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rabl.. 15 Capacity and Case Data Dam Cr..st: RaIses ArnauD t of Raise Caoacity -=~~f PMF ExistinI.': Deve];,;C>oment With Freeboard Cas t in Million $ No Raise Raise 3 ft. Raise 9 ft. Raise 15 ft, Raise 19 ft. 63 70 82 94 100 o 3,7-11,5 7,2-20,7 20,0-34.2 29,3-43.9 bLTERNATIVE 1 - WIDEN EXISTING SPILLWAY This alternative consists of enlarging the existing spillway channel and contains four options. The first option would be too enlal:ge the existing spillway sufficiently to pass the PMF wi thout mod.ifying the dam, The other three options would include enlarging the existing ,;pillway combined with dam raises of 3-. 9-. or IS-feet. Plate 30 shows typical spillway enlargement and dam raise sections. Excavated material from the spillways would be placed on the spillway land between the excavation and the Govertunent boundary for all. spillway widths except the 700- and 925-foot-wide channels. For these t""" widths, there is not sufficient space to place all of the excavation. About 20 percent of the material would be hauled to the downstream embankment slope and placed as sernicompacted embankment berm for these two wi.dths. Rating curves for this alternative were developed by computing water surface profiles from the downstream end of the spillway to the pool. Assumptions include IV on 3H sideslopes, a Mannings n-value of 0.035 and critical depth starting conditions at the downstream end. As shown on table 14, the least: cost option for this alternative would include widening the spillway too 150 feet and raising the dam crest by 15 feet. The construction cost associated with the least cost option is $38,7 million. ALTERNATIVES 2, 3, and 4 - ADD ADDITIONAL SPI1.LWAY Plate 31 presents a typical layout of three spillway alternatives which have been considered in the study. These alternatives have been considered in 49