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<br /> <br /> <br />urban development of the entire high plains. area. In either case considera- <br /> <br />tion was given to floods originating in both the, upper and lower basins <br /> <br />throughout the profile reach as discussed below. <br /> <br /> <br />b. Floods from the Upper Basin,- Floods originating in the upper <br /> <br /> <br />Clear Creek basin determined with the MITCAT model were based on a 400- <br /> <br /> <br />square mile storm centering. Residual inflows from the l75-square mile <br /> <br />lower area were determined with SWMM, combined with flow'from the upstream <br /> <br />area and routed to Derby using the diffusion routing scheme. The 10-year <br /> <br />flood discharge from the upper basin was assumed to have the same magnitude <br /> <br />as that determined arlalytically from the Golden gage record because it, was <br /> <br /> <br />significantly higher than the corresponding discharge estimated with MITCAT. <br /> <br /> <br />This event was assumed to be a snowmelt event and attenuation was considered <br /> <br /> <br />to be minimal as it was routed through the lower basin. <br /> <br />c. Floods from the Lower'Basin. Floods originating in the lower basin <br /> <br />determined with SWMM were based on a l75-square mile storm centering. The <br /> <br />diffusion routing scheme was used to route the SWMM generated hydrographs. <br /> <br />Since these events were treated as being independent of floods in the upper <br /> <br />basin the contribution from that portion of the basin was limited to an esti- <br /> <br />mated coincident snmiIDelt discharge of 950 cubic feet per second which is the <br /> <br />average annual June flow. The flood peaks generated from runoff over the <br /> <br />lower basin dominated all but one of the peak discharge profiles from Ralston <br /> <br />Creek to the mouth. The SOO-year peak computed with SWMM was about 1000 cubic <br /> <br />feet per second lower than the combined upper basin MITCAT and lower basin SWMM <br /> <br />residual results. The apparent inconsistency was the result of higher recession <br /> <br />discharges on the l~Ner basin SWMM residual floods as they were combined with <br /> <br />the upper basin floods for this particular event. <br /> <br />8 <br />