 ft) (NRCS, 2002). Frederick County,
VA climate data included within the model was used. We then used the WinTR-55 model to calculate the peak flow rates, storage volumes, and stages for the design storms over the watershed. Model results are shown in Table 5 and the resulting runoff hydrograph is displayed in Figure 21.
Table 5. Peak flow, storage volume, and stage as outputted by WinTR-55.
Peak Flow (m 3 /s)
Storage Volume (m 3 )
2-yr, 24-hr
0.86 (30.4 ft
3
/s)
2,500 (88,300 ft
3
)
10-yr, 24-hr
1.00 (35.3 ft
3
/s)
8,800 (311000 ft
3
)
Stage (m) 1.46 (4.8 ft) 2.85 (9.4 ft)
Figure 21. Hydrograph of the 2-yr and 10-yr, 24-hr design storms computed using WinTR-55
When comparing the VA DCR recommendation of twice the water quality volume of
1,122 m
3
(39,600 ft
3
) to the storage volume calculated by the model, the water quality volume is well below both values of each storm and shows that the current dry detention basin is capable of withholding the water quality volume.
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Second Site Visit
On February 6, 2009, the team revisited the Friendship Park site to gain information about the depth to bedrock along the planned flow path of the wetland. Four soil cores were taken around the intended area for sediment forebay (Figure 22). The depth of these four soil cores ranged from 1.54 m (5 ft) to 2.29 m (7.5 ft). Because time was limited, the team could not take soil cores throughout the entire site. Therefore, the team gathered depth to bedrock information by driving pins into the ground at various distances from outcrops along the planned flow path and measuring the depth at which the pin hit bedrock. The depth to bedrock along the planned wetland flow path ranged from approximately 4 cm (1.6 in) to greater than 35 cm (13.8 in). Each soil core location and pin location was then surveyed using the total station, and the points were placed into the existing AutoCAD surface for use in the creation of the final design.
Figure 22. Team completing soil borings during second site visit.
On this site visit, the team was accompanied by a geologist, Bob Denton, who offered insight to the Karst conditions on the site. He pointed out areas where seeps are occurring that could be a possible source of water for the wetland during drought conditions. He also suggested that the team look at the areas geo-quadrangle for more information about the sight. Mr. Denton also suggested that the team focus on the forebay design, including a liner, to ensure that if failure due to Karst occurs in another area of the wetland that sediment contaminants will already be settled out of the water.
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IV. Design Criteria and Constraints
Design Criteria
Several criteria will be considered and kept in mind when designing the swale and pocket wetland. The design criteria of pollutant removal, low water level fluctuation, storage volume for the wetland, water transport for the swale, aesthetics, safety and maintenance which were determined to be important based on the literature review and project goals.
Pollutant removal will be one of the more important things to consider. For improved water quality, the design must reduce nitrogen and phosphorous loads, while also promoting sedimentation. The reduction of nutrient loads will be completed through several mechanisms.
As runoff travels through the swale and wetland, the velocity of the water must be decreased and length of flow must increase so that the sediment bound nutrients can settle out to the bottom.
Nutrients can also be reduced through the uptake of plants. Because plants require nitrogen and phosphorous to grow, they will naturally absorb the dissolved nutrients from the water they absorb through their roots. As mentioned above, sedimentation will occur through the settlement of solids, mainly in the sediment forebay and any other pools that will be created in the design.
In addition to pollutant removal, the design must manage stormwater volumes and peak flow rates from its drainage area, so that flooding does not occur. To achieve this, a hydrologic analysis will determine the water quality volume and peak flow rates and volumes for the 2- and
10-year, 24 storms. With both calculated, the BMPs will be designed to be able to treat the water quality volume and have enough storage volume to hold the peak flow rates and volumes of the design storms.
Minimizing water level fluctuations will be important to maximize the wetland functions and vegetation diversity within the wetland. The vegetation chosen is adapted to grow in wet or mostly wet conditions. Large fluctuations in the water levels and changes between dry and wet conditions will reduce the vigor and growth of the vegetation. The reduced vigor will affect plant establishment, nutrient uptake, and sedimentation. To limit fluctuations, the chosen swale design will have the ability to slow the flow and convey the most water to the wetland, and the wetland must increase the detention time and flow path of water.
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In addition, water transport and capacity are important for this project. Water transport is an important criterion for the swale because the swale must carry the water to the wetland in order to support the wetland plants. The capacity of the wetland is important because the wetland must hold the design storm so that flooding does not occur and affect the adjacent property.
Aesthetic value is also a criterion for this project. Friendship Park is a public park in a residential community, so changes to the park should increase its aesthetic value. This will help attain community satisfaction which is a project objective.
The site is located within Friendship Park and down the street from an elementary school.
These factors will ensure that kids will be present around the proposed site, bringing in concerns of safety. To ensure the safety of children and other people, different methods will be implemented. Trees and fences will discourage people from entering the swale and wetland.
Signs posted informing the public of the BMPs and safety concerns will convey the safety issues.
Signs may also be posted within deep pools of the wetland to warn people of their dangers.
Maintenance is another important alternative in this design. As mentioned in the Wetland
Maintenance section of the literature review, maintenance will be important for the establishment of native vegetation and ensure the swale and wetland will function properly. The wetland that is the easiest to maintain over time will be given a higher score in the decision matrix.
Design Constraints
Design constraints are factors that limit the design. There were many design constraints for the Friendship Park project that were considered before the constructed wetland and swale designs could be finalized. The design constraints for this project were: 1) time; 2) safety; 3) site limitations; and 4) economic feasibility.
Time was an important constraint that must be considered for this project. For the
Friendship Park wetland and swale design to be implemented by April 2009 as planned, the team adhered to a strict schedule to ensure that the design is completed, permits and materials are attained, and the construction work is contracted out in a timely manner.
