The costs for a newly designed reservoir project in northern Clarke County are, to say it lightly, eyebrow-raising.
Mark Duben, water section manager and vice president of HDR Inc. of Des Moines, presented a cost analysis and background information on the task order during a Clarke County Reservoir (CCRC) meeting Thursday, Jan. 21.
Duben reported the alternative spillway concept resulted in an estimated cost increase of nearly $22 million, including structural and overall construction contingencies.
“Trying to do something significantly smaller at the same site and handling the same flows is the result of this analysis,” Duben said. “It’s a lot more money for a lot less water stored.”
Basically, the Squaw Creek watershed area was chosen for a reason with the original reservoir design, and significantly altering it while trying to still gain the same results, can skew the costs.
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.
New task order findings
This is the task order for hydrology and hydraulics for a reduced-size pool at dam site 4B.
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. These constraints 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 alternative 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 changes
The total construction costs for the original reservoir design was set at $728,000. The alternative design jumped to $22,690,000.
Another drastically changed design feature was the volume of the available water supply volume, measured in acres-feet. The original design had 11,780 acres-feet. The new modified design has 3,850 acres-feet.
The available water supply in million gallons per day went from 2.2 million to 1.2 million.
“(The) bottom line is it’s about $22 million of concrete extra – above and beyond the original project,” Duben said. “So, you’ve got about half the capacity of water supply in the pool for $22 million.”
For next meeting
CCRC members approved the findings of the analysis, but since it was so in-depth, the commission agreed to table discussion on the analysis to take the time to properly evaluate it for next month’s meeting.
That meeting is scheduled 9 a.m. Thursday, Feb. 18, at Clarke County Development Corporation.
However, Bill Trickey, CCRC member, voiced concerns about the problems with West Lake, including the algae-bloom treatment that is done every year.
There have also been problems with taste and odor issues at West Lake.
“We have always kind of thought that we would build a supplemental water supply to West Lake, but I’m not sure we can count on West Lake long term,” he said. “So, to build a reservoir that is 1.2 million gallons, which would not allow it to replace West Lake as our primary water supply, and spending an additional $22 million in taxpayer money to do that, doesn’t make a lot of sense to me.”