A statistical analysis that estimates the probability of a flood of a given magnitude occurring in any given year. It is used to design infrastructure and assess flood risk.
Flood frequency analysis is a statistical method used to estimate the likelihood that a flood of a particular magnitude will occur within a specified time period. It involves fitting probability distributions, such as the Log-Pearson Type III, Generalized Extreme Value (GEV), or Gumbel distribution, to a series of annual maximum streamflow records. The analysis produces a flood frequency curve that relates peak discharge to exceedance probability or return period. For example, the 100-year flood has a 1% probability of being equaled or exceeded in any given year. In the United States, the USGS Bulletin 17C provides the standard methodology for flood frequency analysis, which includes procedures for handling outliers, historical floods, and regional skew coefficients. Results are used extensively in the design of bridges, culverts, levees, dams, and stormwater systems, as well as in floodplain mapping and insurance rate setting by FEMA. Climate change, land use change, and non-stationarity in flood records present ongoing challenges to traditional frequency analysis methods.