Operation and maintenance of stormwater wetlands

Overview

Maintenance is necessary for a stormwater wetland to operate as designed on a long-term basis. The pollutant removal, channel protection, and flood control capabilities of stormwater wetlands will decrease if:

• Wetland pool elevations fluctuate dramatically
• Debris blocks the outlet structure
• Pipes or the riser are damaged
• Invasive plants out-compete the wetland plants
• Sediment accumulates in the stormwater wetland, reducing the storage volume
• Slope stabilizing and desirable wetland vegetation is lost
• The structural integrity of the embankment, weir, or riser is compromised.

Stormwater wetland maintenance activities range in terms of the level of effort and expertise required to perform them. Routine stormwater wetland maintenance, such as mowing and removing debris or trash, is needed multiple times each year, but can be performed by citizen volunteers. More significant maintenance, such as removing accumulated sediment, is needed less frequently but requires more skilled labor and special equipment. Inspection and repair of critical structural features such as embankments and risers, needs to be performed by a qualified professional (e.g., structural engineer) that has experience in the construction, inspection, and repair of these features.

Design Phase Maintenance Considerations

The following references may be consulted for more information on stormwater wetland maintenance:

• City of Plymouth, MN, Pond Maintenance Policy. 2005.
• Center for Watershed Protection, 2004. Stormwater Pond and Wetland Maintenance Guidebook.
• GIC, 1999. “Storm Water Management Facility Sediment Maintenance Guide”. TheToronto and Region Conservation Authority, Ontario Ministry of the Environment SWAMP Program. By: Greenland International Consulting Inc. August 1999.

Implicit in the design guidance in the previous sections, many design elements of stormwater wetland systems can minimize the maintenance burden and maintain pollutant removal efficiency. Primarily, providing easy access (typically 8 feet wide) to stormwater wetlands for routine maintenance is Required.

Mosquito control is of particular concern in the case of stormwater wetlands. They can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations, but no design will eliminate tem completely. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat. See Chapter 6 for an in-depth discussion of mosquito concerns in stormwater management.

Construction Phase Maintenance

The construction phase is another critical step where O&M issues can be minimized or avoided.

Inspections during construction are needed to ensure that the stormwater wetland is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used 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. An example construction phase inspection checklist is provided in Appendix D.

Post Construction Operation and Maintenance

Proper post-construction maintenance is important to the long-term performance of a stormwater wetland. Potential problems due to lack of maintenance include:

A clogged outlet structure can increase water levels, killing vegetation and reducing the wetland’s ability to attenuate and store floods. Water quality can be compromised by not providing adequate storage time.

Excess sediment can reduce storage volumes leading to many of the problems outlined above.

Nuisance issues such as beaver and muskrat burrows/dens can threaten the integrity of embankments.

Some important post construction maintenance considerations are provided below. A more detailed checklist of maintenance activities and associated schedules is provided in Appendix D. More detailed maintenance guidance can be found in the Pond and Wetland Maintenance Guidebook (CWP, 2004).

It is Required that a legally binding and enforceable maintenance agreement be executed between the practice owner and the local review authority.

Adequate access must be provided for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles. It is Recommended that a maintenance right of way or easement extend to ponds from a public or private road.

It is Highly Recommended that stormwater wetlands be inspected annually during winter freeze periods to look for signs of improper operation.

It is Highly Recommended that sediment removal in the forebay occur every 2 to 7 years or after 50% of total forebay capacity has been lost. In areas where road sand is used, an inspection of the forebay and stormwater wetland should be scheduled after the spring melt to determine if clean-out is necessary.

Sediments excavated from stormwater wetlands that do not receive runoff from confirmed hotspots are generally not considered toxic or hazardous material, and can be safely disposed by either land application or land filling. Sediment testing may be necessary prior to sediment disposal when a confirmed hotspot land use is present (see also Chapter 13).

Periodic mowing of the stormwater wetland buffer is Highly Recommended along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year), prairie, or forest.

General maintenance activities and schedule are provided in Table 12.WETL.5.