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<br />indirect discharge estimates have been used to determine the amount of <br />rainfall that occurred during a storm. This may lead to inaccurate <br />estimates of rainfall and associated flood discharges that are used in <br />the design of flood control structures. <br />In mountainous regions, one morphologic result of debris flows is the <br />formation of debris fans at the mouths of small channels. These areas have <br /> <br />. <br /> <br />consistently been favored as building sites because they sit up on valley <br />sides. Current examples in Colorado include developments in Georgetown, <br />Ouray, and Telluride. Although mud and debris flows are covered under the <br />National Flood Insurance Program, they are not conducive to conventional <br />floodplain hazard analysis. Methods to determine frequency of occurrence, <br />and identification of areas of inundation are very poorly developed today. <br />In a recent floodplain study of the Uncompahgre River which drains the San <br /> <br />Juan Mountains in southwestern Colorado, two small tributaries to the <br /> <br />Uncompahgre River were identified where conventional procedures of hydraulic . <br /> <br />backwater-profile analysis were unusable (A & S Consultants, 1978, p. 12): <br /> <br />As the nature of flooding of Portland and Cascade Creeks was <br />studied, it became increasingly apparent that the flooding did <br />not follow patterns which could be evaluated by normal <br />hydraulic methods. After evaluating other techniques which <br />might be applied to rivers carrying high loads of silt or debris, <br />a basic conclusion was reached--the floodplains of Portland and <br />Cascade Creeks were variable, unpredictable, and could not be <br />defined. <br /> <br />These basins appear to be typical of many small basins in the mountains <br /> <br />of Colorado and the problem idenitified in the flood-insurance study is not <br /> <br />unique in this area. The accurate identification of a "flood plain" across <br /> <br />a debris fan, using conventional hydraulic and hydrologic procedures is not <br />possible (Magura and Wood, 1980). Channel blockage and debris-flow deposition <br />result in continually changing channel patterns and locations of deposition <br /> <br />. <br /> <br />-4- <br />