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<br /> <br />INTRODUCTION <br /> <br />25 <br /> <br />THE cOMMITIEE'S DEFINITION OF ALLUVIAL FAN FLOODING <br /> <br />This committee was asked to provide a new definition of alluvial fan flooding. Ideally, this <br />definition could be used to divide flood hazard areas into two categories: those subject to alluvial <br />fan flooding and those not. Of course, not all cases will fall clearly into one category or another. <br />Although it is traditional to be conservative in the interest of safety, wholesale adoption of the <br />alluvial fan flooding paradigm may not be a good thing because this approach brings its own set of <br />weaknesses and regulatory traps. Furthermore, such blind caution could result in hundreds of <br />cases where flooding sources that have been successfully mapped and managed as ordinary rivers <br />will need to be completely reassessed for the sole purpose of consistently applying the definition. <br />This would be unnecessary in many cases and could, ironically, undermine the original motivation <br />for creating a new category of flood hazard for those cases that would otherwise be dealt with <br />inadequately. <br />Because alluvial fans are the place where there is a strong historical connection to the <br />breakdown of the riverine flooding paradigm, these landforms provide a regulatory partition that <br />allows FEMA to concentrate on the most serious cases. This committee has therefore chosen to <br />restrict the term alluvial fal/ floodil/g to apply only for alluvial fans. Floods with characteristics <br />that fit the alluvial fan flooding concept but occur in nonalluvial fan environments are discussed as <br />a separate, broader category of flooding. <br />The Committee on Alluvial Fan Flooding proposes the following definition, which <br />considers the objectives of the original NFIP definition, the administrative concerns of the NFIP, <br />and the criteria necessary to establish 100-year recurrence interval alluvial fan flooding as a <br />distinct hazard: <br /> <br />Alluvial fan floodil/g is a type of flood hazard that occurs only 01/ alluvial <br />fans, It is characterized by flow path uncertainty so great that this ul/certainty <br />canl/ot be set aside iI/ realistic assessments of flood risk or in the reliable <br />mitigation of the hazard. An alluvial fan flooding hazard is il/dicated by three <br />related criteria: (a) flow path uncertainty below the hydrographic apex, (b) <br />abrupt depositiol/ and ensuing erosion of sediment as a stream or debris flow <br />loses its competel/ce to carry material eroded from a steeper, upstream source <br />area, and (c) aI/ environment where the combinatiol/ of sediment availability, <br />slope, al/d topography creates an ultrahazardous condition for which elevatiol/ 01/ <br />fill willl/ot reliably mitigate the risk (Figure 1-7), <br /> <br />Alluvial fan flooding begins to occur at the hydrographic apex, which is the highest point <br />where flow is last confined, and then spreads out as sheetflood, debris slurries, or in multiple <br />channels along paths that are uncertain. The hydrographic apex may be at or downstream of the <br />topographic apex and may change during a flood event due to deposition or erosion. Such <br />flooding is characterized by sufficient energy to carry coarse sediment at shallow flow depths. The <br />abrupt deposition of this sediment or debris strongly influences hydraulic conditions during the <br />event and may allow higher flows to initiate new, distinct flow paths of uncertain direction. Also, <br />erosion strongly influences hydraulic conditions when flood flows enlarge the area subject to <br />flooding by undermining channel banks or eroding new paths across the unconsolidated sediments <br />of the alluvial fan. Flow path uncertainty is aggravated by the absence of topographic <br />