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<br />FLOOD HYDROLOGY <br /> <br />The discharge of the Arkansas River at Las Animas has been <br />measured by the U.S. Geological Survey since 1939. The gaging <br />station was located on the north bank, 1600 feet downstream from <br />the highway bridge, until 1966 when it was relocated to the down- <br />stream. south side of the highway bridge. In 1973, the gage was <br />relocated to the upstream south side of the highway bridge. <br />The annual peak discharges for the Las Animas gaging station <br />as reported by the Geological Survey are listed in table 2 for <br />the period 1939-1972. These peak discharges were ordered by <br />magnitude and plotted as a logarithmic distribution on figure 4. <br />Figure 4 also contains the flood-frequency curve published by <br />the Corps of Engineers for the Arkansas River at Las Animas and <br />a log Pearson Type III curve from unpublished data of the <br />Geological Survey. The Corps of Engineers reports that the <br />lOa-year flood for the Arkansas at Las Animas as modified by <br />Pueblo Dam is 81,000 cfs. The lOa-year discharge determined by <br />the Geological Survey is 34,500 cfs and this figure has not been <br />modified to account for flood regulation by Pueblo Dam. <br />The trend of the data points on figure 4 does not support <br />either of the frequency curves of the federal agencies. The <br />Corps of Engineers modified curve exceeds the recorded discharge <br />at all frequencies, while the Geological Survey unmodified curve <br />exceeds the lower discharges and is less than the high discharges. <br />Based upon the discharge data as plotted on figure 4, a log- <br />normal curve (sulid line) was drawn to determine the unmodified <br />flood frequency for the Arkansas River at Las Animas. This curve <br />indicates a lOa-year frequency flood discharge of 66,000 cfs. <br />A second curve (dashed line) was drawn to indicate the flood <br />frequency as modified by Pueblo Dam. This curve indicates a <br />lOa-year flood discharge of 47,000 cfs. <br />The actual effect of Pueblo Dam on peak reduction at Las <br />Animas is an uncertainty. As indicated by figure 3, floods can <br />occur downstream of Pueblo while there are insignificant flows <br />at Pueblo. In general however, these tributary floods do not <br />produce large peaks at Las Animas (ie. less than 20,000 cfs). <br /> <br />1 1 <br />