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<br />I I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Four on-stream lakes are located within the Dry Creek Basin. Flood <br />flows will be significantly attenuated through the two larger <br />lakes, Reservoir No. 15 and Douglas Lake. Douglas Lake is the <br />largest of these on-stream lakes and is located downstream of the <br />other three. Flood flows were routed through Douglas Lake <br />initially. The results of these calculations show that the other <br />on-stream lakes have no effect on peak discharges at the study <br />site in Fort Collins. <br /> <br />PRECIPITATION <br /> <br />The primary source of flood producing runoff in the Fort Collins <br />area is the summer thunderstorm. The rainfall data used in the <br />development of peak discharges on Dry Creek was taken from the <br />National Oceanic and Atmospheric Administration Atlas, Volume III, <br />Colorado (Ref. 1). six-hour and 24- hour rainfall depths are given <br />in Table I. Aerial adjustments to this rainfall data were made <br />as required in the NOAA Atlas. <br /> <br />Table I <br />Dry Creek Rainfall Data <br /> <br />3~luS <br /> <br />6-hour (inches) <br />2.2 <br />3.0 <br />3.4 <br /> <br />24-hour (inches) <br />3.2 <br />4.3 <br />4.8 <br /> <br />la-year <br />50-year <br />lOa-year <br /> <br />3.'1 ~ <br /> <br />RUNOFF COMPUTATIONS <br /> <br />Since flood .hydrographs were required to route flows through the <br />reservoir, the SCS method of developing emergency spillway hydro- <br />graphs was used for this study. This method is an extension of the <br />SCS curve number method and is outlined in Chapter 21 of the <br />SCS National Engineering Handbook (Ref. 2). <br /> <br />The effective drainage area above Douglas Lake is 46.4 square miles. <br />A weighted curve number of 79 was calculated for this basin using <br /> <br />-2- <br />