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<br />It appears. based on the above observations. that the <br />potential for major flood damages occurring in the future on <br />alluvial fans is great. The value of developments "at risk" <br />is likely to rise rapidly as populations increase in the <br />southwestern states. While alluvial fan flooding was of no <br />consequence two decades ago, it is now recognized as a <br />serious problem. Unless effective measures are taken to <br />prevent damages, the. liability of federal and state disaster <br />agencies will increase rapidly. <br /> <br />3.4 Recommended Aooroach 1Q Fan Classification <br /> <br />The diversity of fan and watershed characteristics and the <br />highly variable nature of alluvial fan flooding make a clear <br />case for careful analysis of each fan on an individual <br />basis. Because flood behavior is inextricably linked to <br />these characteristics, the first step in flood analysis.must <br />be determination of fan and watershed conditions. Once the <br />general geologic history and present condition of the fan <br />are known, the severity of flood ha~ards can be estimated. <br /> <br />The following general approach to the identificaion of <br />characteristics is suggested: <br /> <br />1. perform a field survey of the watershed and <br />fan, identifying soil types, vegetation, <br />slopes, topography, existing stream channels, <br />and recent fires; <br /> <br />2. develop detailed topographic mapping of the fan <br />to determine morphology and channel paths; <br /> <br />3. obtain the flood history of the fan, when <br />available, and have a geologist estimate the <br />geologic history of the fan; <br /> <br />35 <br />