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<br />Cross section data and topographic mapping having a <br />contour Interval of 2 feet and a scale of I Inch equals 200 feet <br />were established by photogrammetrlc methods from flights made In <br />April 1975. These data were furnished by Larimer County and <br />Boulder County In cooperation with the Colorado Water COnservation <br />Board. . U. S. Geological Survey 7.5 minute quadrangle maps with <br />10 foot contour Intervals at a scale of I Inch equals 2,000 feet <br />were also used In the study. Bridge and roadway survey data were <br />provided by the Colorado Water COnservation Board. <br /> <br />FLOOD SEASON AND FLOOD CHARACTERISTICS <br />The Big Thompson River basin Is located within an area <br />which can receive very Intense rainfall of cloudburst magnitude. <br />Floods have resulted from storms covering large areas receiving <br />heavy general rainfal I as well as from storms covering smal I areas <br />but producing extremely Intense ralnfal I. Floods generally occur <br />n <br /> <br />the <br /> <br /> <br /> <br />Floods In the Big Thompson River basin can have consid- <br />erably different characteristics depending upon the location within <br />the basin. Floods In the mountainous region of the basin typically <br />have rapid rises, high maximum discharges, and relatively short <br />durations. Floods In the plains region, generally, are of lower <br />peak discharge and have longer duration. Floods within the study <br />reaches generally have a duration of about 1 day. Flood peaks <br />from thunderstorms covering small areas are generally rapidly atten- <br />uated In the study reaches by valley storage of floodwaters. <br /> <br />FACTORS AFFECTING FLOODING AND ITS .I~lPACT <br />Obstructions to floodflows,- There are 6 roadways and <br />13 roadways which cross the Big Thompson River flood plain and <br /> <br />12 <br />