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Filter Strips

Cross section of a slope showing how trees and plats alow ground absorption of rain water

 

Filter strips are vegetated areas that are designed to receive runoff from adjacent impervious surfaces. They work by slowing runoff speed, trapping sediment and other pollutants, and providing some absorption. While frequently planted with turf grass, filter strips may also employ native vegetation, which is more effective in removing nutrients.

Filter strips can reduce both the rate and volume of stormwater runoff on a site. This is achieved principally by absorbing runoff into the soil.

Well maintained filter strips can be very effective in reducing runoff volumes, particularly when the impervious drainage area is not overly large (such as more than 4 to 5 times the filter strip area.)Filter strips are most effective in reducing surface runoff volumes – by up to 40 percent – for small storm events (storms up to the magnitude that may occur, on average, once every year or every other year).

 

Filter strips remove suspended solids through settling and filtration. Dissolved pollutants are removed and/or transformed as runoff infiltrates into the ground. Effectiveness is improved when there is dense vegetation. The use of native vegetation can provide additional benefits for pollutant filtering and runoff absorption.

The plants selected should be able to withstand flowing water, and both wet and dry periods. A properly designed and maintained filter strip may remove up to 70 percent to 95 percent of suspended solids and metals (such as cadmium and lead), 25 percent to 65 percent of nutrients (such as phosphorus and nitrogen), and biochemical oxygen demand (the degree of organic pollution in water leading to the depletion of oxygen). However, soluble inorganic compounds (most notably road salt) are generally not well removed in the soil and will eventually migrate downstream or into deep groundwater.

Filter strips function best when applied on gentle slopes, thereby keeping runoff speed low and maximizing opportunities for absorption of runoff and filtering of pollutants. The longer the water moves through a treatment such as this, the more it can be absorbed and the cleaner it will get. Filter strips must disperse the flow as evenly as possible to avoid straight, deep channels, which can reduce effectiveness. Where feasible, a filter strip width of at least 20 feet is recommended, although narrower widths can be effective on flat slopes.

Applicability
Roof runoff and parking lot runoff can be distributed over the width of lawn areas to promote absorption and filtering. Filter strips are strongly recommended in buffer zones between developed areas and sensitive aquatic environments. They are particularly appropriate as buffers for land uses that generate high pollutant loads, such as roadways and parking lots, and are useful in controlling erosion and sediment wash off during construction. Filter strips are probably most appropriate on developments where there are significant expanses of pervious areas (green spaces) adjacent to impervious surfaces (such as parking lots). Specifically, they may be used in the following applications: residential (roof runoff), commercial (roof and parking lot runoff) and vegetated buffers (adjacent to stream or wetland areas).

Maintenance Considerations
Typically, maintenance involves normal activities such as mowing, trimming, removal of invasive species and additional planting if necessary.

Photo of a filter strip near a parking lot.

Cost Considerations
In most cases there is no additional cost associated with establishing filter strips. Typically, all that is required is to direct runoff to an open vegetated area rather than a storm sewer. If the runoff is concentrated, a level spreader (fanning out the water from the immediate source through the use of a wide-mouthed gutter or small culvert into gravel) may be necessary to evenly spread runoff water. Eliminating the need for a local storm sewer may offset the cost of this device. Although periodic cleaning may be required, filter strips should never need to be "replaced." Since filter strips remove sediment and other pollutants, they should lower maintenance costs for downstream catch basins, detention basins and absorption devices.

Local Examples
When used appropriately, disconnected downspouts in residential and commercial buildings will cause the roof runoff to route over a filter strip in a back, side, or front yard.