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<br />are expected to be equaled or exceeded once on the average during any <br />10-, 50-, 100-, or 500-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance rates. These events, commonly termed the 10-, 50-, 100-, and <br />500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equaled or exceeded during any 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 periods greater than 1 year are considered. For example, the risk <br />of having a flood which equals or exceeds the 100-year flood (1 percent <br />chance of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 in 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect <br />flooding potentials based on conditions existing in the community at the <br />time of completion of this study. Maps and flood elevations will be <br />amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for each flooding SOUrce studied in detail <br />affecting the community. <br /> <br />Flood hydrographs and peak discharges for the 10-, 50-, 100-, and <br />500-year flood events on the North Fork Republican River were based <br />on statistical analyses of streamflow records of several drainage <br />basins in the same geographical region as the North Fork Republican <br />River basin. <br /> <br />A frequency-discharge curve for floods on the North Fork Republican <br />River was established using log-Pearson Type III methods outlined <br />by the U.S. Water Resources Council (Reference 5). The hydrologic <br />analysis is described in greater detail in the Technical Addendum <br />of the COE Flood Plain Information Report (Reference 6), where the <br />analysis is referred to as the Upper Republican River Basin Regional <br />Frequency Study. <br /> <br />Peak discharge-drainage area relationships for the North Fork <br />Republican River are shown in Table 2. <br /> <br />There are two gaging stations maintained by the U.S. Geological <br />Survey on the North Fork Republican River. The Colorado-Nebraska <br />State line station, located approximmately 10 miles east of Wray, <br />was established in March 1931 and has been in continuous operation. <br />The station is situated on the right bank, 100 feet east of the <br />ColoradO-Nebraska State line and approximately one-half mile north <br />of U.S. Highway 34. Intermittent records are also available for <br />the Wray gaging station, located approximately 5 miles east of <br />Wray. This station was establiShed in March 1937 and operated <br />until October 1946. In October 1951, the station was reestablished <br /> <br />B <br />