VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 13

CONSTRUCTED WETLANDS

VERSION 1.6
September 30, 2009

SECTION 1: DESCRIPTION

Constructed wetlands are shallow depressions that receive stormwater inputs for treatment. Wetlands are typically less than one foot deep (although they have deeper pools at the forebay and micropool) and possess variable microtopography to promote dense and diverse wetland cover (Figure 1). Runoff from each new storm displaces runoff from previous storms, and the long residence time allows multiple pollutant removal processes to operate. The wetland environment provides an ideal environment for gravitational settling, biological uptake, and microbial activity. Constructed wetlands are the final element in the roof to stream runoff reduction sequence, and should only be considered after all other upland runoff reduction opportunities have been exhausted, and there is still a remaining water quality or channel protection volume to manage. The overall stormwater functions of constructed wetlands are summarized in Table 1.

Figure 1: Plan View Constructed Wetland Basin

Figure 2. Plan and Cross-Section of Shallow Wetland

Figure 4. Cross Section of Linear Wetland Cell

SECTION 4:  PHYSICAL FEASIBILITY AND DESIGN APPLICATIONS

Constructed wetlands are subject to several site constraints when it comes to design;

Constructed wetlands are designed based on three major factors- the desired plant community (emergent wetland, a mix of emergent and forest or emergent/pond), the contributing hydrology (groundwater, surface runoff or dry weather flow) and its landscape position (linear or basin).  Table 3 shows the recommended combinations of these three factors to ensure an effective stormwater wetland.

To simplify design, two basic design variations are presented for constructed wetlands:

IMPORTANT NOTE:  Two wetland designs that have been referenced in past design manuals (Schueler, 1992) are no longer allowed or are highly constrained. These include the extended detention (ED) wetland (with more than 1 foot of vertical extended detention storage) and the pocket wetland (unless it has a reliable augmented water source, such as the discharge from a rain tank).

Common design applications for the pond wetland combination are in moderately to highly urban areas where space is a premium. The critical design factor is the depth of ED ponding above the wetlands if contained in the same cell. A preferred design is illustrated in Figure 5 with the wetland cells independent of the ED ponding. Constructed wetland basins can be used at the terminus of a storm drain pipe or open channel (usually after upland opportunities for runoff reduction have also been applied).

Figure 5.  Pond/Wetland Combination

The pond-wetland combination design involves an on-line wet pond cell that discharges to a series of off-line constructed wetland cells.

SECTION 5: DESIGN CRITERIA

5.1. Sizing of Constructed Wetlands

Constructed wetlands should be designed to capture and treat the treatment volume (TV) remaining from the upstream runoff reduction practices, and the channel protection storm (if needed) using the accepted local or state runoff reduction method.

Runoff treatment credit can be taken for:

To qualify for the higher nutrient reduction rates for Level 2 design, constructed wetlands must be designed with a treatment volume equal to 1.50(Rv)(A). Research has shown that larger constructed wetlands with longer residence times enhance nutrient removal rates.

5.2. Water Balance: Sizing for Minimum Pool Depth

If the hydrology for the constructed wetland is not supplied by groundwater or dry weather flow inputs, a simple water balance must be performed to assure deep pools will not go completely dry during a 30 day summer drought, using the Hunt equation in Table 4.

5.3. Geotechnical  Testing

Soil borings should be taken below the proposed embankment, in the vicinity of the proposed outlet area, and in at least two locations within the planned wetland treatment area. Soil boring data is needed to determine the physical characteristics of excavated material, determine its adequacy for use as structural fill or spoil, provide data for structural designs for outlet workers (e.g., bearing capacity and buoyancy), determine compaction/composition needs for the embankment, define the depth to groundwater and/or bedrock and evaluate potential infiltration losses (and the consequent need for a liner).

5.4. Pretreatment Forebay

Sediment forebays are considered an integral design feature of all stormwater wetlands, and must be located at all major inlets to trap sediment and preserve the capacity of the main wetland treatment cell.

5.5. Conveyance and Overflow

5.6. Internal Design Geometry

Research and experience have shown that the internal design geometry and depth zones are critical in maintaining thepollutant removal capability and plant diversity of a stormwater wetland.  Wetland performance is enhanced when the wetland has multiple cells, longer flowpaths, and a high surface area to volume ratio. Whenever possible, constructed wetlands should be irregularly shaped with a long, sinuous flow path. The following design elements are required for every stormwater wetland.