Safety was also an important constraint on this project. Since Friendship Park is located in a residential area with a school and playing fields in close proximity to the proposed BMP site location, it is important that the BMPs are designed with safety in mind. Therefore, it is
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important that the wetland pools are shallow enough to prevent drowning, and that visibility can be maintained throughout the site to discourage night prowlers that may take shelter there.
There are many important site limitations that served as constraints on the final design.
Size is one of the biggest site limitations at Friendship Park. Constructed wetlands are often created for large areas, and there are regulations as to how large a constructed wetland should be to achieve specific pollutant removal efficiencies. However, the Friendship Park project will be a retrofit BMP on a parcel that is small in comparison to most constructed wetlands. Therefore, the design will have to be extremely efficient because it will have to be effective at removing pollutants in a small amount of space. Another important site limitation that will affect the final swale and constructed wetland design at Friendship Park is the depth to bedrock. As shown in
Figure 23, bedrock is exposed at the surface of the North West corner of the pre-existing dry pond. It is likely that the depth to bedrock is very shallow throughout the site. This will limit the amount of excavation that can occur to create the flow path and pools within the wetland final design. Another site limitation that was considered as a constraint was existing underground utilities. However, recently acquired GIS information from the City of Winchester shows that utility lines do not run under the site, so this is no longer a concern.
Figure 23. Exposed bedrock on the site is a primary constraint for this project.
Since Karst topography is a concern in the City of Winchester, precautions must be taken to ensure that the designed wetland and swale do not exacerbate future problem. The current dry pond and channel are not clay lined, but the final wetland pools must be lined with a plastic liner in order to ensure that groundwater contamination and sink holes do not occur. While clay liners are often used, this method will be inappropriate for the site due to the minimal dept to bedrock
Economic feasibility is also a constraint for this project, although it is not the most important constraint. The department has a budget for BMP implementation in the Opequon
Creek watershed through the Targeted Watershed Grant Program. We estimated that $20,000 is
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available to spend on the wetland, and $10,000 is available for the swale. We also estimated that grants will be available to offset the cost of vegetation, which is often a costly portion of the
BMP.
V. Design Alternatives
Water Quality Swales
The primary functions of the water quality swale in the stormwater retrofit of Friendship
Park are water quality treatment and water conveyance. Typical cross-sectional designs for water quality swales are shown in Figure 24 (ASABE, 2006). Because the storm water regulations for the State of Virginia specify trapezoidal cross-section for channels (VA DCR, 1999), no further research into the parabolic or triangular cross-section designs was taken.
Using a trapezoidal cross section for all designs, five alternatives for the swale where developed. The five alternatives were: 1) conventional grassed swale; 2) wet water quality swale;
3) a dry water quality swale; 4) a dry water quality swale with an underdrain; and 5) a dry water quality swale with checkdams.
Figure 24. Typical grassed waterway cross sections (from ASABE, 2006).
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A decision matrix developed by the design team was developed to evaluate each of the five alternatives against seven factors determined to be the most important in the design selection. The decision matrix developed can be seen in Table 6. Each of the seven factors was given a weight corresponding to its importance in the overall design. Because water quality treatment and water transport are the primary goals of the swale, they were given weights of 20% and 25% of the overall weights, respectively. Aesthetics and safety were included because of the residential location of the park. Area residents would be very concerned with the overall appearance of the swale, as well as with the safety of the swale. Maintenance was included because the swale will be located on City property and will have to be maintained by the City.
The site poses construction limitations in regards to the existing bedrock and the underlying
Karst topography for the potential swale. The swale would have to be constructed without disturbing the existing bedrock in the area. Because of the concern of infiltration into the groundwater Table, also swale designs would have to include a clay liner. The clay liner will allow the swale to function properly, but will prevent stormwater from infiltrating directly into the groundwater Table. Cost was the lowest weighted factor because the grant providing the funding for the project has allotted $10,000 for the construction of the swale.
The grassed swale would be the best alternative for transporting water to the constructed wetland, but it has virtually no water quality treatment functions. The wet water quality swale has a very high water quality treatment, but it has very poor water transport capabilities. The wet water quality swale is also not conducive to residential areas because of standing water and the likelihood of breeding mosquitoes. The dry water quality swale has fairly good capabilities of both water treatment and water transport. Adding checkdams to the dry water quality swale increases the water treatment capabilities of the swale. But, checkdams inherently decrease the ability of the swale to transport water because of the ponding created behind each checkdam. By adding an underdrain to the dry water quality swale, the water treatment capabilities of the dry water quality swale is preserved, but added water transport capabilities are provided by the underdrain transporting all collected waters to the constructed wetland.
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Table 6. Decision matrix for the bioswale at Friendship Park, Winchester, Va.
Criteria/Constraint Weight Grassed Swale Wet W.Q. Swale
Dry W.Q.
Swale
Dry W.Q. w/Underdrain
Dry W.Q. w/Checkdam
Pollutant Removal 20 1 20 5 100 4 80 4 80 5 100
Water Transport
Aesthetics
25
10
5 125
3 30
1 25
2 20
3 75
3 30
5 125
3 30
1 25
1 10
Safety
Maintenance
Site Limitations
Cost
TOTAL
15
15
8
7
100
3 45
5 75
5 40
5 35
370
1 15
4 60
4 32
4 28
280
3 45
4 60
4 32
4 28
350
3 45
3 45
3 24
3 21
370
2 30
3 45
4 32
3 21
263
From the decision matrix, the conventional grassed swale, the dry water quality swale, and the dry water quality swale with an underdrain scored comparably well. The design team has chosen to pursue the dry water quality swale with an underdrain as the desired design due to its high capabilities to both treat water and transport water.
Wetland Design
Proper wetland design is critical for the desired nutrient removal efficiency to be attained.
Therefore, the team developed three alternative designs for the flow path through the wetland.
