Design
This page provides a summary of the following aspects of stormwater wetland design and construction::

• Wetland Zones
• Vegetation and Soil Type
• Sizing
• Outlet structures and Construction

Wetland Zones: There are typically five zones in a wetland: the forebay, deep pools, shallow water,temporary inundation, and upland zones. These zones are generally differentiated by water level, and each has a specific role in the wetland's intended function.

Wetland zonation: typical cross-section and plan views. From"Stormwater Wetland Design Update: Zones, Vegetation, Soil, and Outlet Guidance" (Hunt et. al., 2007)

1. The Forebay is a pool located at the inlet of the wetland into which runoff initially enters. The forebay area generally represents 10 to 20 percent of the total wetland area. The forebay has two main roles: (1) disperse the energy from runoff inflows and (2) remove large quantities of sediment and other solids from runoff influet. By reducing the energy and sediment concentration of runoff, the forebay helps to extend the life of the rest of the wetland. To maintain the forebay's function, it needs to be cleaned out as part of a regular maintenance program, generally after accumulated sediment sediment volume exceeds 50% of the forebay volume or when average sediment depth is 1 ft from forebay water surface.

2. Like the forebay, Deep Pools also disperse runoff energy and and promote sediment removal through deposition. Pools also provide habitat for mosquito predators such as dragonfly larvae and fish. Deep pools generally represent 10% of the wetland area in addition to the forebay, and are deep enough to retain water during droughts (usually at least 18-in deep). By not drying completly during droughts, deep pools provide a refuge for mosquito predators. Pools should ideally be distrubuted throught the wetland to ensure that these mosquito predators have access to a greater portion of the wetland when return to ensure that mosquito larvae do not proliferate.

3. Shallow Water ones are shallow enough to support rooted emergent macrophytes (up to 6 inches deep, though 2 to 4 inches will support a more diverse plant community) and represent about 40% of the total wetland surface area. This zones may dry up during periods of extended drought. The shallow water zone plays an important role in maintaining the water treatment services provided by stormwater wetlands. Shallow water plants may directly filter solids from runoff and directly uptake nutrients. Even more importantly, these plants pump oxygen into their root zone and provide an abundant carbon (food) source, creating an ideal environment for the bacteria responsible for nitrogen removal. The plants present in shallow water zones also serve to attract some species of dragonflies, which are known to feed on insect pests such as mosquitoes. Carbon sequestration by stormwater wetlands is also largely dependent upon the plant community in shallow water zones.

4. Temporary Inundation) zones refer to the area ranging from the shallow water area up to 1 foot above the elevation of the permanent pool. The temporary inundation zone provides storage above the permanent pool to capture a required volume of stormwater runoff. As the name implies, this zone is temporarily submerged during runoff events and then dries over a period of 2 to 5 days as runoff is slowly discharged from the wetland. Because it is not permanently inundated, a greater variety of vegetation is adapted to life in this zone, including edible and flowering species such as blackberries, hibiscus, and marsh mallow.

5. Upper Bank egion of the wetland is the area that surrounds the temporary inundation zones, and is sloped as needed to tie the wetland into the surrounding landscape. This area is not typically inundated, and can support a variety of upland plants.

Choosing Vegetation and Soil Type in the Stormwater Wetland

It is important to select native vegetation that is adapted to the climatic zone in which you live for any vegetated stormwater system. However, since these systems are prone to frequent variations in water level following rainfall events, stormwater wetlands present a challenging environment for plant establishment to which not all natives are adapted to handle. Research by the NCSU Stormwater team has identified native wetland plants which are able to tolerate the relatively dramatic hydroperiod (defined as the frequency and duration of inundation) and pollutant loads characteristic of stormwater wetlands. In addition to selecting natives, it is also important to avoid planting aggressive species (such as Cattail in North Carolina) which tend to crowd out other plant species and form low-diversity monocultures. The table below lists plants native to North Carolina that are commonly recommended for stormwater wetlands. Prior to planting, 2 to 4 inches of an appropriate soil amendment (such as top soil) should be spread on the wetland surface to provide nutrients for plant growth and improve plant establishment.

