Flood Frequency Analysis

Flood frequency analysis estimates how often floods of a given magnitude occur at a river site by fitting an extreme-value probability distribution to the record of annual maximum discharges and then inverting it to read off design floods for specified return periods. The classical approach uses the Gumbel distribution, the limiting form for maxima of light-tailed variables; the more general Generalized Extreme Value (GEV) distribution adds a shape parameter that lets the tail be lighter or heavier, while the log-Pearson Type III distribution is the U.S. federal standard codified in USGS Bulletin 17C. Hosking, Wallis, and Wood's 1985 work on probability-weighted moment estimation of the GEV made robust at-site fitting practical, and Bulletin 17C (England et al., 2018) sets out the modern operational procedure. The output — the 100-year flood, the 500-year flood — underpins dam design, floodplain mapping, and infrastructure standards worldwide.