MS4 inspection guidelines from the Minnesota Pollution Control Agency require communities to do the following:

  • Inspect stormwater basins once every five years
  • Develop a standard operating procedure for inspecting and maintaining stormwater BMPs
  • Determine treatment effectiveness of stormwater basins

(See the WSBpedia article “Stormwater Ponds: Tips for inspecting them, and what to do when you find something”, for more details on what goes into inspecting a stormwater basin or pond.)

A pond can be scheduled for maintenance due to inlet/outlet concerns, vegetation management, or erosion issues.  However, it is most often due to sediment accumulation. Over time as stormwater ponds accumulate sediment, the volume that is meant for water quality treatment is reduced and the ponds begin to fall short of their potential pollutant removal capabilities. WSB has developed a cost-effective design process for communities to perform maintenance projects on their stormwater basins that results in improved water quality and volume control. 

Step 1

First, stormwater ponds suspected of sediment accumulation during routine inspections are surveyed.  This survey includes average water depth, average sediment accumulation based on record drawings, inlet and outlet sizes and location, and wet pond area.

Steps 2 & 3

Maintenance is prioritized and budgeted based on the survey results from Step 1. Communities who utilize WSB’s Stormwater Asset Management Program (SWAMP) can update their SWAMP data for the inspected stormwater ponds (see the WSBpedia article Stormwater Asset Management Program (SWAMP) for more details). SWAMP assists in prioritizing maintenance for ponds based on pollutant removal, cost benefit analysis, and other parameters. 

Once ponds are selected for maintenance, sediment from the stormwater ponds is tested and compared to MPCA’s Remediation Division soil reference values (SRVs) to determine if excavated sediment can be re-used or if it must be disposed of at landfill. For more details on the MPCA’s guidelines for managing stormwater sediment. Testing stormwater sediment is an important step in designing stormwater pond maintenance construction plans because, depending on the SRV, it can greatly affect the cost of excavation for the project.

Step 4

Construction plans are developed based on the pollutant removal and cost/benefit analysis. The plans reflect the final pollutant removal goals based on a measured amount of excavated material. Plans also take into account factors such as access location, staging area, temporary erosion and sediment control BMPs, tree and vegetation removal, inlet/outlet improvements, dewatering/deicing, and final restoration of the pond.

Step 5

Once the plans are created, construction can be completed. Winter construction is often recommended as it can reduce risk of flooding and erosion caused by rainfall. Additionally, frozen ground can make for easier access for trucks and heavy machinery.  Dewatering is often less complicated in winter (if needed at all) due to ponds being iced over and the frozen ground reduces the potential for turbidity of water during dewatering. 

Performing stormwater pond maintenance projects where necessary is a key way to meet measurable goals and uphold the good housekeeping responsibilities required for MS4 permit holding communities. When maintained properly, stormwater ponds not only have the ability to improve water quality in a watershed, but can also provide a desirable aesthetic feature to a neighborhood.

References

https://www.pca.state.mn.us/sites/default/files/wq-strm4-05.pdf

https://www.pca.state.mn.us/sites/default/files/wq-strm4-16.pdf