North Reservoir Dam Improvements – Rehabilitation of a High-Hazard Dam at Full Pool

North Reservoir is a high-hazard embankment dam owned by the Ohio Department of Natural Resources (ODNR) that is part of the Portage Lakes State Park System. The dam includes three separate embankments with a maximum height of 14 feet and a total length of 670 feet. The project was constructed in 1908 solely for recreational purposes. Since then, significant residential development has flourished around the three embankments. However, the spillway is severely deficient and can only pass about 25% of the design storm (PMF). As a result, overtopping protection solutions were considered to bring the dam into compliance with the state’s dam safety regulations.
Because of the recreational use of the facility and adjacent residential development, ODNR included two constraints for Tetra Tech’s design work: the pool would have to be maintained throughout construction and impacts to adjacent residents had to be minimized. Armoring the existing embankments would have required lowering the pool and would have required acquisition and demolition of several existing homes. Consequently, it was determined the best solution for the two design constraints was to construct an overtopping structure within the reservoir, against the upstream slopes of the existing embankments.
After considering several upstream alternatives, the most economical solution was to use deep soil mixing techniques to construct a gravity dam and cutoff wall within a berm. A sand berm was placed into the reservoir against the upstream slopes of the embankments. This berm extended above the normal pool elevation, providing a working platform for the soil-mixing equipment. The soil-cement gravity section was sized using the Corps' Gravity Dam Design requirements. Global stability, as well as overturning and sliding, were all considered in the design. This innovative overtopping design addressed the spillway deficiency, allowed construction to occur at full pool, and minimized impacts to the adjacent landowners.