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<br />~,.., <br /> <br />several sub-alternative construction scenarios, as described below. Based on the <br />cost for lining a 200-foot section ofthe spillway (Alternative 2a), it was determined <br />to not be feasible to line the entire 3,800-foot length of the spillway channel from <br />the reservoir to the South Platte River. Due to the expected rare, if ever, discharge <br />of flood flows into the spillway, RRLC is willing to accept the possible erosion <br />damage of the spillway outlet channel should severe flooding occur. No damage <br />to the dam or loss of reservoir storage would occLir, <br /> <br />2a. Construct a lined open channel spillway. An open channel spillway section <br />would be constructed through the dune sand adjacent to Riverside Dam. The <br />spillway channel would be lined for a minimum length of 200 feet with a soil <br />cement armor. The armor would be 2.0-footthick, with 1 O-foot thick grade control <br />walls incorporated at 1 OO-foot intervals. A full depth cutoff to erosion resistant soil <br />would not be provided. <br /> <br />This alternative would provide significant protection against moderate erosion ofthe <br />spillway channel during frequent storm events and would protect against the <br />potential loss of reservoir storage due to erosion under severe storm events. During <br />severe flooding, erosion of the unprotected portions of the lower spillway channel <br />section would be expected, and some partial failure of the spillway may also occur <br />due to undermining of the shallow depth grade control walls. Reservoir records <br />indicate that during the lifetime of the reservoir, there has not been an inflow flood <br />which would have produced any significant spillway discharge. Because of the low <br />probability for significant spillway flow in the future, providing a fully armored <br />spillway for an extremely rare occurrence is not justified. This alternative would be <br />a costly alternative due to the significant amount of channel armoring proposed, <br />and is therefore not the preferred alternative. <br /> <br />2b. Construct an unlined open channel spillway with sheet piling grade/erosion <br />control cutoff walls. An open channel spillway section would be constructed <br />through the dune sand adjacent to Riverside Dam. This alternative would also <br />include a sheet piling cutoff wall (constructed to bedrock if depths are not too de~p) <br /> <br />8 <br /> <br />. <br /> <br />. <br /> <br />. <br />