Vegetated filter strips receive stormwater runoff as sheet flow from impervious areas.
They are excellent for the treatment of stormwater runoff from small and frequent storms, effectively directing stormwater to landscape areas as passive irrigation.
Filter strips may include existing vegetation and may support a combination of herbaceous plants, shrubs or trees. Selection of vegetation is dependent on characteristics of the
site and on stormwater
runoff quantities and velocities. Water quality treatment performance will vary in accordance with the type and maturity of vegetation and the diversity and depth of soil layers.
Depending on the residence time, a filter strip may function as a sole stormwater treatment practice, or alternatively as a primary treatment practice capturing moderately coarse particles within a treatment train. Some portion of stormwater
runoff may also infiltrate into the ground.
The key to the performance of filter strips is an even dispersal of flows across vegetated areas in order to optimise contact with soil and plants and to avoid concentrated flows (which may lead to the formation of rills and localised erosion). Filter strips may employ a level spreader, contour drain or exfiltration trench at the point of stormwater discharge to ensure an even dispersal of flows.
Critical elements for the function of filter strips include the length and steepness of the slope. Filter strips are generally less than 5% grade, unless terracing or level spreaders can be incorporated mid-slope. Grass filter strips should be mown relatively long (>100 mm high grass). Periodic inspection is also recommended for identification of non-biodegradable materials, erosion or clogging by heavy sediment.
As for other WSD practices, filter strips can be integrated into existing or proposed landscape elements. Typically filter strips are located along
impervious surface boundaries such as carparks and roadways. The buffering function of these practices can be important for protecting the microclimate and interior
habitat of remnant vegetation and streams. Filter strips can also provide a suite of environmental benefits for water and air quality, interception of dust, and cooling of ambient temperatures.