C153629 Feasibility Study - Laserfiche WebLink

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I i II I I 'I I I I I I I I I I I I I I I height. The spillway cut through the left abutment ridge was the con- trolling factor, since spillway construction is a major part of the total cost. A spillway for a lower dam would require a deeper excava- tion and, for a higher dam, a higher concrete overflow section or a gated spillway would be needed. Spillway costs would therefore be higher for alternative dam crest elevations either higher or lower than that chosen. The layout consisted of a dam with crest at El. 8695 with crest of the ungated spillway at El. 8675 providing a reservoir storage capacity of 39,600 acre-feet. Assumptions made for layout development include: o Diversion through a large conduit embedded in a trench under the dam. This would be a major cost item because the con- struction of the dam would probably require more than one season and the conduit and cofferdam would have to be sized for high springtime flows to prevent overtopping of the par- tially completed fill. o Spillway - Ungated ogee crest with concrete lined chute sized for a peak PMF discharge. o Outlet consisting of free-standing tower with access bridge in the reservoir upstream of the dam, conduit under the fill and a valve structure at the downstream toe. No rockfill dam alternative was attempted at the downstream site since the topography does not favor a spillway on either abutment. 3. Arch Dam - Since topography and geology at the downstream site appeared favorable, a concrete arch dam alternative was investigated. A double-curvature arch dam layout was developed with the crest eleva- tion the same as the intermediate height RCC alternative. Design assumptions were similar to those for the RCC alternatives. Comparison of the computed concrete volumes indicated that the ratio of arch dam volume to RCC dam volume for dams of equal height is about 50 percent. 3 - 4 68l9W/502OW