Sizing a Stormwater Wetland

To size a stormwater wetland, one must first determine the volume of runoff that will enter the wetland for a given amount of rainfall. In North Carolina, stormwater wetlands should be sized to capture the runoff generated by the first 1 to 1.5 inches of rain. The NRCS Curve Number Method is among the most common methods used to estimate the runoff depth from small urban watersheds. The Curve Number method uses the following equations to calculate the runoff depth:

Runoff depth (in inches) = (P – 0.2S)2/(P + 0.8S) where,

P = precipitation depth (inches)

S = maximum storage = 1000/CN – 10 (inches)

CN = Curve Number, the value of which is dependent on landcover and soils (see CN Table below)

A simple example of wetland sizing using the curve number method is outlined below:

Size a stormwater wetland to capture runoff from the first 1-in of rainfall.  The area draining to the wetland is 20 acres (871,200 ft2) in area.  75% of the drainage area is occupied by impervious surfaces.  The remainder is occupied by grass with good cover (> 75%).  Soils are class C.    

Step 1. Calculate the depth of runoff using the NRCS Curve Number Method
CN = 98(0.75) + 74(0.25) = 92
S = 1000/CN – 10 = 1000/92 – 10 = 0.87 in
Runoff depth = (P – 0.2S)2/(P + 0.8S) = (1 – 0.2*0.87)2/(1+0.8*0.87) = 0.4 in

Step 2. Calculate runoff volume
Runoff Volume = Area x Runoff depth
Runoff Volume = 20 ac x 0.4 in = 8 ac-in (29,040 ft3)

Step 3. Calculate wetland surface area
Wetland area = Runoff volume ÷ avg. wetland depth (9 -10 in)
Wetland area = 8 ac-in/(9 in) = 0.9 ac (38,720 ft2)

In general, the wetland surface area ranges from 1% of the total watershed area in watersheds with low curve numbers up to 7% of the watershed area in highly impervious watersheds. 

Outlet Structures in Stormwater Wetlands

The outlet structure provides three primary functions: (1) maintain the permanent pool, (2) safely pass runoff that exceeds the volume for which the wetland was designed and (3) allow the wetland to be drained for planting or maintenance. Common components of an outlet structure include an orifice that determines the elevation of the permanent pool and an overflow weir that bypasses flows in excess of the design runoff volume. The storage depth, or the distance between the permanent pool orifice and the overflow weir, should ideally be 12 inches or less to promote plant health and diversity. lthough a variety of outlet structures can be used to meet these objectives, the flashboard riser (pictured below) affords one of the most flexible and cost-effective designs.  The “flashboards” can be constructed from to tongue-in-groove boards that can be removed or inserted to adjust the permanent pool and storage height as desired.  See the Urban Waterways series Stormwater Wetland Design Update for more details on outlet design configurations. 

In North Carolina, the permanent pool orifice is sized such that this maximum storage depth is drained from the wetland over a 2 to 5 day period. The orifice size required to achieve this drainage period can be calculated using the orifice equation:

Solving for A in this equation requires an iterative process, an example of which is provided by Hunt et al. (2000). For a small wetland, a single orifice 2-in or less in diameter is often required. Due to the small size of many outlet orifices, regular maintenance is required to remove debris that could clog the outlet. The liklihood of clogging can be reduced by attaching a down-turned elbow to the outlet orifice to draw water from deeper in the outlet pool and prevent clogging with surface debris.

*All information taken from:

“Urban Waterways Designing Stormwater Wetlands for Small Watershed” (Hunt et al., 2000)

“Urban Waterways Stormwater Wetland Design Update: Zones, Vegetation, Soil, and Outlet Guidance” (Hunt et al., 2007)