The expected average time between events of a specific magnitude. Synonymous with recurrence interval, it is used to express the probability of extreme hydrological events.
The return period is the statistically expected average time interval between occurrences of a hydrological event of a given magnitude or greater. It is mathematically equivalent to the reciprocal of the annual exceedance probability and is the preferred term in many international standards and guidelines. Return periods are used to characterize the severity of design events for hydraulic structures, floodplain delineation, and risk management. Common design return periods include 2-10 years for urban drainage, 25-50 years for highway culverts, 100 years for floodplain regulation, and 500-10,000 years for dam spillways, depending on the consequences of failure. The choice of design return period reflects a balance between the cost of construction and the acceptable level of risk. Non-stationarity in hydrological records due to climate change and land use modification challenges the traditional assumption that return periods remain constant over time. Emerging approaches use time-varying frequency analysis or incorporate climate projections to estimate future return periods that may differ significantly from those based on historical data.