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<br />. <br /> <br />native grass may establish in the spillway over time. The proposed spillway width is 270 <br />feet, with 2.0 (horizontal) to 1.0 (vertical) side slopes. The east edge of the spillway <br />control section would coincide with the west abutment of the Riverside Dam. A detailed <br />review of the spillway hydraulic calculations along with the reservoir's design flood routings <br />and the maximum acceptable routed water surface in the reservoir may allow a smaller <br />spillway width than that proposed herein. <br /> <br />. <br /> <br />A structure is required to prevent erosion of the fine sands which constitute the spillway <br />floor, and to prevent head cutting of the spillway outfall channel upstream and into the <br />reservoir, which otherwise would produce a release of stored reservoir water, and which <br />would constitute a "partial failure" of the dam (according to DWR personnel). It is <br />proposed to construct a wall of sheet pile, reinforced with tieback and deadman devices <br />at regular intervals, to provide a barrier which will prevent the scour and head cutting from <br />extending into the reservoir. The design of the sheet piling wall would consider the <br />potential for scour of the sand material down to the toe of the sheet pile, which would lead <br />to loss of the downstream soil support. The location and alignment of the sheet piling <br />cutoff wall would be near the alignment of the dam crest as shown in the Proposed <br />Spillway location drawing in Appendix B. <br /> <br />The spillway grade would be set by the top of the proposed sheet piling. The east <br />abutment of the spillway control structure would be tied into the existing west end of <br />Riverside Dam, and would be protected from erosion with additional sheet piling. The west <br />end of the spillway would be protected with sheet piling and sand excavated from the <br />spiiJway, The spillway floor would be unarmored natural sand. <br /> <br />The flood hydrology report which describes the development of the reservoir inflow design <br /> <br /> <br />flood, the spillway requirements and the spillway hydraulic calculations accompanies this <br /> <br /> <br />report as Appendix C. <br /> <br />Acquisition of approximately 3,000 acres of BlM land underlying the reservoir and <br />approximately 100 acres adjacent to the reservoir will be induded as a part of this project. <br /> <br />. <br /> <br />11 <br />