The various alternative flow path designs are: 1) wetland cells; 2) wetland island(s); and 3) meandering flow. These alternatives were discussed in detail in the Literature Review. The decision matrix for the wetland flow path is shown in Table 7.
Table 7. Decision matrix for the wetland flow at Friendship Park.
Criteria/Constraint Weight Cells Island(s) Meandering Flow
Pollutant Removal 25 5 125 3 75 4 100
Low WLF
Aesthetics
Safety
20
10
15
5 100
3 30
3 45
4 80
5 50
4 60
4 80
4 40
4 60
Maintenance
Site Limitations
Cost
10 2 20 2 20 3 30
15 3 45 2 30 4 60
5 3 15 5 25 4 20
TOTAL 100 380 340 390
The first alternative design for the Friendship Park wetland that was considered was the creation of wetland cells. In this system, the high degree of micro-topography and varying depths creates high removal efficiency and low water level fluctuation. Therefore, this design scored
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highest in these categories. While the wetland cells scored lower than the other wetland alternatives in the aesthetics category because of flow path and vegetation, this design would still be a visual improvement to the existing stormwater dry pond. The wetland cells also scored lower than the other alternative designs in the safety category because accidents, such as falling into a deep pool, is more likely in the cells than in the other designs. The most limiting constraint for the use of wetland cells is long term maintenance. The micro-topography of the wetland cells may be difficult for inexperienced or uneducated workers to preserve, but it is critical that the cells are maintained at their proper depths to maintain the low WLF and pollutant removal. The site limitations also limit the use of this design because excavation will be required to create the deep cells. This design will also be the most costly because excavated materials will need to be moved offsite, and the pools will require clay liners because of the Karst topography.
Although innovative, the island wetland design scored the lowest between the three alternatives. Pollutant removal was ranked the lowest amongst the three because the other two designs are more capable of increasing flow path, storage volume, and detention time. Low water level fluctuations and aesthetics were ranked high because it can maintain water levels and the island can provide a wide variety of vegetation. It also ranked high in safety because the island will be disconnected from the mainland, making it hard to access. However, for this same reason, it ranked the lowest in maintenance. Because the island is usually designed for larger wetlands
(CWP, 2008) and some vegetation could be limited from the bedrock outcrops present at the site, this design ranked the lowest in the site limitations category. This design is the most cost efficient, however, because any material excavated would be used for the island and no other excavation would be needed.
Figure 25 shows the meandering flow path alternative design. This design scored the highest in the decision matrix, as shown in Table 6. Based on research, the meandering flow received the second highest score (100) for the pollutant removal efficiency. If designed according to Virginia standards and properly maintained, this design should only have moderately fluctuating water levels. The meandering flow was also considered to be aesthetically pleasing because the high marsh areas and meandering flow provides visual appeal that mimics a natural stream. This design also scored high on safety because the vegetated high marshes could provide a barrier that would keep children away from the flow (VA DCR, 1999). The meandering flow design scored higher than the other two wetland alternatives for the site
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limitations category because it was assumed that the meandering flow could be diverted around areas where the depth to bedrock was shallow, but it may cost more than the island wetland design because the excavated material will have to be removed offsite.
Figure 25. Meandering flow alternative design for the wetland at Friendship Park (from CWP, 2008).
According to the design matrix, the best alternative design was the meandering flow with a score of 390, and the second best alternative design was the wetland cells. Since the team wants to create an innovative design, we have chosen to integrate both of these flows into the final design to create a meandering/cell flow wetland .
A second wetland decision matrix was created to determine whether the final wetland design should contain forested or emergent, non woody vegetation. From the literature review, the team found that forested wetlands are not often constructed. However, there are many benefits of a forested wetland, so the team decided to consider a forested wetland for the final design as an alternative to the typical emergent constructed wetland. This decision matrix is shown in Table 8.
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Table 8. Decision matrix for the vegetation of the wetland at Friendship Park.
Criteria/Constraint Weight Forested Emergent
Pollutant Removal
Low WLF
Aesthetics
Safety
Maintenance
Site Limitations
Cost
TOTAL
25
20
10
15
10
15
5
100
5 125 3 75
3 60 4 80
5 50 3 30
3 45 2 30
3 30 5 50
2 30 4 60
3 15 4 20
355 345
As shown, the forested and emergent wetlands ranked very closely in the decision matrix, with the forested wetland scoring 355 and the emergent wetland scoring 345. While the forested wetland scored much higher in pollutant removal and aesthetics, it also scored lower than the emergent wetlands in other areas. Since the mature trees will uptake water at a faster rate than emergent vegetation, this will increase the water level fluctuation in the forested wetland. The forested wetland scored higher than the emergent wetland on safety because it was assumed that the trees would provide a barrier that would keep people away from the deep pools. However, trees also pose a safety concern for residents who fear that prowlers may hide behind the trees.
The forested wetland will be much harder than the emergent wetland to maintain, especially if deciduous trees are used. The trees will be harder to establish than the emergent vegetation, so more maintenance will be required in the first years after installation, and the organic matter from the lost trees will contribute to the aging of the wetland and require more frequent dredging of the pools.
In addition, the depth to bedrock may limit the ability for adequate root growth for trees, so this is a limiting factor on the forested wetland design. Trees are also much more expensive than emergent vegetation, so the emergent vegetated wetland scored higher under the cost constraint.
Based on the decision matrix and the literature review, the team would like to incorporate the forested wetland vegetation into the final design. However, discussions with stakeholders and cost and site limitations may prove this option impossible for the Friendship Park project.
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Natural Area Alternative
At the request of the Natural Resources Advisory Board (NRAB), the team created a
“Nature Area” alternative design. The NRAB wanted to have an alternative design that required no further engineering of the dry detention basin area.
