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Operation and Maintenance of Vegetated Filter Strips

Overview of Typical O&M Issues

Vegetated filter strips, also known as buffer strips or buffers, are vegetated land areas between a pollutant source and a surface water body. They can be stand-alone practices or used as a pre-treatment practice to other practices like bioretention or ponds. They reduce the flow velocity of water and filter and infiltrate pollutants such as sediment from stormwater. Vegetated filter strips may be subject to high public visibility, trash loads, sedimentation, pedestrian traffic, and even vehicular traffic or loads.

Vegetated filter strips require dedicated and regular maintenance to ensure proper and long-lasting operation, and in most cases a vegetated filter strip should be designed to be effective for at least 10 years (NRCS, 2010). The most frequently cited O&M concerns for vegetated filter strips include:

  • Flow channelization and erosion.
  • Insufficient/inadequate vegetative cover.
  • Sediment and debris accumulation leading to practice clogging.

The sections below describe best practices to prevent or minimize these common problems.

Design Phase O&M Considerations

Designers should design these practices in ways that prevent or minimize O&M issues. Examples include:

  • Using a maximum drainage area to filter strip area ratio of 6:1
  • Limiting the lateral slope to less than or equal to 1%
  • Checking that the maximum groundwater level is 2 to 4 ft below the filter strip
  • Providing a vegetation design plan, emphasizing erosion resistant and native plantings (see Plants for Stormwater Design) to enhance pollinator and wildlife habitat, improve infiltration and evapotranspiration, reduce urban heat island effect, provide optimized carbon sequestration, and provide climate adaptation. Site-specific plant selections should take into account sun exposure, shade, proximity to traffic corners (visibility issues), interior vs exterior plantings, salt-tolerant plants, etc. The selection of plantings suitable to their immediate surroundings will minimize long-term care and replacement frequency.
  • Designing the filter strip length based on the hydrologic soil group, slope, and vegetation (PADEP 2006).
  • Providing pretreatment (e.g. gravel trench spreader) if necessary. Pretreatment can be used to dampen the effects of high or rapid inflow, dissipate energy, provide additional storage, and prevent the BMP from becoming overloaded by sediment. Pretreatment is a required part of infiltration and filtration practices covered under the Minnesota Construction Stormwater General Permit. Vegetated filter strips are often used as a pretreatment practice to meet the intent of the stormwater permit, as identified in sections 16 and 17, but are designed with a different set of standards when they are a standalone structural practice.
  • Installing berms where necessary to direct the flow and prevent bypassing.
  • Prohibiting any kind of traffic through the installation of a visible barrier or signage.
  • Providing educational signage to increase public awareness.

Designers should consult and include any local requirements regarding green infrastructure. O&M considerations often depend on whether the practice is located on public land, private land, or in the public right of way. For example, plantings in the public right of way that conflict with any traffic safety considerations could require increased O&M, such as pruning or complete removal.

Designers should also recognize the need to perform frequent maintenance to remove trash or excess sediment, check for clogging, and if necessary perform practice cleaning. Designers can incorporate design solutions to facilitate maintenance activities. Examples include:

  • Incorporating multiple and easy site access points
  • Providing recommendations of vegetation appropriate to the location

The designer should also provide a site-specific O&M plan that includes the following:

  • Construction inspection schedule and checklists
  • Post-construction routine maintenance schedule and checklists
  • Operating instructions for the practice (if applicable)

For more design information for vegetated filter strips, click here.

Construction Phase O&M Considerations

Proper construction methods and sequencing play a significant role in reducing O&M problems. Some key items during the construction phase include:

  • Before construction begins:
    • Check the grading where the practice will be placed to enable sheet flow from the level spreader and throughout the filter strip. Grading must be uniformly perpendicular to the direction of flow to prevent channelized runoff.

Protect the proposed filter strip from the surrounding area by using upstream sediment traps or barriers and diversions to direct stormwater flows from the contributing drainage area away from the practice. Install any needed erosion and sediment controls in your construction site and prepare a storm water pollution prevention plan (SWPPP). Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control. Hold a pre-construction meeting with the designer and the installer to review the construction plans and the sequencing of construction. During construction: Construct any pretreatment devices first. Implement soil stabilization measures until permanent vegetation is established. Use as much of the existing topsoil on the site as possible to enhance plant growth. Prevent soil compaction by marking boundaries of the practice to ensure no construction traffic occurs on the practice. Foot traffic should be minimized if possible. Ensure that the plant and vegetation mix conforms to the vegetation design plan. Inspect the practice during construction to ensure that the vegetated filter strip practice is built in accordance with the approved design and standards and specifications. After construction: Verify that the vegetated filter strip practice was built in accordance with the approved design and standards and specifications. Verify that the filter strip area is fully stabilized with vegetation prior to removing any barriers, diversions, or erosion and sediment control measures. Verify that the practice directs flow as intended over the entirety of the practice. This serves as a verification that the practice is functional at the time of construction. Use a detailed inspection checklists that include sign-offs by qualified individuals at the completion of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided further down below. Review and discuss the plant warranty/establishment period with the plant provider to understand the conditions under which failing plants will be replaced. Determine if stormwater should be kept offline from the practice until the seedlings are established. The design/construction team should provide the O&M team with the following information to be included in the O&M plan: The plant warranty. The “as-built” plans of the practice A list of conditions that might cause failure of the practice if not properly maintained.