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<br />of Colorado River, the 1884 flood of record was selected as the <br />Standard Project Flood in the Grand Junction area. Frequency <br />curves for Colorado River at Glenwood Sprinqs (U.S. Geolooical <br />Survey gage No. 09072500) and Cunnison River above Crand Junction <br />(U.S. Geological Survey gage No. 09152500) indicated that the <br />1921 flood was a suitable event to pattern the standard Project <br />Flood. <br /> <br />The Standard Project Flood is defined as the flood resulting from <br />the most severe combination of meteorological conditions that <br />are considered reasonably characteristic of the geographical area <br />in which the stream basin is located. The 1921 flood was one <br />of the largest on both rivers and was a late season flood (maximum <br />peak on June 15) which would make it a critical flood with regard <br />to Blue Mesa Reservoir routings. Comparison of the 1884 flood <br />with the frequency curves indicated that the 1884 flood was 150 <br />percent that of the 1921 flood. The Standard Project Flood on <br />Colorado River upstream of Grand Junction was determined by multi- <br />plying the 1921 discharge at the Palisade (U.S. Geological Survey <br />gage No. 09106000) by 1.5. The Standard Project Flood on Gunnison <br />River above Grand Junction was computed by multiplying the 1921 <br />discharge (as determined at gage No. 09152500) by 1.5. For Colorado <br />River, the effects of upstream regulation were taken into account <br />by subtracting 8000 cfs from the flows of the Standard Project <br />Flood at Palisade. The flows of the Standard Project Flood on <br />Gunnison River were divided into runoff at Blue Mesa Reservoir <br />and local runoff below the reservoir. Runoff above Blue Mesa <br />Reservoir (55 percent of total) was computed and routed through <br />the structure. The outflow from the reservoir was routed to Grand <br />Junction, and the local runoff below the reservoir was added. <br />The flows of the Standard Project Flood of the two rivers were <br />combined to give a total flow downstream of their confluence. <br />Peak discharges for floods of the 10-, 50-, 100-, and 500-year <br />recurrence intervals were derived by computing percentages of <br />the Standard Project Flood based on the slope of the frequency <br />curves. <br /> <br />For Indian Wash, the principal cause of flooding has resulted <br />from intense cloudburst storms. Snowmelt flooding is relatively <br />insignificant due to the low altitude. As no stream gage data <br />are available for Indian Wash and few are available for the general <br />area, and, also, due to the presence of the flood-retention struc- <br />ture, a rainfall-runoff analysis was conducted on the watershed <br />to determine tbe flood discharges. This was accomplished by using <br />the U.S. Army Corps of Engineers HEC-l rainfall-runoff computer <br />program (Reference 7). For this analysis, basin characteristics <br />which define the size, shape, and runoff characteristics of the <br />watershed, as well as rainfall amounts based on the selected recur- <br />rence intervals, are used to compute flood hydrographs for various <br />design points in the basin. <br /> <br />15 <br />