ESTIMATING PROBABLE MAXIMUM FLOOD PROBABILITIES 1

Methods of computing probabilities of extreme events that affect the design of major engineering structures have been developed for most failure causes, but not for design floods such as the probable maximum flood (PMF). Probabilities for PMF estimates would be useful for economic studies and risk assessments. Reasons for the reluctance of some hydrologists to assign a probability to a PMF are discussed, and alternative methods of assigning a probability are reviewed. Currently, the extrapolation of a frequency curve appears to be the most practical alternative. Using 46 stations in the Mid‐Atlantic region, the log‐gamma, log‐normal, and log‐Gumbel distributions were used to estimate PMF probabilities. A 600,000‐year return period appears to be a reasonable probability to use for PMFs in the Mid‐Atlantic region. The coefficient of skew accounts for much of the variation in computed probabilities.