Water quality swales are shallow vegetated open channels designed to treat and convey stormwater runoff. Water quality swales provide higher pollutant removal than traditional grass drainage channels, which are designed strictly for conveyance.

Wet water quality swales (also referred to as “wet swales”) temporarily store and treat stormwater runoff from the water quality storm. However, unlike Dry Water Quality Swales, wet swales are constructed directly within existing soils and are not underlain by a bioretention soil media or underdrain system. Wet swales store stormwater runoff within a series of cells within the channel, which may be formed by berms or check dams. Wet swales are designed to remain saturated, maintaining wetland plants and conditions. The pollutant removal mechanisms in wet swales are similar to those of Stormwater Wetland BMPs, which rely on sedimentation, adsorption, and microbial breakdown.

Wet water quality swales are primarily used for treatment and conveyance of stormwater runoff. They do not provide stormwater retention, runoff volume reduction, or groundwater recharge because they are constructed in groundwater and are not designed for infiltration. Wet swales may also be used to provide stormwater quantity control when designed as on-line facilities.