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I I I I I I I I I I I I I I I I I I I ENGINEERING EV ALUA TION To evaluate the four alternatives a hydrologic analysis was conducted to determine the spillway size required, and a slope stability and seepage analysis was conducted to determine the embankment stability. The hydrologic analysis included an evaluation of the 100 year storm and the 100% PMP (probable maximum precipitation) storm event, for both the Local and General storms, as outlined in the Design of Small Dams. The USBR procedure was utilized for development of the unit hydrograph and the flood routing determined using the computer program HEC-1. The seepage conditions were analyzed using ?EEP-W from GEO-SLOPE which is a fInite element analysis which considers the pressures, flow, and location of the phreatic surface. The slope stability analyses were conducted using computer program SB-SLOPE by GEOSYSTEM. Hydrology. The breach was designed for the 100 year storm with the breach width as required to limit ponding in the reservoir to less than 5-feet as required by the State Engineer, Rule 7, The three embankment alternatives were analyzed for the 100% PMP event and the spillway sized to prevent overtopping. A final design of the dam sections may allow utilizing a 75% PMP event after a hazard evaluation is completed. The drainage basin map and basin characteristics are shown on Figure 4. The following table summarizes the results of the hydrologic analyses including the spillway size required for the four alternatives. Alternative Spillway Spillway Peak Stage Width Side Slopes 1. Breach 40 2.5:1 5' - max pool 100-year Flood depth 2, Dam Height 8242' 100 6:1 8242' 100 % PMP Flood 3, Dam Height 8252' 50 6: 1 8252' 100% PMP Flood 4, Dam Height 8267' 50 6:1 8267' 100% PMP Flood TABLE 2 SPILLWAY SUMMARY Carl Smith Dam Feasibility Page 11