DRAFT BAY-WIDE STORMWATER DESIGN SPECIFICATION No. 13: CONSTRUCTED WETLANDS

DRAFT BAY-WIDE STORMWATER DESIGN SPECIFICATION No. 13: CONSTRUCTED WETLANDS

DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 13

CONSTRUCTED WETLANDS

VERSION 1.5

Note to Reviewers of the Stormwater Design Specifications

The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact Development (LID) practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.

Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:
Status as of 9/24/2008
# / Practice / Notes / Status 1
1 / Rooftop Disconnection / Includes front-yard bioretention / 2
2 / Filter Strips / Includes grass and conservation filter strips / 2
3 / Grass Channels / 2
4 / Soil Compost Amendments / 3
5 / Green Roofs / 1
6 / Rain Tanks / Includes cisterns / 2
7 / Permeable Pavement / 1
8 / Infiltration / Includes micro- small scale and conventional infiltration techniques / 2
9 / Bioretention / Includes urban bioretention / 3
10 / Dry Swales / 2
11 / OPEN
12 / Filtering Practices / 2
13 / Constructed Wetlands / Includes wet swales / 2
14 / Wet Ponds / 2
15 / ED Ponds / 2
1 Codes: 1= partial draft of design spec; 2 = complete draft of design spec;
3 = Design specification has undergone one round of external peer review as of 9/24/08

Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.

Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:

The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community. This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.

The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009. The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.

The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.

As with any draft, there are several key caveats, as outlined below:

Many of the proposed design standards and specifications lack graphics. Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.

There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions.

While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.

Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.

These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an email copy of your comments, etc., to Scott Crafton (). The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.

Thank you for your help in producing the best stormwater design specifications for the Commonwealth.

DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 13

CONSTRUCTED WETLANDS

VERSION 1.5

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.

In addition, constructed wetlands are the final element in the roof-to-stream runoff reduction sequence. However, they should only be considered after all other upland runoff reduction techniques have been exhausted, and there is still a remaining water quality or channel protection volume to manage.

Figure 1: Plan View of a Constructed WetlandBasin

(NOTE: Graphics derived from non-Virginia sources will need to be edited to eliminate erroneous references to figure numbers, etc..)

SECTION 2. PERFORMANCE CRITERIA

Table 1: Summary of Stormwater Functions Provided by Infiltration
Stormwater Function / Level 1 Design / Level 2 Design
Annual Runoff Reduction / 0% / 0%
Total Phosphorus Removal 1 / 60% / 65%
Total Nitrogen Removal 1 / 25% / 55%
Channel Protection / Moderate (?)
RRv can be subtracted from CPv(?)
Flood Mitigation / Partial (?)
Reduced Curve Numbers and Time of Concentration (?)
1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate.
Sources: CWP and CSN (2008) and CWP (2007).

SECTION 3: PRACTICE APPLICATIONS ANDFEASIBILITY

Constructed wetlands are subject to the following site constraints when it comes to design:

Adequate Water Balance: The wetland must have enough water supply from groundwater, runoff or baseflow so that wetland micropools will not completely go dry after a thirty day summer drought. A simple water balance must be performed (see Section 6.2).

Contributing Drainage Area (CDA): The contributing drainage area must be large enough to sustain a permanent water level within a stormwater wetland. Several dozen acres of drainage area are typically needed to maintain constant water elevations if the only source of wetland hydrology is stormwater runoff. Smaller drainage areas are acceptable if the bottom of the wetland intercepts the groundwater table or if designers and approving agencies are willing to accept periodic wetland drawdown.

Space Requirements: Constructed wetlands normally require a footprint of about 3% of the contributing drainage area, depending on the average depth of the wetland and the extent of its deep pool features.

Available Hydraulic Head: The depth of a constructed wetland is usually constrained by the hydraulic head available on the site. The bottom elevation is fixed by the elevation of the existing downstream conveyance system to which the wetland will ultimately discharge. Head requirements for constructed wetlands are typically less than for wet ponds because of their shallow nature – a minimum of 2-4 feet of head is usually needed.

Minimum Setbacks: Local ordinances and design criteria should be consulted to determine minimum setbacks to property lines, structures, utilities, and wells. As a general rule, the edges of constructed wetlands should be located at least 10 feet from property lines, 25 feet from building foundations, 50 feet from septic system fields and 100 feet from private wells.

Depth to Water Table: The depth to the groundwater table is not a major constraint for constructed wetlands, as a high water table can help to maintain wetland conditions. However, designers should keep in mind that high groundwater inputs may reduce pollutant removal rates and increase excavation costs.

Soils: Geotechnical tests should be conducted to determine the infiltration rates and other subsurface properties of the soils underlying the proposed wetland (see Section 6.3). If soils are permeable or karst geology is a concern, it may be necessary to use an impermeable liner (see Section 11.1).

Trout Streams: The use of constructed wetlands in watersheds containing trout streams is generally NOT recommended due to the potential for stream warming, UNLESS (a) all other upland runoff reduction opportunities have been exhausted (b) the channel protection volume has not been provided, and (c) a linear/mixed wetland design is applied to minimize stream warming.

