Vegetative filter strips are used as a pretreatment BMP and are designed to treat sheet flow from adjacent gently sloping impervious surfaces. Filter strips function by slowing runoff velocities and filtering out sediment and other pollutants, and by providing some infiltration into underlying soils. The vegetation of a filter strip 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; they can accent adjacent natural areas or provide visual buffers within developed areas.
To be effective, filter strips should receive sheet flow across the entire strip. The filter strip must be designed to withstand the full range of storm events without eroding. Filter strips can be as simple as a strip of turf between a roadway or parking lot and a filtration basin or swale. Filter strips typically do not provide adequate removal of pollutants to act as a stand-alone BMP. Filter strips cannot treat high-velocity flows. They are ideal for areas where flow is dispersed into smaller catchment areas, which can be an effective practice on PWA sites.
Filter strips slow runoff but do not significantly reduce runoff volume; there is some minor reduction due to infiltration and depression storage. With proper design and maintenance, filter strips can remove pollutants. One challenge associated with filter strips is that it is difficult to maintain sheet flow, so the practice may be short-circuited by concentrated flows receiving little or no treatment. Channelized flow significantly reduces the effectiveness of a filter strip because flow velocities are minimally reduced and the effects of filtration through plant stems and accumulated thatch on the ground surface are lost.
Vegetated filter strips are commonly used for treating runoff from roads and small parking lot drainage areas, and as a pretreatment in combination with bioretention.
The ability to retrofit a site to incorporate a vegetative filter strip is highly dependent on existing site conditions and how well the filter strip conforms to design requirements. Vegetated filter strips are most effective if they receive sheet flow and if the filter strip has a low gradient. A retrofit may therefore require additional grading or repaving to create sheet flow. 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.
Filter strips are effective as an “outer zone” for a shoreland buffer. A filter strip can also be an enhanced natural buffer along a shoreline, where the capability of the natural buffer to remove pollutants is improved through grading and erosion control, plant selection, and maintenance activities.
While the design of vegetated filter strips can include 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 natural cover for habitat.
During the winter season, vegetated filter strips can become covered with 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 vegetation. Furthermore, the frozen ground eliminates the minor benefit of infiltration.
Properly designed vegetated filter strips slow runoff velocities and allow sediment in the runoff to settle or be filtered by the vegetation. The water quality benefit of a vegetated filter strip is dependent on the length of flow through the filter strip, the effectiveness in achieving sheet flow and avoiding channelized flow, and the characteristics of the contributing watershed. Filter strips can remove pollutants in runoff, including small particulates, hydrocarbons, heavy metals, and nutrients such as phosphorus and nitrogen. Studies have examined the effectiveness of vegetated filter strips in treating runoff from urban settings, including highways and parking lots; however, these studies have been too few in number or have too many varying characteristics (watershed variables and vegetated filter strip design) to determine strong correlations between design components and specific water-quality benefits.
Critical components for an effective vegetated filter strip:
Vegetated filter strips are most effective for storms up to the 1- to 2-year event. The effectiveness of filter strips is reduced for flow depths that exceed 1 inch and flow velocities that exceed 0.5 feet per second. Current State of Minnesota guidelines are consistent with other published guidelines.
One significant benefit of vegetated filter strips is that construction is relatively inexpensive since it is limited to grading and vegetation establishment. Maintaining sheet flow into the filter strip is critical to its effectiveness; therefore, construction must result in an evenly graded site. In some instances a level spreader (e.g., pea gravel diaphragm, filter strip diagram SW-160) may be used at the top of the slope to capture sediment and to maintain sheet flow as runoff enters the filter strip. Care should also be taken to avoid excessive compaction in the filter strip area to maintain healthy soils for vegetation and to preserve the infiltration capacity of the soils.
If designed properly and if they receive sheet flow, vegetated filter strips require minimal maintenance. Depending on the contributing watershed characteristics, common maintenance needs include periodic removal of accumulated sediment and debris (particularly at the upstream end of the filter strip) and monitoring to ensure channels and preferential flow paths have not developed. Contributing watersheds with high sediment concentrations require more frequent maintenance of the vegetated filter strip to manage sediment buildup. Maintenance may also include mowing, trimming, or burning the plant community, depending on specific needs of the vegetation.