Forebay: Eroded soil is captured from incoming runoff before entering the wetland. Over time, build-up of eroded soils can be removed from the forebay, allowing for easier maintenance. This prevents damage to the wetland and plants while increasing overall longevity of the wetland.

Microtopography: A series of small berms and depressions designed to increase the distance water has to travel. This “stormwater maze” forces water to weave slowly through the wetland promoting pollutant removal. 

Areas of Shallow Water: Varying depths of water promote plant growth allowing for biological uptake which helps remove pollutants

Pools: Deep pools reduce the suspension of sediment, reduce thermal pollution, and increase habitat.

Outlet: A primary function of a stormwater wetland is to help slow down stormwater. Aligning with this goal, the outlet of the wetland is intentionally designed to release water slowly to improve downstream environmental conditions.

Stormwater wetlands can be designed to address both stormwater quality and quantity using the unified sizing criteria. Refer to the Iowa Stormwater Management Manual (ISWMM) for complete design guidelines and calculation worksheets.

Design Highlights:

• Be able to pass or manage all volumes of storm events safely through the wetland
• Minimum treatment is for water quality volume storage in the permanent pool
• Channel protection volume can usually be managed within the same footprint as a WQv and is highly recommended for downstream protection
• Pre-treatment at each inflow is critical to minimize sediment loading into the wetland, along with increasing the lifespan of the practice

Stormwater wetlands are able to treat flow at the end of a drainage swale or stormwater pipe. Many urban areas have limited space to install retrofit practices. Stormwater wetlands can be located in parks or open-space areas at the outlet of stormwater pipes or drainage swales. Unlike other options, they can be used to treat runoff from large drainage areas with a single end of pipe practice. That creates efficiencies by treating a larger area with only one stormwater practice to inspect and maintain. 

A public area allows the community to control the installation, maintenance and use of the wetland and buffer area. They are perfect for greenways and park space, and provide additional community benefits such as recreation, habitat, and education.

Surface Drainage Guidance:

10 acres minimum drainage area

3-5% of drainage area needed to manage large flood events

6-12% of drainage area needed to manage large flood events

Look for pipe outlets and drainage swales that flow to open space or public areas near a stream

Check the sites to be sure there is enough elevation change to bring the stormdrain water to the surface and into the wetland and to a wetland outlet. 

Avoid existing wetlands and sensitive areas.

Once a potential wetland site is identified:

• determine the drainage area to check if wetland site is adequate
• an engineer can further study the site feasibility