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Estimating the frequency of extreme flood events



Ever since large dams and reservoirs were first constructed, extreme flood estimates have been produced for use in designing spillway capacities. For this purpose, the National Weather Service, the Corps of Engineers, and the Bureau of Reclamation have codified a methodology to estimate an extreme flood, which is commonly referred to as the Probable Maximum Rainfall and resulting Probable Maximum Flood event. The combined method uses historic data to extrapolate meteorologic and hydrologic conditions to extreme levels to produce the maximum flood estimates. As more data are collected, the estimates of the higher limits of extreme floods have increased.

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