VI. Final Design
Water Quality Swale
The design of the proposed water quality swale was based on recommendations in the
Virginia SWM Handbook for a dry water quality swale. The design involved converting the current riprap channel into a water quality swale to encourage better infiltration and nutrient uptake. A schematic of a typical cross-section of the water quality swale is shown in Figure 26.
The water quality swale will be reshaped where needed to provide the necessary bottom width of
2 m and the necessary 3:1 side slopes. The 3:1 side slopes were necessary to decrease the risk of erosion along the banks of the swale and to increase the storage volume of the swale. The swale was designed to handle the peak flows from both the 2-year and 10-year storms. The entire length of the 104 m (340 ft) swale will be converted from the riprap channel to a dry water quality swale. The water quality swale will connect directly to pool 1 of the wetland.
The design of the swale included leaving a 0.5 m depth of the existing riprap in the channel. The riprap will be left in place to act as an underdrain for the swale. Dry water quality swales typically contained a perforated pipe that acts as an underdrain for the swale. This underdrain will collect infiltrated stormwater in the pipe and then convey it to the outlet of the swale. To help convey infiltrated stormwater from the designed swale to the wetland, some of the riprap will be left in place. Because of the void space present in the riprap, infiltrated water will be able to travel down the swale through the riprap. By leaving the riprap in place, the need to install an actual pipe underdrain will be unnecessary.
Above the riprap, 0.5 m of a permeable sand/soil mixture will be placed. This permeable soil mixture was important for encouraging infiltration, nutrient uptake, and plant growth.
Between the riprap and soil, a geotextile fabric liner will be placed. This liner was necessary to prevent sediment from entering the riprap layer and clogging it.
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Figure 26. Cross section of final water quality swale design.
For the water quality swale to function properly, appropriate vegetation must be established along the banks and the swale bottom. Grasses and shrubs that can withstand the force of the moving water in the swale have been selected to be planted. Properly establishing vegetation in the swale will decrease erosion along the swale, decrease the velocity of entering stormwater, and increase the nutrient uptake from the stormwater.
Wetland Design
After determining that a combination of cells and meandering flow path wetlands was most appropriate for the site, the team created the final design. Due to the constraints of the bedrock depth, the final design could only include three pools rather than the suggested four micro-pools. The final design also includes a berm to increase flow path and filtering capacity within the wetland (Figure 27).
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Berm Pool 2
Pool 3
Flow Path
Pool 1
Figure 27. AutoCAD design for the retrofit wetland designed for Friendship Park.
The flow path between the pools is designed to vary 5.08-10.16 cm (2-4 in) to create microtopography that will encourage nutrient attenuation.
Pool 1
During the soil depth investigation, the depth within the planned pool 1 area was 1.52 –
2.29 m (5 – 7.5 ft). Pool 1 was designed to act as the sediment forebay for the wetland.
According to the Virginia Stormwater Management handbook, the depth of the sediment forebay should be between 1.22 – 1.83 (4 – 6 ft) to limit the resuspension of settled particles (VA DCR,
1999). The recommended forebay design should also hold a minimum volume of 0.254 – 0.635 cm (0.1 – 0.25 in) of runoff over the impervious area (VA DCR, 1999). The recommended sediment forebay volume was calculated to be between 112.2 - 280.5 m³ (3,962.3 – 9,905.7 ft³).
Based on NRAB feedback, the final water depth in any pool could not exceed 0.61 m (2 ft) because of the problems that would arise with community approval and safety concerns.
However, the pool will be excavated to a depth of 1.069 m (3.5 ft) with 3:1 side slopes, lined
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with a plastic liner, and then re-filled with topsoil to meet the requested maximum depth. The pool will be lined with 278 m² of 45 mil EDPM pond liner (Pond Depot, 2009) to prevent possible infiltration and piping in Karst areas. The liner will be inserted into the side slopes at a depth of 0.457 m (1.5 ft) from the bottom of the pool. After the liner is placed, the liner will be covered with previously excavated topsoil at a depth of 45.72 cm (18 in) from the bottom of the pool, and the bottom of the pool will be seeded with a wetland grass mix to encourage microbial transformation of nutrients. Therefore, the final depth of water in this pool will be 0.61 m (2 ft) and will not exceed NRAB recommendations. The surface area was limited to 210 m² based on site constraints, so the total volume of this pool is 71.8 m³. While this is less than the suggested minimum of 112.2 m³ (3962.3 ft³) for a sediment forebay, the stakeholders requested that the depth of any pool be limited to 0.61 m (2ft). Therefore, this design will probably not remove as much nutrients as originally hoped, but the sacrifice of some pollution removal efficiency had to be made for the project to move toward implementation. The final design for the pool 1 profile is shown in Figure 28.
Pool 2
Pool 2 was designed to increase microbial processes for nitrogen removal and enhance settlement of sediment and sediment bound phosphorous. It is shown in Figure 29. This pool is designed with a 0.3 m (1 ft) depth, and 3:1 side slopes. The top surface area of this pool is 139 m². This pool is also lined with 278 m² of 45 mil EDPM pond liner (Pond Depot, 2009) to prevent Karst issues. The liner will be inserted into the side slopes at a depth of 15.24 cm (6 in) from the bottom of the pool. Due to shallow depth to bedrock, this pool will not be filled with soil and seeded.
Pool 3
Pool 3 was also designed to increase microbial processes (Figure 30). This pool was designed with a 0.25 m (0.8 ft) depth, and 3:1 side slopes. The top surface area of this pool will be 122 m².
This pool will be lined with 244 m² of 45 mil EDPM pond liner (Pond Depot, 2009), which will be inserted into the side slopes at a depth of 12.5 cm (0.4 ft). Due to the extremely shallow depth to bedrock in this area, pool 3 will not be filled with soil and seeded.
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Figure 28. AutoCAD profile of pool 1.
Figure 29. AutoCAD profile of pool 2.
Figure 30. AutoCAD profile of pool 3.
