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<br />In 1974, congressional authorization was given to the COE to <br />investigate the feasibility of certain flood protection measures <br />for Oak, Coal, and Chandler Creeks in Fremont County, Colorado. <br />Preliminary investigations carried out by the COE have included <br />site investigations, hydrologic studies, and hydraulic analyses of <br />each of these three flooding sources. Specifically, the project <br />proposals include flood control dams on Coal and Oak Creeks and a <br />diversion dam on Chandler Creek. None of these flood control <br />measures were considered in the hydrologic and hydraulic analyses <br />conducted herein; however, if these measures were achieved, they <br />would greatly reduce, and in most cases eliminate, flood hazards <br />from storms of frequencies as low as the SOO-year recurrence <br />interval. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magnitude which are expected to be equaled or exceeded once on the <br />average during any 10-, 50-, 100-, and 500-year period (recurrence <br />interval) have been selected as having special significance for <br />floodplain management and for flood insurance rates. These events, <br />commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, <br />1-, and 0.2-percent chance, respectively, of being equaled or exceeded <br />during any year. Although the recurrence interval represents the long- <br />term average period between floods of a specific magnitude, rare floods <br />could occur at short intervals or even within the same year. The risk <br />of experiencing a rare flood increases when periods greater than one <br />year are considered. For example, the risk of having a flood which <br />equals or exceeds the 100-year flood (l-percent chance of annual <br />exceedence) in any 50-year period is approximately 40 percent (4 in 10), <br />and, for any 90-year period, the risk increases to approximately <br />60 percent (6 in 10). For the Arkansas River and Chandler, Oak, and <br />Coal Creeks, the 10-, 50-, 100-, and 500-year recurrence interval floods <br />were considered, while for the NE Canyon Drainage area only the 100-year <br />recurrence interval flood was considered. The analyses reported herein <br />reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood <br />elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were completed to establish the peak <br />discharge-frequency relationships for floods of the selected <br />recurrence intervals for the Arkansas River. A regional <br />relationship of basin area to peak discharge was used for the <br />Arkansas River. Gaging stations located within the upper Arkansas <br />basin were used to determine the f1oodflow-frequency relationship <br />based on a log-Pearson Type III distribution for each selected <br />gaging station. <br /> <br />5 <br />