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<br /> <br />2) <br />k <br />, <br /> <br />Preface <br /> <br />> <br /> <br />') <br /> <br />;t'> <br /> <br />:',) <br /> <br />People have long elected to build in flood-prone areas-whether because they sought easy <br />access to the waterways that were once our main transportation routes, because they offer <br />relatively flat building sites, or because of their aesthetic appeal. As the population increases and <br />people search for desirable locations to live, they sometimes come into conflict with those who <br />regulate construction on floodplains. In the western United States, some of the most intense <br />conflicts revolve around development on alluvial fans, which can be susceptible to a particularly <br />catastrophic type of flooding. Controversy over alluvial fan flooding issues led the Federal <br />Emergency Management Agency (FEMA) to ask the National Research Council (NRC) for help. <br />As a result, the NRC established the Committee on Alluvial Fan Flooding with a membership <br />composed of eight engineers and earth scientists, all of whom have experience with alluvial fan <br />morphology and processes. <br />The committee was charged to revise the existing definition of alluvial fan flooding, to <br />develop criteria to determine if an area is subject to alluvial fan flooding, and to provide examples <br />of the application of the definition and the criteria used. The committee recognized immediately <br />that in addition to "alluvial fan flooding," there exists a broader category termed "uncertain <br />f10wpath flooding" that requires further consideration by FEMA. Confusion caused by linking <br />two aspects of the flood hazard (i.e" land fonn type and uncertainty in flood processes) is part of <br />the reason for the controversy on this subject. This committee cannot claim to have the final word <br />on what it considers to be a complex technical and regulatory issue, hence we may not have <br />achieved everything desired by FEMA. It has, however, provided significant guidance for <br />characterizing how floods occur on alluvial fans and describing how FEMA might more <br />consistently administer the National Flood Insurance Program on such land fonns, which <br />comprise large areas of the western United States and elsewhere, <br />In order to more fully understand the problems associated with alluvial fan flooding, the <br />committee met at three locations in Arizona, California, and Utah, where different alluvial fans <br />could be visited and evaluated in the field. The examples ranged from typical large alluvial fans in <br />Arizona and California to small debris flow fans in Utah, Fans ranged from fully active, where <br />flooding or debris flows could occur anywhere on the fan, to incised, where the bulk of the fan is <br />not subject to flooding. Hence, not only was the varied expertise of the committee brought to bear <br />on the problem, but the members were exposed in the field to new and different situations. <br /> <br />;! <br />~ <br /> <br /> <br />vii <br />