Biofiltration

A vegetated swale is a broad shallow channel with a dense stand of vegetation covering the side slopes and bottom, typically located along property boundaries along a natural grade. Gravel and coarse sandy soils that cannot easily support dense vegetation are not appropriate for swales. Alkaline soils and subsoils should be used to promote the removal and retention of metals. Soil infiltration rates should be greater than 0.2 millimeters per second (one-half inch per hour).

Swales, one of the stormwater BMPs, are designed to trap particulate pollutants (suspended solids and trace metals), promote infiltration, and reduce the flow velocity of stormwater runoff. Vegetated swales are most effective when used in conjunction with other BMPs, such as wet ponds, infiltration strips, wetlands, etc.. Swales can be used in place of curbs and gutters along parking lots in residential, industrial, and commercial areas with low stormwater flow and smaller populations.

The advantage of vegetated swales comparing to the ordinary storm sewer system are:

• Reduction of peak flows

• Removal of pollutants

• Promotion of runoff infiltration

• Lower capital costs

The disadvantage of vegetated swales are the following:

• Limitation of application on flat grades, steep topography, or wet or poorly drained soils

• Limited effectiveness and may even erode when flow volumes and/or velocities are high

• Pollution off groundwater from the infiltrated stormwater

• Creating an environment for potential drowning, odor, and mosquito breeding problems from standing water

Click here for the detailed design of vegetated swales from the EPA’s Storm Water Technology Fact Sheet: Vegetative Swales .

Click here to view the removal efficiencies of vegetated swales systems.

Source: Image of EPA Biofiltration System

Copyright @ January,2005 Prepared by Transportation Environmental Resource Center (TERC)