After hiring engineering firm Mead and Hunt last month to design a crest gate for installation in between the Indianford dam powerhouse and spillway, the Rock-Koshkonong Lake District (RKLD) Board of Commissioners approved a change in plans. The board opted instead for the removal of two wicket gate carousels and their associated mechanics from within the powerhouse, replacing them with six slide gates.
The RKLD board met Jan. 6 at the Edgerton Public Library. Commissioner Steve Proud was not in attendance. Commissioners in attendance approved wicket gate removal unanimously.
The board will next apply for a Department of Natural Resources (DNR) sponsored dam rehabilitation reimbursement grant and then bring the dam modification plan before the district’s electors for approval of funds. Both steps are needed before any construction can begin, RKLD Chairman Alan Sweeney said.
The change came after Mead and Hunt Project Engineer Jeff Anderson told RKLD commissioners that while building the exterior crest gate would be less costly than removing the interior wicket gates, it would also have less capacity to move water. Installation of the exterior crest gate configuration would also require dredging, Anderson said. Board members believed, they said, dredging would likely become a repetitive process as silt over time moved downriver, adding maintenance costs.
Using a slide presentation, Anderson presented three alternatives, one of which revolved around the crest gate spillway concept, as approved by the board in December, and two of which involved powerhouse retrofits. The first involved retrofitting one turbine pit or bay within the powerhouse, including the removal of one wicket gate carousel and its associated mechanics, and replacing it with three slide gates, spanning a total of 30 feet. The second employed the same concepts, but involved retrofitting both bays within the powerhouse, using a total of six slide gates and spanning 60 feet, to control water flow.
Identifying project goals as increasing discharge capacity, reducing the number of slow-no-wake days, improving debris-handling capability and limiting costs, Anderson compared the three options.
The crest gate option, a diagram showed, involved removal of 16 feet of existing concrete between the powerhouse wall and the dam’s spillway. When the gate was open, debris and water would pass through. Installation of the gate would require some excavation to lower the riverbed, Anderson said. The gate itself was 10 feet high, he said.
Looking at both the single bay and double bay retrofit, Anderson said, in both cases, slide gates, similar to those used on the river’s east bank, and new trash racks would be installed within each bay. Trash racks would have larger, 12-inch spacing between bars to allow larger debris to move through the powerhouse. Current trash racks at the powerhouse have 4-inch spaces between bars, Anderson said.
Trash racks would still be needed to keep large debris, like trees, from lodging in the slide gates, he said.
Anderson said the powerhouse structure could effectively be removed with the proposed system, but the option was costly. Commissioners further noted that the building had historical significance, which might make removal undesirable. Exploring the option also could hinder plans to stay within a timeline that would allow the board to apply for the DNR dam rehabilitation grant. The application for grant funding is due by the end of February.
“There are limitless options of what you can do. None are perfect,” Anderson said.
Looking at costs, he said, the crest gate spillway option came with an estimated price tag of $1.09 million. The single bay powerhouse retrofit option would cost approximately $1.34 million. Retrofitting both bays brought an estimated cost of $2.25 million.
Using data supplied by MAR-EOR engineer Rob Montgomery, Anderson said the crest gate spillway approach might be expected to produce as many as five fewer slow-no-wake days (between May and September) and reduce water levels on Lake Koshkonong by 0.27 feet. Using the single bay retrofit model, Anderson said there might be about six fewer slow-no-wake days and a lake water level reduction of 0.38 feet. Retrofitting both bays might be expected to produce 10 fewer slow-no-wake days and a lake water level reduction of 0.49 feet.
Montgomery said: “The dam doesn’t do much at flood stage; it does a lot at low flows.”
Answering questions posed by audience members about the effects on flow and water levels that might have resulted from blockages when debris was allowed to accumulate in trash racks, Montgomery said his analysis was not “fine-tuned” to represent such occurrences, adding: “We didn’t have that data.”
Looking at a “benefits and drawbacks” analysis, Anderson said, using the crest gate option, benefits included: lowest project cost, gate is “particularly useful for debris management,” and a simplified project operation. Drawbacks included: “smallest improvement in discharge capacity,” construction unknowns that might escalate costs, and a potential for debris to collect upstream of the powerhouse.
Looking at the cost and drawbacks of the retrofit options, Anderson said both options (one or two bay retrofits) would increase discharge capacity by “significantly more” than the crest gate, produce fewer project unknowns and less potential for cost escalation, and improve handling of small debris. Drawbacks included a higher cost than that associated with the crest gate while leaving unaddressed issues with large debris.