The average time period between occurrences of a hydrological event of a given magnitude. A 100-year flood has a recurrence interval of 100 years, meaning a 1% chance of occurring in any year.
The recurrence interval, also called the return period, is the average length of time between occurrences of a hydrological event (such as a flood or extreme rainfall) that equals or exceeds a specified magnitude. It is the reciprocal of the annual exceedance probability; for example, a flood with a 100-year recurrence interval has a 1% probability (1/100) of being equaled or exceeded in any given year. The concept is widely used in engineering design and risk assessment to specify design events for infrastructure such as bridges, dams, culverts, and stormwater systems. A common misconception is that a 100-year event occurs exactly once every 100 years; in reality, it is a probabilistic statement, and multiple 100-year events can occur within a short time span. The probability of experiencing at least one 100-year event over a design life of n years is calculated as 1-(1-1/T)^n, where T is the return period. For a 100-year flood over a 30-year mortgage period, there is a 26% chance of occurrence. Recurrence intervals are estimated from historical records using flood frequency analysis techniques and are subject to uncertainty, particularly when record lengths are short relative to the return period of interest.