In order to prevent erosion before the vegetation establishes within the wetland, a perimeter was established around each pool with a 1 m (3.28 ft) wide biodegradable fiber mat. BioD 70
(RoLanka, 2007) woven coir mat is utilized in this design and is shown in Figure 31.
Figure 31. BioD 70 woven coir mat (put company info here), a biodegradable fiber mat, will be used to prevent erosion around the pools before vegetation develops (from RoLanka, 2007).
Berm
To increase the residence time and flow length, a berm will be installed within the wetland between the last two pools. The berm will be designed as a check dam and not as an earthen embankment to allow runoff to flow through it.
The material that will be used to build the berm will be riprap from the current swale at the site and VDOT #1 coarse aggregate (VA DCR, 1992). The coarse aggregate will be placed facing the natural flow path. To make sure the aggregate will not fail due to high flow rates, the riprap will be placed behind it for support (VA DCR, 1999).
For the site’s drainage area, a maximum height of 0.91 m (3 ft) for the berm will be used
(VA DCR, 1992). Because of the exposed bedrock at the site, the berm will only be able to extend 15 m (49.2 ft) from the embankment rather than span the width of the site.
To stabilize the berm, several practices will be used. To least disturb the current embankment, the berm will be anchored into the side wall a minimum of 0.61 m (2 ft) (VA
DCR, 1992). As mentioned above, the riprap from the channel will also add stability to the berm by supporting the aggregate. Although 2:1 side slopes are used for check dams (VA DCR, 1992), side slopes of 3:1 will be utilized for further stabilization.
To protect the back end of the berm from falling water and erosion, riprap will be placed at the bottom as toe protection (VA DCR, 1992). Plastic liner will also be placed between the berm and the soil interface and below the toe protection to prevent further erosion and infiltration
(VA DCR, 1999). Figure 32 shows the downstream and side views of the berm that was designed.
Figure 32. Downstream view and side view of constructed berm (from VA DCR, 1992).
Planting Scheme
Our proposed planting scheme will include trees, shrubs, grasses, and wetland plants. A diverse population of vegetation is necessary in reducing nutrient concentrations in runoff and providing wildlife habitat (EPA, 2000). Each plant chosen is native to Virginia and readily available (VA DCR, 1999). Some plants have also been utilized in nearby constructed wetlands and have proven to be successful. The appropriate vegetation are listed in Table 9, and the general planting scheme is shown in Figure 33.
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Table 9. List of wetland plants, grasses, trees, and shrubs to be planted.
Wetland Plants Grasses Trees and Shrubs
Fox Broom Sedge Kentucky 31 Tall Fescue Eastern Cottonwood
Soft Rush
Blue Flag
Redtop
Reed Canary
Switchgrass
Blue Grass
Blackgum
Buttonbush
Black-Eyed Susan
Square-Stemmed
Monkey Flower
Partridge Pea
Common Three Square
Shadbush
Ironwood/
Hophornbeam
Silky Dogwood
Blackgum
Elderberry
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Figure 33. General planting scheme.
Due to the bedrock present at the site and their large root zones, the area outside of the swale and wetland will primarily contain trees and shrubs. Trees and shrubs will promote safety by preventing people from entering the site. The swale will contain grasses and shrubs with shallow root systems. The vegetation in the swale will slow down runoff and promote deposition and infiltration. In the bottom of the first pool there will be more grasses planted. Around the first pool will be hardy and thorny shrubs such as buttonbush, fringe tree, and shadbush. The shrubs will be located here to prevent people from entering the pool. The berm will also have grasses growing on it to cover the coarse aggregate and riprap. The rest of the site will contain wetland plants that will promote de-nitrification, deposition, and infiltration, further improving the water quality of the effluent.
When it is time to plant, trees and shrubs will be planted between March 15 and June 30 or between September 15 and November 15 (VA DCR, 1999). The grasses and wetland plants
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will be planted in early spring (EPA, 2000). The trees and shrubs will come from nurseries where they have already started growing. The spacing between them will be 3.04 and 5.45 m (10 to 18 ft) (ISU, 2004). It is important not to plant any trees and shrubs on any embankments, since their larger root systems can cause failer (VA DCR, 1992). Before the other plants can be established, the soil within the constructed wetland and swale will be disked or harrowed. This will break up compacted soil that can make establishment difficult (EPA, 2000). With a planting area of 0.52 ha (1.27 ac), the number of trees and shrubs that can be planted are between 180 and 550 with the given spacing. For the wetland plants and grasses, a seed mix will be utilized. Within the wetland, a recommended spreading rate of 16.8 kg/ha (15 lb/ac) results in 11.43 kg (25.2 lb) of wetland seed mix that will be spread throughout the site. For the grass seed mix, a recommend spreading rate of 48.8 kg/ha (43.6 lb/ac) will result in 4.23 kg (9.32 lb/ac) of mix needed.
Nature Area Design
The “Nature Area” design includes the same swale design as the final engineered design because the NRAB considered this as essential in the improvement of the visual appeal of the area. Although the engineered wetland design should not exacerbate Karst problems since plastic liners are included in the pool designs, the Board suggested a non-engineering option to prevent problems with Karst and community approval. The “Nature Area” option for the dry detention basin is a planting scheme based on the same planting scheme used in the final engineered design, but the “Nature Area” option does not include a berm or any of the three pools that are included in the final engineered design. A sketch of the “Nature Area” option is shown in Figure
34. For more information on the specific plants for the “Nature Area” vegetation scheme, refer to the final engineered design vegetation scheme.
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Figure 34. Vegetation scheme for the “Nature Area” alternative. Wetland plants will be planted in the area shown in blue. Swale grasses will be planted in the area shown in yellow, and trees and shrubs will be planted in the area shown in green.