5.7.  Microtopographic Features

Stormwater wetlands shall have internal structures that createvariable microtopography, which is defined as a mix of above pool vegetation, shallow pools and deep pools that promote dense and diverse vegetative cover. Designers will need to incorporate at least two of the following internal design features to meet the microtopography requirements for Level 2 design:

*See Design Specification #11 (Wet Swales) for standard details.

5.8.  Maintenance Reduction Features

Some important maintenance problems can be avoided during the design of constructed wetlands:

5.9. Wetland Landscaping Plan

An initial wetland landscaping plan is required for any stormwater wetland and should be jointly developed by the engineer and a wetlands expert or experienced landscape architect. The plan should outline a detailed schedule for the care, maintenance and possible reinforcement of vegetation in the wetland and its buffer for up to ten years after the original planting. More details on preparing a wetland landscaping plan can be found throughout this specification.

The plan should outline a realistic, long-term planting strategy to establish and maintain desired wetland vegetation. The plan should indicate how wetland plants will be established within each inundation one (e.g., wetland plants, seed-mixes, volunteer colonization, and tree and shrub stock) and whether soil amendments are needed to get plants started. At a minimum, the plan should contain the following:

The local review authority will usually establish the vegetative goals to achieve in the wetland landscaping plan. The following general guidance is offered:

      Zone 1:  -6 inches to -12 inches below normal pool
      Zone 2:  -6 inches to normal pool
      Zone 3:  normal pool to + 12 inches
      Zone 4:  +12 inches to + 36 inches (i.e., above ED Zone)

Note that the low marsh zone (-6 inches to -18 inches below normal pool) has been dropped since experience has shown that few emergent wetland plants flourish in this deeper zone.

5.10. Constructed Wetland Material Specifications

Wetlands are generally constructed with materials obtained on-site except for plantings, inflow and outflow devices such as piping and riser materials, possibly stone for inlet and outlet stabilization, and filter fabric for lining banks or berms. The basic material specifications for earthen embankments, principal spillways, vegetated emergency spillways and sediment forebays shall be as specified in Appendices A-D of the Introduction to the New Virginia Stormwater Design Specifications, as posted on the Virginia Stormwater BMP Clearinghouse web site, at the following URL:

http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html

Plant stock should be nursery grown unless otherwise approved and should be healthy and vigorous native species, free from defects, decay, disfiguring roots, sun-scald, injuries, abrasions, diseases, insects, pests, and all forms of infestations or objectionable disfigurements as determined by the local approving authority.

SECTION 6:  REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

6.1. Karst Terrain

Even shallow pools in active karst terrain can increase the risk of sinkhole formation and groundwater contamination. Designers should always conduct geotechnical investigations in karst terrain to assess this risk in the planning stage. If they are employed, designers must:

6.2. Coastal Plain

Constructed wetlands are an ideal practice for the flat terrain, low head and high water table conditions found at many coastal plain development sites. The following design adaptations can make it work more effectively:

• Shallow, linear and multiple cell wetland configurations are preferred.
• Deeper basin configurations, such as the pond/wetland system and the ED wetland have limited application in the coastal plain.
• It is acceptable to excavate up to 6 inches below the seasonally high water table to provide the requisite hydrology for wetland planting zones, and up to 3 feet below for micropools, forebays and other deep pool features.
• The volume below the seasonably high water table is acceptable for the water quality or treatment volume as long as the other primary geometric and design requirements for the wetland are met (e.g., flow path and microtopography).
• Plant selection should focus on species that are wet-footed and can tolerate some salinity.
• A greater range of coastal plain tree species can tolerate periodic inundation, so designers should consider forested wetlands, using species such as Atlantic white cedar, bald cypress and swamp tupelo.
• The use of flashboard risers is recommended to control or adjust water elevations in constructed wetlands in flat terrain.
• The regenerative conveyance system is particularly suited for coastal plain situations where there is a significant drop in elevation from the channel to the outfall location (see Design Specification #11, Wet Swales).

6.3. Steep Terrain

Constructed wetlands are not an effective practice at development sites with steep terrain. Some adjustment can be made by terracing wetland cells in a linear manner (using a 1 to 2 foot armored drop between individual cells. Terracing may work well up to about 10% longitudinal slopes.

