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USGS Water Science Centers are located in each state.

Techniques for Estimating Magnitude and Frequency of Floods for Rural Basins in South Carolina

Project Number: 2519-CH001
Project Chief: Toby Feaster
Cooperator: South Carolina Department of Transportation
Period of Project: May 1, 2006 to February 2, 2009

SUMMARY

Flood-frequency estimates are required for the design of bridges, highway embankments, culverts, levees, and other structures near streams. In addition, such estimates are needed for the effective planning and management of land and water resources, to protect lives and property in flood-prone areas, and to determine flood-insurance rates. Flood-frequency estimates are often needed at locations where no streamflow information is available. Regional methods provide a way to estimate flood magnitudes and frequencies at such ungaged sites. South Carolina, Georgia, and North Carolina are involved in a unique investigation that has the potential to improve flood-estimate techniques in our states and enhance methods nationally. Such important work is made possible by cooperation between the U.S. Geological Survey (USGS) and our state partners.

PROBLEM

Equations to estimate the magnitude and frequency of selected recurrence-interval peak flows are part of a technically based framework for flood-plain delineation and hydraulic-structure design in South Carolina. The South Carolina Department of Natural Resources (SCDNR) uses the equations for flood-plain management and flood-insurance studies. The South Carolina Department of Transportation (SCDOT) also uses these equations in the design of bridges, culverts, and road embankments. Currently, procedures documented in Feaster and Tasker (2002) are used to estimate selected-year peak flows. These procedures were developed based on data through the 1999 water year. Water year is the 12-month period beginning October 1 and ending on September 30 of any given year, and designated by the calendar year in which the water year ends.

Flood-frequency estimates made at gaged sites and regional flood-frequency equations developed from the gaged-site estimates contain varying degrees of uncertainty based on numerous factors such as length of record, number of stations available for regionalization, range of basin characteristics, and so on. To provide simple methods of estimating flood-peak estimates, the USGS has developed and published regression equations for every State, the Commonwealth of Puerto Rico, and a number of metropolitan areas in the United States (http://water.usgs.gov/osw/programs/nffp.html). These investigations are typically not coordinated between neighboring states and are therefore based on various periods of record and sometimes result in flood regions that are not contiguous at the state boundaries. Thus, if flood frequency estimates are needed for a basin that crosses state boundaries, engineers and water resources managers may have to deal with multiple equations based on an assortment of basin characteristics with results that have varying degrees of uncertainty.

What is a 100-year Flood?

OBJECTIVES AND SCOPE

For this flood-frequency investigation, the USGS South Carolina, North Carolina, and Georgia Water Science Centers will be coordinating efforts. Each state will be updating the flood-frequency estimates at appropriate gaging stations and will then coordinate efforts with respect to a regional regression analysis.

The objectives for South Carolina will be:

• Update magnitudes and frequencies of peak streamflow at unregulated streams in South Carolina where adequate data are available and at regulated streams where appropriate.
• In coordination with the USGS North Carolina and Georgia Water Science Centers, establish physiographic regions that are consistent at the state boundaries.
• In coordination with the USGS North Carolina and Georgia Water Science Centers and the USGS Office of Surface Water, develop a method for obtaining generalized skews using skew coefficients from gaging stations having 25 or more years of record.
• Coordination and concurrence with the North Carolina and Georgia Water Science Centers on the explanatory variables for the flood regions at the state boundaries.
• Update the regional rural-flood-frequency equations for the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence interval flows.

APPROACH

The project objectives will be accomplished jointly with the USGS North Carolina and Georgia Water Science Centers. Peak-flow data will be retrieved from the USGS files. Quality-assurance and quality-control procedures will be used to ensure the accuracy of the peak-flow file with the National Water Information System (NWIS) database. Similar procedures will be applied to the Georgia and North Carolina peak-flow files. The frequency curves will be fitted mathematically to data using the log-Pearson Type III distribution. For some streams, if the log-Pearson Type III fit to the data is unacceptable, then a graphical curve-fitting procedure may be used. A minimum of 10 years of peak-flow data at continuous- or partial-record gages through the 2006 water year is required for the analysis.

Prior to the development of regression equations, the three states will update basin characteristics using consistent geographical information system (GIS) methods and where possible, the same GIS coverages. In addition, the Water Science Centers will establish consistent physiographic regions at the state boundaries. Based on the review of the streamflow and basin-characteristics data, sub-areas within the physiographic regions may be identified.

The 2-, 5-, 10-, 25-, 50-, 100-, and 500-year peak flows of rural, unregulated streams will be related to basin characteristics, such as stream length, slope, drainage area, and so on, by log-linear regression programs. Regression equations will be developed, which can be used to estimate peak flow at ungaged sites. As done by Feaster and Tasker (2002), a combination of ordinary least squares (OLS) and generalized least squares (GLS) regression techniques will be used to develop the equations. The OLS techniques will be used to select the explanatory variables to be used in the final equations and to determine the regionalization scheme for the states. The GLS techniques will be used to compute the final coefficients and to measure the accuracy of the regression equations. Generalized least squares regression equations have been determined to be more accurate than OLS regression equations when streamflow data at gaging stations are of different and widely varying lengths (Stedinger and Tasker, 1985).

PLANNED REPORTS

The results of the project will be documented in a USGS Scientific Investigations Report, tentatively titled "Magnitude and frequency of floods in South Carolina, 2006." A link to the report will be posted on the South Carolina Water Science Center web site.

REFERENCES

Feaster, T.D., and Tasker, 2002, Techniques for estimating the magnitude and frequency of floods in rural basins of South Carolina, 1999: U.S. Geological Survey Water-Resources Investigations Report 02-4140, 34 p.

Pope, B.F., Tasker, G.D. and Robbins, J.C., 2001, Estimating magnitude and frequency of floods in rural basins of North Carolina-revised: U.S. Geological Survey Water-Resources Investigations Report 01-4207, 44 p.

Stamey, T.C., and Hess, G.W., 1993, Techniques for estimating magnitude and frequency of floods in rural basins of Georgia: U.S. Geological Survey Water-Resources Investigations Report 93-4016, 75 p.

Stedinger, J.R., and Tasker, G.D., 1985, Regional hydrologic analysis 1. Ordinary, weighted, and generalized least square compared: Water-Resources Research, v. 21, no. 9, p. 1421-1432.