MANSFIELD — How deep is the Clear Fork Reservoir, the 4.4 billion gallon body of water that supplies most of the drinking water for the City of Mansfield?
The best answer is: It depends on where you measure.
Thanks to a bathymetric survey planned this year by Stantec Consulting Services, the city will have a good idea of exactly what’s under the water at the reservoir, which was completed four years after the end of World War II (1949).
The city’s Board of Control on Tuesday morning approved a $149,779 contract with the southwest Ohio company to study what sediment has settled over the years at the bottom of the reservoir seven miles southwest of the city.
City engineer Bob Bianchi said the funding was previously approved by City Council and is in the 2025 water fund budget.
“We want to determine how much sedimentation has occurred over the life of the reservoir for the past 75 years … looking at the sedimentation rates we know what we can expect going forward,” he said.
A bathymetric survey, sometimes referred to as a fathometric survey, is a type of hydrographic (water-based) survey that maps the depths and shapes of underwater terrain to illustrate the land that lies below.
The reservoir supplies about 60 percent of the city’s drinking water on an average day with the remainder coming from city-owned wells.
“We want to look at our water storage capacity and determine if we have any concerns moving forward,” Bianchi said. “The reservoir is very important and vital to our ability to convey drinking water to the city.”
Bianchi said he had found evidence in the city engineering department regarding discussions of sediment in the reservoir in the 1970s. No in-depth study was ever done, he said.
The work on the 997 acres of surface water will take about 10 months to complete. What comes next depends on the results, including identifying the potential need to remove sediment from the bottom.
“The study will highlight options, if (sediment) removal is necessary … how would you go about removing the sedimentation, whether it be by dredging or other means.
“They will also provided a high-level cost estimate, as well as different techniques for removal,” Bianchi said.
An estimate of such costs would be influenced by the kinds of material found in the sediment.
Stantec was the only company to submit “qualifications” for the reservoir projects, according to Bianchi.
“We have worked with Stantec before, as well. They helped us on the dam seepage project in 2023,” he said in November.
Bianchi said the four goals of the study would be to determine how much silt has collected, the composition of the silt, whether it makes sense to remove it, and the best method to use in removing it.
He said the silt enters the low-flow reservoir from streams and tributaries within a 35-mile drainage area.
“There is a lot of energy in the streams to carry that silt, but there is not as much energy in the reservoir, so the silt will drop to the bottom,” Bianchi said in November.
“With the silt, we are losing water storage and that is a drinking water facility. We want to make sure we have enough drinking water for the next 100 to 200 years to serve the Mansfield area,” he said.
