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Scientific Investigations Report 2009–5158

Prepared 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)

Magnitude and Frequency of Rural Floods in the Southeastern United States, through 2006: Volume 2, North Carolina

By J. Curtis Weaver, Toby D. Feaster, and Anthony J. Gotvald

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Reliable estimates of the magnitude and frequency of floods are required for the economical and safe design of transportation and water-conveyance structures. A multistate approach was used to update methods for estimating the magnitude and frequency of floods in rural, ungaged basins in North Carolina, South Carolina, and Georgia that are not substantially affected by regulation, tidal fluctuations, or urban development. In North Carolina, annual peak-flow data available through September 2006 were available for 584 sites; 402 of these sites had a total of 10 or more years of systematic record that is required for at-site, flood-frequency analysis. Following data reviews and the computation of 20 physical and climatic basin characteristics for each station as well as at-site flood-frequency statistics, annual peak-flow data were identified for 363 sites in North Carolina suitable for use in this analysis. Among these 363 sites, 19 sites had records that could be divided into unregulated and regulated/ channelized annual peak discharges, which means peak-flow records were identified for a total of 382 cases in North Carolina. Considering the 382 cases, at-site flood-frequency statistics are provided for 333 unregulated cases (also used for the regression database) and 49 regulated/channelized cases. The flood-frequency statistics for the 333 unregulated sites were combined with data for sites from South Carolina, Georgia, and adjacent parts of Alabama, Florida, Tennessee, and Virginia to create a database of 943 sites considered for use in the regional regression analysis.

Flood-frequency statistics were computed by fitting logarithms (base 10) of the annual peak flows to a log-Pearson Type III distribution. As part of the computation process, a new generalized skew coefficient was developed by using a Bayesian generalized least-squares regression model.

Exploratory regression analyses using ordinary least-squares regression completed on the initial database of 943 sites resulted in defining five hydrologic regions for North Carolina, South Carolina, and Georgia. Stations with drainage areas less than 1 square mile were removed from the database, and a procedure to examine for basin redundancy (based on drainage area and periods of record) also resulted in the removal of some stations from the regression database.

Flood-frequency estimates and basin characteristics for 828 gaged stations were combined to form the final database that was used in the regional regression analysis. Regional regression analysis, using generalized least-squares regression, was used to develop a set of predictive equations that can be used for estimating the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent chance exceedance flows for rural ungaged, basins in North Carolina, South Carolina, and Georgia. The final predictive equations are all functions of drainage area and the percentage of drainage basin within each of the five hydrologic regions. Average errors of prediction for these regression equations range from 34.0 to 47.7 percent.

Discharge estimates determined from the systematic records for the current study are, on average, larger in magnitude than those from a previous study for the highest percent chance exceedances (50 and 20 percent) and tend to be smaller than those from the previous study for the lower percent chance exceedances when all sites are considered as a group. For example, mean differences for sites in the Piedmont hydrologic region range from positive 0.5 percent for the 50-percent chance exceedance flow to negative 4.6 percent for the 0.2-percent chance exceedance flow when stations are grouped by hydrologic region. Similarly for the same hydrologic region, median differences range from positive 0.9 percent for the 50-percent chance exceedance flow to negative 7.1 percent for the 0.2-percent chance exceedance flow. However, mean and median percentage differences between the estimates from the previous and current studies vary substantially depending on the region and whether or not additional data were collected.

Revised February 20, 2015

First posted September 15, 2009

For additional information contact:
Director, USGS North Carolina Water Science Center
3916 Sunset Ridge Road
Raleigh, NC 27607

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Suggested citation:

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.




Purpose and Scope

Previous Studies in North Carolina

Description of Study Area


Data Compilation

Peak-Flow Data

Physical and Climatic Basin Characteristics

Estimation of Flood Magnitude and Frequency at Gaged Stations

Flood Frequency

Skew Coefficient

Generalized Skew Analysis

Gaged Stations Affected by Regulation

Estimation of Flood Magnitude and Frequency at Ungaged Sites

Regression Analysis

Regionalization of Flood-Frequency Estimates

Regional Regression Equations

Accuracy and Limitations

Analysis of Gaged Basins within Multiple Hydrologic Regions

Comparison of Results with Previous North Carolina Study

Maximum Floods

Application of Methods

Estimation for a Gaged Station

Estimation for an Ungaged Site near a Gaged Station

Summary and Conclusions

Selected References

Appendix—Development of Generalized Skew Coefficient

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