Vegetated filter strips are a pre-treatment Best Management Practice (BMP) designed to remove solids from stormwater runoff. The vegetation can consist of natural and 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, but 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 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 is lost.
Contents
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 the filter strip conforms 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. A retrofit may therefore 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 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 ability of the vegetation. Furthermore, the frozen ground eliminates the minor benefit of infiltration.
Special receiving waters suitability
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.
Design restrictions for special waters
Water quality benefits
Properly designed vegetated filter strips slow runoff velocities and allow sediment in the runoff to settle or be filtered by the vegetation. In addition to filtering solids, vegetated filter strips can also remove portions of other pollutants in runoff, including small particulates, hydrocarbons, heavy metals and nutrients such as phosphorus and nitrogen. The removal mechanisms include sedimentation, filtration, adsorption, infiltration, biological uptake, and microbial activity.
The water quality benefit of a vegetated filter strip is dependent on the length of flow through the BMP, the effectiveness in achieving sheet flow and avoiding channelized flow, and the characteristics of the contributing watershed. 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.
Water quantity benefits
Vegetated filter strips are not a primary means of water quantity control; however, there are minor volumetric losses from infiltration and evapotranspiration.
Design considerations
There are five critical components for an effective vegetative filter strip design:
- contributing drainage area;
- type of vegetation;
- filter strip length;
- filter strip slope; and
- filter strip soils.
Vegetated filter strips are most effective for storm events 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.
The flow length leading to the vegetated filter strip is a primary design factor because if the flow length is too long, there is a greater chance that the sheet flow will become concentrated flow and require additional design to re-establish sheet flow entering the vegetated filter strip. The flow length into the vegetated filter strip should be limited to 75 feet from impervious surfaces and 150 feet from pervious surfaces.
Vegetated filter strips are most effective if the vegetation is healthy and dense. Grasses are most effective in shorter filter strips while woody species may be suitable for longer filter strips.
A properly designed filter strip length is dependent on the slope, the underlying soils, and the type of vegetation to be used. Filter strips that are too short will have reduced effectiveness. Filter strips with steeper slopes will require more length to be equally effective as a filter strip with a lower slope; slopes less than 5 percent are most effective. The current state of Minnesota guidelines are consistent with other published guidelines.
Guidelines for filter strip pre-treatment sizing
Link to this table
| Parameter | Impervious Parking Lots | Residential Lawns | ||||||
|---|---|---|---|---|---|---|---|---|
| Maximum Inflow Approach Length (ft) | ||||||||
| Filter Strip Slope | =<2% | >2% | =<2% | 2% | =<2% | 2% | =<2% | 2% |
| Filter Strip Minimum Length | 10' | 15' | 20' | 25' | 10' | 12' | 15' | 18' |
Construction
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 in the filter strip is critical to its effectiveness; therefore, construction must result in an evenly graded site. Care should also be taken to avoid excessive compaction in the filter strip area to maintain healthy soils for a healthy vegetation community and to preserve the infiltration capacity of the soils.
Maintenance
If designed properly and if they receive sheet flow, vegetated filter strips require minimal maintenance. More maintenance is required of vegetated filter strips that receive channelized flow from the contributing watershed or concentrated flow from curb cuts. Systems that redistribute concentrated or channelized flow into sheet flow may require periodic maintenance to ensure the redistribution system maintains its design function. 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 or preferential flow paths have not developed. Contributing watersheds with high sediment concentrations require more frequent maintenance in the vegetated filter strip to manage sediment build-up. Maintenance may also include mowing and trimming or burning the plant community depending on specific needs of the vegetation.