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The following documentation was taken from:

U.S. Geological Survey Water-Resources Investigations Report 94-4002: Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites, 1993

TENNESSEE

STATEWIDE RURAL

Summary

Tennessee is divided into four hydrologic areas (fig. 1). The regression equations developed for these areas are for estimating peak discharges (QT) having recurrence intervals T that range from 2 to 500 years. Drainage area (A), in square miles, the only explanatory basin variable, can be measured from topographic maps. The regression equations were developed from peak-discharge records for 304 gaging stations in Tennessee and adjoining States with each of these stations having 10 or more years of record of unregulated flows as of 1986. The average regional standard errors of prediction of the regression equations range from 32 to 48 percent. The report by Weaver and Gamble (1993) also includes peak discharges for selected recurrence intervals and maximum known discharges at gaging stations and at miscellaneous sites.

Procedure

Topographic maps, the hydrologic area map (fig. 1), and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

Reference

Weaver, J.D., and Gamble, C.R., 1993, Flood frequency of streams in rural basins of Tennessee: U.S. Geological Survey Water-Resources Investigations Report 92-4165, 38 p.

MEMPHIS URBAN

Summary

The regression equations developed for estimating peak discharges (QT) for urban streams in Memphis have recurrence intervals T that range from 2 to 100 years. The explanatory basin variables used in the equations are drainage area (A), in square miles; and channel condition (P). Channel condition (P) ranges from 1 to 2 and is the average of conditions at 100 percent, 75 percent, 50 percent, and 25 percent of the drainage area. If the channel is paved with concrete, assign a value to 2; if unpaved, assign a value of 1. The equations were developed from peak-discharge records for 25 stations and are recommended for estimating flood magnitudes for ungaged, urban streams in the Memphis area. The standard errors of estimate of the regression equations range from 13 to 18 percent.

Procedure

Topographic maps, knowledge of channel conditions (paved or unpaved), and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

Reference

Neeley, B.L., Jr., 1984, Flood frequency and storm runoff of urban areas of Memphis and Shelby County, Tennessee: U.S. Geological Survey Water-Resources Investigations Report 84-4110, 51 p.

STATEWIDE URBAN
(EXCEPT MEMPHIS AREA)

Summary

The regression equations developed for estimating peak discharges (QT) for urban streams in Tennessee (except the Memphis area) have recurrence intervals T that range from 2 to 100 years. The explanatory basin variables used in the equations are drainage area (A), in square miles; the percentage of the contributing drainage area occupied by impervious area (IA); and the 2-year 24-hour rainfall (P2_24), in inches. The variables A and IA can be measured from topographic maps and P2_24 can be determined from figure 2. The regression equations were developed from peak-discharge records for 22 stations having drainage areas of 0.21 to 24.3 square miles in municipalities with populations between 5,000 and 100,000. The peak-discharge records were extended by the use of a rainfall-runoff model. The equations are applicable to urban streams having drainage areas less than 24.3 square miles in Tennessee except for streams in the Memphis area, which have the specific equations described above. The standard errors of estimate range from 25 to 32 percent.

Procedure

Topographic maps, the 2-year 24-hour rainfall map (fig. 2), and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

Reference

Robbins, C.H., 1984, Synthesized flood frequency for small urban streams in Tennessee: U.S. Geological Survey Water-Resources Investigations Report 84-4182, 24 p.

Figure 1. Flood-frequency area map for Tennessee. (PostScript file of Figure 1.)

Figure 2. The 2-year 24-hour rainfall in Tennessee. (PostScript file of Figure 2.)