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NEED FOR THE PROJECT <br />Riverside Dam and Reservoir has been classified by the Colorado Division of Water <br />Resources (DWR), Office of the State Engineer, Dam Safety Branch as a Large Size, High <br />Hazard structure. This classification denotes that failure of the dam would cause extensive <br />property damage and probable loss of human life. <br />Riverside Dam and Resenroir currently is not equipped with an emergency spillway, which <br />is required by the DWR's Ru/es and Regulations for Dam Safety and Dam Construction. <br />Construction of a spillway on the dam will reduce the risk of dam failure by overtopping <br />during an extreme flood event, enhancing the safety of downstream residents and <br />property. The RRLC is undertaking this project to enhance the safety of Riverside Dam <br />and Reservoir by providing an emergency spillway to allow the safe routing of the inflow <br />design flood without the overtopping of the dam embankment. The inflow design flood is <br />that flood produced by 75% of the probable maximum precipitation. The inflow design - <br />flood has been approved by the State Engineer for Riverside Dam. The State Engineer's <br />letter approving the inflow flood is included in Appendix C. <br />ALTERNATIVES EVALUATION <br />ENGINEERING DESIGN ISSUES <br />Riverside Dam is a homogenous sand fill embankment structure constructed on a sandy <br />foundation, with a concrete slab upstream slope protection providing seepage control <br />and wave erosion protection. The locations available for siting the spillway all require <br />construction of the spillway through the dam embankment. The potential west abutment <br />sites are too remote from the natural drainage way, and the west abutment sites require <br />flood easements from multiple property owners and would have high spillway gradients <br />relatively close to the reservoir. <br />The design considerations and construction issues associated with constructing an over- <br />the-dam spillway on sandy embankment and foundation materials are complex. The <br />design standard in the engineering industry is to try to avoid the use of over-the-dam <br />spillways unless other viable options are unavailable. <br />8 <br />