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<br />The runoff from snowmelt typically sustains periods of high flows <br />and marked daily fluctuation. <br /> <br />The steep rocky tributaries to the Uncompahgre River in the vicinity <br />of Ouray have had a long history of intense and devastating floods. <br />The earliest documented report was in 1874, and incidents of serious <br />flooding have been reported on at least 25 occasions since then. <br /> <br />Flow records are not available for all flooding events; however, <br />newspaper accounts show that major floods occurred in August 1909, <br />June 1921, July 1927, July 1929, July 1965, and July 1973. The <br />1921 floodflow reached a volume of 2,000 cubic feet per second <br />(cfs) at Ouray, 2,400 cfs below Ouray, and 4,080 cfs at Colona. <br /> <br />Examples of flooding and flood damage in Ouray are shown in Figures 2 <br />through 5. <br /> <br />2.4 Flood Protection Measures <br /> <br />Within the City of Ouray, Cascade and Portland Creeks flow through <br />small concrete flumes or channels that traverse the alluvial fan <br />in a westerly direction. The flumes are generally located at high <br />points on the alluvium, such that flows exceeding the tops of the <br />flumes flow away from the channel rather than being contained in a <br />larger flood plain. During violent cloudbursts, flows carrying <br />debris frequently clog the channel. The flows and their debris <br />are thus diverted out of the channel and seek their own flow paths <br />through the city to the Uncompahgre River. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the conununity, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />10-, 50-, 100-, or sOO-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 />sOO-year floods, have ala, 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 lOa-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 /> <br />5 <br />