Washburn County hired Ayres Associates to complete a feasibility study at the Minong Flowage Dam. This 29-foot-high concrete gate and powerhouse structure impounds a 1,600-acre-feet reservoir and supports one of Wisconsin’s best wild rice habitats. As part of the feasibility study, dam removal options were evaluated, including cost estimates, construction dewatering, sequencing, and environmental risks.
Based on a full dam failure analysis, Ayres Associates classified the dam as a high hazard dam and determined it did not have adequate spillway capacity nor the required structural stability.
After receiving a $400,000 grant for dam safety repairs and a $100,000 grant for continued power production, Washburn County selected Ayres Associates to design repairs at the dam to meet the high hazard dam regulations. Ayres assisted the County in winning the $400,000 grant.
These improvements included adding 50% more spillway flow capacity; replacing three slide gates and two flashboard bays; reconstructing six spillway piers; stabilizing the steep embankments; repairing an inoperable sluiceway gate; replacing sections of a deteriorated sluice gate trash rack; adding a new stepped spillway; and replacing the old apron with a dentated endsill.
The County had only four of the 22 original 1936 design drawings, so many design details had to be field-modified for differing site conditions. The construction contractor also overcame numerous setbacks, such as a 10-year flood during construction, the discovery of 5-foot-high by 40-foot-wide voids under the existing dam, very high phreatic surfaces in the embankments, large spillway toe boils, and a stuck gate opening that threatened to drain one of the state’s premier wild rice flowages and cold water fisheries. The project was completed for a total cost of $1.77 million.