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<br />interval) have been selected as having special significance for <br />floodplain management and for flood insurance premium rates. These <br />events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a <br />10, 2, 1 and 0.2 percent chance, respectively, of being equaled or <br />exceeded during any year. Although the recurrence interval represents <br />the long-term, averS2e period between floods of a specific magnitude, <br />rare floods could occur at short intervals or even within the same year. <br />The risk of experiencing a rare flood increases when periods greater <br />than 1 year are considered. For example, the risk of having a flood <br />which equals or exceeds the lOO-year flood (1 percent chance of annual <br />occurrence) in any 50-year period is approximately 40 percent (4 in 10); <br />for any 90-year period, the risk increases to approximately 60 percent <br />(6 in 10). The analyses reported herein reflect flooding potentials <br />based on conditions existing in the community at the time of completion <br />of this study. Maps and flood elevations will be amended periodically <br />to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge- <br />frequency relationships for floods of the selected recurrence <br />intervals for each flooding source studied by detailed methods <br />affecting the community. <br /> <br />Peak discharges for floods of the 10-, 50-, 100-, and SOa-year <br />recurrence intervals for the Gunnison and Uncompahgre Ri vers were <br />obtained from previous reports conducted by the U.S. Army Corps of <br />Engineers (References 5 and 6). In the U.S. Army Corps of <br />Engineers reports, a regional floodflow-frequency analysis of <br />gaging station records was made to obtain generalized relationships <br />of the mean, standard deviation, and skew coefficients to develop <br />floodflow-frequency relationships. This analysis included a <br />separate determination of rainflood and snowmelt peaks. <br /> <br />The floodflow-frequency curves for the Gunnison Ri ver were based <br />primarily on streamflow data collected at gages located near <br />Somerset (length of record, 1934 to present), near Grand Junction <br />(1897 to 1960), and below Gunnison Tunnel (1906-1960), and <br />secondarily on values adopted from other gages in the area. The <br />peak flows recorded since the regulation of flow by Paonia <br />Reservoir and Blue Mesa Reservoir were also used in the <br />determination of the peak flows. The present condition peak flow- <br />frequency curves for the ungaged sites on the Gunnison River were <br />developed by applying the difference between the pre-regulated and <br />present conditions for the gage located near Somerset and the gage <br />located below the Gunnison Tunnel to the pre-regulated peak flow- <br />frequency curves for the ungaged sites. <br /> <br />In 1988, hydrologic data for the Gunnison River upstream of Delta <br />County were updated for a water control manual for Blue Mesa Dam <br />and Reservo iron the Cunni son Ri ver ( Reference 9). The new da ta <br />were only slightly higher than the data in the 1983 Flood Insurance <br />Study and the 1980 Flood Hazard Information reports. Since the new <br />flow estimates fell wi thin the 90-percent confidence interval of <br /> <br />9 <br />