Operation and maintenance (O&M) of stormwater treatment wetland practices
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Operation and maintenance (O&M) of stormwater treatment wetland practices

Green Infrastructure: Wetlands are an important consideration for stormwater management considerations. Because of the diverse environment that can inhabit a wetland, they can be beneficial for improving water quality through sediment settling, groundwater infiltration, and attenuating flow velocity.

Example of a stormwater wetland in a suburban area.

This page provides guidance for operation and maintenance (O&M) of stormwater treatment wetland practices. These include any constructed basin designed to treat stormwater through sedimentation but that additionally provide other green infrastructure benefits.

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 retain stormwater long enough to allow pollutants to settle out of stormwater and into the sediment. The vegetated pools can provide excellent wildlife and aquatic habitat, enhance nutrient cycling, sedimentation, 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 the following.

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.

Post-construction phase O&M

Effective short and long-term operation of stormwater treatment wetland practices requires dedicated and routine maintenance. Proper maintenance will not only increase the expected lifespan of the facility but will improve ecological function, aesthetics, and property value. Important post-construction considerations are provided below.

A site-specific Operations and Maintenance Plan should be prepared by the designer prior to putting the stormwater practice into operation. This plan should provide any operating procedures related to the practices. The plan should also provide clear maintenance expectations, activities, and schedules. Include photos if possible. Be clear about who is responsible for the maintenance and the type of expertise that will be needed for distinct O&M activities. The O&M plan should include an anticipated budget for O&M activities.The O&M plan should also include an example O&M inspection checklist and an example maintenance report. Example O&M plans and inspection checklists are provided further down below.
A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority. Example maintenance agreements are provided further down below.
Inspection and maintenance activities are distinct and can be done as separate activities or together. Inspection will typically assess the practice for any O&M issues, whereas maintenance will address the O&M issues identified by the inspection. A dedicated inspection effort on a large number of BMPs can help prioritize maintenance activities.
Maintenance activities should be careful not to cause compaction or damage to the vegetation. No vehicles or stockpiling should be allowed within the footprint of the practice. Foot traffic should be kept to a minimum.
Maintenance activities should apply to all parts of the treatment wetland, including the pre-treatment devices, the main storage and pooling area, the vegetation, the soil, and any conveyance or discharge pipes.
Wetlands areas generally should not be used as dedicated snow storage areas.

Overview and schedule of general maintenance activities for wetlands

Link to this table

First Year of Operation
Activity	Frequency	Time Period	Level of Effort	O&M Benefit^a
Check that there is no ponding in the pre-treatment device.	At least twice after storm events > 0.5 inches	Within the first 6 months	1-2 hours	1
Check for evidence of clogging in the pre-treatment device and/or in any conveyance structures	At least twice after storm events > 0.5 inches	Within the first 6 months	1-2 hours	1
Maintain proper water level for establishing vegetation (may require supplemental source)	1/week initially	During first 2 months	1-2 hours	2,3,4,5,6
	As needed	First growing season	1-2 hours	2,3,4,5,6
Remove any stormwater diversion or barriers once seedlings are established.	Once	When plants are sufficiently established	1-2 hours	1,2,3,4,5
Install protective barriers (e.g. chicken wire) to protect growing vegetation from wildlife	As needed	First growing season	8 hours	1,2,3,5
Check for signs of burrowing animals and incorporate gravel or rock in problem areas to discourage burrowing.	As needed	First growing season	1-2 hours	5
Check that water is moving as planned through the wetland. Identify and fix any stagnant zones.	As needed	During first 2 months	4 hours	1
Mow dikes frequently to establish deep rooted ground cover	Biweekly or as needed	First growing season		3,4
Remove and replace dead plants	As needed	First growing season	2-4 hours	2,3,4,5,6
Spot reseeding of bare patches and eroding areas	As needed	First growing season	2-4 hours	1,2,3,4,5,6
After First Year of Operation
Activity	Frequency	Time Period	Level of Effort per Visit	O&M Benefit^a
Check that there is no ponding in the pre-treatment device	Biannually	Any time when ground is not frozen	1-2 hours	1
Check for evidence of clogging in the pre-treatment device and/or in any conveyance structures.	Biannually	In spring and fall	1-2 hours	1
Remove trash and debris from the pre-treatment device and/or in the main treatment area.	Monthly	All year long	1-2 hours	1,5
Inspect for and remove excess sediment in the pre-treatment device and/or in the main treatment area.	Monthly	All year long	4 hours if removal is needed	1
Check and repair eroded areas	Annually	In fall or spring when vegetation has died down	4 hours if repairs are needed	1,2,3,4,5
Weed and remove invasive plants^b	Twice during growing season	During growing season	1-2 hours	2,3,4,5
Inspect plant composition and health and replace as needed to maintain >50% coverage.	Biannually	In fall or spring	4-8 hours, more if plant replacement is needed	2,3,4,5,6
Remove woody vegetation on or near embankments, forebays, spillways, and outlets	Annually	In fall or spring	4 hours	1,2,3,4,5,6
Check for signs of burrowing animals and incorporate gravel or rock in problem areas to discourage burrowing.	As needed	During growing season	1-2 hours	1,2,5
Inspect for and repair broken inlets, pipes, or other water control structures	Biannually	After high flows and ice breakup	2 hours, more if repairs are needed	1
Mow dikes and remove any shrubs or trees growing on dikes	As needed	During growing season	1-2 hours	1,5
Monitor operating range of water level	Daily (collect data continuously, use information to diagnose issues)	Any time	1-2 hours	1,2,4,6
Check that water is moving as planned through the wetland. Identify and fix any stagnant zones	Annually	During growing season	4 hours	1,2,3,4,5
Inspect sediment accumulation in forebay and treatment areas. Remove if hydrology is impacted or if less than 50% of forebay capacity remains.	Annually	Fall	2 hours	1,4,6
Harvest wetland plants	Once every 2-3 years	After growing season	4-8 hours	1,4,5,6
Remove sediment from forebay	Once every 2-7 years, or when less than 50% forebay capacity remains	As needed	Varies	1,4,5,6
After 5+ Years of Operation (non-routine maintenance)
Activity	Frequency	Time Period	Level of Effort per Visit	O&M Benefit^a
Remote inspection of all conveyance structures (e.g. pipes, underdrains, etc.)	Every 10 years	Any time	Varies	1
After long term operation of the practice, some occasional and infrequent maintenance activities might be required, such as bigger repairs or redesign of key elements of the practice. Replacement of concrete components (e.g. pipes, risers, may be required after 25-50 years.	As needed	As needed	Could be significant depending on the activity	1,2,3,4,5,6
^aKey to Maintenance Benefits: Proper stormwater flow and infiltration Creation and maintenance of wildlife habitat Creation and maintenance of pollinator habitat Nutrient cycling and storage Aesthetics and public enjoyment Carbon sequestration ^bRemove invasive species immediately and replace vegetation as needed. As a general rule, control of undesirable invasive species (e.g., cattail and Phragmites) should commence when their coverage exceeds more than 15% of a wetland cell area. Although the application of herbicides is not recommended, some types, such as Glyphosate, have been used to control cattails with some success. Extended periods of dewatering may also work, since early manual removal provides only short -term relief from invasive species.

