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<br />r] <br /> <br />It was determined that the 100 year storm peak discharge for Ralston <br />Creek at the point where the channel enters the NW 1/4 of Section 25, T2S, <br />RI1W, is approximately 6990 c.f.s. <br />Table 2.2-2 <br />Assumptions for flood Hydrograph Computation a <br />1. 100 yr. - 6 hr. storm - 3.8 inches b <br />2. Drainage Area (upstream from Ore Sorter) - 38 square miles <br />3. Average Watershed Slope - 220 feet per mile or 4% <br />4. Approximate Drainage Basin Length - 16.5 miles <br />5. Hydrologic Soil Groups - Dc <br />6. Antecedent Moisture Condition - II (Average) <br />7. Estimated Runoff Curve Number - 79~ <br />8. Time of Concentration - 7.4 hours <br />9. Type II storm distribution above 8000 ft. elevation <br />a See discussion pertaining to the Probable Maximum Flood in <br />Section 2.2-2, above. <br />• b Precipitation frequency atlas of the Western U. S., Volume 3, <br />National Oceanic and Atmospheric Administration. <br />c As defined by SCS National Engineering Handbook, Section 4, Hydrology, <br />chapters 7-10, 15, and 21. <br />The peak discharge value of 6990 cfs. determined by the method described <br />by King and Wilkes compares favorably with the 100 year storm discharge <br />calculated for the design of Ralston Reservoir. (Denver Urban Drain,~ge and <br />Flood Control District, 1978.) The Ralston Reservoir calculation assumed a <br />watershed of approximately 46 square miles and resulted in a peak dig;charge <br />value of 7200 c.f.s. <br />A flood level analysis using Manning's Equation with the discharge <br />value of 6990 c.f.s. was made for the Ralston Creek channel in the vicinity <br />of the ore sorter. (See Figure 2.2-1) <br />lJ <br />2-7 <br />