Constructed Stormwater Wetlands

From the Massachusetts Storm Water Handbook

Description

Constructed stormwater wetlands are stormwater wetland systems that maximize the removal of pollutants from stormwater runoff through wetland vegetation uptake, retention and settling. Constructed stormwater wetlands temporarily store runoff in shallow pools that support conditions suitable for the growth of wetland plants. Like extended dry detention basins and wet basins, constructed stormwater wetlands must be used with other BMPs, such as sediment forebays. There is also an innovative constructed wetland—the gravel wetland—that acts as a filter. Information on the gravel wetland is presented at the end of this section.

Ability to Meet Massachusetts Stormwater Management Standards

Standard	Description
2 - Peak Flow	If properly designed, can provide peak flow attenuation.
3 - Recharge	Provides no groundwater recharge.
4 - TSS Removal	Provides 80% TSS removal when combined with sediment forebay for pretreatment
5 - Higher Pollutant Loading	May be used as treatment BMP provided basin bottom is lined and sealed
6 - Discharges near or to Critical Areas	Do not use near cold-water fisheries. Highly recommended for use near other critical areas.
7 - Redevelopment	Suitable if sufficient space is available.

Advantages/Benefits

• Relatively low maintenance costs.
• High pollutant removal efficiencies for soluble pollutants and particulates.
• Removes nitrogen, phosphorus, oil and grease
• Enhances the aesthetics of a site and provides recreational benefits.
• Provides wildlife habitat.

Pollutant Removal Efficiencies

• Total Suspended Solids (TSS) - 80% with pretreatment
• Total Nitrogen - 20% to 55%
• Total Phosphorus - 40% to 60%
• Metals (copper, lead, zinc, cadmium) - 20% to 85%
• Pathogens (coliform, e coli) - Up to 75%

Disadvantages/Limitations

• Depending upon design, more land requirements than other BMPs.
• Until vegetation is well established, pollutant removal efficiencies may be lower than anticipated.
• Relatively high construction costs compared to other BMPs.
• May be difficult to maintain during extended dry periods
• Does not provide recharge
• Creates potential breeding habitat for mosquitoes
• May present a safety issue for nearby pedestrians
• Can serve as decoy wetlands, intercepting breeding amphibians moving toward vernal pools.

Maintenance

Activity	Frequency
Inspect wetland during both the growing and non- growing seasons	Twice a year for the first three years of construction,
Clean out forebays	Once a year
Clean out sediment in basin/wetland systems	Once every 10 years

Special Features

There are five basic types of constructed stormwater wetlands: shallow marsh systems, basin/wetland systems, extended detention wetlands, pocket wetlands, and gravel wetlands.

Like other stormwater BMPs, constructed stormwater wetlands may not be located within natural wetland areas other than riverfront area, land subject to coastal storm flowage, isolated land subject to flooding or bordering land subject to flooding.

The Operation and Maintenance Plan for constructed stormwater wetlands must include measures for monitoring and preventing the spread of invasive species.

The Five Basic Types of Constructed Stormwater Wetlands

Like wet basins, most constructed stormwater wetlands require relatively large contributing drainage areas and dry weather base flows. Ten acres is the minimum contributing drainage area, although pocket type wetlands may be appropriate for smaller sites, if sufficient groundwater flow is available. There are five basic constructed wetland designs: 1) Shallow Marsh, 2) Basin/Wetland (formerly Pond/Wetland) 3)Extended Detention (ED) Wetland, 4) Pocket Wetland, and 5) Gravel Wetlands. In addition to these designs, there is a sixth type known as a subsurface gravel wetland. However, due to the lack of performance data, MA currently does not recognize subsurface gravel wetlands as having a presumed TSS removal credit.

Shallow marsh systems

Most shallow marsh systems consist of pools ranging from 6 to 18 inches deep during normal conditions. Shallow marshes may be configured with different low marsh and high marsh areas, which are referred to as cells. Shallow marshes are designed with sinuous pathways to increase retention time and contact area. Shallow marshes may require larger contributing drainage areas than other systems, as runoff volumes are stored primarily within the marshes, not in deeper pools where flow may be regulated and controlled over longer periods of time.

Basin/wetland systems (formerly pond/wetland system)

Multiple cell systems, such as basin/wetland systems, use at least one wet basin along with a shallow marsh component. The first cell is a sediment forebay that outlets to a wet basin, which removes particulate pollutants. The wet basin also reduces the velocity of the runoff entering the system. Stormwater then travels to the next cell, which contains a plunge pool. The plunge pool acts as an energy dissipator. Shallow marshes provide additional treatment of runoff, particularly for dissolved pollutants. These systems require less space than the shallow marsh systems and generally achieve a higher pollutant removal rate than other stormwater wetland systems.

