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The changes are coming in, and some are pretty significant.
Clarke County Reservoir Commission (CCRC) was presented an engineering task order during its meeting Thursday, Dec. 17, for planning a reduced-size reservoir.
“So, we’ve spent the last few months really getting the NRCS the information, updating their information, adding new and really doing a new analysis with the previous watershed work that NRCS had done,” said Mark Duben, water section manager and vice president of HDR Inc. of Des Moines, while presenting the task order.
Original plan
The original plan was for CCRC to build a reservoir project located in northwest Clarke County.
The reservoir was to provide a water supply for Osceola and Southern Iowa Rural Water Association (SIRWA) with an 816-acre lake, which could provide 2.2 million gallons of water per day.
West Lake, the current water source, doesn’t meet the needs of Clarke County and SIRWA.
Funding for the project includes many sources, especially funding from the local-option sales tax.
However, there have been many legislative setbacks and issues with landowners and eminent domain, which has caused CCRC to look into reducing the design of the reservoir.
Modified design
This is the task order for hydrology and hydraulics for a reduced-size pool at dam site 4B that was presented during the CCRC meeting.
Principal and auxiliary alternatives were investigated to determine the maximum size of a normal pool that can be feasibly developed given the commission’s current land ownership constraints at dam site 4B, which may require the original design top of dam elevation of 1,021 be reduced to constrain the maximum pool level to downstream of 180th Avenue and Osage Street.
This corresponds to a top of dam elevation of approximately 1,005 feet. A reduced top of dam elevation results in reduced flood storage for a given normal pool elevation and requires higher principal and auxiliary spillway capacities to safely pass the design rainfall events.
A hydraulic analysis of principal and auxiliary spillway configuration was performed and the maximum feasible normal pool elevation was determined to be approximately 990 feet. This results in a normal pool area of approximately 509 acres and safe withdrawal capacity of approximately 1.2 million gallons per day (MGD).
The modified principal and spillway design includes two 96-inch principal spillway conduits and a concrete labyrinth weir auxiliary spillway. A labyrinth weir structure allows for increased flow across the spillway per unit length when compared to the original grassed spillway design.
Big change
One of the most drastically changed design features was the volume of the available water supply (acres-feet). The original design had 11,780. The new modified design has 3,850.
“That’s unacceptable,” said Mayor Fred Diehl, who represents the city of Osceola on the CCRC.
The available water supply in million gallons per day went from also went from 2.2 million to 1.2 million.
“Our concern was we don’t go to the expense and trouble of building a reservoir was to make sure our children aren’t looking at us 20 years from now saying why in the hell did they build that reservoir? Excuse my language. Why didn’t they build that reservoir larger? What were they thinking if they were going to all that trouble?” said CCRC member Bill Trickey.
A full report, including expense, costs and background, should be available at the next CCRC meeting, which is planned 9 a.m. Thursday, Jan. 17.
“This was kind of our attempt at what still was maybe within the realm of feasibility,” Duben said. “It’s still a lot more expense, but to get anything beyond what we’ve got here is — takes us even to another level of expense and extreme design.”