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
• 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.

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 in the table below.

Stormwater pond/wetland - construction inspection checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Project:
Location:
Site Status:
Date:
Time:
Inspector:
Construction Sequence	Satisfactory/Unsatisfactory	Comments
1. Pre-Construction/Materials and Equipment
Pre-construction meeting
Pipe and appurtenances on-site prior to construction and dimensions checked
1. Material (including protective coating, if specified)
2. Diameter
3. Dimensions of metal riser or pre-cast concrete outlet structure
4. Required dimensions between water control structures (orifices, weirs, etc.) are in accordance with approved plans
5. Barrel stub for prefabricated pipe structures at proper angle for design barrel slope
6. Number and dimensions of prefabricated antiseep collars
7. Watertight connectors and gaskets
8. Outlet drain valve
Project benchmark near pond site
Facility location staked out
Equipment for temporary de-watering
Temporary erosion and sediment control in place
2. Subgrade Preparation
Area beneath embankment stripped of all vegetation, topsoil, and organic matter
3. Pipe Installation
Method of installation detailed on plans
A. Bed preparation
Installation trench excavated with specified side slopes
Stable, uniform, dry subgrade of relatively impervious material (If subgrade is wet, contractor shall have defined steps before proceeding with installation)
Invert at proper elevation and grade
B. Pipe placement
Metal / plastic pipe
1. Watertight connectors and gaskets properly installed
2. Anti-seep collars properly spaced and having watertight connections to pipe
3. Backfill placed and tamped by hand under “haunches” of pipe
4. Remaining backfill placed in max. 8 inch lifts using small power tamping equipment until 2 feet cover over pipe is reached
Concrete pipe
1. Pipe set on blocks or concrete slab for pouring of low cradle
2. Pipe installed with rubber gasket joints with no spalling in gasket interface area
3. Excavation for lower half of anti-seep collar(s) with reinforcing steel set
4. Entire area where anti-seep collar(s) will come in contact with pipe coated with mastic or other approved waterproof sealant
5. Low cradle and bottom half of anti-seep collar installed as monolithic pour and of an approved mix
6. Upper half of anti-seep collar(s) formed with reinforcing steel set
7. Concrete for collar of an approved mix and vibrated into place (protected from freezing while curing, if necessary)
8. Forms stripped and collar inspected for honeycomb prior to backfilling. Parge if necessary.
C. Backfilling
Fill placed in maximum 8 inch lifts
Backfill taken minimum 2 feet above top of antiseep collar elevation before traversing with heavy equipment
4. Riser / Outlet Structure Installation
Riser located within embankment
A. Metal riser
Riser base excavated or formed on stable subgrade to design dimensions
Set on blocks to design elevations and plumbed
Reinforcing bars placed at right angles and projecting into sides of riser
Concrete poured so as to fill inside of riser to invert of barrel
B. Pre-cast concrete structurer
Dry and stable subgrade
Riser base set to design elevation
If more than one section, no spalling in gasket interface area; gasket or approved caulking material placed securely
Watertight and structurally sound collar or gasket joint where structure connects to pipe spillway
C. Poured concrete structure
Footing excavated or formed on stable subgrade, to design dimensions with reinforcing steel set
Structure formed to design dimensions, with reinforcing steel set as per plan
Concrete of an approved mix and vibrated into place (protected from freezing while curing, if necessary)
Forms stripped & inspected for honeycomb prior to backfilling; parge if necessary
5. Embankment Construction
Fill material
Compaction
Embankment
1. Fill placed in specified lifts and compacted with appropriate equipment
2. Constructed to design cross-section, side slopes and top width
3. Constructed to design elevation plus allowance for settlement
6. Impounded Area Construction
Excavated / graded to design contours and side slopes
Inlet pipes have adequate outfall protection
Forebay(s) constructed per plans
Pond benches construction per plans
7. Earth Emergency Spillway Construction
Spillway located in cut or structurally stabilized with riprap, gabions, concrete, etc.
Excavated to proper cross-section, side slopes and bottom width
Entrance channel, crest, and exit channel constructed to design grades and elevations
8. Outlet Protection
A. End section
Securely in place and properly backfilled
B. Endwall
Footing excavated or formed on stable subgrade, to design dimensions and reinforcing steel set, if specified
Endwall formed to design dimensions with reinforcing steel set as per plan
Concrete of an approved mix and vibrated into place (protected from freezing, if necessary)
Forms stripped and structure inspected for honeycomb prior to backfilling; parge if necessary
C. Riprap apron / channel
Apron / channel excavated to design cross-section with proper transition to existing ground
Filter fabric in place
Stone sized as per plan and uniformly place at the thickness specified
9. Vegetative Stabilization
Approved seed mixture or sod
Proper surface preparation and required soil mendments
Excelsior mat or other stabilization, as per plan
10. Miscellaneous
Drain for ponds having a permanent pool
Trash rack / anti-vortex device secured to outlet structure
Trash protection for low flow pipes, orifices, etc.
Fencing (when required)
Access road
Set aside for clean-out maintenance
11. Stormwater Wetlands
Adequate water balance
Variety of depth zones present
Approved pondscaping plan in place and budget for additional plantings
Plants and materials ordered 6 months prior to construction
Construction planned to allow for adequate planting and establishment of plant community (April-June planting window)
12. Final Inspection
Construction sediment removed from settling basin
Contributing drainage area stabilized
Vegetation established per specifications
Inlet and outlet structures operational
Comments:
Actions to be Taken:

Post-construction operation and maintenance

Operation to address frozen conditions

It is Highly Recommended that the O&M 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).

Maintenance

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 BMP 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 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 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 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 Chapter 13).
• 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.
• It is Required that OSHA safety procedures be followed for maintenance activities within enclosed areas, such as outlet structures.

The next section in the chapter addressing stormwater ponds is Monitoring. To proceed to that section, click here.

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