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• peak runoff rates ranged from 3.05 to 20.7 cfs and from 32.6 to 188 cfs for the 10 yr/24 hr and 100 yr124 hr <br />storms, respectively. <br />Storm cross-section plots were developed from a 1" = 400' scale Trapper topographic map with a contour <br />interval of 5 feet. The cross-section location was taken at the lower end of each basin. The cross-sections <br />may have been moved slightly upstream or downstream of the basin mouth in order to measure a uniform <br />section. Figure 2.7-15c shows the cross-sections at the mouth of the Pyeatt Gulch sub-basins and Figures <br />2.7-15d and 2.7-15e show the cross-sections at the mouth of the Flume Gulch sub-basins. The cross- <br />sections generally show wide parabolic shaped channels. <br />Peak flow velocities for both the 10 yr/24 hr and 100 yr/24 hr storms were computed for these cross-sections <br />using Manning's equation for velocity, namely: <br />v = (1.49/n)R~"' S'"~ <br />where: v = veloci ry in feet/sec <br />R =hydraulic radius, ft. <br />S =slope percent <br />n = Manning's n <br />• A Manning's n of 0.03 is considered representative for these channels. The resulting velocities are given in <br />Table 2.7-12a. The peaF; velocities for the 10 yr/24 hr storm for the drainages range between 3.02 to 10.4 <br />ft/sec. Two high velocity values of 7.15 and 10.4 ft/sec occur for the two larger drainages. Velocities from <br />6.21 to 21.2 ft/sec are predicted for the 100 yr/24 hr event. <br />As noted on Table 2.7-12:b, results presented in this section are derived from storm inputs assuming 1.7" of <br />rainfall for the 10 yr/24 hr event and 2.5 inches for the 100 yr/24 hr event. Subsequent determinations by the <br />National Oceanographic and Atmospheric Administration (NOAA) defined the 10-year, 24 hour rainfall event <br />as 1,4"and the 100-year, 24 hour rainfall event as 2.2". <br />Peak flow rates were not ~sstimated for the south-facing drainage basins, <br />r 1 <br />L_J <br />2-414h <br />tJ <br />,nA, o~. ~ _ ~ ~ <br />