Water Resources of the United States
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
Mississippi is divided into four regions (fig. 1). Three of these regions are defined by geographic boundaries and one by drainage area magnitude (streams outside the Mississippi River Delta having drainage areas greater than 800 square miles). The regression equations developed for these subgroups are for estimating peak discharges (QT) having recurrence intervals T that range from 2 to 500 years. The explanatory basin variables used in the equations are drainage area (A), in square miles; channel slope (S), in feet per mile, defined as the difference in altitude between points located at 10 and 85 percent of the main-channel length divided by the channel length between the two points; and main-channel length (L), in miles, from the point of discharge to the drainage divide as measured in 0.1 mile increments on topographic maps. At a stream junction, the branch draining the largest area is considered the main channel. The regression equations were developed from peak-discharge records available at 312 stations with 10 or more years of record. The standard errors of prediction of the equations range from 15 to 45 percent and the equations are applicable to floods for all natural drainage basins in Mississippi, except for the Pearl River main stem. A graphical relation of flood-frequency discharge to drainage area, with an adjustment for basin shape, is presented in the report for the Pearl River main stem. The report by Landers and Wilson (1991) includes flood-frequency discharges and basin characteristics for 330 gaged streams.
A user would select: (1) the Mississippi River Delta equations, if the stream is in the Delta; (2) the GT800 equations, if the stream is outside the Delta with drainage area greater than 800 square miles (GT800); or (3) the East or West equations, based on stream-site location (fig. 1), regions 1, 2, and 3 are the East, West, and Delta regions, respectively. The Delta and West boundary is crossed by stream basins sloping westward down the abrupt, dissected escarpment. For ungaged sites located in the Delta part of these basins, it is suggested that two discharges be estimated for each frequency by assuming all of the basin lies in each region and then averaging the discharges by areal weighting.
Topographic maps, the hydrologic regions map (fig. 1), and the following equations are used to estimate peak discharges QT, in cubic feet per second, having selected recurrence intervals T. Peak discharges for drainage basins affected by urbanization should be estimated using the equations from Sauer and others (1983), with the rural peak discharge estimated from the appropriate equation as shown below.
Figure 1. Flood-frequency region map for Mississippi. (PostScript file of Figure 1.)