Estimating Probabilities of Extreme Floods for Spillway Design

The Probable Maximum Flood (PMF) is used as a criterion for safe spillway capacity by many federal and state dam safety agencies. The PMF is a deter­ministic, rather than probability-based, concept. The process of estimating the PMF generally consists of three components: first, determining the Probable Maximum Precipitation (PMP) at the site for a range of areas and durations; second, selecting the temporal and spatial arrangment of the PMP which produces the most severe flooding at the point of interest; and third, applying this storm (the PMS) to the watershed in a rainfall-runoff model to determine the resulting streamflow hydrograph at the point of interest. By definition, the PMF is the largest flood that could occur at that site from any combination of hydrologic and meteorologic conditions. Therefore, the probability of any naturally occurring flood that is greater than the PMF is, theoretically, zero. Floods approaching the size of the computed PMF for their watersheds have oc­curred in some parts of the U.S.A. (Bullard, 1985). Because our knowledge of the most extremehydrologic and meteorologic conditions possible is incomplete, many computed PMFs probably represent not an unsurpassable flood, but only an extraordinarily unlikely one.