Common problems and how to troubleshoot them for wetlands
Link to this table

Symptom	Possible causes	Solution
Vegetation will not establish properly	Water levels need to be maintained per species specific growth patterns during establishment. A secondary water source and flow routing may be necessary to establish vegetation for a year. Soils may not have been properly decompacted during construction. Influent pollutant loads may be harming unestablished vegetation. Wildlife may harm unprotected vegetation.	Consult with the designer to check what water depths should be maintained for the planted vegetation. Consult an engineer to ensure proper decompaction occurred during construction. Install chicken wire or other barriers to deter wildlife from young plantings. Reroute or bypass some of the stormwater carrying harmful pollutants while plantings mature (if pollution is deemed the primary cause). Review nearby winter maintenance activities to see if runoff with high concentrations of chloride is reaching the practice. Vegetative maturation may take two growing seasons.
Stagnant water	Significant erosion that has impacted hydraulics. Clogs in the conveyance network. Grading issues.	Remove any sediment or soil that has sluffed into the treatment pools due to erosion. Check and remove debris or clogs from all conveyance structures. Consult the designer or an engineer to identify grading or other hydraulic issues.
Mosquitos	Optimal breeding conditions	Address stagnant water Modify water level controls to increase velocity Introduce predators (fish and/or invertebrates)
Wetland capacity is reduced	Sediment build up or vegetative overgrowth	Remove accumulated sediment Harvest vegetation
Flow or water imbalance (more water coming in than going out), lower than expected water levels.	Unanticipated infiltration	Check the integrity of any impermeable barriers or linings that were installed beneath the growth media.

Maintenance costs

Maintenance costs will vary on a number of factors, including but not limited to:

Size of the practice and its contributing drainage area
Type of plantings used
Site visit frequency
Level of maintenance needed
Local weather conditions
Staffing needs (number of staff, external vs. internal staff, etc)
Travel time between sites
Efficiencies of scale (single GI vs. a cluster of GI)
Equipment needed

Preventative maintenance is key to minimizing major costs associated with repairs. A general rule of thumb for annual GI maintenance is 3%-6% (Clary, 2017). Maintenance costs may be higher the first few years, while plants are being established or modifications to the design are required. Sediment removal may also be the most costly maintenance activity. A 2015 report by E2 Storm Project in Europe recently estimated maintenance costs to be between $41 and $72 per cubic foot of storage based on case studies, including those from the United States (Torres, 2015).

Useful resources

References

Torres, A.M., Perales-Momparler, S., Jefferies, C., Andrés-Doménech, I., Escuder-Bueno, I., Gómez, Á.P.N., López, E.P., Solar, D.A., Wade, R., Berwick, N. and Duffy, A., 2015. Report on stormwater management.

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