Stormwater Wetland

A stormwater wetland is an engineered system that mimics the functions of a natural wetland to treat stormwater runoff. These shallow marsh systems use settling, filtration, and biological uptake by wetland plants and microorganisms to remove pollutants. As stormwater flows through the wetland’s pools and vegetated areas, sediments and associated contaminants settle out, while dissolved nutrients are taken up by plants. Stormwater wetlands are among the most effective practices for pollutant removal and can also provide significant aesthetic and habitat value.

Unlike natural wetlands, which are protected ecosystems, stormwater wetlands are constructed specifically as treatment facilities. They are designed to handle the flashy, often contaminated flows from urban and suburban landscapes. While they support less biodiversity than their natural counterparts, they are robust systems capable of improving water quality before it reaches downstream water bodies. Several design variations exist, differing in the configuration of shallow and deep water zones and the use of extended detention storage. For communities unsure which practice is best for their site, a BMP selector tool can help compare options like wetlands, ponds, and swales.

Applicability

Stormwater wetlands are a versatile and widely applicable management practice, though some site constraints and regional factors can limit their use.

Regional Suitability

Wetlands can be implemented in most regions of the United States. The primary exception is arid climates, where high evaporation rates make it difficult to maintain the permanent pool of water necessary for wetland vegetation to survive. In semi-arid regions, a careful water balance analysis is required to confirm that inflows are sufficient to offset evaporative losses. In cold climates, special design considerations are needed to manage spring snowmelt, prevent freezing of outlet structures, and select salt-tolerant vegetation where road salt is applied.

Ultra-Urban Environments

Due to their relatively large footprint, typically 3% to 5% of their contributing drainage area, stormwater wetlands are often challenging to site in dense, ultra-urban settings. They are most feasible where a larger downstream parcel is available for a regional treatment facility. In space-constrained new development or redevelopment projects, other practices like bioretention may be more practical.

Stormwater Hotspots

Stormwater wetlands can treat runoff from stormwater hotspots—areas with higher-than-normal pollutant loads, such as gas stations or industrial sites. However, if the wetland is expected to intersect the water table, an impermeable liner may be necessary to prevent groundwater contamination. Care must also be taken to avoid the bioaccumulation of certain pollutants in the local food web.

Retrofit Applications

Wetlands are an excellent option for stormwater retrofits, particularly when a larger, watershed-level treatment solution is desired. They can be constructed to manage runoff from previously uncontrolled areas, providing water quality treatment, channel protection, and habitat benefits. It is also possible to add wetland features and plantings to existing dry or wet detention basins to enhance their performance and ecological value.

Cold Water Streams

The shallow, slow-moving water in a stormwater wetland can be warmed by the sun, posing a moderate risk to downstream cold-water fisheries. Research has shown that runoff passing through a wetland can increase in temperature by several degrees (Galli, 1990). While this effect may be less than that of a deeper wet pond in some cases, the large, shallow surface area has significant potential for thermal enrichment. Designs should incorporate shading at the outlet channel to help mitigate temperature increases.

Design Criteria

Successful stormwater wetland design requires careful consideration of site feasibility, conveyance, pretreatment, treatment volume geometry, and landscaping. The following criteria establish a framework for creating an effective and sustainable facility.

Feasibility

• Drainage Area: A minimum drainage area is needed to sustain the permanent pool. In humid regions, approximately 25 acres is a common guideline, though smaller areas may be sufficient for pocket wetlands that intersect the groundwater table.
• Slope: Wetlands can be sited in areas with upstream slopes up to 15%, but the facility itself must be constructed on a relatively flat grade. A minimum of three to five feet of hydraulic head is required from the inflow to the outflow.
• Soils and Geology: Wetlands are adaptable to most soil types. In karst topography, an impermeable liner is required to prevent sinkhole formation and groundwater contamination.
• Water Balance: A water balance calculation should be performed to demonstrate that the wetland can maintain its permanent pool during extended dry periods, especially in regions with high summer evaporation rates.
• Location: Stormwater wetlands must not be constructed within existing jurisdictional natural wetlands.

Conveyance

• Flow Path: The flow path from inlet to outlet should be maximized to increase treatment time. A length-to-width ratio of at least 2:1 is required. This is often achieved by using internal berms or grading.
• Outfall Protection: The outlet channel must be stabilized to prevent erosion and scour from concentrated flows.
• Emergency Spillway: A stabilized emergency spillway must be included to safely pass large storm events that exceed the primary outlet’s capacity.

Pretreatment

A sediment forebay is required at all major inflow points. This small, deep pool dissipates energy and captures coarse sediment particles before they enter the main wetland area. The forebay should be sized to hold approximately 10% of the total water quality volume (WQv). This feature simplifies maintenance by concentrating the majority of sediment accumulation in an easily accessible location.

