Evaluating Dam Safety Retrofits With Uncertain Benefits: The Case of Mohawk Dam (Walhonding River, Ohio)

Mohawk Dam, part of the Muskingum basin flood control system, was built in 1938 and is operated by the U.S. Army Corps of Engineers (Corps). Since this high-hazard dam could not survive a probable maximum flood (PMF), the Corps conducted a study to determine the least expensive means of enabling the dam to survive a PMF. Applying a previously proposed framework to select the social cost minimizing capacity of a dam, we show that Mohawk Dam had sufficient capacity that any retrofit has a social cost larger than expected benefits. Sensitivity analyses were performed adjusting the peak flow distribution, the costs of modification, and downstream flood damage, as well as the possibility of loss of life. For any reasonable value of these variables the conclusion does not change that the structure already met so high a safety goal regarding extreme floods that no retrofit is needed. Using risk-based methods to perform reservoir safety evaluations, as recommended by a National Research Council committee in 1985, is indeed feasible. Furthermore, their use provides valuable insight and guidance into the selection of strategies to enhance the safety of dams.