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I I I 1 I I I I I I I I I I I I To establ; sh a common basi s for compari son, the desi gn al ternati ves were developed for the full decree of 460 acre-feet. The first alternative design for Twin Lakes Dam is summarized in Table V.l. This design focused on providing a long term, low maintenance solution by: (1) replacing the existing embankment dam, outlet works, and spillway with an RCC dam and (2) constructing an earth dike across the present spillway channel. The technical confidence for each major component of this alternative design is high. Long-term 0 & M expenditures would be SUbstantially reduced. Future 0 & M activities would primarily include riprap and grass cover repairs at the dike and seepage monitoring. =============================================================================== Table V;l; Twin Lakes Dam Rehabilitation Alternative No.1 o Replace dam and outlet works with RCC dam. o Grout dam foundation rock. o Construct earth dike in existing spillway site. o Increase water storage level from elevation 9295 to 9303 to obtain storage capacity of approximately 460 acre-feet. o Set crest of dam at elevation 9306. Provide spillway overflow at el evati on 9303 wi thi n RCC dam for lOO-year flood. Dam overtops for greater fl oods. o Set crest of earth dike at elevation 9308 to provide freeboard during fl oodi n9. o Provide outlet works within RCC structure. =============================================================================== Alternative design No.2 for Twin Lakes Dam is summarized in Table V.2. This design focused on: (1) raising the earth dam crest to provide additional storage and freeboard; (2) constructi ng an RCC di ke/overfl ow spi 11 way across the existing spillway channel; (3) reducing, collecting, and controlling seepage; (4) stabilizing the downstream slope; and (5) replacing the outlet works structure. -64-