Annual exceedance probability is the chance that a flood or rainfall of a given size is equalled or exceeded in any one year.
Annual exceedance probability, or AEP, is the chance that an event of a given size will be equalled or exceeded in any single year. It is usually given as a percentage. A 1% AEP flood has a 1 in 100 chance of occurring each year, and is the same event often called the 100-year flood. AEP is approximately the inverse of the return period T. Many agencies now prefer AEP because "100-year flood" wrongly suggests that such a flood happens only once a century. The risk over a longer period is 1 - (1 - AEP)ⁿ for n years. A 1% AEP flood has about a 26% chance of occurring at least once in 30 years, and about a 63% chance in 100 years. A building with a 30-year life therefore faces a real risk from a "100-year" flood. The simple inverse holds for annual maximum series. For partial duration series, AEP = 1 - exp(-1/T), which differs noticeably only for frequent events. AEP values come from fitting a distribution such as Gumbel or Log-Pearson III to observed data, so they carry uncertainty, particularly for short records and under a changing climate.