Finding Depths

If you've seen Cuyahoga SWCD staff out and about with a fishing rod in your community lately, never fear, we aren't fishing for our lunch, we are actually assessing storm water basin depths. With the expansion of our storm water facility long-term operation and maintenance program this year we have had a significant upturn in follow-up assistance requests. One of the common requests we have received is for an estimate of basin depth.

Storm water basins are designed to hold a volume that can control for typical storm events. Once the basin accumulates over 20% sediment, it loses the capacity to treat its required storm water volume and needs to be dredged. As flood control basins reach a lifespan of 10-20 years, dredging may be needed due to accumulation of sediment. Unlike some of the routine maintenance needed for storm water facilities, dredging is a non-routine maintenance need that can be costly and should be budgeted for accordingly. So it isn't surprising that basin owners want to know how you can tell when the basin needs dredging.

As a courtesy to basin owners, Cuyahoga SWCD invested in a depth finder this year and can provide a rapid field assessment to get an estimate of basin depths, which can then be compared to basin design depths. This depth finding tool can provide a rough estimate of the amount of sediment that has accumulated in the basin, which can help basin owners decide whether they should do a formal bathymetric survey, start looking for dredging contractors, or start a rainy day fund for dredging a few years in the future.

While the depth finder does not substitute for a formal bathymetric survey, it does provide a good estimate to get a basin owner started. The tool is based on sonar, and looks like a fishing bobber. You can toss the bobber out and reel it back in or use it behind a boat to map the basin waterbed. The bobber is used with an app to map the waterbed. The app creates a screen shot of depth in feet versus length in feet (based on your reel rate) while the bobber is reeled in.

Cuyahoga SWCD saves the waterbed map and records data about location of the measurements and the date measurements were taken. It is also important to note the water level on the day measurements are taken versus a known elevation in the basin (like an inflow or outflow structure). The use of this data with the plans for the basin can help estimate the amount of sediment accumulation in the basin.

As many basins in the community are reaching that 10-20 year lifespan, and many basins in-line with streams are experiencing higher sedimentation rates, Cuyahoga SWCD has found the depth finder to be a useful tool for basin owner assistance.

Blog Author, Elizabeth Hiser, Natural Resouces Coordinator

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