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<br />equalled or exceeded once on the average during any 10, 50, 100, and 500 <br />year period (recurrence-interval) have been selected as having special <br />significance for flood plain management and for flood insurance premium <br />ra tes. These events commonly termed the 10-, 50-, 100- and SOD-year <br />floods, have aID, 2, 1 and 0.2 percent chance, respectively, of beina <br />equalled or exceeded during anyone year. Although the recurrence <br />interval represents the long term average period between floods of a <br />specific magnitude, rare floods could occur at short intervals or even <br />within the same year. The risk of experiencing a rare flood increases <br />when peri ods greater than 1 year are consi dered. For exampl e, the ri sk <br />of having a flood which equal s or exceeds the 100-year flood (1 percent <br />chance of annual occurrence) in any 50-year period is approximately 40 <br />percent (4 in 1O), and, for any gO-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported here reflect <br />flooding potential s based on conditions existing in the county at the <br />time of completion of this study. Maps and flood E'levations will be <br />amended periodically to reflect future chan,ges. <br /> <br />3.1 HydrOlogic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak <br />discharge-frequency relationships for floods of the selected <br />recurrence interval s for each stream studied in detail and to <br />compa re results between previous reports for the fl oodi ng sources <br />under study here. <br /> <br />A regional relationship of basin area to peak discharge was used <br />for the Arkansas River, King Arroyo and Anderson Arroyo. Gaging <br />stations located within the upper Arkansas basin were used to <br />establish the regional relationships. The method developed by the <br />u. S. Water Resources Council (Reference 4) wa s used to determi ne <br />the fl oodflow- frequency relationship, based on a Log-Pearson <br />Type III di stribution for each sel ected gaging station. Gaging <br />stations used for developing the regional relationship were <br />selected according to the following criteria: (1) stations having <br />10 or more years of record, and (2) stations where peak flows are <br />not affected by regulations and diversion. The annual peak <br />discharges were obtained from U.S. GeOlogical Survey Water-Supply <br />Paper Nos, 1681, 1921, and 3721 (References 3, 5, and 6, <br />respectively) and from Water Resources Data for Colorado (Reference <br />7). Regional relationships of basin area to peak discharge were <br />determined for the 10-, 50-, 100-, and 500-year floods. <br /> <br />Stream gaging record analysis was used to determin€' a peak <br />discharge-frequency relationship for the Arkansas River, King <br />Arroyo and Anderson Arroyo in La Junta. The results were compared <br />to the U.S. Army Corps of Engineers Flood Hazard Study (Reference <br />I) and after review showed a reasonable comparison. Therefore, the <br />resul ts of the COE Fl ood Hazard Study were adopted for thi s Flood <br />Insurance Study. <br /> <br />Peak discharge and drainage areas for the Arkansas River at La <br />Junta, Anderson Arroyo and King Arroyo are shown on Table 1. <br /> <br />7 <br />