Extended detention wetlands

Extended detention wetlands provide a greater degree of downstream channel protection. These systems require less space than shallow marsh systems, because temporary vertical storage substitutes for shallow marsh storage. The additional vertical storage area also provides extra runoff detention above normal elevations. Water levels in the extended detention wetlands may increase by as much as three feet after a storm, and return gradually to normal within 24 hours of the rain event. The growing area in extended detention wetlands expands from the normal pool elevation to the maximum surface water elevation. Wetlands plants that tolerate intermittent flooding and dry periods should be selected for the extended detention area above the shallow marsh elevations.

Pocket wetlands

Use these systems for smaller drainage areas from one to ten acres. To maintain adequate water levels, excavate pocket wetlands to the groundwater table. Pocket wetlands that are supported exclusively by stormwater runoff generally will have difficulty maintaining marsh vegetation during normal dry periods each summer.

Gravel Wetland

The gravel wetland consists of a series of horizontal flow through treatment cells preceded by a sediment forebay. The University of New Hampshire (UNH) has developed specifications that allow the gravel wetland to treat the required water quality volume; 10% in the forebay and 45 % in each treatment cell. The UNH design calls for excess runoff to overflow into an adjacent swale with side slopes graded at 3:1 or flatter.

Treatment occurs in each cell as stormwater passes horizontally through the microbe rich gravel substrate. The wetland is designed to continuously saturate at a depth that begins four inches below the treatment’s surface. This design permits treatment and vegetation growth. To generate this condition, UNH designs the device with an outlet pipe that has an invert 4 inches below the surface.

Applicability

Never use constructed stormwater wetlands to manage runoff during site grading and construction. Site constraints that can limit the suitability of constructed stormwater wetlands include inappropriate soil types, depth to groundwater, contributing drainage area, and available land area. Soils consisting entirely of sands are inappropriate unless the groundwater table intersects the bottom of the constructed wetland or the constructed stormwater wetland is installed over the sand to hold water. Where land area is not a limiting factor, several wetland design types may be possible. Consider pocket wetlands where land area is limited.

Do not locate constructed stormwater wetlands within natural wetland areas. These engineered stormwater wetlands differ from wetlands constructed for compensatory storage purposes and wetlands created for restoration or replication. Typically, constructed stormwater wetlands will not have the full range of ecological functions of natural wetlands. Constructed stormwater wetlands are designed specifically to improve water quality. Note that constructed stormwater wetlands do not create any additional wetland resource area or buffer zones as discussed in Volume 1, Chapter 2.

Before designing and siting constructed stormwater wetlands, investigate soil types, depth to bedrock, and depth to water table. Medium-fine texture soils (such as loams and silt loams) are best at establishing vegetation, retaining surface water, facilitating groundwater discharge, and capturing pollutants.

At sites where infiltration is too rapid to sustain permanent soil saturation (such as sandy soils), consider using an impermeable liner. Liners are also required where the potential for groundwater contamination from runoff is high, such as from sites with high potential pollutant loads.

At sites where bedrock is close to the surface, high excavation costs may make constructed stormwater wetlands infeasible. Table CSW.1 lists the recommended minimum design criteria for constructed stormwater wetlands.

Effectiveness

A review of the existing performance data indicates that the removal efficiencies of constructed stormwater wetlands are significantly higher than the removal efficiencies of dry extended detention basins. Indeed constructed stormwater wetlands are among the most effective treatment practices.

To preserve their effectiveness, MassDEP requires placing a sediment forebay as pretreatment for all constructed stormwater wetlands.

Studies indicate that removal efficiencies of constructed stormwater wetlands decline when they are covered by ice or receive runoff derived from snow melt. Performance also declines during the non-growing season and the fall when vegetation dies off. Expect lower pollutant removal efficiencies until vegetation is re-established.

One preferred wetland installation is to combine an off-line stormwater wetland design, for runoff quality treatment, with an on-line runoff quantity control, because large surges of water can damage wetlands. Further, the shallow depths required to maintain the wetlands conflict with the need to store large volumes to control runoff quantity.

