Pardee Dam-South Spillway Interim Modifications

Pardee Dam's South Spillway, in Calaveras County, California has been modified in response to issues raised during the Federal Energy Regulatory Commission (FERC)mandated 5-year safety inspections. The spillway was originally designed for a maximum flow of 120,000 cfs. Current studies indicate a probable maximum flood (PMF) discharge of 184,000 cfs. The calculated factors of safety for the South Spillway, under both the PMF and Maximum Credible Earthquake (MCE) loading conditions, were below levels considered acceptable by the FERC, and previous flood flows had scoured large holes at the toe of the spillway that threatened the integrity of the spillway. Modifications to the structure were therefore necessary. Woodward-Clyde, under contract to the East Bay Municipal Utility District (EBMUD), designed modifications to improve the spillway. In the summer and fall of 1996, the interim modifications to the South Spillway were successfully constructed with both FERC and California Division of Safety of Dams (DSOD) approval. This paper will summarize the design and construction issues, and discuss the lessons learned during modifications to this 69-year-old structure. 12 pp., 2 references.