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<br />debris fans. During cloudbursts flood flows are deep and fast because of the <br />(~small floodplain area and steep channels. Valley storage is not significant, so <br />reaches of damage can extend a long way downstream before flood peaks attenuate. <br />The greatest natural disaster in Colorado history occurred in 1976 when severe <br />thunderstorms in the Big Thompson Canyon north of Denver produced an enormous flood <br />which resulted in $30 million in property damages and claimed 139 lives (Grozier <br />and others, 1976). The area of maximum rainfall was nine miles upstream of the <br />canyon mouth, but the peak discharges were nearly identical. The geomorphic setting <br />of foothill channels results in a much greater potential for loss of life and <br />property damage in such areas. <br />On the basis of gaging station records, rainfall/radar data, and geomorphic <br />investigations, the upper limit to cloudburst rainfalls in the Southern Rocky <br />Mountains is about 7500 feet. Above this elevation cloudburst storms are rare, <br /> <br />and the major cause of flooding is snowmelt. Snowmelt flood peaks have broad <br />!~YdrograPhS that vary diurnally. Peaks are much flatter than cloudburst storms. <br />although volumes of flood waters are generally greater. Flooding can also occur <br />when landslides dam channels and are subseQuently breached. <br />One recently identified problem in mountain channels is that debris flows have <br />been confused with rainfall-produced water floods (Costa and Jarrett, 1981). Debris <br />flows are a body of granular solids, water, and air that flows like wet cement. <br />Velocities range from two to 50 feet per second, and volume concentration of solids <br />range from about 25 to 80 percent. The distinction between debris flows and water- <br />floods is important because (a) mitigating procedures for waterf100ds, such as <br />channelization and damming, may not be effective for debris flows, (b) peak discharge <br />estimates made from evidence of debris flows lead to excessive estimates of floods <br /> <br />in the Rocky Mountains, and (c) because of sparse rainfall data in mountain regions, <br /> <br />fe <br /> <br />-3- <br />