6.4. Winter Performance

Wetland performance decreases when snowmelt runoff delivers high pollutant loads. Shallow constructed wetlands can freeze in the winter, which allows runoff to flow over the ice layer and exit without treatment. Inlet and outlet structures close to the surface may also freeze, further diminishing wetland performance. Salt loadings are higher in cold climates due to winter road maintenance. High chloride inputs have a detrimental effect on native wetland vegetation, and can shift the wetland to more salt-tolerant species such as cattails (Wright et al., 2006). Designers should choose salt-tolerant species when crafting their planting plan and consider reducing salt application in the contributing drainage area. The following design adjustments are recommended for stormwater wetlands installed in higher elevations of the Bay watershed, such as New York, Pennsylvania, and West Virginia.

• Treat larger runoff volumes in the spring by adopting seasonal operation of the permanent pool (see MSSC, 2005).
• Plant salt-tolerant wetland vegetation.
• Do not submerge inlet pipes and provide a minimum 1% pipe slope to discourage ice formation.
• Locate low flow orifices so they withdraw at least 6 inches below the typical ice layer.
• Angle trash racks to prevent ice formation.
• Oversize riser and weir structures to avoid ice formation and freezing pipe.
• Increase forebay size if road sanding is prevalent in the contributing drainage area.

6.5. Linear Highway Sites

Wet swales, linear wetland cells and regenerative conveyance systems are particularly well suited to treat runoff within open channels located in the highway right of way, and are considered a preferred practice.




SECTION 7: CONSTRUCTED WETLAND
CONSTRUCTION SEQUENCE AND INSPECTION

The construction sequence for stormwater wetlands depends on site conditions, design complexity and the size and configuration of the proposed facility. The following two stage construction sequence is recommended for installing an on-line wetland facility and establishing vigorous plant cover

7.1. Construction Sequence:  Stage 1 Facility Construction

Step 1.  Stabilize Drainage Area.  Stormwater wetlands should only be constructed after the contributing drainage area to the wetland is completely stabilized.  If the proposed wetland site will be used as a sediment rap or basin during the construction phase, the construction notes should clearly indicate that the facility will be de-watered, dredged and re-graded to design dimensions after construction is complete.

Step 2.  Assemble Construction Materials on-site, make sure they meet design specifications and prepare any staging areas.

Step 3.  Clear and Strip the project area to desired sub-grade.

Step 4.  Install Project Erosion and Sediment (E&S) Control, including temporary dewatering devices, sediment basins, and stormwater diversion practices prior to construction. All areas surrounding the wetland that are graded or denuded during construction of the wetland are to be planted with turf grass, native plant materials or other approved methods of soil stabilization.  Grass sod is preferred over seed to reduce seed colonization of the wetland. During construction, the wetland must be separated from the contributing drainage area so that no sediment flows into the wetland areas.  In some cases, a phased or staged ESC plan may be necessary that diverts flow around the stormwater wetland area until installation and stabilization are complete.

Step 5.  Excavate the Core Trench for the Embankment and Install the Spillway Pipe.

Step 6.  Install the Riser or Outflow Structure and ensure that the top invert of the overflow weir is constructed level and at the proper design elevation (e.g., flashboard risers are strongly recommended- Hunt et al, 2007).

Step 7.  Construct the Embankment and any Internal Berms in 8 to 12-inch lifts and compacted with appropriate equipment.

Step 8.  Excavate/Grade until the appropriate elevation for the bottom of the wetland is reached and desired contours and acceptable side slopes are achieved. This is normally done by “roughing up” the interim elevations with a skid loader or other equipment to achieve the desired topography across the wetland.  Spot surveys should be made to ensure that the interim elevations are three to six inches below the final elevations for the wetland.

Step 9.  Install MicroTtopographic Features and Soil Amendments within wetland area. Since most stormwater wetlands are excavated to deep sub-soils, they often lack the nutrients and organic matter needed to support vigorous growth of wetland plants. It is therefore essential to add sand, compost, topsoil or wetland mulch to all depth zones in the wetland. The importance of soil amendments in excavated wetlands cannot be over-stressed; poor survival and future wetland coverage are likely if soil amendments are not added (Bowers, 1992). The planting soil should be a high organic content loam or sandy loam, be placed by mechanical methods, and spread by hand.  A soil depth of at least four inches should be used for shallow wetlands.  No machinery should be allowed over the planting soil during or after construction.  Planting soil should be tamped as directed in the design specifications, but not overly compacted.  After the planting soil is placed, it should be saturated and allowed to settle for at least one week prior to installation of plant materials.