Planning Considerations

Carefully evaluate sites when planning constructed stormwater wetlands. Investigate soils, depth to bedrock, and depth to water table before designing, permitting, and siting constructed wetlands. Proponents must consider a "pond-scaping plan" for each constructed stormwater wetland. The plan must contain the location, quantity and propagation methods for the wetland plants as well as site preparation and maintenance. The plan should also include a wetland design and configuration, elevations and grades, a site/soil analysis, estimated depth zones, and hydrological calculations or water budgets. The water budget must demonstrate that a continuous supply of water is available to sustain the constructed stormwater wetland. Develop the water budget during site selection and then check it after the preliminary site design. The water budget analysis must be based on the Thornwaite method, arranging data in a "bookkeeping" or "spreadsheet" format. The water budget must take into account prcipitation, runoff, evaporatranspiration, soil moisture, and groundwater inputs. Drying periods of longer than two months adversely affect the richness of the plant community, so make sure that the water budget confirms that the drying time will not exceed two months.

Table CSW.1

Recommended Design Criteria for Stormwater Wetlands Designs

Design Criteria	Shallow Marsh	Basin/Wetland	ED Wetland	Pocket Wetland	Gravel Wetland (Surface)
Minimum Drainage Area (acres)	≥ 25	≥ 25	≥ 10	≥ 1 to 10	See specifications
Constructed Wetland Surface Area/Watershed Area Ratio1	≥ 0.02	≥ 0.01	≥ 0.01	≥ 0.01
Length to Width Ratio (minimum)	≥ 2:1	≥ 2:1	≥ 2:1	≥ 2:1
Extended Detention (ED)2	NOT ALLOWED	OPTIONAL	YES	OPTIONAL
Allocation of WQv Volume (wet pools3/low and high marsh/ED) in %	30/70/0	70/30/02	20/30/50	20/80/02
Allocation of Surface Area (wet pools3/low marsh/ high marsh/semi- wet) in %	15/40/40/5	45/25/25/5	10/40/40/10	10/45/40/5
Sediment Forebay4	REQUIRED	REQUIRED	REQUIRED	REQUIRED
Micropool	REQUIRED	REQUIRED	REQUIRED	REQUIRED
Outlet Configuration	Reverse slope pipe or hooded broad crested weir	Reverse slope pipe or hooded broad crested weir	Reverse slope pipe or hooded broad crested weir	Hooded broad- creasted weir
Target Allocations	Shallow Marsh	Basin/Wetland	ED Wetland	Pocket Wetland
	% Surface Area
Sediment Forebay4	5%	0%5	5%	5%
Micropool	5%	5%	5%	5%
Deep Water Channel	5%	40%	0%	0%
Lo Marsh	40%	25%	40%	45%
High Marsh	40%	25%	40%	40%
Semi-Wet	5%	5%	10%	5%
	% WQv Volume
Sediment Forebay4	10%	0%5	10%	10%
Micropool	10%	10%	10%	10%
Deep Water Channel6	10%	60%	0%	0%
Lo Marsh	45%	20%	20%	55%
High Marsh	25%	10%	10%	25%
Semi-Wet	0%	0%	50% (ED)	0%

Notes:
1. Constructed Wetland Surface Area includes wet pool, deep water channel, marshes, and semi-wet zone.
2. ED volume shall be an additional volume above the WQv (except for the ED Wetland)
3. Wet Pool = Forebay+Micropool+Deep Water
4. Sediment Forebay for 1/2-inch WQv is 20% of WQv. Only 10% of that Volume may be included in the Constructed Wetland.
5. Basin Wetland Forebay: Forebay sizing must not be counted as part of WQv. Sediment Forebay Volume = 0.1-inch x Impervious area
6. Includes "basin" volume in Basin/Wetland Design

Design

Constructed stormwater wetlands may be designed as on-line systems with permanent pools for both treatment and storage of peak flows. Constructed stormwater wetlands can also be designed as off- line systems with high flows routed around the wetland. The basic constructed stormwater wetland design sizing criteria is set forth in Table CSW.1. Whether designed as an on-line or off-line system, a constructed stormwater wetland must be sized for the required water quality volume.

The ratio of the surface area of the constructed stormwater wetland to longer flow paths through the constructed stormwater wetlands to the contributing watershed area must meet the criteria specified in Table CSW.1. The reliability of pollutant removal tends to increase as the ratio of constructed stormwater wetlands area to watershed area increases.

Design the constructed stormwater wetlands with the required proportion of "depth zones." Each of the constructed wetland designs other than the gravel wetland, has depth zone allocations, which are given as a percentage of the stormwater wetland surface area. Target allocations for these constructed wetland designs are listed in Table CSW.1. The four basic depth zones are (see figure below):

Deepwater zone

From 1.5 feet to six feet below normal pool elevation. This zone supports little emergent vegetation, but may support submerged or floating vegetation. This zone can be further broken down into forebay, micropool and deepwater channels.

Low marsh zone

Ranges from 6 inches to s18 inches below the normal pool elevation. This area is suitable for growing several emergent wetland plant species.

