A stormwater wetland is a man-made management poractice that provides a natural way to treat and remove pollutants from stormwater runoff before it enters a stream, river, or lake. As stormwater is captures in the wetland, pollutant removal is achieved through various mechanisms. Vegetation aids in this process by helping to slow, settle, and uptake water.
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.
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:
Removes pollutants through settling
Eroded soil is a major pollutant prevalent in Iowa waters. When stormwater is slowed down in the stormwater wetland, eroded soil settles out and improves water clarity.
Slows down Stormwater
Instead of rapidly flowing directly into a water resource, the stormwater from a contributing water course now slowly flows through the wetland providing temporary water storage and other benefits
Removes pollutants through Biological Uptake
Plants present in wetlands have special chemical and biological functions that can help remove pollutants through biological uptake. These biological processes remove nutrients such as phosphorus and nitrates, and common pollutants in Iowa waterways.
Supports Wildlife and Habitat
Wetlands support a wide variety of plant and animal life. Typical wetland species have sturdy stems, leaves, and flowers that provide a great habitat for birds, animals, and invertebrates.
Polk Soil and Water Conservation District
1513 North Ankeny Blvd., Suite 3
Ankeny, IA, 50023
Polk Soil and Water Conservation District is an Equal Opportunity Employer.
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