Use of or Discharges to Natural Wetlands: It can be tempting to construct a stormwater wetland within an existing natural wetland, but this should never be done unless it is part of a broader effort to restore a degraded urban wetland and is approved by the local or state wetland review authority. Constructed wetlands may not be located within jurisdictional waters, including wetlands, without obtaining a section 404 permit from the appropriate state or local agency (Virginia Department of Environmental Quality, or DEQ). In addition, designers should investigate the status of adjacent wetlands to determine if the discharge from the constructed wetland will change the hydroperiod of a downstream natural wetland (see Cappiella et al., 2006b, for guidance on minimizing stormwater discharges to existing wetlands).

Regulatory Status: Constructed wetlands built for the express purpose of stormwater treatment are not considered jurisdictional wetlands in most regions of the country, but designers should check with their wetland permit authority to ensure this is the case.

Perennial Streams: Locating constructed wetlands along or within perennial streams is strongly discouraged, and will require both a Section 401 and Section 404 permit from the state or federal permitting authority (DEQ).

SECTION 4. COMMUNITY AND ENVIRONMENTAL CONSIDERATIONS

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 should 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 retrofit 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 5. DESIGN APPLICATIONS AND VARIATIONS

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

Pond-wetland combination (see Table 2)
Constructed wetland basins
Linear wetland cells (includes wet swale and regenerative conveyance systems)

IMPORTANT NOTE: Two wetland designs that have been referenced in past design manuals are no longer allowed (Schueler, 1992). These are the extended detention (ED) wetland (with more than one 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, and all upland runoff reduction opportunities have been exhausted. Constructed wetland basins can be used at the terminus of a storm drain pipe or open channel after all upland opportunities for runoff reduction have been exhausted. Linear wetland cells are installed within the conveyance system or zero-order stream channels.

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 1 shows the recommended combinations of these three factors to ensure an effective stormwater wetland. Table 2 shows the design elements of a pond-wetland combination.

Table 2: Wetland Configurations Based on Hydrology and Desired Plant Community
Contributing Hydrology / Desired Plant Community
Emergent Wetland / Mixed / Emergent/Pond
Groundwater / Linear
Basin
Wet Swale / Linear
Basin
RCS / Basin
Stormwater Runoff / Linear
Basin / Linear
Basin
RCS / Basin Drainage area limits may apply
Dry Weather Flow / Linear or Basin / Linear or Basin / Basin
RCS= Regenerative Conveyance System (see Appendix X)

(NOTE: Table 1 does not show any applications for the pond-wetland combination. Is that intentional? Regarding Table 2, another way to do that would be to create a sub-heading in this section entitled “Design Elements of the Pond-Wetland Combination.” Then have the illustration identified as “Figure 1”, with the text following to describe the configuration of the practice. That approach would maintain a consistency of labeling graphics as Figures and tabular information as Tables.)

Table 2: Design Elements of the Pond-Wetland Combination
This design involves an on-line wet pond cell that discharges to a series of off-line constructed wetland cells.

The wet pond cell can be sized to store up to two-thirds of the water quality storage volume through a permanent pool and temporary ED. The wet pond cell will have variable water levels, but a minimum drawdown pool depth of at least three feet (to provide a steady supply of flow to sustain the wetland cells).

The wet pond cell has four primary functions:
Pretreatment to capture and retain sediments before they reach the wetland cell;
Provide some initial removal of other pollutants;
Provide a small but steady supply of flow to support wetland conditions in between storms; and
Be the cell where larger storms (such as the channel protection, overbank and flood control design events) are routed into the downstream conveyance system, and water quality storms are split into the wetland cell for additional treatment.

The discharge from the pond cell to the wetland cell should consist of two reverse slope-pipes (one for the water quality storm and a lower, smaller diameter pipe to provide trickle flow to the wetland in between storms. The discharge structure from the pond cell that handles larger storm events (i.e., channel protection and flood control design storms) should be configured to discharge into the final wetland cell.

No extended detention is allowed within the wetland cell to prevent water level fluctuations from reducing the diversity and function of wetland cover. It is expected that the wetland cell will experience no more than 6-8 inches of water elevation during the maximum water quality storm event.

Initially, it is recommended that there be no minimum drainage area requirement to the system, although it may be necessary to calculate a water balance for the wet pond cell when it’s CDA is less than ten acres in area.

The wetland cells should generally be a long, linear feature with a length to width ration of at least 3:1. The wetland should be divided into at least four internal subcells of different elevation. Cells are formed by sand berms (anchored by rock at each end), with back-filled coir fiber logs or forested peninsula extending as wedges over 95% of the width. The vegetative target is to ultimately achieve a 50:50 mix of emergent and forested wetland vegetation within all four cells.

The first cell is deeper, and is used to accept runoff from the pond cell and distribute it as sheetflow into successive cells. The second cell has an elevation of the normal pool elevation, and may contain a forested island or a sand wedge channel to promote flows into the third cell which is 3 to 6 inches below the normal pool. The purpose of the cells is to create an alternating sequence of aerobic and anaerobic conditions to maximize nitrogen removal. The fourth wetland cell is located at the junction of a zero order stream and the discharge point from the wetland cell. The perimeter of the wetland cell shall be armored with appropriately-sized stone.

SECTION 6. SIZING AND TESTING GUIDANCE