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Operation and Maintenance of Stormwater Treatment Wetland Practices

Overview of Typical O&M Issues

Stormwater treatment wetland practices are constructed depressions and pools planted with emergent aquatic or wetland vegetation. Wetlands typically remain wet due to relatively impervious sublayers and have retention times that allow pollutants to settle out of stormwater and into the sediment. The vegetated pools can provide excellent wildlife habitat, enhance nutrient cycling, sequester carbon, and offer aesthetic improvements to urban and rural areas if properly maintained.

Wetlands can provide stormwater and other ecosystem services for many years if they are properly maintained. Typical lifespans range between 25 and 75 years, depending on local conditions and ongoing maintenance to the systems (Torres et al., 2015). The most common O&M concerns for stormwater wetland practices include:

  • Erosion in the storage pools causing turbid effluent.
  • Clogged inlets and outlets. Clogs may occur from accumulation of trash, debris, and sediment.
  • Poor vegetative establishment.
  • Overgrowth of unwanted vegetation.

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

Design Phase O&M Considerations

Designers should design stormwater treatment wetland practices in ways that prevent or minimize O&M issues. In general, wetland designs should mimic natural wetlands and introduce few complex components. Design phase O&M considerations include:

  • Providing pre-treatment and trash racks to prevent clogging or trash accumulation.
  • Designing for weather and climate extremes.
    • Flooding and droughts can impact the ecosystem health if not anticipated.
    • Freezing conditions should be planned for in Minnesota (see Cold Climate Suitability). Inlets should not be submerged to avoid freezing. Outlet designs should include baffle weirs or other features to avoid ice formation.
  • Anticipating a multi-year establishment phase.
  • Shaping the wetland using existing topography and geology, if possible.
  • Introducing plants with deep roots along slopes to prevent erosion, emphasizing native species (see Plants for Stormwater Design).
  • Selecting plantings suitable for the designed water depth and climate patterns.
  • Considering the land use in the drainage area and selecting pollution tolerant species (e.g. salt tolerant species if heavy winter maintenance occurs in the drainage area).
  • Completing a planting list with species that will enhance green infrastructure co-benefits, such as pollinator and wildlife habitat, improve infiltration and evapotranspiration, reduce urban heat island effect, provide optimized carbon sequestration, and provide climate adaptation.
  • Considering whether vegetation will be established with seeds, seedlings, or transplants. The establishment period will be influenced greatly by this decision.
  • Providing easy access to infrastructure that will receive routine maintenance (e.g. inlet and outlet pipes).
  • Providing easy access points for all monitoring equipment. Right-of-way access should be designated when the wetland is setback from public or private roads. Right-of-way access should be at least 12 feet wide and minimal slope in order to stage larger vehicles and equipment.
  • Providing educational signage to increase public awareness.
  • Installing measures like low fencing to prevent damage from pedestrian foot traffic.

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, exterior plantings in the public right of way that conflict with any traffic safety considerations (e.g. sight lines) could require increased O&M, such as pruning or complete removal.

Designers should also recognize the need to perform frequent landscaping maintenance to remove trash, check for clogging, and maintain vigorous and healthy vegetation. Designers can incorporate design solutions to facilitate maintenance activities. Examples include:

  • Incorporating multiple and easy site access points
  • Installing observation wells
  • 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 stormwater wetlands practices, see the “design criteria for stormwater wetlands” page.

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:
    • Develop a planting schedule that meets the needs of the selected species. Planting during fall dormancy or early spring is usually successful.
    • Ensure that the contributing drainage area is fully stabilized with vegetation prior to the beginning of construction. Also make sure that impervious areas in the contributing drainage area are clean. If this is not possible, use barriers or 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 pre-treatment devices before constructing the main wetland area.
    • Decompact and prepare treatment beds. Allow an extended wetted settling period to soften up soil if possible.
    • Ensure heavy equipment does not enter the footprint of the practice after any vegetation has been seeded or planted.
    • Store any soil or gravel media downstream of the practice footprint to avoid filling in any depressions and clogging any inlets and pools. If this is not possible, store soil or gravel media in some type of covered or contained structure.
    • Inspect the practice during construction to ensure that it is built in accordance with the approved design and standards and specifications. This includes verification of the media composition and depths. Use a detailed inspection checklists that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. Example construction phase inspection checklists are provided further down below.
    • Ensure that the plant and vegetation mix conforms to the vegetation design plan, particularly if the vegetation was selected to provide ecological function (such as pollinator habitat).
  • After construction:
    • Verify that the wetland was built in accordance with the approved design and standards and specifications, including the pre-treatment devices as well as the main practice.
    • Verify that the contributing drainage area and any slopes are fully stabilized with vegetation prior to removing any barriers, diversions, or erosion and sediment control measures.
    • Verify that the inlet structures capture the intended runoff. Conduct a full inundation test to inspect the outflow and any bypass functions.
    • Verify that the practice reduces nutrient loads. Collect inflow and outflow storm water samples and have them analyzed for nutrient concentrations.
    • Use a detailed inspection checklist that includes 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. **Example construction phase inspection checklists are 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.