Bioswales are sloped drainageways designed to convey stormwater runoff efficiently. A bioswale has an engineered subgrade with amended soil, aggregate and subsurface drain that is designed to treat the water quality volume (1.25” rainfall) and convey larger rainfall events typically to flood management practices (wetlands, retention or detention basins). They consist of a series of vegetated cells with level bottoms that have check dams spaced between them. They are an effective option when water can be routed through a channel long enough to allow flow to pass at slow velocities for the desired residence time.

Bioswales differ from biocells in that they are in sloped areas so that when larger storms occur they are safely conveyed to flood management practices such as constructed wetlands, retention or detention basins before draining into the nearest stream or waterbody. It is critical that bioswales are inspected and maintained periodically to ensure their continued performance.

Design Components

1. Plants. Vegetation should provide critical erosion protection and be aesthetically pleasing.
2. Berms. Berms ensure water is slowed down and infiltrated, helping remove pollutants common to stormwater runoff. Berms can be constructed of earthen material or rocks.
3. Modified Soil. Amended soils are placed into the bioswale to facilitate infiltration of water during rainfall and runoff events.
4. Washed Rock. The subdrain tile is bedded in clean washed rock.
5. Perforated Subdrain. Tile Drainage ensures water can percolate down through the soil. If water can’t move down through the soil profile, it will move to the subdrain and be conveyed to a stream or other outlet site in a controlled manner.