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<br />, <br /> <br />. <br /> <br />.'! <br /> <br />\ <br /> <br />3.0 <br /> <br />; <br /> <br />, <br /> <br />, <br /> <br />In August 1955, a brief cloudburst, lasting approximately 30 minutes, <br />damaged four bridge and culvert crossings and deposited several <br />inches of mud in local residences. <br /> <br />The town was also flooded in 1965, and again in May 1969, following <br />three days of heavy snow and rain, The floodwaters left the <br />normal channel, destroying a number of buildings and the town <br />water supply. Photos of the 1969 flood are shown in Figures 2, <br />3, 4, and 5. <br /> <br />Readings by members of the University Institute of Arctic and <br />Alpine Research showed snow content or rainfalls of 6.56 inches <br />at 10,000 f~et, 9.10 inches at 8,500 feet, and 8.90 inches at <br />7,200 feet (Reference 1). Flood damage estimates in a 9-county <br />area were $7,000,000 including $700,000 to roads and bridges <br />in Boulder county alone. <br /> <br />2.4 Flood Protection Measures <br /> <br />No flood protection structures or measures exist or are planned <br />for the flooding sources of Jamestown. <br /> <br />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 flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equalled or exceeded once on the average during <br />any 10-, 50-, 100-, or 500-year period (recurrence interval) have been <br />selected as having special significance for flood plain management <br />and for flood insurance premium rates. These events, commonly termed <br />the 10-, '50-, .100-, and 500-year floods, have ala, 2, 1, and 0.2 percent <br />chance, respectively, of being equalled or exceeded during any year. <br />Although the recurrence interval represents the long term average period <br />between floods of a specific magnitude, rare floods could Occur at <br />short intervals or even within the same year. The risk of experiencing <br />a rare flood increases when periods greater than 1 year are considered. <br />For example, the risk of having a flood which equals or exceeds the <br />100-year flood (1 percent chance of annual occurrence) in any 50-year <br />period is approximately 40 percent (4 in 10), and, for any 90-year <br />period, the risk increases to approximately 60 percent (6 in 10). <br />The analyses reported here reflect flooding potentials based on condi- <br />tions existing in the community at the time of completion of this study. <br />Maps and flood elevations will be amended periodically to reflect future <br />changes. <br /> <br />5 <br />