Vegetated filter strips are a pre-treatment BMP designed to remove solids from stormwater runoff. The vegetation can consist of natural to established vegetation communities, and can range from turf grass to woody species with native grasses and shrubs. Because of the range of suitable vegetation communities, vegetated filter strips can be easily incorporated into landscaping plans; in doing so, they can accent adjacent natural areas or provide visual buffers within developed areas. They are best suited for treating runoff from roads, parking lots, and roof downspouts.
Their primary function is to slow runoff velocities and allow sediment in the runoff to settle or be filtered by the vegetation. By slowing runoff velocities, they help to attenuate flow and create a longer time of concentration. Filter strips do not significantly reduce runoff volume; however, there are minor losses due to infiltration and depression storage.
Filter strips are most effective if they receive sheet flow and the flow remains uniformly distributed across the filter strip. Channelized flow significantly reduces their effectiveness because flow velocities are minimally reduced and the effects of filtration through plant stems and accumulated thatch on the ground surface is lost. Vegetated filter strips are included in the "Infiltration/Filtration" category (Section III.D.1) of the MPCA Construction Stormwater General Permit and will usually require such a permit as part of the overall construction project. Properly designed vegetated filter strips also satisfy permit requirements for pre-treatment for certain stormwater quality BMPs. Retrofits are cases in which construction of a vegetated filter strip may not require a MPCA Permit, although the permit’s standards may still provide design guidelines. Additional permits may be necessary depending on the jurisdiction and the proposed location of the vegetated filter strip.
Permit applicability
Vegetated filter strips are included in the "Infiltration/Filtration" category (Section III.D.1) of the MPCA Construction Stormwater General Permit and will usually require such a permit as part of the overall construction project. Properly designed vegetated filter strips also satisfy permit requirements for pre-treatment for certain stormwater quality BMPs. Retrofits are cases in which construction of a vegetated filter strip may not require a MPCA Permit, although the permit’s standards may still provide design guidelines. Additional permits may be necessary depending on the jurisdiction and the proposed location of the vegetated filter strip.
Retrofit applicability
The ability to retrofit a site to incorporate a vegetative filter strip is highly dependent on existing conditions and how well they conform to design requirements. As noted in the overview and design guidelines, vegetated filter strips are much more effective if they receive sheet flow and if the filter strip has a low gradient; therefore, a retrofit may require additional grading or repaving to create sheet flow and/or create a grade suitable for effective pretreatment. The availability of space for the filter strip will impact its effectiveness, particularly if the available space is smaller than design guidelines stipulate. In such cases, the pretreatment needs should be evaluated to determine if a potentially undersized filter strip is the most appropriate pretreatment option.
Cold climate suitability
During the winter season, vegetated filter strips can become covered in snow and ice, causing runoff to find an alternate flow path or possibly flow on top of ice-covered ground and away from the filtering ability of the vegetation. Furthermore, the frozen ground eliminates the minor benefit of infiltration. Design restrictions for filtration BMPs hold true for vegetated filter strips specifically. While the design of filter strips can utilize a variety of vegetation communities, native vegetation with robust root systems should be used immediately adjacent to water bodies to provide additional bank stabilization and provide natural cover for habitat.