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4-21 The method described in the manual consists of a series of • regression equations for several flood frequencies for each region. The regression equations relate flood peak dis- charges to basin and climate parameters. The mountain equations relate drainage area and mean annual precipitation to peak discharge; the southern plateau equations relate drainage area to peak discharge. The equations are derived from long term records at gauged sites within each region. Each equation has a corresponding error of estimate which measures how well the equation fits the data from which it is derived. The East Salt Creek basin lies in both the mountain and southern palteau regions as shown on Figure 4.2-15. Runoff records at the gauging station on East Salt Creek near Mack indicate that monthly flows generally peak in May; however, the extremes of record have occurred in September, 1972 (2,160 cfs by slope-area measurement) and July, 1974 (2,630 cfs) (Hydro- . Triad, 1979). Technical Manual No. 1 describes a method for computing peak discharges in the transitional region. Application of this method to the parameters of the East Salt Creek basin shows that the peak discharges from the Southern Plateau Region are much larger than the Mountain Region Peak Flows. There- fore, the Southern Plateau Region Discharges are used. These discharges for several drainage areas in the East Salt Creek basin are given in Table 4.2-85, (Hydro-Triad, 1980). 4.3.4.2 Hydraulic Analysis of 100-Year Flood In order to determine the approximate flooding limits of the 100-year flood on East Salt Creek at the central facilities site, the U.S. Army Corps of Engineers HEC-2 Water Surface Profile computer model was used. This model uses input para- meters of peak discharge, streambed and floodplain topography • as well as roughness characteristics of the stream channel and floodplain to model hydraulic characteristics (HEC, 1981).