Stormwater Management Fact Sheet: Grassed Filter Strip

Description

Grassed filter strips (a.k.a., vegetated filter strips, filter strips, and grassed filters) are vegetated areas that are intended to treat sheet flow from adjacent impervious areas. Filter strips function by slowing runoff velocities and filtering out sediment and other pollutants, and providing some infiltration into underlying soils. Filter strips were originally used as an agricultural treatment practice, and have more recently evolved into an urban practice. With proper design and maintenance, filter strips can provide relatively high pollutant removal. One challenge associated with filter strips, however, is that it is difficult to maintain sheet flow. Consequently, urban filter strips are often "short circuited" by concentrated flows, which results in little or no treatment of stormwater runoff.

Applicability

Filter strips are applicable in most regions of the country, but are restricted in some watersheds where land is not available to install them. Filter strips are best suited to treating runoff from roads and highways, roof downspouts, very small parking lots, and pervious surfaces. They are also ideal components of the "outer zone" of a stream buffer (for more information see the Buffer Zone Fact Sheet), or as pretreatment to another stormwater treatment practice. Recent stormwater manuals do not consider the filter strip as a treatment practice, but do offer stormwater volume reductions, in exchange for using filter strips to treat some of a site (MDE, 2000).

Regional Applicability
Filter strips can be applied in most regions of the country with the exception of arid areas, where the cost of irrigating grass outweighs its water quality benefits.

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. Filter strips are often impractical in these areas since they require more space than is often available.

Stormwater Hotspots
Stormwater hotspots are areas where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater. A typical example is a gas station. In general, filter strips should not accept hotspot runoff, since the infiltrated runoff could cause groundwater contamination. In addition, it is doubtful whether filter strips can reliably remove high levels of pollutants.

Stormwater Retrofit
A stormwater retrofit is a stormwater management practice (usually structural) put into place after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. Filter strips, however, generally a are poor retrofit option because they consume a relatively large amount of space, and cannot treat large drainage areas.

Cold Water (Trout) Streams
Cold water species, such as trout, are sensitive to changes in stream temperature. While some treatment practices can warm streams substantially (such as wet ponds; see the Wet Ponds Fact Sheet for more information), filter strips do not pond water on the surface for long periods of time, and are not expected to contribute to stream warming. Thus, filter strips are a good stormwater option for cold water streams.

Siting and Design Considerations

Siting Considerations

Designers need to ensure that filter strips are actually feasible for the development site in question. The following section provides basic guidelines for locating filter strips.

Drainage Area
Typically, filter strips are used to treat very small drainage areas. The limiting design factor, however, is not the drainage area the filter strip treats but rather the length of flow contributing to it. As stormwater runoff flows over the ground's surface, it changes from sheet flow to concentrated flow. That is, rather than moving uniformly over the surface, it forms rivulets which are slightly deeper and cover less area than the sheet flow. When flow concentrates, it moves too rapidly to be effectively treated by a grassed filter strip. As a rule, flow concentrates within a maximum of 75 feet for impervious surfaces, and 150 feet for pervious surfaces (CWP, 1996).

Slope
Filter strips should be designed on slopes between 2% and 6%. Greater slopes than this would encourage the formation of concentrated flow. Except in the case of very sandy or gravelly soil, runoff tends to pond on the surface of filter strips. Slopes flatter than 2%, may create mosquito breeding habitat.

Soils /Topography
Filter strips should not be used on soils with a high clay content because they require some infiltration for proper treatment. Another possible limiting factor would be very poor soils that cannot sustain a grass cover crop.

Groundwater
Filter strips should be separated from the groundwater by between two and four feet to prevent contamination, and to ensure that the filter strip does not remain wet between storms.

Design Considerations
Filter strips appear to be a minimal design practice, because they are basically no more than a grassed slope. However, some design features are critical to ensure that the filter strip provides a minimum amount of water quality treatment (for more information, see Manual Builder Category).

• Use a pea gravel diaphragm at the top of the slope. The pea gravel diaphragm (a small trench running along the top of the filter strip) serves two purposes. First, it acts as a pretreatment device, settling out sediment particles before they reach the practice. Second it acts as a level spreader, maintaining sheet flow as runoff flows over the filter strip.

• The filter strip should be designed with a pervious berm of sand and gravel at the toe of the slope. This feature provides an area for shallow ponding at the bottom of the filter strip. Runoff ponds behind the berm and gradually flows through outlet pipes in the berm. The volume ponded behind the berm should be equal to the water quality volume. The water quality volume is the amount of runoff that will be treated for pollutant removal in the practice. Typical water quality volumes are the runoff from a 1" storm or �" of runoff over the entire drainage area to the practice.

