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<br />commonly termed the 10-, 50-, 100-, and 500-year floods, have ala, 2, 1 <br />and 0.2 percent chance, respectively, of being equaled or exceeded <br />during any year. Although the recurrence interval represents the long- <br />term, avera2e period between floods of a specific magnitude, rare floods <br />could occur at short intervals or even within the same year. The risk of <br />experiencing a rare flood increases when periods greater than 1 year are <br />considered. For example, the risk of having a flood which equals or <br />exceeds the 100-year flood (l percent chance of annual exceedence) in <br />any 50-year period is approximately 40 percent (4 in 10); for any <br />90-year period, the risk increases to approximately 60 percent (6 in <br />10). The analyses reported herein reflect flooding potentials based on <br />conditions existing in the community at the time of completion of this <br />study. Maps and flood elevations will be amended periodically to <br />reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge- <br />frequency relationships for each flooding source studied by <br />detailed methods affecting the community. <br /> <br />A hydrology report on the Colorado River dated May 1989, was <br />prepared by J.F. Sato and Associates, Inc. (Reference 7). In this <br />study, the peak discharges for the 10-, 50-, 100-, and 500-year <br />floods on the Colorado River from DeBeque Canyon to the Utah State <br />line were based on a statistical analysis of annual peak discharges <br />recorded at the following USGS gaging stations: <br /> <br />Ga2iml: Stat ion <br /> <br />Period of Record <br /> <br />Colorado River <br />near Cameo <br />near Palisade <br />near Fruita <br />near Colorado-Utah State Line <br /> <br />1934-1985 <br />1902-1933 <br />1911-1923 <br />1951-1985 <br /> <br />Gunnison River <br />near Grand Junction <br />(after Blue Mesa Dam) <br /> <br />1965-1985 <br /> <br />The frequency analysis was performed using the log-Pearson Type III <br />Method, in accordan~e with criteria outlined in Bulletin 178 <br />(Reference 8). <br /> <br />The peak flow series that were analyzed represent primarily <br />snowmelt floods which peak between late April and June. Following <br />the COE guidelines, a regional skew of zero was used for the Cameo <br />gage. The results of the computation compared favorably with the <br />results of the COE hydrology report on the Colorado River published <br />in 1985 (Reference 9) and are shown in Table 2. <br /> <br />To estimate peak discharges for the various recurrence intervals on <br />the Gunnison River near Grand Junction, records from the long-term <br />gage on the Gunnison River were analyzed, using the log-Pearson <br /> <br />11 <br />