What is flood recurrence interval?

Answer 1

The Recurrence Interval is an estimate of the average time between past occurrences of random events. Typically, the period of record of these events must be at least 10 years to be statistically significant. Statistics are used to estimate the probability of the occurrence of some event. The following is from a USGS water science publication: "Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equalled or exceeded in any given year." The equation for figuring Recurrance Interval(RI) is: RI = (n+1)/m where n = number of years in the record, m = magnitude ranking For example; A flood event that had a peak stream discharge of 75,800 cfs ranked as the 3rd largest discharge over a 30 year record period. Therefore the RI is: RI = (30+1)/3 = 11yrs. ;meaning the likelihood of a flood event of that magnitude recurring within 11 years is fairly high. To estimate the exact probability of this event actually ocurring requires one to apply statistical analysis of probability; that is' P = 1 - (1 - 1/RI) (expression in parenthesis is raised to the X power where X is the number of years - 11 in this case)