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<br />-3- <br /> <br /> <br />yields a mean annual flood of 1,300 cubic <br />feet per second (cfs) and a 100 year flood <br />discharge of 2,730 cfs. (See Figues 2 and <br />3 and calculations in Appendix) <br /> <br />Approach #2 <br /> <br />Since the lower portions of the basin are <br />subject to cloudburst runoff events, an <br />approach was taken whereby a 100 year <br />cloudburst runoff could be superimposed <br />on the mean annual discharge expected from <br />the upper portions of the basin. Runoff <br />from the portion of the basin above ele- <br />vation 7,500 feet was again estimated <br />using Magnitude and Frequency of Floods <br />in Colorado, Part 9, but based on a <br />higher mean elevation of 9,700 feet. <br />The expected 100 year runoff from a cloud- <br />burst event on the lower portions of the <br />basin below elevation 7,500 feet was <br />developed using the envelope curves from <br />an Internal Office Memorandum, Garfield <br />and Mesa Counties, Flood Insurance Study <br />Hydrology, November, 1975, Sacramento <br />District Corps of Engineers. This ap- <br />proach yielded a 100 year flood discharge <br />of 2,380 cfs. (See Figure 4 and calcula- <br />tions in Appendix) <br /> <br />Approach #3 <br /> <br />The third approach was to apply the envelope <br />curves for snowmelt runoffs developed by the <br />Corps of Engineers in the Internal Office <br />Memorandum, Garfield and Mesa Counties, <br />previously citen. Treating the Sweetwater <br />basin as an intermediate point between <br />mountain and plateau regions of Colorado, <br />a discharge of 2,370 cfs was obtained. <br />(See Figure 5 and calculations in Appendix) <br /> <br />As noted earlier, the Canyon Creek above New Castle and <br />Elk Creek at New Castle gauge records are available to <br />compare to the results of the regionalized approaches <br />used for these calculations. As a part of the Internal <br />Office Memorandum prepared by the Sacramento District <br />Corps of Engineers, the 100 year flood discharges for <br />these 2 gauges were calculated and are plotted as points <br />No. 10 and lIon Figure 5. As can been seen, these fall <br />at or below the intermediate line between mountain region <br />and plateau region utilized for Approach #3. Also, their <br />