High marsh zone

Ranges from the normal pool elevation to 6 inches below normal pool elevation. This zone will support a greater density and diversity of emergent wetland species than the low marsh zone. The high marsh zone must have a higher surface area to volume ratio than the low marsh zone (see table CSW.1).

Semi-wet zone

This zone includes those areas above the normal pool elevation that are intermittently inundated and that can be expected to support wetland plants.

Design each constructed stormwater wetland with the required proportion of treatment volumes, which have been represented as a percentage of the three basic depth zones (pool, marsh, extended detention). Table CSW.1 specifies the allocations of treatment volume per zone.

Increase the contact time over the surface area of the marsh, thereby improving treatment efficiency. The constructed stormwater wetland must be designed to achieve a dry weather flow path of 2:1 (length: width) or greater.

Prepare a water budget to demonstrate that the water supply to the constructed stormwater wetland is greater than the expected loss rate. The water budget must be based on the Thornwaite method.

Provide extended detention (ED) for smaller storms. Schueler 1992 lists the following design standards for ED wetlands:

• The volume of the extended detention must be no more than 50% of the total treatment volume.
• The target ED detention time for this volume must be 12 to 24 hours.
• Use V-shaped or proportional weirs to ensure constant detention time for all storm events.
• Extended detention is defined here as the retention and gradual release of a fixed volume of stormwater runoff. For ED wetlands less than 100 acres, the extended detention volume can be assumed to fill instantaneously for design purposes.
• Use a reverse slope pipe and increase the actual diameter of the orifice to the next greatest diameter on the standard pipe schedule. The pipe must be equipped with a gate valve.
• Protect the ED orifice from clogging.
• Make the maximum ED water surface elevation no greater than three feet above the normal pool elevation.

Design each constructed stormwater wetland with a separate cell near the inlet to act as a sediment forebay. Design the forebay with a capacity of at least 10% of the total treatment volume, normally 4 to 6 feet deep. Provide a direct and convenient access for cleanout.

Surround all deep-water cells with a safety bench that is at least ten feet wide, and zero to 18 inches below the normal water depth of the pool.

Place above-ground berms or high marsh wedges at approximately 50-foot intervals, and at right angles to the direction of the flow to increase the dry-weather flow path within the wetland.

Include a four- to six-foot deep micropool before the outlet to prevent the outlet from clogging. Provide a micropool capacity of at least ten percent of the total treatment volume. Use a reverse slope pipe or a hooded, broad-crested weir for outlet control. Locate the outlet from the micropool at least one foot below the normal pool surface.

To prevent clogging, install trash racks or hoods on the riser. To facilitate access for maintenance, install the riser within the embankment. Install anti-seep collars on the outlet barrel to prevent seepage losses and pipe failures. Install a bottom drainpipe with an inverted elbow to prevent clogging and to facilitate complete draining of the wetland for emergency purposes or routine maintenance. Fit both the outlet pipe and the bottom drainpipe with adjustable valves at the outlet ends to regulate flows. Design embankments and spillways in accordance with the state regulations and criteria for dam safety.

All constructed stormwater wetlands must have an emergency spillway capable of bypassing runoff from large storms without damage to the impounding structure.

Provide an access for maintenance, with a minimum width of 15 feet and a maximum slope of 15%, through public or private rights-of-way. Make sure this access extends to the forebay, safety bench and outflow structure and never crosses the emergency spillway, unless the spillway has been designed and constructed for this purpose.

Locate vegetative buffers around the perimeter of the constructed stormwater wetland to control erosion and provide additional sediment and nutrient removal for sheet flow discharging to the constructed stormwater wetland.

Construction

A seven-step process to prepare a wetland bed prior to planting (Shueler 1992):
1. Prepare final pond-scaping and grading plans for the constructed stormwater wetland. At the same time, order wetland plant stocks from aquatic nurseries.
2. Once the constructed stormwater wetland volume has been excavated, grade the wetland to create the major internal features (pool, aquatic bench, deep water channels, etc.).
3. Because deep subsoils often lack the nutrients and organic matter needed to support vigorous plant growth, add topsoil and/or wetland mulch to the wetland excavation. If available, wetland mulch is preferable to topsoil.
4. After the mulch or topsoil has been added, grade the constructed stormwater wetland to its final elevations. Temporarily stabilize all wetland features above the normal pool. After final grading, close the pool drain to allow the pool to fill. MassDEP recommends evaluating the wetland elevations during a standing period of approximately six months to assess how the constructed stormwater wetland responds to storm flows and inundation, where the pond-scaping zones are located, and whether the final grade and micro-topography will persist over time.
5. Before planting, measure the constructed stormwater wetland depths to the nearest inch to confirm planting depth. If necessary, modify the pond-scape plan at this time to reflect altered depths or availability of plant stock.
6. Aggressively apply erosion controls during the standing and planting periods. Stabilize the vegetation in all areas above the normal pool elevation during the standing period (typically by hydroseeding).
7. Dewater the constructed stormwater wetland at least three days before planting, because a dry wetland is easier to plant than a wet one.

