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I I I I I I I I I I For these values, la = 0.4", S = 2, and the related curve number is 83. For comparison, we computed the CN by taking the BFITA qp = 484*A*Q/Tp, and computing Q, then S, and eN for each watershed. For Chase, the CN from BFIT A qp values is 78, We would expect the CN values to be closer, so it is likely that there are other adjustments made in the peak q determination. From this information it appears that the CN used was somewhere in the range of 78 to 83. If other adjustments were made th.at would impact the watershed variables such as time to peak or drainage area, then the CN values could be outside of the range observed here. Grel!orv Gulch Similar to Chase Gulch, the Gregory Gulch 100-year 6-hour hydrograph and hyetograph are shown on page 2-8 ofBFITA. The drainage area from page 2-6 is 3.47 square miles. The peak runoff value is 1,240 cfs. The runoff starts at hour 3 of the storm where 0.35" oftotal rainfall is shown, so Ia = 0.35". For these values, Ia = 0.35", S = 1,75, and the related curve number is 85. For comparison, we computed the CN by taking the BFITA qp = 484*A*Qrrp, and computing Q, then S, and CN for each watershed. For Gregory Gulch the eN from BFIT A qp values is 8 L Again, we would expect the CN values to be closer, so it is likely that there are other adjustments made in the peak q determination. From this information it appears that the CN used was somewhere in the range of 81 to 85. If other adjustments were made that would impact the watershed variables such as time to peak or drainage area, then the CN values could be outside of the range observed here. For Gregory Gulch, the triangular hydrograph for time = 3 hours was checked. Reading the peak q = 700 cfs from the plotted triangle, area A = 3.47 sq. mi., and Tp = 0.68 hours from the triangle, incremental Q, runoff inches = 0.283". The incremental rainfall for that period was 1.16 inches. From NEH 4, figure 10,1, the intersection of the rainfall and runoff occurs at CN 85. So the triangular hydrograph was definitely developed with this curve number. Graphically summing the triangular hydro graphs yields a value for the I DO-year peak flow between the curves shown on the plotted hydro graph. It appears that either plotting errors occurred or adjustments were made and reflected in the plotted hydro graph for the IOO-year flows, but not in the triangular hydrographs. From the available information we cannot determine which of these impacts may have occurred.