EDGERTON — A Design Documentation Report created by engineering firm Mead and Hunt, outlining a draft of discharge capacity upgrades for the Indianford Dam, was presented Tuesday to the Rock-Koshkonong Lake District (RKLD) Board of Commissioners.
The board met at the Edgerton City Hall.
After receiving the presentation, the board unanimously approved the draft, giving Mead and Hunt the approval to send it to the Wisconsin Department of Natural Resources (DNR) for its final approval.
The draft will be sent to the DNR by Jan. 15, Jeff Anderson, Mead and Hunt project engineer, said.
The DNR set the January deadline, extending it from one initially set in December, to allow the lake district time to continue its application for a grant to help fund its water control project, which will be achieved, after DNR approval, by making the drafted modifications at the dam.
After receiving the draft, the DNR will have 30 days to review the materials and give the project the green light, Anderson said. Once DNR approval is achieved, other next steps will include a bidding process and selecting a contractor to perform the work.
He anticipated that a bidding selection process could be completed by mid- to late March. He further anticipated that construction activities could commence at the powerhouse, as river conditions allow, as early as April.
The RKLD contract with Mead and Hunt is for engineering design services, associated investigations, bidding support and assistance with the grant application process, for a total amount of $183,300, Anderson indicated. As of Tuesday, the company has expended $166,000 of that budget.
During his presentation, Anderson said modifications to the dam’s powerhouse will affect both wicket gate bays and increase the dam’s overall discharge capacity.
The proposed modifications include removing the two wicket gate carousels from within the powerhouse, including draft tube cones and associated mechanicals; reinforcing the east/west walls and dividers in the powerhouse to allow for removal of portions of the turbine pit floors; demolition and removal of a portion of the turbine pit floors, replacing areas within turbine pits and draft tubes with new concrete to improve discharge hydraulics; removal of upstream bulkheads and bulkhead support frames; fabrication of six new slide gates and installation of the gates in the existing bulkhead slots; procurement of dedicated rising-stem operators for all six slide gates; upgrades to existing electrical service to provide sufficient power for the gate operators;
Also, construction of a new operator support frame and integral operating platform, which also will include the construction of an interior staircase to access the operators; reconfiguring the existing debris boom to improve the handling of large debris; removal of the existing trash racks to allow leaves and other small debris to be sluiced through the powerhouse; constructing a permanent gravel equipment pad from which larger machinery can operate while removing large, waterlogged debris that passes under the debris boom; repairing underwater concrete surfaces at intake piers and abutments, and installing a steel frame with grating over the existing turbine pit holes in the powerhouse floor.
Looking ahead at the construction process, Anderson said Mead and Hunt anticipates that a hired contractor would be looking to use the new slide gates as a means by which to de-water the powerhouse, allowing construction of the proposed structural modifications.
He therefore anticipated a sequence of work as follows: initiate fabrication and procurement of slide gates and their associated operators, which, he said, have an anticipated six- to eight-month lead time; remove the existing bulkhead support frames; fabricate and install the operating platform and stairway access; complete electrical service upgrades; install slide gates and gate operators; de-water the powerhouse and complete structural modifications, with that work commencing one bay at a time; reconfigure the debris boom and remove the existing trash racks.
Anderson said that lead times for some of the work likely would extend construction of the project into 2022. RKLD intends to give contractors the option of a phased construction approach, which, he said, Mead and Hunt believes will help attract more favorable bids for the project.
A construction contract for the project will state that work must be completed by Oct. 31, 2022, Anderson said.
Within the Mead and Hunt design documentation report, under a heading of “Hydraulics,” a section called “Estimated discharge capacity and approach velocity” presents calculations used to estimate the discharge capacity of the powerhouse and “approach velocities upstream of the powerhouse intake for the normal pool condition and for flood conditions.”
The report states that under normal pool conditions, a calculation of headwater elevation of 775.3 feet and a tail water elevation of 770.6 feet are assumed. Maximum discharge will occur during normal pool conditions due to the lesser degree of submergence when compared to flood conditions.
The report further states that calculations show that the cutout/opening in the turbine pit floor and the draft tube act in tandem as the hydraulic control through the structure under normal pool conditions.
Assuming all six slide gates at the powerhouse are open, the resulting total discharge through the powerhouse is approximately 4,200 cubic feet per second. Based on that discharge, the report continues, “a velocity of 6.3 feet per second was computed for just upstream of the powerhouse intake.”
The report further calculated discharges and upstream velocities at the powerhouse for both the 10-year and 100-year flood event.
Using headwater and tail water elevations for both events as provided by the 2015 Flood Insurance Study for Rock County, Mead and Hunt provided the following data: during a 10-year flood event, headwater elevation was calculated at 779.1 feet and tail water at 778.1; during the 100-year event, headwater calculations were 782.2 feet and tail water was 781.9.
For both conditions analyzed, the passageway through the dam would be completely submerged; therefore, the report states, it assumes that the intake opening would act as the hydraulic control since it creates the smallest opening through the structure, acting like a submerged orifice.
The report also calculates the resulting discharges and approach velocities upstream of the powerhouse intake. During a 10-year flood event, the discharge would be 2,560 cubic feet per second and the approach velocity would be 2.8 feet per second. During the 100-year flood event, the discharge was calculated at 1,400 cubic feet per second and the approach velocity is 1.2 feet per second.
Looking more closely at mechanicals, the report states that each of the six slide gates installed at the powerhouse, three in each bay, will be raised and lowered by a dedicated electrically-powered hoist. The mechanism will be able to lift the gates upward, but also push them downward, Anderson said, to compensate for “the hydraulic forces and the anticipated coefficient of friction between the gate leaf bearing strips and the steel-lined slots.”