• The filter strip should be at least 25' long to provide water quality treatment.

• Designers should choose a grass that can withstand relatively high velocity flows, and both wet and dry periods.

• Both the top and toe of the slope should be as flat as possible to encourage sheet flow and prevent erosion.

Regional Variations

In cold climates, filter strips provide a convenient area for snow storage and treatment. If used for this purpose, vegetation in the filter strip should be salt tolerant, (e.g., creeping bentgrass), and a maintenance schedule should include the removal of sand built up at the bottom of the slope. In arid or semi-arid climates, designers should specify drought-tolerant grasses (e.g., buffalo grass) to minimize irrigation requirements.

Limitations

Filter strips have several limitations related to their performance and space consumption.

• There is very little data to suggest that the practice can achieve high pollutant removal.
• Filter strips require a large amount of space, typically equal to the impervious area they treat, making them often infeasible in urban environments where land prices are high.
• If improperly designed, filter strips can become a mosquito breeding ground.
• Proper design requires a great deal of finesse, and slight problems in the design, such as improper grading, can render the practice ineffective in terms of pollutant removal.

Maintenance Considerations

Filter strips require similar maintenance to other vegetative practices (see Grassed Swales Fact Sheet for more information). These maintenance needs are outlined below. Maintenance is very important for filter strips, particularly in terms of ensuring that flow does not short circuit the practice.

Effectiveness

Stormwater treatment practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal (see the Manual Builder Category for more information). The first two goals, flood control and channel protection, require that a stormwater practice be able to reduce the peak flows of relatively large storm events (at least 1- to 2-year storms for channel protection and at least the 10- to 50-year storms for flood control). Filter strips do not have the capacity to detain these events, but can be designed with a bypass system to route these larger flows around the strip to a supplementary practice.

Filter strips can provide a small amount of groundwater recharge as runoff flows over the vegetated surface and ponds at the toe of the slope. In addition, it is believed that filter strips can provide modest pollutant removal. Studies from agricultural settings suggest that a fifteen foot wide grass buffer can achieve a 50% removal rate of nitrogen, phosphorous and sediment, and that a 100-foot buffer can reach closer to 70% removal of these constituents (Desbonette et al., 1994). It is unclear how these results can be translated to the urban environment, however. The characteristics of the incoming flows are radically different both in terms of pollutant concentration and the peak flows associated with similar storm events. To date only one study (Yu et al., 1992) has investigated the effectiveness of a grassed filter strip to treat runoff from a large parking lot. The study found that the pollutant removal varied depending on the length of flow in the filter strip. The narrower (75 foot) filter strip had moderate removal for some pollutants and actually appeared to export lead and nutrients (see Table 2).

Cost Considerations

Little data is available on the actual construction costs of filter strips. One rough estimate can be the cost of seed or sod, which is approximately 30� per square foot for seed or 70� per square foot for sod. This amounts to between $13,000 and $30,000 per acre for a filter strip, or the same amount per impervious acre treated. This cost is relatively high compared with other treatment practices. However, the grassed area used as a filter strip may have been seeded or sodded even if it were not used for treatment. In these cases, the only additional costs are the design, which is minimal, and the installation of a berm and gravel diaphragm. Typical maintenance costs are about $350/acre/year (adapted from SWRPPC, 1991). This is relatively inexpensive, and again, this may overlap with regular landscape maintenance costs.

The true cost of filter strips is the land that they consume, which is higher than for any other treatment practice. There are some situations where this land is available as wasted space beyond back yards or adjacent to road sides, but this practice is cost prohibitive when land prices are high and could be used for other purposes.

References

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1997. Maryland Stormwater Design Manual. Prepared for: Maryland Department of the Environment. Baltimore, MD. http://www.mde.state.md.us/environment/wmh/stormwatermanual/mdswmanual.html

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.

Desbonette, A., P. Pogue, V. Lee, and N. Wolff. 1994. Vegetated Buffers in the Coastal Zone: A Summary Review and Bibliography. Coastal Resources Center. University of Rhode Island. 72 pp.

Magette, W., R. Brinsfield, R. Palmer and J. Wood. 1989. Nutrient and Sediment Removal by Vegetated Filter Strips. Transactions of the American Society of Agricultural Engineers. 32(2): 663-667.

Yu, S., S. Barnes and V. Gerde. 1993. Testing of Best Management Practices for Controlling Highway Runoff. Virginia Transportation Research Council. FHWA/VA 93-R16. 60 pp.