Maintenance Plan
Maintenance is one of the most important aspects to ensuring the wetland and the swale function properly. Maintenance should be broken down roughly into two general schedules. The initial maintenance schedule involves the initial three years after instillation. The second maintenance schedule involves the remaining life of the SWM facility. During maintenance inspections, plant species distribution, fatality rates, sediment accumulation, water elevations, and the outlet condition should all be documented.
Initial Three Years
Establishing vegetation in the swale and wetland is the most important aspect of having a functioning project. The first three years after implementation are the most important for establishing this vegetation. It is likely that some plants will die after their initial planting and these most be removed and replanted with new plants. During the first three years, the plants may need to be watered during dry conditions to help promote growth. The vegetation should not
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be expected to reach full growth during the first few years. The vegetation should be monitored on a monthly basis, and more frequently during extremely wet or dry conditions. During these monthly inspections, the site should also be monitored to ensure water flow through the swale and wetland is functioning properly. The coconut fiber mats should be checked to ensure stabilization along the pool banks. Trash and other debris should also be removed from the site during the monthly inspections. Because of the nature of a constructed wetland and water quality swale, the site should not be routinely mowed. It is not expected that accumulated sediment will need to be removed from pool 1 during the first 3 years.
Remaining life of Swale and Wetland
After the initial three years, if properly maintained, the vegetation should be fully established in the swale and the wetland. Inspections should occur on a bi-annual basis. During the inspection, vegetation health should be monitored. If necessary, dead and dying plants should be removed and replanted with new plants. Minimal pruning of the vegetation should be done to prevent excessive overgrowth, but routine mowing is not necessary. Trash and other debris should also be removed from the site. Roughly every 3 to 4 years (or when 0.15 – 0.3 m of accumulated sediment has occurred) pool 1 should be dredged of accumulated sediment to ensure proper storage volume. If necessary, accumulated sediment may need to be removed from the swale at the same time.
VII. Community Outreach
Natural Resources Advisory Board Meetings
On January 12, 2009, the team presented the Friendship Park stormwater retrofit plan to the Winchester NRAB. Overall, the board was impressed by the plan and gave the team useful feedback. For the next Winchester presentation, the board advised that the team include more information about the Winchester MS4, the Opequon TMDL, and specific design specifications.
They also informed the team that the Frederick/Winchester Service Authority would be accepting maintenance responsibility for the Friendship Park natural area for the first three years after construction and that maintenance instructions must be incorporated into a city agreement.
NRAB also suggested that when presenting to the public in the future, the team refer to the project as the “Friendship Park Nature Area” because they have found that negative connotations are associated with the term “wetland.” They also advised the team to emphasize that this project
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would be paid for by a grant rather than by city resources. The main concerns presented at the meeting were of the cost, mosquitoes, Karst, depth of water, and pathway construction through the wetland area. The team kept these issues in mind to ensure that the topics were well covered in the following presentations for NRAB, Public Service Board, and Council meetings.
The team met again with NRAB and other stakeholders including members of
Winchester Parks and Recreation Services on April 23, 2009. This meeting was held primarily to present the final project design and gain more feedback before completing the final report. At this meeting, the team was ensured that Winchester was still interested in implementing the design for the Friendship Park site. While most of the final design specifications were generally approved, the consensus was that the deepest pool should not exceed 0.61 m (2 ft) to ensure community approval. Due to budget constraints, there was also question as to whether or not the
Frederick/Winchester Service Authority would be able to accept maintenance responsibility as originally thought. NRAB also offered many suggestions for the pamphlet which are reflected in the final version in this report.
Before the final project can be implemented on the site, all portions of the design must be approved by NRAB, the city engineer, and the Public Service Board before the project can go on to be approved at the final Town Council meeting.
Outreach Plan
The team has developed a three-fold plan for community outreach for the project. The first component of this plan is the community education pamphlet. This pamphlet was designed to educate the average citizen about the purpose of the project and the benefits of having the wetland and water quality swale, or so-called “Nature Area” implemented at Friendship Park. At the request of NRAB, the primary focus of this pamphlet is on the design and benefits to the citizens, with water quality being of secondary focus in the pamphlet.
The next component of the plan is a Nature Board Walk through Friendship Park. By marketing the area as a place for walking, bird watching, and public access to nature, the team feels that the community will be more accepting of the project. This plan component is very well endorsed by NRAB.
The third component of the outreach plan is to create a community outreach organization called “Friends of Friendship Park.” This organization would be comprised of local citizens who are interested in using and preserving the many benefits the “Nature Area” offers. This group
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would be citizen run, so they would have the final say in the purpose and extent of “Friends of
Friendship Park,” but the idea is that this group would sponsor minor vegetation maintenance or planting days, Nature Area clean ups to remove trash, and organize activities to allow local youth to explore the amenities of the Nature Area.
VIII. Cost Analysis
Through the Targeted Watershed Grant, the construction of the designed wetland and swale will be funded. Below, Table 10 displays the different materials that will be used in construction, quantity, unit prices, total prices, and total cost of the design.
Table 10. Construction materials, unit prices, quantities, and total price.
Description Unit Price per Unit Quantity Total Price
Backhoe Rental
Geotechnical
Liner
Fiber Mat
Soil
Plastic Liner
VDOT #1 Coarse
Aggregate
Coconut Fiber
Grass Seed Mix
Wetland Plant
Mix
Shrubs
Day kg kg ea
Equipment
$150
Swale Materials
1.8 m x 109.7 m
(6 ft x 360 ft)
1.9 m x 50.3 m
(6.5 ft x 165 ft) m 3
$213
$250
$44.57
($34/yd 3 ) m 2
Ton
1 m x 25.3 m
(3.28 ft x 83 ft)
Wetland Materials
$5.92
($0.55/ft 2 )
$3
$96
Vegetation
$7.91
($8.59/lb)
$162
($367.75/lb)
$1.88
1
1
8
338
87.52
53.9
6
4.23
11.43
365
TOTAL =
$150
$213
$2000
$15031
$518
$170
$576
$80
$1800
$685
$21223
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From the Table, the total construction cost for these BMPs is $21,223. It is important to note that these prices are estimates and will change in the future if different quantities are needed, different vendors are chosen, or if different designs are utilized. Prices not considered include the prices for the trees and the riprap used for the berm. The trees are to be provided by the Natural Resources Advisory Board (NRAB) free of charge. The riprap for the berm will not have to be bought because the riprap from the channel will be used to support the coarse aggregate.
