National Flood Frequency Bulletin
Issue No. 2
The purpose of the NFF mailing list is to provide information on the U.S. Geological Survey (USGS) National Flood Frequency (NFF) program. NFF is a computer application that is used to estimate peak discharges for unregulated streams. Values (discharges) derived by the program often serve as input for other applications (such as hydraulic computer models) that are used to technically support end-products (map revisions) of the National Flood Insurance Program (NFIP).
The National Flood Frequency Program and USGS Regression Equations
Introduction
The US Geological Survey (USGS) has developed regional regression equations to estimate
the flood frequency and magnitude at ungaged locations of a watershed.
These equations are based on flood frequency and magnitude from
gaged watersheds. Regression equations transfer flood characteristics
from gaged sites to ungaged sites through the use of topographic,
physical, and climatic characteristics of the ungaged watershed.
The USGS regression equations published up to September 30, 1993, are summarized in the USGS Water Resources Investigations Report 94-4002, titled Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods for Ungaged Sites, 1993. Detailed USGS Water Resources Investigations Reports, summarizing the development of the regression equations for each State or Commonwealth, are published by the respective USGS district office. The USGS Water Resources Investigations Reports for many States/Commonwealths have been revised since the aforementioned report. These reports are available from the USGS.
The USGS has also developed the National Flood Frequency (NFF) computer program that estimates the flood frequency and magnitude for ungaged sites by the application of the appropriate regional regression equations. NFF was released in 1993; however, it does not incorporate any revisions to regional regression equations that occurred after September 30, 1993. NFF is being revised and will be released soon.
Applicability
Because the majority of stream gage data used in developing the regression equations
were collected from rural watersheds, the applicability of the regression
equations is generally restricted to rural watersheds.
Urban watersheds exhibit different flow characteristics than rural watersheds. Therefore, regression equations for urban watersheds have been developed for some States/Commonwealths. These urban equations adjust the peak-flow discharges computed using rural regression equations to reflect the effects of urbanization of watersheds. Urban equations have characteristics similar to those of rural equations; however, they attempt to include a measure of urbanization by using such parameters as the percentage of impervious areas and urban development factors.
Urban regression equations are available for the following states: Alabama, Georgia, Montana, North Carolina, Ohio, Oregon, Tennessee, Texas, and Wisconsin. The USGS and several States/Commonwealths are actively developing urban equations. Users should check with the USGS district office for up-to-date information. If State- or Commonwealth-specific urban regression equations are not available, the general guidance provided by the 1983 USGS Water-Supply Paper 2207, Flood Characteristics of Urban Watersheds in the United States, should be used to establish the flood magnitude and frequency of ungaged sites in urban watersheds.
Generally, USGS regional regression equations are not appropriate for estimating flood magnitude and frequency of ungaged locations or watersheds subjected to flow regulation. Watersheds with flow regulation contain streams whose flow characteristics are affected by such flood-control measures as reservoirs. The regression equations may not be applicable to heavily mined areas or karst areas where excessive runoff is diverted into or outside the basin. Regression equations should not be applied to streams where variables fall outside the range of variables used to develop the equations.
Advantages and Limitations
USGS rural regression equations are available for all 50 States/Commonwealth, Puerto Rico,
and American Samoa. The rural and urban regression equations are
based on observed peak-flow discharges and sound statistical methodologies.
The regression equations are simple to use and are accepted by the
Federal Emergency Management Agency (FEMA) and many Federal agencies.
Due to the relatively limited number of years of stream gage data availability, large estimation errors are inherent in the peak-flow discharges estimated using USGS regional regression equations. The number of years of stream gage data developed from urban watersheds is even more limited than that for rural watersheds. This leads to standard errors associated with urban regression equations being larger than those for rural regression equations.
For some States/Commonwealths, the regression equations used to estimate the 500-year flood magnitude are not defined; thus a graphical procedure should be used to determine the 500-year peak-flow discharges. The graphical procedure is outlined in the 1994 USGS Water Resources Investigations Report 94-4002, titled Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods for Ungaged Sites, 1993.
Next Month: Discussion on Specific Application of USGS Regression Equations to the National Flood Insurance Program
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Last Modified: Friday, 22-Jun-2007 11:57:20 EDT