Bioswales go by many names—including vegetated swales, grassy swales, bioretention areas, and filter strips—and serve many functions depending on how they are engineered.
In their simplest forms, bioswales are linear rain gardens planted with native vegetation that receive and absorb stormwater runoff from impervious surfaces.
Bioswales are sometimes planted on a gentle slope so that runoff flows along the length of the swale, the vegetation slowing and filtering the water as it infiltrates the soil. Sloping bioswales may have check dams to help slow and detain the flow.
More complex bioswales (usually installed with underdrains and infiltration trenches) are used to manage and treat stormwater runoff from larger developments and parking lots. These bioswales are designed to filter pollutants, regulate flows, and increase infiltration.
Studies have found that properly designed and constructed bioswales are able to achieve excellent removal of heavy metals, total suspended solids, and oil/grease. 1 Bioswales remove suspended solids through settling and filtration. Dissolved pollutants are removed and/or transformed as runoff infiltrates into the ground. 2
Retention and Infiltration
Bioswales can be used to temporarily store runoff water and increase infiltration. Resulting benefits can include:
- Reduces impervious runoff volumes and rates.
- Recharges groundwater and sustains stream base flows.
- Reduces detention needs. 3
Bioswales can also reduce thermal pollution. Stormwater can rise in temperature as it washes across impervious surfaces, e.g. hot parking lots in summer. Heated stormwater flowing into streams can impact fish and other wildlife that depend on cold water streams to live and breed.
Heated runoff from impervious surfaces can be cooled as it filters through a bioswale. One study observed a temperature drop of 12°C between stormwater entering a bioswale and stormwater filtering out of the bioswale. 4
Larger bioswales planted with native flowering perennials can be attractive landscaping features that provide food and shelter for birds and butterflies.
- Unit costs range from $3 to $10 per square foot depending on complexity of system and planting plan.
- Replaces storm sewers within parking lots.
- Reduces cost premium where parking islands are already required.
- Reduces detention volume and land area necessary for detention. 5
Bioswales are often coupled with other watershed friendly landscaping practices like permeable hardscaping. See the
green parking section of our permeable hardscaping web page. Also see the visit a bioswale near you section to learn about the bioswale/porous asphalt combination at the Ryerson Welcome Center in Deerfield, IL and the bioswale/permeable paving combination at the Morton Arboretum.
The City of Portland, Oregon's has developed many "green streets" technologies that utilize the principles of bioretention in a variety of forms including curb cuts, flow-through planters, and infiltration planters. Click here to access case studies of these green streets projects.
A number of laboratory and field experiments have been conducted by the University of Maryland in conjunction with Prince George's County Department of Environmental Resources and the National Science Foundation in order to quantify the effectiveness of bioswales in terms of pollutant removal. A web site dedicated to this work can be found at www.ence.umd.edu/~apdavis/Bioret.htm