Bioswales are shallow, open channels that are designed to reduce runoff volume through infiltration. Additionally, bioswales trap trash and remove polllutants such as some chemicals by filtering water through vegetation and soil within the channel. Swales can serve as conveyance for stormwater and can be used in place of traditional curbs and gutters. The primary objective of a bioswale is infiltration and water quality enhancement rather than conveyance. However they can convey stormwater volumes to a discharge point, but unlike ditches, they intentionally promote the slowing and filtering of polluted runoff along the way. In addition to reducing the mass of pollutants in runoff, when properly maintained, bioswales can enhance the aesthetics of a site.

Bioswale vegetation is typically lawn grasses, but many low-volume swales are now being built with a combination of grasses, perennials, shrubs, groundcover and trees.  A typical swale bottom is flat in cross section, 600 to 2400 mm wide, with a 1-2% longitudinal slope, or dished between weirs on steeper slopes. Bioswale side slopes are usually no more than 3:1, horizontal to vertical.

Bioswales are effective for removing

  • Runoff volume
  • TSS
  • Trash/debris
  • Organics
  • Bacteria
  • Nutrients
  • Heavy metals
  • Oil and grease


Siting and Design:

Drainage AreaSoil Infiltration RateWater Table SeparationDepth to bedrockFacility slopeInflow rate
2 ac> 0.5 in/hr(if < 0.5 in/hr,install UD)> 10 ft(if > 2 but < 10 ft,install UD)> 10 ft(if > 2 but < 10 ft,install UD)< 2%Mulch: 1 cfs,
Grass: 3 cfs



TaskFrequencyMaintenance Notes
Pruning1-2 times/yrNutrients in runoff often cause bioswale vegetation to flourish.
Mowing2-12 times/yrFrequency depends on location and desired aesthetic appeal.
Mulching1-2 times/yrRecommend maintaining 1"–3" uniform mulch layer.
Mulch removal1 time/2–3 yearsMulch accumulation reduces available water storage volume. Removal of mulch also increases surface infiltration rate of fill soil.
Watering1 time/2–3 days for first 1–2 months;Sporadically after establishment 
Fertilization1 time initiallyOne time spot fertilization for first year vegetation.
Remove and replace dead plants1 time/yearWithin the first year, 10% of plants can die. Survival rates increase with time.
Inlet inspectionOnce after first rain of the season, then monthly during the rainy seasonCheck for sediment accumulation to ensure that flow into the bioswale is as designed. Remove any accumulated sediment.
Outlet inspectionOnce after first rain of the season, then monthly during the rainy seasonCheck for erosion at the outlet and remove any accumulated mulch or sediment.
Miscellaneous upkeep12 times/yearTasks include trash collection, plant health, spot weeding, removing invasive species,and removing mulch from the overflow device



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