Jeremy Siudzinski, Michael Spies, & Tony Graham
City of Minneapolis Department of Public Works, Surface Water & Sewers Division, Regulatory Team
Logo for City of Minneapolis
Background
- Beyond flood control, urban stormwater ponds are designed to capture and retain sediment, nutrients, and other pollutants.
- Sedimentation rates vary between stormwater ponds both across the landscape and through time as contributing areas are impacted by construction and development.
- Close monitoring and adequate maintenance are critical to successful stormwater management.
- Sediment accumulation is the primary driver of maintenance for stormwater ponds, since the storage capacity is what allows ponds to provide volume, rate, and water quality treatment.
- Bathymetric surveys are an effective way to monitor sediment accumulation over time.
Goal: Conduct a bathymetric survey of City-owned stormwater ponds in Minneapolis to identify sites of sediment accumulation and prioritize maintenance activities.
Methods
- Fishnet grids were created in ArcGIS Pro for each stormwater pond. Cell sizes were based roughly on pond area.
- Benchmark points were selected from the City of Minneapolis Survey Monument Viewer or NOAA’s National Geodetic Survey Map and surveyed using a Trimble R2 GPS unit to confirm the unit’s accuracy.
- Water surface elevations were measured using the Trimble unit at each stormwater pond
- Using the Trimble and an iPad running ArcGIS Field Maps, each fishnet grid cell was accessed by boat or using waders.
- Depth to sediment and depth to refusal measurements were taken in each cell using a survey rod.
- Recorded depths were converted to elevations using the water surface elevation as a reference.
- TINs were created in ArcGIS using sediment elevations and refusal elevations.
- Volumes were calculated and compared with contractor-surveyed bathymetric conditions from 2020.
Results
A map shows the location of the stormwater ponds surveyed. Caption reads: “Figure 1. Stormwater pond sites surveyed”
Two photos of pond surveying, one showing two people on the bank of a pond with survey equipment, and the other showing a person on a boat. Caption reads: “Figure 2. Inspectors collecting depth to sediment and depth to refusal measurements using survey rod and boat”
An aerial photo of Camden pond displays the data points collected. Caption readds: “Figure 3. Fishnet grid and locations of collected data points at Camden Pond”
An aerial photo of Camden pond displays elevation of the pond bottom. Caption reads: “Figure 4. Refusal elevation TIN showing bottom of Camden Pond”
An aerial photo of Campdon pond displays sediment thickness in feet. Caption reads: “Figure 5. Sediment accumulation TIN showing difference between refusal elevation and sediment elevation at Camden Pond”
A table displays data collected in 2020 and 2025. In 2020, measured total pond volume in cubic feet was 797,664, measured sediment volume in cubic feet was 49,418, and measured percent accumulation (sediment volume/pond total volume) was 6.40%. In 2025, measured total pond volume in cubic feet was 815,353, measured sediment volume in cubic feet was 65,816, and measured percent accumulation (sediment volume/pond total volume) was 8.07%. Caption reads: “Figure 6. Comparison of surveyed volumes of Camden Pond.”
Discussion and Next Steps
- Sedimentation measurements will inform the City’s Capital Improvement Plan (CIP) and allow us to prioritize dredging, retrofits, and other projects. Maintenance timelines and strategies will be data-driven.
- Comparing sediment volumes and pond capacities with previous survey measurements allows us to estimate accumulation rates.
- Future bathymetric surveys will continue to improve our understanding of these stormwater ponds.
- This project used the same methodology as the 2020 survey. Future surveys and supplemental field verifications may explore new technologies.