Filter Strip

Filter strips are densely vegetated areas that collect and slow runoff, thereby filtering out sediments and infiltrating insoluble pollutants. Filter strips are typically used to treat runoff from impervious areas that generate significant pollutants, such as parking lots. They should be uniformly graded at shallow slopes to encourage sheet flow through their full flow length and are best used in conjunction with other management practices, as they do not significantly reduce peak flows or runoff volume in large storms.

Advantages

• Relatively Low Cost
• Easy to construct and maintain
• Remove sediment and insoluble pollutants
• Increase the infiltration of runoff
• Can provide habitat for wildlife

Disadvantages

• Ineffective for high velocity runoff
• Require a large amount of land area
• Reduced effectiveness with large storm events

Maintaining sheet flow into the filter strip is critical for them to work effectively. To ensure sheet flow, the maximum allowable run-on flow length is 100 feet from impervious surfaces, with a maximum slope of 5%. Filter strips cannot be used to treat concentrated flow from downspouts unless a level spreader is used. Filter strips are not intended to treat runoff from pervious areas.

Each filter strip should be sized according to the individual characteristics of the site, taking into account the size of the area to be drained and the slope of the land that they are located on. Filter strips that border impervious surfaces should stretch the entire width of the intended treatment area and have a minimum flow length of 10 feet . Increased lengths enhance the treatment ability of the practice by increasing detention time. However, lengths greater than 50 feet may concentrate flow, so are not allowed without additional flow dissipation.

The necessary flow length of the filter strip is dependent on the slope of the site. Longitudinal slopes of 1-2% are recommended, while the maximum allowable is 5%. Steeper slopes encourage concentrated flow and may lead to channelization, while slopes flatter than 1% may result in ponding.

If the required contributing area run-on lengths cannot be met or flow is concentrated, a level spreader may be constructed at the top of the filter strip. These devices disperse flows over a wide area, dissipating the energy of the runoff and creating sheet flow. Level spreaders constructed with level, durable surfaces (such as concrete) are required to ensure sheet flow into the filter. The figure below illustrates a swale that fills up before flowing over the level spreader, onto a 1 foot (or larger) gravel diaphragm, then into the filter strip. Incoming flow should enter the swale parallel to the swale length. The designer should calculate the peak rate for the 1-yr storm and multiple that result by 10 feet per cfs to determine the required width of the level spreader, with a maximum width of 100 ft. This requirement helps ensure water is released into the filter strip at a non-erosive velocity. If soil conditions warrant, an underdrain should be installed to assist the swale drainage after a storm.

Filter strips require dense vegetation to work effectively. “Dense” is defined as vegetation that uniformly covers at least 90% of a representative 1 square yard plot. Plant species selected for filter strips should meet the following criteria:

• Species should be resistant to matting and form a dense cover
• Species should be tolerant to frequent inundation, as well as extended dry periods
• Native species may be used with careful selection
• No exotic, noxious, or invasive species

• The ground must be uniformly sloped perpendicular to the flow
• At least 6 inches of loose topsoil with appropriate soil amendments is required
• Minimize and reduce soil compaction
• Establish vegetation as quickly as possible
• Stabilize with erosion control matting

• Vegetation type and height must be maintained in accordance with that specified in the stormwater plan.
• Mowing should only be performed during dry periods using lightweight equipment to prevent soil compaction and damage to vegetation.
• Filter strips should be inspected weekly and after all major storm events to ensure they are operating properly and to check for any potential problems, such as the formation of rills and gullies, bare spots, or excessive sediment accumulation.
• Minimize foot and vehicle traffic on the filter strip.
• Filter strips should be included in a site’s deed-recorded stormwater maintenance agreement and their location shown on a simple site map.

WinSLAMM must be used to calculate the efficiency of filter strips. The impervious area must be modeled as “directly connected”. A dynamic infiltration rate of 0.01 in/hr can be assumed unless a soil texture evaluation or infiltration test indicates a higher infiltration rate is appropriate. Compost may be amended into the soil to improve the infiltration rate to the next soil type (as listed in WinSLAMM). The selected vegetation height and retardance class should be consistent with the expected land use and maintenance.

WinSLAMM will reduce the efficiency of a filter strip if the ratio of contributing area to filter strip area is greater than 5:1. WinSLAMM also reduces the modeled filter strip flow length based on the longitudinal slope of the filter strip. See the WinSLAMM Model Algorithms document for additional information.

• Filter Strip Guidance for Owners
• North Caroline Department of Environmental Quality Stormwater Manual: Level Spreader-Filter Strip