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<br />. <br /> <br />modification of facilities for Alternate C. The most economical <br />design would consist of a combination of spillway enlargement and <br />dam enlargement. Most of the material excavated from the spillway <br />would be used for placement of fill on the downstream face of the <br />dam. The preliminary spillway design would consist of an enlarge- <br />ment of the existing side channel bathtub inlet and discharge chute. <br />The spillway shown in Figure 12would be capable of discharging about <br />64,000 cfs at a reservoir water surface elevation of 9473 (gage 115). <br />The flood inflow hydrograph and spillway flood routing is discussed <br />in the flood hydrology report, Appendix C. The spillway crest would <br />consist of about 260 feet of ogee crest or weir crest. The inflow <br />would discharge into a 240 foot long side channel bathtub where the <br />flow would be accelerated toward the spillway chute. The bathtub <br />width would vary from about 30 feet at the upper end to about 120 <br />feet at the lower end. The bathtub floor would slope from elevation <br />9440 to elevation 9435. The spillway chute width downstream of the <br />dam crest would gradually narrow to about 23 feet at the lower end. <br />For reservoir water surface elevations below 9460 feet, spillway <br />discharge would be limited by flow over the crest into the side channel <br />bathtub. Above 9461 feet elevation, the inflow would be affected by <br />weir submergence conditions. At the maximum discharge at 9472.7 feet <br />elevation, the crest at the upper end of the bathtub would be almost <br />completely submerged, with the degree of submergence control I ing the <br />inflow rate. <br />Since the enlarged spillway would only be needed during extreme <br />rainfall events, it would not be necessary to increase the capacity <br />of the lower portion of the spillway. Flows in excess of the existing <br />capacity would spillover the concrete walls but should not endanger <br />the dam. Additional hydraulic calculations should be made prior to <br />final design to confirm that the spilled water would cascade safely <br />into the canyon downstream of the dam toe. <br />As shown in Figure 12, the spillway would be widened primarily <br />toward the northern side, minimizing the rock cut in the south abutment. <br /> <br />. <br /> <br />. <br /> <br />-46- <br />