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*

Florida is divided into four hydrologic regions (fig. 1) in one of which floods are undefined. The regression equations developed for these regions 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 (SL), in feet per mile; and the area of lakes and ponds (LK) as percentage of drainage area. The constants of 3 and 0.6 are added to LK in the computer application of the regression equations. The user should enter the actual value of LK. These variables can be measured from topographic maps. The regression equations were developed from peak-discharge records for 182 gaging stations. The regression equations are applicable to natural-flow streams and they do not apply to the undefined area shown on figure 1. The standard errors of estimate of the regression equations range from 40 to 60 percent for Region A, 60 to 65 percent for Region B, and 44 to 76 percent for Region C. The report by Bridges (1982) includes a graph showing relations of flood characteristics to drainage area along a reach of the Apalachicola River.

Topographic maps, the hydrologic regions 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.

Bridges, W.C., 1982, Technique for estimating magnitude and frequency of floods on natural-flow streams in Florida: U.S. Geological Survey Water-Resources Investigations Report 82-4012, 45 p.

A separate flood-frequency analysis was performed using data for urban streams in the western part of Hillsborough County and all of Pinellas County near Tampa. The regression equations developed for this area are for estimating peak discharges (QT) having 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; basin development factor (BDF) (defined earlier in this report in the section entitled Urban Flood Frequency Techniques); main-channel slope (S), in feet per mile; and detention storage area (DTENA), which is the percentage of the drainage area covered by natural lakes or ponds, detention basins, and retention basins. A constant of 0.01 is added to DTENA in computer application of the regression equations. The user should enter the actual value of DTENA. These variables can be measured from topographic maps. The regression equations were developed from peak-discharge records of 9 streams and rainfall records at 13 sites and are applicable to urban streams in the Tampa Bay area draining less than 10 square miles with DTENA less than 5 percent. The standard errors of estimate of the regression equations range from 32 to 42 percent.

Topographic maps and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

Lopez, M.A., and Woodham, W.M., 1982, Magnitude and frequency of flooding on small urban watersheds in the Tampa Bay area, west-central Florida: U.S. Geological Survey Investigations Report 82-42, 52 p.

A separate flood-frequency analysis was performed using data for urban streams in Leon County, Florida. Two sets of regression equations were developed for estimating peak discharges (QT)--one set for streams in Lake Lafayette basin, and the other for streams in Leon County, outside Lake Lafayette basin. The range of recurrence intervals T for the regression equations range from 2 to 500 years. The explanatory basin variables used in the equations are drainage area (DA), in square miles; and impervious area (IA) as a percentage of the drainage area. These variables can be measured from topographic maps and, in the case of IA, from aerial photographs. The regression equations were developed from peak-discharge records based on rainfall-runoff modeling at 15 gaged streams in the county and are applicable to developing basins in Leon County. The drainage areas ranged from 0.2 to 16 square miles and impervious area ranged from 5.8 to 54 percent. The standard errors of estimate of the regression equations range from 18 to 30 percent.

Topographic maps and (or) aerial photographs and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

LAKE LAFAYETTE BASIN

Franklin, M.A., and Losey, G.T., 1984, Magnitude and frequency of floods from urban streams in Leon County, Florida: U.S. Geological Survey Water-Resources Investigations Report 84-4004, 37 p.

Figure 1. Flood-frequency region map for Florida. (PostScript file of Figure 1.)