C153835A Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I .. The recommended alternative for Westerdoll Dam and Spillway consists of a 100-year spillway and irrigation outlet as shown in Figure IV-7, The 100-year spillway consists of an uncontrolled 48-inch concrete box drop structure with a 36-inch concrete encased steel outlet pipe. The embankment would have a 60-foot wide emergency spillway at elevation 4973.5 sized to pass a flood produced by 50% of the probable maximum precipitation (pMP) storm event. The downstream face of the embankment would be protected by roller compacted concrete (RCC). The recommended alternative for Horseshoe Spillway consists of lowering the dam crest between Horseshoe and Boyd Reservoirs 1.5 feet, from elevation 4980.00 to elevation 4978,5 as shown in Figure IV-9, The lowered section would serve as an emergency spillway between the two reservoirs, capable of passing the 75% PMP storm event. The existing spillway would remain and enable the dam to pass an 100-year storm without utilizing the emergency spillway. The existing roadway would be demolished and removed, 1II1d then regraded to elevation 4978,5 and repaved, Approximately 1,000 feet of the dam crest would be lowered, The existing road and dam crest would be sloped from elevation 4980 to the spillway crest elevation of 4978,50 at a 3 :H:I V slope. Training walls would be constructed along the downstream sides of these slopes to channel the water into the discharge chute, In this alternative, a 1000-foot wide roller compacted concrete slope protection chute would be constructed along the downstream face of the dam, It would begin at elevation 4978,50 at the heel of the spillway crest, and end at the road running along the toe of the dam. The RCC would follow the existing slope of the downstream embankment face, The recommended alternative for Boyd Lake Spillway involves constructing a 75% PMP spillway at Horseshoe Dam between the two reservoirs and a 75% PMP spillway at Boyd Lake Dam, Boyd Lake Dam would be raised by 2 feet and a 300-foot wide spillway to accommodate the 75% PMP storm event would be constructed. The spillway would be placed along the southeast comer of the existing Boyd Lake Dam embankment which would return the flow back to its original drainage basin, As shown in Figures IV-ll and IV-12, the spillway would consist of a 300-foot concrete control structure with a crest elevation of 4961.60. The dam crest would be sloped down from elevation 4966.00 to the spillway crest elevation of4961.60 at a 6H:IV slope to allow service E:\G&LREPORWPD Hi Marchl',1997