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and Regulations for Dam Safety and Dam Construction, the spillway was required to safely route the runoff generated by 75% of the probable maximum storm as defined by the National Weather Service. URS in their 2006 feasibility report estimated that the current spillway has a capacity of 42,000 cfs, but the routed reservoir outflow from the 75% PMP event was over 100,000 cfs and would overtop the dam by approximately 8 feet. Since that report, the SEO has promulgated new Rules and Regulations for Dam Safety and Dam Construction that allow reductions in the NWS PMP values for altitude and the use of a site specific weather model developed by the SEO for high altitude precipitation referred to as the Extreme Precipitation Analysis Tool (EPAT). Reductions of 40% of the Probable Maximum Precipitation (PMP) values have been observed in some basins with the use of this tool. Over 90% of the 640 square mile drainage basin flowing into the reservoir is above 6,000 ft. MSL, the lower limit for use of the EPAT process. Based on the new permitted reductions in PMP values, it is feasible to permanently ]ower the existing spillway to an elevation that could safely route the required inflow design flood. There is sufficient elevation difference between the existing concrete ogee crest spillway, the reservoir basin and the rock cut spillway discharge channel to lower the emergency spillway 25 vertical feet. This elevation roughly corresponds to the current restricted storage level of gauge height 100 feet. Lowering the spi]]way to this level would produce 57,000 acre-feet of flood routing storage and significantly increase the spillway capacity. The goal would be to only lower the spillway to an elevation that could safely pass the required inflow design flood and maintain as much reservoir storage as possible. A detailed basin and reservoir hydrologic model would be developed in conjunction with the EPAT tool to determine the required spillway control elevation. While there are significant structural issues with the present dam embankment that make permanent reservoir storage at higher elevations not feasible, the dam has performed adequately for the past twenty years at the current restricted level of gauge height 100 feet. No additional embankment consolidation or displacement has been measured and only minor increases in seepage have been noted at the maximum restricted storage level. Permanently lowering the spillway and restricting the maximum storage level could have an additional benefit by reducing the hazard classification from high to significant hazard. A Failure Modes and Consequences Analysis study is planned to evaluate the structural issues and the need for a stability / seepage control berm along the downstream toe of the dam will be considered, particularly if storage levels are increased above the current restricted level. A key component toward rehabilitating the existing dam at a permanently reduced storage level would be to improve outlet operation reliability and a significantly improved instrumentation and monitoring program. Cucharas Feasibility Study ~ Project Alternatives