This page provides information on construction specifications for pretreatment vegetated filter strips.
Contents
Construction
During construction, the ground must be stabilized, accurately graded, protected, and made suitable for healthy vegetation establishment.
Site erosion and sediment control
During construction, exposed soil will require soil stabilization and sediment-control measures. Erosion control is particularly important if the pretreatment vegetated filter strip is being installed upstream of an existing full-treatment BMP. If sediment is not contained within the construction boundaries, excess soil will clog the downstream BMP and require additional cleanup and maintenance. Measures that can be considered for soil stabilization include mulching and erosion-control blankets. When mulching is used, either straw or woodchips should be used while achieving at least 90 percent soil coverage. Erosion-control blankets and mulch are also used to provide structural stability to the soil during seed application to aid in vegetation establishment. For detailed construction site stabilization requirements, see the MPCA construction stormwater program page online.
Sediment-control measures will be used along with soil stabilization and includes BMPs such as silt fences, rock checks, bio rolls, drainage swales, and sediment traps. In addition to soil stabilization and sediment-control measures, managing soil stockpiles and sweeping tracked sediment on pavement near the construction site is required. If construction occurs in late summer to fall and is expected to carry into the winter, ensure that all of the exposed areas are covered before the first freeze. For more information on general erosion-control practices, see the Erosion Prevention Practices website.
Areas that will receive flows from impervious surfaces need to receive additional erosion-control protection to ensure that flows do not cause erosion while vegetation is being established. An aggregate of #57 stone with a depth of 3 to 4 inches North Carolina State Cooperative Extension Service, 2006 may be used at the leading edge or entrance to the filter strip as a buffer to slow flow velocity but should not be included in the vegetated filter strip length.
Grading
For pretreatment vegetated filter strips to be effective, the ground must be uniformly sloped perpendicular to the flow (i.e. zero lateral slope) to prevent channelization of runoff. This slope includes the slope upstream of the pretreatment vegetated filter strip (or installing a level spreader), as well as the pretreatment vegetated filter strip itself. During the grading process, efforts should be taken to minimize and reduce soil compaction to the best degree, as well as stabilizing any exposed soil. See the section on Site erosion and sediment control.
Compaction prevention and alleviation
To prevent soil compaction, ensure that no construction traffic occurs on the site of the pretreatment vegetated filter strip and that foot traffic is minimized as much as possible. Marking the boundaries of the pretreatment vegetated filter strip with paint and/or stakes can help keep away traffic. If compaction occurs or soil at the site is already compacted, measures can be used to alleviate the soil compaction. Methods indicated by the MPCA include soil ripping, scarifying, or mixing organic matter with the soil. These methods are often combined. For additional information, see the MPCA’s website.
Vegetation establishment
Establishing full and healthy vegetation is vital to ensuring that the pretreatment vegetated filter strip will perform as designed. For the best results, weeds should be removed from the bare soil before planting. To ensure that vegetation will become established and survive the winter freeze, a minimum of 60 days before the first expected frost should be allowed for the vegetation to grow. Soil stabilization practices such as erosion-control blankets should be used during vegetation establishment. To avoid damaging the emerging vegetation and causing site compaction, vehicle and foot traffic should not be allowed during the vegetation establishment phase. Achieving a minimum of 80 percent cover on the pretreatment vegetated filter strip, per the design guidelines, will ensure that the site is stabilized after construction. Following rain events, additional inspections and possible maintenance on the full-treatment BMP downstream of the pretreatment vegetated filter strip is recommended during the vegetation establishment phase.
Post-construction
After construction is completed, the boundary that marks the extents of the pretreatment vegetated filter strip must be maintained during vegetation establishment to keep away vehicle and foot traffic. Maintaining this boundary during vegetation establishment will prevent soil compaction and damage to the vegetation. Once the vegetation is fully established, the boundary can be removed, but foot traffic should still be minimized.