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Spillway Design RequirementsSpillway Design Requirements„„A spillway is designed to pass the A spillway is designed to pass the ““inflow design inflow design floodflood””, a flood determined through state, a flood determined through state--ofof––thethe--art art techniques, Precipitation, runoff and infiltration and techniques, Precipitation, runoff and infiltration and flood storage such that the dam is not overtopped. flood storage such that the dam is not overtopped. overtopping and spillway failure account for 49% of overtopping and spillway failure account for 49% of all dam failures. all dam failures. (UNICIV 1998) (Piping accounts for 47% of all failures)(UNICIV 1998) (Piping accounts for 47% of all failures)„„The The ““inflow design floodinflow design flood””, IDF, used to determine the , IDF, used to determine the spillway capacity requirements, is often characterized spillway capacity requirements, is often characterized as a percentage of the probable maximum as a percentage of the probable maximum precipitation (PMP). However there are other methods precipitation (PMP). However there are other methods and analysis available for determining the IDF and and analysis available for determining the IDF and necessary spillway capacity.necessary spillway capacity.