Return period
A return period denotes a recurrence interval. It is a statistical measure of how often an event of a certain size is likely to happen. Within hydrology return period is important in relating extreme discharge to normal discharge. A 10 year flood is a large enough discharges that we expect it only to occur every 10 years. Equally, a 100 years flood is such a big discharge that we expect it only to occur every 100 years. It is important, however, to understand that 2 events of floods with 100 years return period may occur in 2 subsequent years.