Underseepage Analyses at Huffman Dam

Huffman Dam is a 65-foot high hydraulic fill dam (3,320 feet long) located on the Mad River just north of Dayton, Ohio. The dam was constructed between 1919 and 1921 as part of a program by the Miami Conservancy District (District) to control flooding in the Great Miami River Basin. The dam was constructed on glacial outwash deposits 240 feet deep with only a nominal 20 foot deep cut-off trench. The “dry” dam functions purely as a flood control structure, retaining no permanent storage reservoir. A low-level outlet and emergency spillway structure on the left abutment controls outflow from the reservoir during significant runoff events. The dam has stored flood waters to a maximum height of 24 feet over the valley floor in its 72 year history. This level was equivalent to 48 percent of the design Official Plan Flood level (50 feet of head) and 37 percent of the Probable Maximum Flood level (65 feet of head). Significant flows from relief wells installed near the downstream toe of the dam and seepage from low areas downstream were also recorded during high storage events. Instrumentation indicated significant rises in observation well readings with rapid response to reservoir levels. Investigation of the rises in observation wells indicated that high underseepage gradients might occur near the toe of the dam, producing unstable conditions at reservoir levels below the design level. A finite element underseepage study was conducted to estimate gradients at various reservoir levels and to establish if possible the maximum reservoir level which the dam could safely impound. 14 pp. (author intro)