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<br />! <br />I- <br />I <br /> <br />A - Soil moisture at wilting point <br />Late summer or fall <br />DTHETA = 0.35 <br />B - Soil moisture at tield capacity <br />Early summer <br />DTHETA = 0.25 <br />C - Soil moistur,e at wet condl tion <br />Spring after snowmelt or after <br />previous .>torm <br />DTHETA = 0.10 <br />D - Soil moisture at saturation <br />Theoretical limit since saturation is not <br />a practical condition <br />DTHETA = 0.0 <br /> <br />peak Dischargg, <br />8,280 cis <br /> <br />9,735 ds <br /> <br />12,690 cfs <br /> <br />17,730 cis <br /> <br />Comparison of the values listed above with the accepted values indicates <br />that a 100-year flood peak of 11,600 cfs could occur with an antecedent soil <br />moisture that is slightly dryer (more drained) than the soil moisture immedi- <br />ately after snowmelt. That would be a reasonable assumption for conditions <br />that often exist in the watershed. A graph of thl! accepted flood frequency <br />values and the flood peak discharges from the model under the four assumptions <br />of antecedent soil moisture is shown in Figure 2. <br /> <br />FINAL HODEL CONHGURlITION <br /> <br />The model was run for I:he condition of 2 percent imperviousness (RTIMP = <br />2). This increased the peak discharge for the 100-year event from 9,735 cts <br />to 10,000 cfs, a 3 percent increase. This was not jUdged to be significant <br />and the imperviousness at B,lch subbasin '.as left as shown in Table 1. <br /> <br />The 100-year flood hydrographs from the model (both for DTHETA = 0.10 and <br />0.25) were plotted on a graph of the 100--year hydrograph that was d,eveloped by <br />the Corps of Engi neers. Th:ls is shown in Figure 3. The hydrographs f rom the <br />HEC-l model is similar to the Corps' hydrograph. That is, the rising and <br />falling limbs of the hydrographs are parallel and the peaks are comparable. <br /> <br />6 <br />