Step 10. Construct the Emergency Spillway in cut or structurally stabilized soils.

Step 11.  Install Outlet Pipes, including the downstream rip-rap apron protection.

Step 12.  Stabilize Exposed Soils with temporary seed mixtures appropriate for a wetland environment. All wetland features above the normal pool should be temporarily stabilized by hydro-seeding or seeding over straw.

7.2. Stage 2 Construction Sequence: Establishing the Wetland Vegetation

Step 13.  Finalize the Eetland Landscaping Plan.  At this stage the engineer, landscape architect, and wetland expert work jointly to refine the initial wetland landscaping plan after the stormwater wetland has been constructed. Several weeks of standing time is needed so that the designer can more precisely predict:

• Where the inundation zones are located in and around the wetland, and
• Whether the final grade and wetland microtopography will persist over time

This allows the designer to select appropriate species and additional soil amendments, based on field confirmation of soils properties, and the actual depths and inundation frequencies occurring within the wetland.

Step 14.  Open Up the Wetland Connection.  Once the final grades are attained, the pond and/or contributing drainage area connection should be opened to allow the wetland cell to fill up to the normal pool elevation. The wetland is to be gradually inundated so as not to erode unplanted features.  Inundation is to occur in stages so that deep pool and high marsh plant materials can be placed effectively and safely.  Wetland planting areas should be at least partially inundated during planting to promote plant survivability.

Step 15.  Measure and Stake Planting Depths at the onset of the planting season. Depths in the wetland should be measured to the nearest inch to confirm the original planting depths of the planting zone. At this time, it may be necessary to modify the plan to reflect altered depth or the availability of wetland plant stock. Surveyed planting zones should be marked on the as-built or design plan, and also located in the field using stakes or flags.

Step 16.  Propagate the Stormwater Wetland.  Three techniques are used in combination to propagate the emergent community over the wetland bed:

• Initial Planting of Container Grown Wetland Plant Stock.  The transplanting window extends from early April to mid-June. Planting after these dates is quite chancy, as emergent wetland plants need a full growing season to build the root reserves needed to get through the winter. If at all possible, the plants should be ordered at least 6 months in advance to ensure the availability of desired species
• Broadcasting Wetland Seed Mixes:  The higher wetland elevations should be established by broadcasting wetland seed mixes to establish diverse emergent wetlands. Seeding of switchgrass or wetland seed mixes as a ground cover is recommended for all zones above three inches below the normal pool. Hand broadcasting or hydroseeding can be used to spread seed depending on the size of the wetland cell.
• Allowing “Volunteer Wetland Plants to Establish on Their Own.  The remaining areas of the stormwater wetlands will eventually be colonized by volunteer species from upstream or the forest buffer within 3 to 5 years.

Step 17.  Install Goose Protection to Protect Newly Planted or Newly Growing Plants.  This is particularly critical for newly established emergents and herbacacious plants, as predation by Canada geese can quickly dessimate wetland vegetation.  Goose protection can consist of netting, webbing, or sting installed in a criss-cross pattern over the surface area of the wetland above the level of the emergent plants.

Step 17.  Plant the Wetland Fringe and Buffer Area.  This zone generally extends from 1 to 3 feet above the normal pool (from the shoreline fringe to about half of the maximum 2 year storm water surface elevation). Consequently, plants in this zone are infrequently inundated (5 to 10 times per year), and must be able to tolerate both wet and dry periods.

7.3. Construction Inspection

Construction inspections are critical to ensure that stormwater wetlands are properly constructed and established. Multiple site visits and inspections are recommended during the following stages of the wetland construction process:

• Pre-construction meeting
• Initial site preparation (including installation of project E&S controls)
• Excavation/Grading (interim/final elevations,)
• Wetland installation (e.g., microtopography, soil amendments and staking of planting zones)
• Planting Phase (with experienced landscape architect or wetland expert)
• Final Inspection (develop punch list for facility acceptance)

A construction inspection form for stormwater wetlands can be accessed at the CWP website at www.cwp.org/postconstruction (see Tool #6 of Managing Stormwater in Your Community: A Guide for Building Effective Post-Construction Programs).

