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<br />, <br /> <br />. <br /> <br />across the spillway control section. The sheet piling would be supported against <br /> <br /> <br />overturning (should the downstream spillway channel section be subject to <br /> <br /> <br />excessive scour) with a wale and tieback system attached to concrete deadman <br /> <br /> <br />buried in the sand upstream of the control section within the reservoir basin. <br /> <br />This alternative will provide for the safe passage of the inflow design flood, <br /> <br /> <br />although erosion damage to the spillway outfall cha'lJrel below the spillway control <br /> <br /> <br />would be expected to occur under moderate and severe flooding. The <br /> <br /> <br />grade/erosion control (cutoff) walls would prevent head cutting into the reservoir <br /> <br /> <br />and the eliminated the possibility of loss of reservoir storage. This alternative is <br /> <br /> <br />feasible and is considered the preferred alternative. <br /> <br />. <br /> <br />2c. Construct an unlined open channel spillway with a soil cement grade/erosion <br />control cutoff wall constructed as a gravity section. An open channel spillway <br />section would be constructed through the dune sand adjacent to Riverside Dam. <br />This alternative would also include the construction of a soil cement "dam" cutoff <br />wall extending from the non-erosive soils underlying the dune sands up to the level <br />of the spillway control. The soil cement section would be constructed as a gravity <br />dam section to resist overturning should the downstream spillway channel section <br />be scoured to the depth of the non-erosive soils. The construction will require the <br />removal of the spillway sands along the alignment of the cutoff wall, and <br />replacement of the material upon completion of the construction. <br /> <br />This alternative will provide for the safe passage of the inflow design flood, <br />although erosion damage to the spillway outfall channel downstream from the <br />gravity section would be exr;ected to occur. The grade/erosion control (cutoff) wall <br />would prevent head cutting into the reservoir and the resulting possible loss of <br />reservoir storage. This alternative becomes very expensive as the depth to bedrock <br />or non-erosive soil increases beyond about 20 feet. This is not the preferred <br />alternative due to high costs. <br /> <br />. <br /> <br />2d. Construct an unlined spillway without grade/erosion control cutoff walls. An <br /> <br />9 <br />