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Vegetated Buffer Strip

General

Buffer strips are densely vegetated areas that collect and slow runoff, filtering out sediments and insoluble pollutants and encourage infiltration. Stormwater flows into a buffer strip over a level spreader, a device that converts concentrated flow into sheet flow. As the runoff flows through the vegetation, its velocity is reduced, releasing its load of suspended solids and promoting infiltration.

Buffer strips are uniformly graded and are located down slope from disturbed or impervious areas or adjacent to waterways. Buffer strips are best used in conjunction with other management practices, however, as they do not significantly reduce peak flows or the volume of runoff.

Advantages

  • Relatively Low Cost
  • Easy to construct and maintain
  • Can be aesthetically pleasing if designed properly
  • Remove sediment and insoluble pollutants
  • Increase the infiltration of runoff
  • Can provide habitat for wildlife
  • Can help stabilize stream banks

Disadvantages

  • Ineffective in areas with high velocity runoff
  • Require a large amount of land area
  • Reduced effectiveness with large storm events
  • Best used in conjunction with other management practices

Design

Level Spreader and Berms

Maintaining sheet flow is critical to the proper operation of buffer strips. To ensure that concentrated flow is eliminated before runoff enters the buffer strip, a level spreader may be constructed at the top of the buffer strip. These devices disperse flows over a wide area, dissipating the energy of the runoff and creating sheet flow. Common types of level spreaders are curb cuts, concrete weirs, and stone weepers or trenches.

Length, Width, and Slope

Each buffer 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.

Buffer strips that border impervious surfaces should stretch the entire width of the surface and have a minimum flow length of at least 25 feet, with a 20-minute detention time. Increased lengths enhance the treatment ability of the practice by increasing detention time. However, lengths greater than 40 feet generally result in channelized flow and require additional flow dissipaters. Regardless of the length, each buffer strip should not drain an area larger than ½ acre. Sites that border bodies of water may have additional requirements beyond this ordinance. For length requirements on this type of site, please contact your local WDNR office.

The length of buffer strips is dependent upon the slope of the site. Slopes of 1-2 percent are recommended and may not exceed 6%. Steeper slopes encourage concentrated flow and may lead to channelization, while slopes flatter than 1 percent may result in ponding. Runoff velocities are determined by the detention time.

Vegetation

Buffer strips only provide effective erosion control once the vegetation is densely established. “Dense” is defined as a stand of 6-8 inch sod-forming vegetation that uniformly covers at least 90% of a representative 1 square yard plot. As a result, until vegetation is firmly established, it shall under no circumstances be relied upon to prevent soil loss from the site.

Plant species selected for buffer strips should meet the following criteria:

  • Native species may be used with careful selection
  • Species should be tolerant to frequent inundation as well as extended dry periods
  • Species should be resistant to matting
  • Species should form a dense cover
  • Avoid exotic, noxious, and invasive species

Construction

  • Buffer strips must be established before construction activity begins
  • In order to be effective, buffer strips must be densely established

Maintenance

  • Grassed vegetation should be cut and removed at least once per year
  • Mowing should only be performed during dry periods using lightweight equipment to prevent soil compaction and damage to vegetation
  • Buffer 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, and sediment accumulation
  • Buffer strips should be inspected for the accumulation of sediment after all major storm events

Method to Determine Practice Efficiency

Buffer strips filter out sediment and other particles by reducing the flow velocity of runoff. The trapping efficiency of this practice is dependant upon the particle size and the flow length of buffer strip. RUSLE2, when available, has the ability to calculate the approximate efficiency of vegetative buffer strips.

Buffer strips help remove suspended sediment from runoff by reducing the flow velocity. As the runoff velocity decreases, the sediment settles out. Buffer strips also help with reducing the amount of pollutants in the runoff since many pollutants are associated with the sediment. Studies have shown a suspended solid removal rate ranging between 40%-90%, with the efficiency of the buffer strip being dependent upon the quantity of runoff, length and steepness of the slope, as well as the vegetation used in the strip and the ability of the soil to infiltrate. Due to the number of variables affecting the performance of buffer strips, it is difficult to determine the exact efficiency of sediment removal for this practice.

References