SECTION 8: CONSTRUCTED WETLAND MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program.  The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.

It is also recommended that the maintenance agreement include a list of qualified contractors that can perform inspection or maintenance services, as well as contact information for owners to get local or state assistance to solve common nuisance problems, such as mosquito control, geese, invasive plants, vegetative management and beaver removal. The CWP (2004) pond and wetland maintenance guidebook provides some excellent templates on how to respond to these problems.

8.2. First Year Maintenance Operations

Successful establishment of constructed wetland areas requires certain tasks be undertaken in the first two years:

Initial Inspections:  For the first 6 months following construction, the site should be inspected at least twice after storm events that exceed 0.5 inches in depth.

Spot Reseeding:  Inspectors should look for bare or eroding areas in the contributing drainage area or around the wetland buffer, and make sure they are immediately stabilized with grass cover.

Watering:  Trees planted in the buffer and on wetland islands and peninsulas need watering during the first growing season, In general, consider watering  every three days for first month, and then weekly during first year (April - October), depending on rainfall.

Reinforcement Plantings:  Regardless of the care taken during the initial planting of wetland and buffer it is probable that some areas will remain un-vegetated, and some species will not survive. Poor survival can result from many unforeseen factors, such as predation, poor plant stock, water level changes, drought, and many other unpredictable factors. Thus, it is advisable to budget for an additional round of reinforcement planting after one or two growing seasons. Construction contracts should include a care and replacement warranty extending at least two growing seasons after initial planting to selectively replant portions of the wetland that fail to fill in or survive.

8.3. Ongoing Maintenance

Ideally, maintenance of constructed wetlands should be driven by annual inspections that evaluate the condition and performance of the wetland (see Table 8). Based on inspection results, specific maintenance tasks will be triggered. An annual maintenance inspection form for stormwater wetlands can be accessed at CWP website at the following URL: www.cwp.org/postconstruction (see Tool #6 of Managing Stormwater in Your Community: A Guide for Building Effective Post-Construction Programs).

A more detailed maintenance inspection form is also available from Appendix B of CWP (2004).

Managing vegetation is an important ongoing maintenance task at every constructed wetland, across each inundation zone. The proposed designs reduce the need for regular mowing to the embankment and access roads. Vegetation within the wetland, however, requires some annual maintenance. Designers should expect significant changes in wetland species composition over time. Invasive plants should be dealt with as soon as they colonize the wetland. Vegetation may need to be periodically harvested if the constructed wetland becomes overgrown. Inspections should carefully track changes in wetland plant species distribution over time.

8.4.  Non-Routine Maintenance

• Sediment Removal:  Frequent sediment removal from the forebay is essential to maintain the function and performance of a constructed wetland. Maintenance plans should schedule cleanouts every 5 years or so, or when inspections indicate that 50% of the forebay capacity has been lost. Designers should also check to see whether removed sediments can be spoiled on-site or must be hauled away. Sediments excavated from constructed wetlands are not usually considered toxic or hazardous, and can be safely disposed by either land application or land filling.

 

• Control Invasive Species:  Designers should expect significant changes in wetland species composition over time. Invasive plants should be dealt with as soon as they colonize the wetland. As a general rule, control of undesirable invasive species, such as cattail and Phragmites, should commence when their coverage exceeds more than 15% of a wetland cell area.  Although the application of herbicides is not recommended, some types, such as Glyphosate, have been used to control cattails with some success. Extended periods of dewatering may also work, since early manual removal provides only short-term relief from invasive species. While it is difficult to exclude invasive species from stormwater wetlands, their ability to take over the entire wetland can be reduced if the designer creates a wide range of depth zones and a complex internal structure within the wetland.
• Thinning and Harvesting of Woody Growth:  Thinning or harvesting of excess forest growth may be periodically needed to guide the forested wetland into a more mature state. Vegetation may need to be periodically harvested if the constructed wetland becomes overgrown. These operations should be scheduled for about five and ten years after initial wetland construction. Removal of woody species on or near the embankment and maintenance access areas should be conducted every two years.

Table 8: Suggested Annual Maintenance Inspection Points for Constructed Wetlands

Activity

Measure sediment accumulation levels in forebays and micropools.

Monitor the growth and survival of emergent wetlands and tree/shrub species. Record species and approximate coverage, and note presence of any invasive plant species.

