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study developed regional equations for Colorado including the Plains and <br /> incorporated basin area and the six-hour 100-year storm precipitation as shown in <br /> NOAA Atlas 2 (reference 17). The equation for one percent annual chance peak <br /> flow in the Plains region of Colorado is shown below. <br /> Q 100 =100 88 A0 63 6P 100298 <br /> Where, <br /> Q =peak flow, cfs <br /> A =drainage area, square miles (0.5<A<2,930) <br /> 6P100 = 6hr 100-year precipitation in inches, 5.1 inches at Holly (2.4<P<5.1) <br /> Flows on Willow Creek were calculated by the U.S. Army Corps of Engineers <br /> using a synthetic unit hydrograph that was developed from a flood event on Salt <br /> Draw near Orla, Texas. The flows were compared to a gage station analysis of <br /> the Arkansas River U.S.G.S. gage at Lamar. The Arkansas River at Lamar gage <br /> has a period of record of 1921 to the current year, with the data from 1944 to the <br /> current year reflecting the operation of John Markin Reservoir. The peak flows <br /> obtained by the U.S. Army Corps of Engineers, in their gage analysis, agreed with <br /> the unit hydrograph analysis. The study contractor checked this gage analysis by <br /> using Reference 18, and obtained peak flow rates within the 90 percent <br /> confidence limits of the frequency-discharge curve obtained in the "Review <br /> Survey for Water Resource Development, Lamar, Colorado, and Vicinity, <br /> Hydrology (Revised) 1978" (Reference 1). <br /> The peak discharges used in this study are shown in Table 1. <br /> 9 <br />