Vegetative Filtering Systems Filter Strips
Uses vegetation to slow runoff velocities and filter sediment and pollutants from stormwater
composed of stone trench, grass strip, and wooded strip
must have presence of sheetflow
- over 150 feet of pervious areas, or
- over 75 feet of impervious areas
non-erosive for 100-year design storm
grass portion is pretreatment for wooded portion
Notes:
Filter strips are grassed practices which accept sheet flow runoff from adjacent surfaces. Filter strips function by slowing runoff velocities and filtering out sediment and other pollutants. The design approach for filter strips involves site design techniques to maintain prescribed maximum sheet flow distances as well as checking to ensure adequate temporary storage for the WQv for a 24 hour period. Filter strips are also designed using a volume based sizing criteria.
It is doubtful that runoff can be maintained as sheet flow over distances beyond 150 feet for pervious surfaces, and 75 feet for paved surfaces. Once runoff concentrates, filtering is reduced or eliminated through short-circuiting, preventing effective treatment. Therefore, the use of filter strips to treat stormwater runoff is primarily a function of limiting the flow path to the filter. One of the main abuses of the past has been draining too much area through the filter strip. In most cases the sheet flow distance limitations will be the controlling factor.