Inspect the condition of stormwater inlets to the wetland for material damage, erosion or undercutting

Inspect upstream and downstream banks for evidence of sloughing, animal burrows, boggy areas, woody growth or gully erosion that may undermine embankment integrity,

Inspect the wetland outfall channel for erosion, undercutting, rip-rap displacement, woody growth, etc.

Inspect condition of the principal spillway and riser for evidence of spalling, joint failure, leakage, corrosion, etc.

Inspect the condition of all trash racks. reverse sloped pipes or flashboard risers for evidence of clogging, leakage, debris accumulation, etc.

Inspect maintenance access to ensure it is free of woody vegetation and check to see whether valves, manholes or locks can be opened or operated.

Inspect internal and external side slopes in the wetland for evidence of sparse vegetative cover, erosion or slumping, and repaired immediately

Cleanups should be scheduled at least once a year to remove trash, debris and floatables

Note: For a more detailed maintenance inspection checklist, see Appendix B in the CWP (2004) Stormwater Pond and Wetland Maintenance Guidebook

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

Constructed wetlands can generate several community and environmental concerns that may need to be addressed during design:

• Aesthetics and Habitat:  Constructed wetlands can create wildlife habitat and become an attractive community feature. Designers think carefully about how the wetland community will evolve over time, since the future plant community seldom resembles the one initially planted.
• Existing Forests:  Given the large footprint of constructed wetlands, there is a strong chance that construction may cause extensive tree clearing. Designers should preserve mature trees during layout, and may want to use a wooded wetland concept to create a forested wetland community (see Cappiella et al., 2006b).
• Stream Warming Risk:  Constructed wetlands have a moderate risk of stream warming. If a constructed wetland will discharge to temperature-sensitive waters, designers should consider the wooded wetland design, and any ED storage should be released in less than 12 hours.
• Safety Risk:  Constructed wetlands are safer than other pond options, although forebays and micropools should be designed with aquatic benches to reduce safety risks.
• Mosquito Risk:  Mosquito control can be a concern for stormwater wetlands if they are under-sized or have a small contributing drainage area. Few mosquito problems are reported for well designed, properly-sized and frequently maintained constructed wetlands, but no design can eliminate them completely. Simple precautions can be taken to minimize mosquito breeding habitat within constructed wetlands, such as constant inflows, benches that create habitat for natural predators, and constant pool elevations (see Walton 2003 and MSSC, 2005).
 



SECTION 10: DESIGN REFERENCES

Cappiella, K., T. Schueler, J. Tasillo and T. Wright. 2005. Adapting watershed tools to protect wetlands. Wetlands and Watersheds Article No. 3. Center for Watershed Protection. Ellicott City, MD.

Cappiella, K., L. Fraley-McNeal, M. Novotney and T. Schueler. 2008. The next generation of stormwater wetlands. Wetlands and Watersheds Article No. 5. Center for Watershed Protection. Ellicott City, MD

Center for Watershed Protection. 2004. Pond and Wetland Maintenance Guidebook. Ellicott City, MD.
 
CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD.

Claytor, R. and T. Schueler.  1996.  Design of Stormwater Filtering Systems.  Center for Watershed Protection. Ellicott City, MD.

Hirschman, D. and J. Kosco. 2008. Managing stormwater in your community: a guide for building an effective post-construction problem. EPA Publication 833-R-08-001, Tetra-tech, Inc. and Center for Watershed Protection. Ellicott City, MD

Hunt, W., M. Burchell, J. Wright and K. Bass. 2007. Stormwater wetland design update: zones, vegetation, soil and outlet guidance. Urban Waterways. North Carolina State Cooperative Extension Service. Raliegh, NC.

Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD 

Schueler, T. 1992. Design of stormwater wetland systems: guidelines for creating diverse and effective stormwater wetlands in the mid-Atlantic region. Metropolitan Washington Council of Governments. Washington, DC.  

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net

Shaw, D. and R. Schmidt. 2003. Plants for stormwater design: species selection for the upper Midwest. Minnesota Pollution Control Agency. St. Paul, MN

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA.

Wright, T., J. Tomlinson, T. Schueler, Karen Cappiella, A. Kitchell and D. Hirschman. 2006. Direct and indirect impacts of land development on wetland quality. Wetlands and Watersheds Article No. 1. Center for Watershed Protection. Ellicott City, MD.