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<br />/. <br /> <br />20 <br /> <br />. <br /> <br />field observations set limits on the. topograph.ic map data, allowing <br /> <br />construction of a maximum and minimum cross sectional area. Two <br /> <br />slopes were determined from the topographic map; the slope of the <br /> <br />channel and of the flood plain. In both cases. an ass-umpt,ion was <br /> <br />made that these gradients' were reasonable approximations of the <br /> <br />energy gradient. Roughness (n) was estimated using the guidelines <br /> <br />set forth by Chow (1959) and Barnes' (1967). <br />Discharges were then calculated using the above information. <br /> <br />Figure '5 shows how the calculation was made for Lone Tree Creek; <br /> <br />Table 3 summarizes data and results, and Plate 1 shows the locations. <br /> <br />Discharge maxima and minima are given in Table 3 because of the <br /> <br />variable nature of, several of the parameters, The maxima and minima <br /> <br />reflect the largest 'possible range 'in values (standard error would <br /> <br />be smaller) and is due primarily to variati?ns in cross sectional <br /> <br />area and roughness. <br /> <br />On Lone Tree Creek (cross section 6, Plate 1), a discharge <br /> <br />of 5,100 cfs :!: 30 percent was calculated, which agrees well with a <br /> <br />flood discharge gaged a,t Nunn, of 5,810 c.f.s. It seems that the <br /> <br />flood mapped may be smaller than flood estimated to be 2,4 times <br /> <br /> <br />the magnitude of a 1/50 probability flood (Table 1). This suggests <br /> <br /> <br />that the area mapped as susceptible to flooding on Lone Tree Creek <br /> <br /> <br />is an area that could be inundated one or more times during a <br /> <br />lifetime, but not as often as eve'ry t~n. years. <br /> <br />Flood discharges for the Cache la Poudre River (cross <br /> <br />sections 1 and 2, Plate 1) were calculated at 63,000 c,f.s. :!: <br /> <br />11 percent (Table 3). Unfortunately, there are no gaged discharges <br />