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<br />damages from the flood were estimated at $602,000. The <br />estimated discharge from this flood was 22,000 cfs at Loveland, <br />larger than the lOa-year discharge of 19,000 cfs (Reference 2). <br /> <br />The most recent flood occurred on July 31, 1976, as torrential <br />rains in the Big Thompson Canyon caused a severe flood, greater <br />than the laO-year flood, A total of 139 people lost their <br />lives. At Loveland, the flood and damage were less severe; <br />however, damages totaled as much as $500,000 for some businesses <br />(Reference 7). The peak discharge at a gaging station located <br />at the mouth of the canyon was estimated at 31,000 cfs, At <br />Loveland, the peak discharge measured approximately 20,000 cfs, <br />approximately the same as the laO-year discharge (Reference 8). <br /> <br />2,4 Flood Protection Measures <br /> <br />No flood protection structures exist on the Big Thompson River; <br />however, some protection ;s provided by weather reports and <br />local watches on stream gages during crucial storm periods <br />(Reference 2). <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the <br />flood hazard data required for this study, Flood events at a magni- <br />tude which are expected to be equalled or exceeded once on the average <br />during any 10-, 50-, 100-, or SOD-year period (recurrence intervals), <br />have been selected as having special significance for flood p~ain <br />management and for flood insurance premium rates. These events, <br />commonly termed the 10-, 50-, 100-, and 500-year floods. have a 10, <br />2,1, and 0.2 percent chance, respectively, of being equalled or <br />exceeded during any year. Although the recurrence interval repre- <br />sents the long-term, avera~e period between floods of a specific <br />magnitude, rare floods cou d occur at short intervals or even within <br />the same year, The risk of experiencing a rare flood increases when <br />periods greater than one year are considered. F<rexample, the risk <br />of having a flood which equals or exceeds the 100-Year flood (one <br />percent chance of annual occurrence) in any 50-year period is about <br />40 percent (4 in 10), and for any gO-year period, the risk increases <br />to about 60 percent (6 in 10), The analyses reported here reflect <br />flooding potentials based on conditions existing in the community at <br />the time of completion of this study. Maps and flood elevations \':'11 <br />be amended periodically to reflect future changes, <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic ana,lyses were carried out to establish the peak <br />discharge-frequency relationships for floods of the selected <br />recurrence intervals for each flooding source studied in detail <br />affecti ng the community. <br /> <br />5 <br />