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This report has been superseded by SIR 2009-5158, Magnitude and frequency of rural floods in the Southeastern United States, through 2006–Volume 2, North Carolina.

Estimating the Magnitude and Frequency of Floods in Rural Basins of North Carolina — Revised

Water-Resources Investigations Report 01-4207
By Benjamin F. Pope, Gary D. Tasker, and Jeanne C. Robbins

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This report revises and supersedes U.S. Geological Survey Water-Resources Investigations Report 99-4114. The revised flood-discharge values are listed in table 2 (Q2, Q5, Q10, Q25, Q50, Q100, Q200, and Q500). The revised flood discharges, for all recurrence intervals, vary by as much as 17 percent from the earlier published values, with 80 percent of all values within 7 percent of the earlier published data. Differences in the values for the 100-, 200-, and 500-year discharges are greater than in the values for the 2-, 5-, 10-, 25-, and 50-year discharges, 80 percent of which are within 3 percent of the earlier published values.

The revised t-year discharges were used to update the regional regression equations and the region-of-influence data base, as indicated in revised text tables 5, 6, and 7 and in appendix table 1. The maximum difference in computed results for the regional regression equations was noted for the Coastal Plain equations, where application of the revised equations to small drainage areas, less than 10 square miles, resulted in discharges that are about 3 to 9 percent greater than those values obtained using equations from the previous report. Computed flood discharges using the revised Blue Ridge-Piedmont equations generally were within about 2 percent of the values from the previously published equations, except for results for drainage areas less than 10 square miles, which ranged from about 3 to 7 percent less than the previously published values. Application of the revised regression equations to the Sand Hills hydrologic area shows results in discharges that are up to 3 percent less than those computed using the equations published in the earlier report. The average error of prediction for the revised equations was nearly the same as for the earlier published Blue Ridge-Piedmont equations, lower for the Coastal Plain equations, and higher for the Sand Hills equations.

As in the previous report, the root mean square error (RMSE) for the region-of-influence method was only marginally better than the RMSE reported for the regional regression equations, resulting in neither method being clearly superior. The revised computer program for computing the estimates of flood-frequency discharges, using either the regional regression equations or the region-of-influence method, and the associated site-specific errors of prediction are available at the North Carolina District Web site wri014207.


A statewide study was conducted to develop two methods for estimating the magnitude and frequency of floods in rural ungaged basins in North Carolina. Flood-frequency estimates for gaged sites in North Carolina were computed by fitting the annual peak flows for each site to a log-Pearson Type III distribution. As part of the computation of flood-frequency estimates for gaged sites, new values for generalized skew coefficients were developed. Basin characteristics for these gaged sites were computed by using a geographic information system and automated computer algorithms. Flood-frequency estimates and basin characteristics for 317 gaged sites were combined to form the data base that was used for this analysis.

Regional regression analysis, using generalized least-squares regression, was used to develop a set of predictive equations that can be used to estimate the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year recurrence interval discharges for rural ungaged basins in the Blue Ridge-Piedmont, Coastal Plain, and Sand Hills hydrologic areas. The predictive equations are all functions of drainage area. Average errors of prediction for these regression equations range from 36 to 65 percent.

A region-of-influence method also was developed that interactively estimates recurrence interval discharges for rural ungaged basins in the Blue Ridge-Piedmont and Coastal Plain hydrologic areas of North Carolina. Regression techniques are used to develop a unique relation between flood discharge and basin characteristics for a subset of gaged sites with similar basin characteristics. This, then, can be used to estimate flood discharges at ungaged sites. Because the computations required for this method are somewhat complex, a computer application was developed that performs the computations and compares the predictive errors for this method. The computer application also includes the option of using the regression equations to compute estimated flood discharges and errors of prediction specific to each ungaged site.

Root mean square errors, computed for each recurrence interval and hydrologic area, are generally only slightly lower for the region-of-influence method than for the regression equations and do not provide sufficient basis for recommending one method over the other. In addition, the region-of-influence method is a new method that is still being improved. As a result, the regional regression equations are considered to be the primary method for computing flood-frequency estimates at ungaged sites.

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North Carolina Water Science Center
U.S. Geological Survey
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Raleigh, North Carolina 27607
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Last update: Wednesday, December 19 2012, 02:10:04 PM