Description of Challenge
Bioswales provide good treatment of stormwater runoff without the extensive maintenance required for some other stormwater BMPs. Pollutant removal rates increase when bioswales are well maintained, and as the residence time of water in a swale increases.A bioswale is an urban landform used to convey surface water in order to enhance infiltration and reduce surface runoff.
Solution
Also known as infiltration swales, biofilters, grassed swales, or in-line biorentention, bioswales are vegetated open channels specifically designed to attenuate and treat stormwater runoff for a defined water volume.
Like open ditches, they convey larger stormwater volumes from a source to a discharge point, but unlike ditches, they intentionally promote slowing, cleansing and infiltration along the way. A sloped base to facilitate this water movement distinguishes bioswales from rain gardens.
There are some design variations of the bioswale, including grassed channels, dry swales and wet swales. These designs may also include an underlying rock reservoir, with or without a perforated underdrain. The specific design features and treatment methods differ in each variation, but all are considered improvements on traditional drainage ditches.
Each type of swale incorporates modified geometry and other design features to allow it to treat and convey stormwater runoff. 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.
Bioswale vegetation is typically lawn grasses, but more and more of the low volume swales being built in North America are finished with a combination of grasses, perennials, shrubs, groundcover and trees in order to meet other community goals in addition to stormwater management.