Wetland Vegetation

Establishing and maintaining wetland vegetation is important when creating a constructed stormwater wetland. Horner et al. (1994) recommend the following actions when constructing stormwater wetlands:

• In selecting plants, consider the prospects for success over the specific pollutant removal capabilities and plant species growing in nearby natural wetlands. Plant uptake is an important removal mechanism for nutrients, but not for other pollutants. The most versatile genera for pollutant removal are Carex, Scirpus, Juncus, and Lemna. Consult the NRCS plant database to determine if the plant is appropriate. The NRCS database lists the plants prohibited for sale in Massachusetts.
• Select native species, avoiding those that are invasive. Because diversification will occur naturally, use a minimum of species adaptable to the various elevation zones within the stormwater wetland.
• Give priority to perennial species that establish themselves rapidly.
• Select species adaptable to the broadest ranges of depth, frequency and duration of inundation (hydroperiod).
• Match site conditions to the environmental requirements of plant selections.
• Take into account hydroperiod and light conditions.
• Give priority to species that have already been used successfully in constructed stormwater wetlands and that are commercially available.
• Avoid using only species that are foraged by the wildlife expected on site.
• Establish woody species after herbaceous species.
• Where applicable, add vegetation that will achieve other objectives, in addition to pollution control.

Plants will develop best when soils are enriched with plant roots, rhizomes, and seed banks. Use "wetlands mulch" to enhance the diversity of the plant community and speed its establishment. Wetlands mulch is hydric soil. This mulch is available where wetland soils are removed during cleaning and dredging of drainage channels, swales, sedimentation basins, dry detention basins, and infiltration basins. Wetland soils are also available commercially. The upper 5.9 inches of donor soil should be obtained at the end of the growing season, and kept moist until installation. Drawbacks to using wetlands mulch are the unpredictable content, limited donor sites, and the potential for the introduction of exotic, opportunistic species. Wetland plants are commercially available through wetland plant nurseries.

Maintenance

Unlike conventional wet basin systems that require large-scale sediment removal at infrequent intervals, constructed stormwater wetlands require small-scale maintenance at regular intervals to evaluate the health and composition of the plant species.

Proponents must carefully observe the constructed stormwater wetland system over time. In the first three years after construction, inspect the constructed stormwater wetlands twice a year during both the growing and non-growing seasons. This requirement must be included in the Operation & Maintenance plan. During these inspections, record and map the following information:

• The types and distribution of the dominant wetland plants in the marsh;
• The presence and distribution of planted wetland species;
• The presence and distribution of invasive wetland species (invasives must be removed);
• Indications that other species are replacing the planted wetland species;
• Percentage of standing water that is unvegetated (excluding the deep water cells which are not suitable for emergent plant growth);
• The maximum elevation and the vegetative condition in this zone, if the design elevation of the normal pool is being maintained for wetlands with extended zones;
• Stability of the original depth zones and the micro-topographic features; and
• Accumulation of sediment in the forebay and micropool; and survival rate of plants (cells with dead plants must be replanted).

Maintenance of Sediment Forebay

Another important maintenance activity is regulating the sediment loading into the constructed stromwater wetland. All constructed stormwater wetlands are required to have a sediment forebay. Sediment accumulating in wetlands reduces water depths, changes the growing conditions for emergent plants, and alters the wetland plant community. Most sediment should be trapped and removed by the forebay or other type of basin before it reaches the wetland. The sediment forebay should be cleaned once a year.

References

Shuler, Thomas, 1992. Design of Stormwater Wetlands Systems: Guidelines for Creating Diverse and Effective Stormwater Wetlands in the Middle Atlantic Regions, Metropolitan Washington Council of Governments, Washington, D.C.

Carleton, J.N., Grizzard, T.J., Godrej, A.N., and Post, H.E., 2001, Factors Affecting the Performance of Stormwater Treatment Wetlands, Water Research, Volume 35, No. 6, pp 1552-1562

UNH Stormwater Center, 2005, Gravel Wetland Fact Sheet, www.unh.edu/erg/cstev/fact_sheets/gravel_wetland.pdf