Ayres Associates proposed and evaluated several alternatives for rehabilitating Grimh Dam, a 30-foot-tall hydroelectric dam in northwestern Wisconsin. Based on the alternatives evaluation, North Central Power Company decided to remove the dam. Ayres Associates developed plans and specifications for staged removal of the concrete powerhouse, concrete spillway structures, and earthen embankments.
Sediment volumes within the 710-acre-feet reservoir were calculated from hydrographic and topographic surveys in conjunction with probe depths to bedrock. A sediment management plan addressed the reduction and mitigation of potential sediment releases. Of the 115,000 cubic yards of sediment behind the dam, approximately 20% was released during the multi-year drawdown and staged removal phases. River channel restoration was specified with native seed mixes and possible future modifications to restore fish passage to the Couderay River.
The project required coordination with the Wisconsin Department of Natural Resources for permits and fisheries issues. The physical removal of Grimh Dam was completed in 2011, with native species and bank restoration work continuing through 2012. Project challenges included discovery of contaminated soils and a 10-year flood that overtopped the cofferdam.