What is return period in hydrology?

Return period (recurrence interval) is the average time between occurrences of a hydrological event of specified magnitude. A 100-year flood has 1% probability of occurring in any given year, not that it occurs exactly every 100 years. Return period T = 1/P, where P is annual exceedance probability. Design return periods vary: 2-10 years for minor drainage, 25-50 years for highways, 100 years for major structures, and up to 10,000 years for large dams. Longer return periods mean rarer, more severe events.