IX. Measurements of Success
There are several points by which the team can measure the overall success of the project.
The most important point for our personal achievement that can be measured by the end of the year is the successful installation of the final design at the site. The team will consider itself successful if the design is approved by our advisors and the City of Winchester engineer, and the design meets all permitting specifications. Since the overall goal of this project is to install a pocket wetland and swale at the site, this is the main measure of success for the team. However, the team will also consider itself successful if the wetland and swale gains community approval.
If the residents surrounding the site are happy with the design and do not express dissatisfaction during the meetings, this will add to the success of the design.
Since this project will be implemented, the overall success of several aspects of the design will not be known until years after the implementation of the final design. The overall design will be successful if measurable nutrient reduction is attained for years to come. It is likely that future graduate work will focus on nutrient reduction from the Friendship Park site.
The design will be considered successful if it reduces phosphorous levels by at least 20% of the pre-existing levels since this is being considered as the new regulation for stormwater retrofits
(Wetland Studies, 2008). The long-term success of this project will also be based on whether or not a high native plant variety is sustained throughout the years. In addition, the project will be successful if low water-level fluctuations are created and sustained, as well as if the maintenance plan is followed in years to come. While these measurements will not be realized until well after this team has disbanded and left the University, they are nonetheless important measurements of the overall success of the final design.
At the NRAB meeting on April 23, 2009, the team was asked to weigh the benefits of implementing the entire wetland design versus only implementing the vegetation scheme within
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the wetland. Implementing the vegetation plan would eliminate the costs of machinery, initial labor, riprap, liner, and coconut fiber for a total cost of approximately $19,809. However, the full cost of implementation is $21,223 , and is covered within the grant, so long term maintenance requirements would be the only cost to the community.
While the initial wetland design implementation would be free for the City of
Winchester, the potential benefits are great. As mentioned in the literature review, total phosphorous removal rates in stormwater wetlands can be as high as 55% (CWP, 2008), but long term phosphorous removal rates can be expected to be approximately 30% (VA DCR, 1999).
The long term phosphorous removal rate of 30% is projected for stormwater wetlands that follow the Stormwater Management Handbook recommendations and are implemented with a sediment forebay (VA DCR, 1999). By vegetating the area without a sediment forebay or micro-pools, the phosphorous removal can be expected to be much less than the expected high of 55% and the long term removal efficiency of 30%. Since the vegetating the area will provide some of the wetland benefits such as plant uptake, stormwater filtration, and reducing runoff velocity to promote sedimentation, implementing the vegetation scheme only is likely to achieve the lower end of the phosphorous removal rate in a stormwater wetland, approximately 15% (CWP, 2008).
For the implementation of the vegetation scheme only, the area may achieve the lower to mid range of nitrogen removal: 0 – 25% (CWP, 2008), since longer residence time and lower flow velocity that encourage nitrogen removal up to 55% (CWP, 2009) will not be achieved without the implementation of the entire wetland design. Bacteria removal for the implemented design should reach the higher range of 60 – 85%, but with only the vegetation scheme it is likely that the area may not even reach the lower removal rate projection of 40%.
X. Conclusion
Although the team has encountered some obstacles throughout the semester, many major accomplishments have also been achieved, and the team is confident that a functioning water quality swale and pocket wetland will be installed by the proposed deadline.
The largest obstacle the team encountered was in the ArcGIS analysis. While required layers were not available and problems with geo-referencing limited further accomplishments, the team was able to make new contacts to find available data and work with this data in GIS to find the required information for the hydrologic analysis.
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In spite of this setback, the team has completed a site survey, created an AutoCAD surface of the site, and GIS analysis of the site and drainage area. The GIS analysis has allowed the team to determine that underground utilities are not a constraint on the design, that storm drains bring water within the drainage area to the site, and begin the hydrologic analysis. In addition to these accomplishments, the team has determined alternative designs for the swale and wetland and chosen the final design alternatives of a dry water quality swale with underdrain and a forested, cell/meandering wetland.
While much has been completed, the team also has multiple goals for next semester. The team plans to complete the hydrologic analysis, and create the final design based on these results.
In addition, the team is on the agenda for a meeting in Winchester to present the plan to residents and other stakeholders. An economic analysis and construction plans will also be completed before the final goal of the project, on-site installation, can be completed in April.
Urban water quality retrofits have the prospect of reducing nutrient levels and mitigating pollution to waterways. Through this design project, an algae covered rip-rap channel and a dry pond will be retrofitted with an innovative bio-swale and wetland which should reduce nutrient levels to Opequon Creek. Potentially, the retrofit design will please residents with its aesthetic quality and serve as a model for nutrient reduction in an urban setting.
XI. Reflections
Kelly
Overall, the team worked very well together, and we were all equally committed to making this project successful. While there were many obstacles in the design process along the way, the team worked hard and benefited from the expertise of the others. We gained an extraordinary amount of experience this year in carrying this project from the conception of the idea to the final design. One especially important aspect of this project was learning how to balance public perceptions and approval of the design with the need to make the design improve water quality as much as possible. This is not taught in classes, but I’m sure that it will prove to be a very helpful lesson for the future. Even after the many setbacks that prohibited us from implementing the design this year, it was very reassuring to know that there is a real possibility that the design will be implemented in the future.