Treatment

• Sizing: The surface area of the wetland at the permanent pool elevation should be at least 1% of the contributing drainage area (1.5% for the shallow marsh design).
• Depth Zones and Microtopography: Creating varied depths is critical for treatment and plant diversity. The design should include:
  • Deepwater Zones: At least 25% of the WQv should be in deepwater zones (greater than 4 feet deep). The forebay and a micropool at the outlet can meet this requirement. These zones provide habitat for fish that prey on mosquito larvae.
  • Low Marsh: At least 65% of the wetland’s surface area should have a depth between 6 and 18 inches.
  • High Marsh: At least 35% of the wetland’s surface area should have a depth of 6 inches or less.
• Outlet: A non-clogging outlet, such as a reverse-slope pipe or a weir with a trash rack, should be used. Orifices should be no smaller than 3 inches in diameter to prevent clogging. A micropool (3-6 feet deep) should be located at the outlet to protect the low-flow pipe.
• Extended Detention (ED): When ED is used to reduce the wetland’s footprint, the ED volume should not exceed 50% of the total WQv. The maximum water surface elevation during ED events should not extend more than 3 feet above the permanent pool.

Landscaping

A detailed landscaping plan is essential for establishing a healthy wetland plant community. The plan must specify planting zones by water depth, plant species for each zone, soil amendments, and planting sequence. A buffer of at least 25 feet, planted with native trees, shrubs, and grasses, should be established around the wetland perimeter. Plant establishment is most successful using nursery stock planted during the appropriate seasonal window. Soils may need to be amended with topsoil or wetland mulch to provide nutrients for vigorous growth. Donor soils for wetland mulch must not be taken from natural wetlands.

Design Variations

• Shallow Marsh: The classic design, with most of its volume in shallow high and low marsh zones. It is effective but requires a large land area.
• Extended Detention (ED) Wetland: Incorporates temporary storage above the permanent pool, allowing a smaller footprint to treat the same volume of runoff.
• Pond/Wetland System: A two-stage system combining a wet pond for initial settling with a shallow marsh for final polishing. This hybrid requires less space than a full shallow marsh design.
• Pocket Wetland: A smaller design for drainage areas under 10 acres that relies on intersection with the groundwater table to maintain its permanent pool.
• Gravel-Based Wetland: A specialized system where runoff flows horizontally through a subsurface gravel bed where wetland plants are rooted. It functions more like a filtration system, such as a wet swale, than a traditional surface-flow wetland.

Pollutant Removal

Stormwater wetlands are highly effective at removing a broad range of stormwater pollutants. Monitored studies show high removal rates for sediment, nutrients, and bacteria. They are particularly effective at removing nitrate through denitrification in the anaerobic soil layers. The table below summarizes typical removal efficiencies based on data from numerous field studies.

Performance can be variable and depends heavily on proper design, particularly the inclusion of adequate pretreatment, a long flow path, and diverse depth zones. For a more detailed look at the data behind these numbers, consult the comprehensive pollutant removal database.

Source: Adapted from Winer (2000)

Construction and Cost Considerations

Stormwater wetlands are a moderately expensive practice, primarily due to the land area they consume. Construction costs are typically about 25% higher than for a wet pond of equivalent volume. A general cost equation developed by Brown and Schueler (1997) and adjusted for wetlands is:

C = 30.6V^0.705

Where C is the construction, design, and permitting cost in dollars, and V is the wetland volume in cubic feet needed to control a large storm event (e.g., the 10-year storm). A stormwater pond design calculator can be used to estimate this volume. Based on this equation, a 1 acre-foot facility might cost around $57,000, while a 10 acre-foot facility could cost nearly $290,000.

Annual maintenance costs are typically estimated at 3% to 5% of the initial construction cost. Despite the initial investment, well-designed and maintained wetlands can become community amenities, and studies on similar water features have shown they can increase the value of adjacent properties.

Maintenance

Regular maintenance is crucial for the long-term performance and health of a stormwater wetland. Key activities include managing vegetation, inspecting structures, and removing accumulated sediment.

Limitations

While highly effective, stormwater wetlands have several limitations. Their large land requirement makes them impractical for many dense urban sites. There can be public perception that they are a breeding ground for mosquitoes, although proper design with deep pools and predator habitat minimizes this risk. The establishment of wetland vegetation requires a careful and well-executed landscaping plan, and failure to establish a robust plant community can compromise performance. Finally, wetlands should not be sited in high-quality natural habitats like existing forests or jurisdictional wetlands.

Frequently Asked Questions