WHAT IS A RECURRENCE INTERVAL?
Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given event. The recurrence interval (sometimes called the return period) is based on the probability that the given event will be equalled or exceeded in any given year. For example, there is a 1 in 50 chance that 6.60 inches of rain will fall in Mecklenburg County in a 24-hour period during any given year. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval (see table below). Likewise, using a frequency analysis (Interagency Advisory Committee on Water Data, 1982) there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft3/s) will occur during any year at Little Sugar Creek at Archdale Drive (site 54, fig. 1). Thus, a peak flow of 15,000 ft3/s at site 54 is said to have a 100-year recurrence interval. Rainfall recurrence intervals are based on both the magnitude and the du
