Vegetated swales are broad, shallow channels designed to convey and infiltrate stormwater runoff. The swales are vegetated along the bottom and sides of the channel, with side vegetation at a height greater than the maximum design stormwater volume. The design of swales seeks to reduce stormwater volume through infiltration, improve water quality through infiltration and vegetative filtering, and reduce runoff velocity by increasing flow path lengths and channel roughness.
Two primary vegetated swale design variations exist. Dry swales are designed with highly permeable soils and an underdrain to allow the entire stormwater volume to convey or infiltrate away from the surface of the swale shortly after storm events. Dry swales may be designed with check dams that act as flow spreaders and encourage sheet flow along the swale. Check dams also retain stormwater. Wet swales are designed to retain water and maintain marshy conditions for the support of aquatic vegetation. Because of their highly permeable soil and conveyance capability, dry swales are more applicable for urban environments.
Stormwater Management Objectives
Infiltration into the underlying and surrounding soils is the mechanism through which vegetated swales reduce stormwater volume. Evapotranspiration further reduces the stormwater volume. Reductions in discharge volume will be most apparent in moderate to small storms. Soils in vegetated swales can be amended to enhance permeability and increase volume reductions.
Peak discharge is decreased because of a decrease in volume and an increase in runoff duration. Dry swales should be sized to store and infiltrate the determined water quality volume of runoff within 24-48 hours.
Vegetated swales improve water quality through two main mechanisms. The vegetation in the channel removes large and course particulate matter from stormwater. Pollutant removal is also facilitated by the infiltration process encouraged through the use of swales. Estimated removal efficiencies are 80 percent for TSS, 50 percent for phosphorus and nitrogen, and 40 percent for metals.