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Dan
The two-semester Senior Design project was a very important experience and one of the highlights of my undergraduate career. I feel our group had a unique opportunity in our project to include, and work with, people from outside the University. We had two presentations with the
NRAB group in Winchester, as well as numerous email communications throughout the project.
Presenting our design to people outside of class allowed the group to receive feedback from a group of people with a different viewpoint than Engineering professors and students. Through this, we learned that the general public is not too concerned with technical engineering details, but much more concerned with details Engineers could easily overlook. The simple question of
“how is this going to look in our community” is much more important to them than the question of “what design storm will this hold and how much will the nutrient loads be reduced.”
Both time management and teamworking skills were two other big things I got out of the project. I learned that task will always take longer to complete than expected. Our team met the every Tuesday and Thursday at the same time since September to work on the project. This consistency was good at keeping all team members on task and up to date as the project moved along. The project taught me the importance of effective communication and the importance of consistency in team management. The yearlong project was an important experience for teamwork that all of us will learn from and apply to our careers.
Kerry
Although there are many guidelines for designing best management practices, I learned that the guidelines available are to be used as references and not as step-by-step instruction manuals. There were many sources that contained much of the same information, but there were some sources that had different information. Overall, I learned that the different design manuals and guidelines are very good resources and should be referenced and that the compilation of different resources are needed to design a complete best management practice.
~ 83 ~
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Appendix A
AutoCAD Surface
~ 90 ~
Appendix B
Original Gantt Chart
Task Name
1 Team Formation and Project Selection
8
9
6
7
4
5
2
3
Preliminary Research
Scope of Work
Literature Review
First Site Visit
Work Plan
Design Criteria and Alternatives
Decision Matrix
Final Semester Report
10 Technical Presentation
11 NRAB Presentation
12 Hydrologic Analysis
13 Second Site Visit
14 Final AutoCAD Design
15 Economic Analysis
16 Second NRAB Presentation
17 Detailed Modifications
18 Permitting
19 Detailed Construction Plans
20 Mid-Term Progress Report
21 Final Report Draft
22 Installation
23 Poster Presentation
24 Final Report
Start
2/5/2009
2/6/2009
3/2/2009
3/17/2009
3/16/2009
3/16/2009
3/16/2009
2/16/2009
2/20/2009
4/10/2009
4/24/2009
4/21/2009
9/16/2008
9/16/2008
9/16/2008
10/6/2008
10/16/2008
11/10/2008
11/13/2008
11/13/2008
11/13/2008
11/20/2008
1/12/2009
1/1/2009
~ 91 ~
Finish
2/6/2009
3/2/2009
3/6/2009
3/17/2009
3/26/2009
3/26/2009
4/1/2009
2/16/2009
4/20/2009
4/15/2009
4/24/2009
5/13/2009
9/16/2008
9/23/2008
9/25/2008
11/20/2008
10/17/2008
11/13/2008
11/17/2008
11/18/2008
12/16/2008
12/2/2008
1/12/2009
2/20/2009
Oct
2008
Nov Dec Jan Feb Mar Apr
2009
May Jun Jul Aug Sep
Actual Gantt Chart
Task Name
1 Team Formation and Project Selection
2 Preliminary Research
3 Scope of Work
4 Literature Review
5 First Site Visit
6 Work Plan
7 Design Criteria and Alternatives
8 Decision Matrix
9 Final Semester Report
10 Technical Presentation
11 NRAB Presentation
12 Hydrologic Analysis
13 Second Site Visit
14 Final AutoCAD Design
15 Determine Vegetation
16 Economic Analysis
17 Maintenance Plan
18 Community Outreach Plan
19 Vegetation Scheme
20 NRAB meeting
21 Poster Presentation
22 Nature Area Option
23 Final Report
Start
1/12/2009
1/1/2009
2/5/2009
2/6/2009
3/16/2009
3/16/2009
4/1/2009
4/1/2009
4/20/2009
4/23/2009
4/24/2009
4/27/2009
4/21/2009
9/16/2008
9/16/2008
9/16/2008
10/6/2008
10/16/2008
11/10/2008
11/13/2008
11/13/2008
11/13/2008
11/20/2008
Finish
1/12/2009
2/20/2009
2/6/2009
5/1/2009
3/26/2009
5/1/2009
5/1/2009
4/24/2009
4/20/2009
4/23/2009
4/24/2009
4/29/2009
5/13/2009
9/16/2008
9/23/2008
9/25/2008
11/20/2008
10/17/2008
11/13/2008
11/17/2008
11/18/2008
12/16/2008
12/2/2008
2008 2009
Oct Nov Dec Jan Feb Mar Apr May
~ 92 ~
Appendix C
Team Skills
Qualifications:
Land and water resources engineering background
Autodesk Civil 3D experience
GIS experience
Required prerequisite courses:
CEE 3104 Introduction to Environmental Engineering
ESM 3024 Introduction to Fluid Mechanics
BSE 3305 Land and Water Resources Engineering I
BSE 3306 Land and Water Resources Engineering II
BSE 3314 CADD for Land and Water Resources Engineers
Required co-requisite courses:
BSE 4304 Nonpoint Source Pollution Modeling and Management
Estimated commitment:
6 – 8 hours per week per team member
Skills that must be developed for successful completion of this project:
Strong team skills
Working knowledge of surveying equipment and procedures
Communication skills, including writing and public speaking
Basic knowledge of hydrology, nonpoint source pollution, and wetland functions
Working knowledge of GIS, AutoCAD, and RUSLE2
Cost analysis procedures
Basic knowledge of the permitting and construction processes
~ 93 ~
Appendix D
WinTR-55 Flow Paths
~ 94 ~
Appendix E
WinTR-55 Land Use Input Data
WinTR-55 Structure Input Data
WinTR-55 Time of Concentration Data
~ 95 ~
WinTR-55 Peak Flow Rates Output
~ 96 ~
Appendix F
Community Outreach Pamphlet
~ 97 ~
~ 98 ~
