Using Bioswales for Passive Stormwater Treatment

Bioswales are one of several passive options available to meet the stormwater management objectives for a development which and can be located wherever traditional landscaped areas are proposed. They are typically located within traffic islands or adjacent to parking lots. Bioswales provide both a water quality and water balance component and are often the preferred choice of developers due to the relatively low cost in comparison to alternatives. Bioswales consist of a filter bed which is a mixture of sand, fines and organic materials with a recommended bed depth between 1 and 1.25m. A 75mm mulch cover is recommended over the filter bed to enhance plant survival, reduce weed growth and provide pre-treatment of the filter bed. Contributing drainage areas directed to bioswales should range between 100 square meters to 0.5ha and should be designed with a level surface to encourage stormwater to spread out evenly over the surface. Pre-treatment of drainage with a vegetated filter strip, or a rip-rap diaphragm, often precedes the bioswale to dissipate the flow and remove particles that could clog the filter bed, reducing maintenance.


Depending on the proposed functionality of the bioswale, the landscape architect will specify drought resistant or water tolerant plants or native grasses. Coordination with the landscape architect is important during the design stage as the proposed planting is critical to the function and appearance of the bioswale and will determine the level of ongoing maintenance required. As bioswales are designed to capture small storm events, an overflow or bypass is necessary to redirect the less frequent storms to the storm sewer system.


As stated above, bioswales can be located wherever traditional landscape areas are proposed but there are design considerations required to overcome site specific constraints. Some of the major constraints are listed below:


  • Stormwater runoff from fuel sites or some industrial operations have the potential to contaminate the existing soils and water table. Additional measures such as impermeable liners are required to ensure runoff is not infiltrated but held within the voids of the filter bed and taken up by the plants.
  • An improperly designed sub-drain system could result in standing water within the bioswale. Designers should target a 24 hour drawdown to eliminate the potential for mosquito larvae.
  • The Ontario Building Code requires a 4.0m setback from buildings to ensure building foundation drainage is not impacted and a minimum of 1.0m is required above the seasonally high water table to reduce the potential failure of the bioswale.
  • In order to achieve a water balance component, native soils must meet the municipality’s minimum percolation rate requirements.


Bioswales have been shown to provide a water balance component by reducing runoff volume through evapotranspiration and infiltration. Factors that can impact water balance benefits are native soil infiltration rate, rainfall patterns, and sub-drain sizing criteria. The quality component is provided by the removal of pollutants as a result of sedimentation, filtering, soil adsorption, microbial processes and plant uptake. It is also important to note that there is a direct benefit relationship between the water balance and water quality functions provided by the bioswale. The percentage of runoff infiltrated or taken up by the plants is equivalent to the reduction of pollutants leaving the site. Other benefits include reduced thermal aquatic impacts and reduced urban heat island effects.


A properly designed and constructed bioswale provides a cost effective and sustainable solution that assists in achieving stormwater management objectives.



Low Impact Development Stormwater Management Planning and Design Guideline, 2010

Toronto and Region Conservation, Credit Valley Conservation



For more information on this topic and how it could relate to your project, contact MGM directly.