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Bioswales, also known as infiltration swales, biofilters, grassed swales, or in-line bioretention, are a low impact development practice which consists of gently sloped channels designed to catch, store and filter stormwater.

They function by taking flowing water (from precipitation or other sources) and slowing it down within the swale, which helps trap pollutants and silt before returning it to the river through our stormwater system.

Bioswales are planted with either lawn grasses or hardy, low-maintenance plants, and generally require deeper layers of amended topsoil. Some designs may also include a reservoir layer of larger rocks below the soil and a perforated drain pipe to transport filtered water. Bioswales, whether natural or manmade, can work in alongside existing curbs, gutters and stormwater systems.

Information about bioswales including specification requirements can be found in the following documents:

How do bioswales work?

Bioswales typically take stormwater runoff from nearby paved surfaces and hold the water long enough to allow it to slowly soak into the deep soil and possible rock drainage layer. Unlike ditches, bioswales purposely slow and filter stormwater before it enters the stormwater system.

As stormwater flows down the length of the bioswale, the natural processes of plants and soils work together to improve water quality by trapping and storing sediment, and by filtering contaminants and nutrients. Excess filtered water not used by the plants infiltrates into the native soil below or collects in the drainage pipe located under the drainage layer. This drainage layer pipe connects to the existing stormwater system to carry excess filtered stormwater back to the river.

Benefits of bioswales

Bioswales offer a number of benefits as they promote:

  • Reduction in peak water flow to our waterways
  • Removal of pollutants from stormwater
  • Improved stormwater infiltration
  • Decreased downstream/riverback erosion
  • Improving aesthetic of the area

Locations for bioswales

Bioswales can be applied in most situations, including residential areas, office complexes, rooftop runoff, parking and roadway runoff, parks and green spaces. Swales are well-suited to treat highway or residential road runoff because they can run parallel to the roadway.

Plants for bioswales

Bioswale plants are typically lawn grasses, however some swales are planted with hearty, low-maintenance plants which meet other community goals of increased aesthetic appeal of the community in addition to stormwater management.