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<br /> <br />Table 4. Comparison of 1979 Flood Peak Estimates to TR20 Model Estimates, 100- <br />Year Event. <br /> <br />Chase and Gre 0 Gulches <br />Design 100, ear eak cfs <br />Point 1979 est. 200 1 Model <br />Cl 1160 1179 <br />C2 N/A 1157 <br />Gl 1240 1248 <br />G2 N/A 619 <br /> <br /> <br />North Clear Creek <br />100- ear ak cfs <br />Point 1979 est. 200 I Model <br />NCt 2694 2702 <br />NC2 2528 2524 <br />NC3 2280 2267 <br />NC4 N/A 1924 <br />NC5 N/A 1311 <br />NC6 N/A 929 <br /> <br /> <br /> <br />Note: See Figure 4 for design points. <br /> <br />IMPACT OF RESERVOIR STORAGE ON FLOOD PEAKS <br /> <br />The impact of the Chase Gulch and Dorothy Lee Reservoirs on flood peak discharges <br />was identified using the TR20 hydrologic models calibrated to reconstitute 1979 lOO-year <br />flood peak estimates. Storage/elevation curves (Figures 5 and 6) were developed from <br />design and as-constructed drawings of the two reservoirs (RMC, 1991, and RMC, 1994). <br />The discharge curve for Chase Gulch Reservoir uses the spillway discharge curve <br />developed for the reservoir shown in the Appendix, The discharge curve for Dorothy Lee <br />Reservoir was calculated using the FHW A HY -8 program in the Appendix. <br /> <br /> <br />The results of the analysis for the 10-, 50-, and IOO-year event are summarized at selected <br />design points on Table 5. The results for the IOO-year event are found in the Appendix. <br />The results indicate detention storage above the Chase Gulch water supply reservoir <br />spillway reduces flood peaks significantly at the outlet. At the outlet, design point C I, <br />IOO-year discharges are reduced from 1179 cfs to 551 cfs. Dorothy Lee Reservoir has <br />only a minor impact on flood peaks during the 100-year event at the outlet of Gregory <br />Gulch. At the outlet, design point Gl, 1 DO-year discharges are reduced from 1248 cfs to <br />1159 cfs. This reduction is less than predicted by a recent HEC-l analysis by ],F. Sato <br />(1996), which assumed a slightly lower curve number causing a lower peak discharge <br />than the current study. In addition, the current study defined the storage and discharge <br />relationship of Dorothy Lee Reservoir using design drawings and hydraulic calculations. <br />The 1996 Sato study used simplified hydraulic relationships and did not examine the as- <br />built information when modeling the reservoir, The TRlO model indicates the reservoir <br />overtops during the IOO-year event. <br /> <br />-10- <br /> <br />