Operation and maintenance of stormwater ponds

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

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

Pond maintenance activities range in terms of the level of effort and expertise required to perform them. Routine pond and wetland maintenance, such as mowing and removing debris or trash, is needed multiple times each year. Owners may consider an “adopt-a-pond” program in which properly trained citizen volunteers perform basic landscape maintenance activities (the City of Plymouth, for example, has instituted such a program). 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.

The following terminology is used throughout this section:

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the bioretention practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for bioretention practice performance but not critical to the design.

Design phase maintenance considerations

Implicit in the design guidance in the previous sections, many design elements of pond systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key maintenance considerations are providing access for inspection and maintenance, and designing all outlets and the principal spillway to minimize clogging.

Warning: Providing easy access (typically 8 feet wide) to all pond components for routine maintenance is Required.

Stormwater ponds can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations. 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 section on mosquito control).

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 practice is built in accordance with the approved design 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 at this link.

Post-construction operation and maintenance

Some important post construction maintenance considerations are provided below. More detailed maintenance guidance can be found in the Pond and Wetland Maintenance Guidebook (CWP, 2004).

Warning: It is Required that a maintenance agreement be executed between the BMP owner and the local review authority. It is Highly Recommended that this agreement be legally binding and enforceable.

• 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 ponds be inspected annually during winter freeze periods to look for signs of improper operation.
• It is Highly Recommended that sediment removal in the forebay and permanent pool occur every 2 to 7 years or after 50 percent of total forebay or permanent pool capacity has been lost. In areas where road sand is used, an inspection of the forebay and permanent pool should be scheduled after the spring melt to determine if clean-out is necessary.
• Sediments excavated from stormwater ponds 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 the section on Case studies).
• Periodic mowing of the pond 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.
• Ponds should not be drained during the spring, as temperature stratification and high chloride concentrations at the bottom can occur, which could result in negative downstream effects.
• Care should be exercised while draining the pond to prevent rapid release and minimize the discharge of sediments or anoxic water. The approving jurisdiction should be notified before draining a pond.

Example operation and maintenance checklist here.

Warning: It is Required that OSHA safety procedures be followed for maintenance activities within enclosed areas, such as outlet structures.

Operation to address frozen conditions

It is Highly Recommended that the Operation and Maintenance plan include a provision to lower the level of the permanent pool in the late fall, to provide additional retention storage for snowmelt runoff and ensure that some permanent pool storage is available above the ice (the permanent pool should not be completely eliminated nor allowed to freeze through completely).