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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />13 <br />that potential incremental downstream consequences <br />are minimized. <br />Furthermore, the design of spillway modifications <br />should include detailed evaluations of foundation <br />conditions and structural stability, including <br />earthquake load conditions. <br />New Spillway Construction <br />Where existing spillway modifications alone are <br />insufficient or impractical, increasing the overall <br />outflow capacity can be accomplished by adding an <br />additional spillway. The placement of an additional <br />spillway is often constrained by site-specific conditions; <br />however, it is common to site new spillways within one <br />of the dam abutments or somewhere along the <br />reservoir rim. <br />These types of spillways are typically earth or rock cuts <br />and can be lined or unlined depending on hydraulic <br />conditions, erosive potential, and risk tolerance. <br />Careful consideration of the impacts both proximate to <br />the dam and new spillway as well as downstream is <br />required to avoid creating unnecessary adverse <br />consequences. <br />It is generally discouraged to site a spillway of any kind <br />(i.e., principal, emergency, etc.) over a dam <br />embankment due to increased dam failure potential <br />resulting from increased embankment seepage and <br />hydraulic structure failure potential. If site conditions <br />constrain selection of a new spillway to over the dam, <br />overtopping protection could be a more desirable <br />alternative than a structural spillway. <br />Overtopping protection of embankment dams can be <br />used to increase the overall outflow capacity in lieu of <br />constructing a new spillway over the dam, abutments <br />or reservoir rim. Overtopping protection can be <br />constructed using a variety of materials, the most <br />common being roller compacted concrete (RCC), <br />conventional concrete, articulated concrete blocks <br />(ACB) and soil cement. Regardless of the overtopping <br />protection material, operation of an overtopping <br />spillway should be limited to infrequent and extreme <br />events to reduce the risk of potential dam failure [7]. <br /> <br />Figure 4. RCC Overtopping Protection Installation <br /> <br />Figure 5. ACB Overtopping Protection <br />Overtopping of concrete dams can be acceptable on a <br />more frequent basis if structural evaluations confirm <br />dam stability and adequate downstream energy <br />dissipation is provided or erosion resistant material is <br />present. <br />The design of new spillways should include detailed <br />evaluations of: <br />• Foundation conditions <br />• Spillway crest types and shapes <br />• Energy dissipation structures <br />• Structural stability, including earthquake load <br />conditions <br />• Potential incremental downstream <br />consequences for more frequent events. <br />Dam Crest Modifications <br />In lieu of constructing modifications to increase overall <br />outflow capacity directly, dam crest modifications are