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<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 />I <br /> <br />5.4 <br /> <br />HYDROLOGY AND HYDRAULICS <br /> <br />5.4.1 PROBABLE MAXIMUM FLOOD <br /> <br />The Hourglass Reservoir drainage basin Is just north of the Rocky Mountain National Park. Its <br />headwaters are located along the north side of the 12,000 foot Mummy Range peaks. <br /> <br />Hourglass Dam and Reservoir are located in a small tributary to Beaver Creek. Beaver Creek <br />Is a tributary to the south fork of the Cache la Poudre River. The drainage basin area is 0.74 <br />square miles and has an average siope of 600 feet per mile. The basin is about 1.8 miles long <br />with an average width of about 0.4 mUes. The Soil Conservation Service dimensionless <br />t <br />hydrograph was used, with a lag time of 1.46. i c' . ~ ~ L" h., <br /> <br />J,',I- <br /> <br />-," I <br />Q, 7 rI ""p'" <br /> <br />/ " <br /> <br />The Hourglass Dam spillways are able to discharge the PMF with 2.8 feet of remaining freeboard <br />on the dam. The usual refinements of determining drainage basin infiltration rates and <br />incremental damage assessment were not needed and were not evaluated. Instead, all of the <br />probable maximum precipitation PMP was assumed to flow into the reservoir. The resulting <br />inflow was routed through the reservoir. <br /> <br />I <br /> <br />l' .~,cv' <br /> <br />d c. '^"'-" "" '/,,-, S '..). '~~ <br />,^,j., I ' <br /> <br />Lot',.' Stt,,-. <br /> <br />The PMP for Hourglass Reservoir. was taken from NOAA HydrometerologicaJ Report No. 55A. <br />The U.S. Army Corps of Engi9illrs HEC-l computer program was used to produce the inflow <br />hydrographs. The general storm was found to produce largest maximum probable flood, PMF. <br />The 6-hour PMP of 10.66 inches is the most severe. The resulting PMF hydrograph has a peak <br />discharge of 1 n2 cubic feet per second at 1.45 hours, and a volume of 3981 acre-feet. :' <br />-~--= L '7 <br />l' c.' 7" ~'. <br />pe,,,, 6' /"'-t 8c.'/'1 ",- <br />5.4.2 FLOOD ROUTING . ~'.~..,,; / <br /> <br />Flood routing was done with a starting elevation of 9366.5 feet, (34.5 gage) the crest of the side <br />channel spillway. The outlet gates were assumed to be closed. The spillway rating curve <br />developed and used is the U.S. Army Corps of Engineers Phase I Dam Inspection Dam <br />Inspection Report. The maximum storage in the reservoir is 1889 acre-feet for a 6-hour storm. <br />Peak stage 9329.2 feet (37.2 gage) at 3.5 hours. This leaves a freeboard of 2.8 feet for the <br />dam at maximum reservoir stage. <br /> <br />5-14 <br />