# Aftermath of Hurricane Fran in North Carolina --

## Recurrence Interval

Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a flow having a given magnitude at a gaging station. The recurrence interval (or sometimes called the return period) of a peak flow is based on the probability that the flow will be equalled or exceeded in any given year. For example, there is a 1 in 100 chance that a flow of 35,500 ft3/s or greater will occur during any year at the Haw River at Haw River (site 21, fig. 2). Thus, a peak flow of 35,500 ft3/s at site 21 is said to have a 100-year recurrence interval. This is not to say that a peak flow of 35,500 ft3/s will not occur more than once during a 100-year period at site 21, but rather there is only a 1 in 100 chance that a flow of this magnitude or greater will occur in any year.

Ten or more years of measured annual peak flows at a gaging station are required to perform a frequency analysis for the station. More confidence can be placed in a frequency analysis based on, for example, 30 years of record at the station than an analysis based on 10 years of record. Recurrence intervals at a gaging station change if there is a significant change in the flow regime at the station caused by an impoundment or major diversion of flow. For example, the 50-year flood at the Neuse River near Clayton (site 10, fig. 2) before the construction of Falls Dam was 21,400 ft3/s, whereas the 50-year flood at site 10 for post-impoundment conditions is 19,000 ft3/s

In table 1, recurrence intervals for unregulated and regulated conditions are given for sites that are downstream from reservoirs. As an example of how this information is to be interpreted, a flow of 24,000 ft3/s or greater was expected to occur at Kinston, on average, once every 10 years before Falls dam was built. With Falls dam in place, a flow of 24,000 ft3/s or greater is expected to occur at Kinston once every 50 years.