North Carolina Flood Frequency Statistics

Reliable estimates of the magnitude and frequency of floods are needed for numerous design and management functions throughout North Carolina. For U.S. Geological Survey (USGS) streamgage sites, where more than 10 years of annual peakflow data have been recorded, estimates of flood frequency can be computed using statistical analysis. Floodfrequency estimates at gaged sites can be regionalized, or extended in space, to develop estimates of floodfrequency at ungaged sites. This Web page was developed in cooperation with the North Carolina Department of Transportation, Division of Highways (Hydraulics Unit) and the North Carolina Department of Crime Control and Public Safety, Division of Emergency Management (Floodplain Mapping Program) for users to view the location and floodfrequency information for specific stream sites by using various maps.
Using peakflow data compiled through 2006, computed floodfrequency estimates are presented for gaged sites with more than 10 years of annual peakflow record. For ungaged sites, the appropriate site information and predictive equations that can be used to calculate floodfrequency estimates are presented. Further details and computational methods can be found in Bulletin 17B (Hydrology Subcommittee of the Interagency Advisory Committee on Water Data, 1981) and Weaver and others, 2009.
Floodfrequency estimates for a given stream site typically are presented as a set of peak flows and the associated exceedance probabilities or recurrence intervals. Exceedance probability of a peak flow is the probability of that flow being equaled or exceeded in a 1year period and is expressed as a decimal fraction less than 1.0. The recurrence interval of a peak flow is the number of years, on average, in which the specified flow is expected to be equaled or exceeded one time. Exceedance probability and recurrence interval are mathematically inverse of each other; thus, an exceedance probability of 0.01 is equivalent to a recurrence interval of 100 years. For example, a peak flow with a 100year recurrence interval will, on average, be equaled or exceeded once every 100 years and has an exceedance probability of 0.01 (a 1percent chance of being exceeded in a given year). Recurrence intervals refer to the average number of occurrences over a long period of time; for example, a 100year flood is statistically expected to occur about 10 times in a 1,000year period, rather than exactly once every 100 years. Additionally, it should be noted that the occurrence of a flood of a given recurrence interval in a given year does not affect the probability of such a flood occurring again the next year.
References
Weaver, J.C., Feaster, T.D., and Gotvald, A.J., 2009, Magnitude and frequency of rural floods in the Southeastern United States, through 2006—Volume 2, North Carolina: U.S. Geological Survey Scientific Investigations Report 2009–5158, 111 p.
Hydrology Subcommittee of the Interagency Advisory Committee on Water Data, 1981, Guidelines for determining flood frequency: Reston, VA., U.S. Geological Survey Bulletin 17B, Office of Water Data Collection, 183 p. (corrected editorially in March 1982).