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<br />12 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br /> <br />A number of professionals in the field have taken exception to the formulation ofFEMA's <br />risk delineation procedure (e.g., French et aI., 1993; Zhao and Mays, 1993), asserting, for <br />example, that alluvial fans do not necessarily behave randomly and that floods are more likely to <br />follow previous flow paths; that flows do not occur at critical depth; that the width-to-depth ratio <br />is different from the assumed value of200; or that the method, as adopted, is unrealistically simple <br />and therefore easy to misapply. Although specific applications of the method may be deserving of <br />such criticism, the original formulation of the problem (Dawdy, 1979, equation 6) is quite general <br />and sound for calculation of a conditional risk. This same mathematical approach has been used <br />by the U. S. Army Corps of Engineers to analyze the flood risk of a levee failure while dealing <br />directly with the uncertainties inherent in such an occurrence (US ACE, 1992, 1994). If the <br />conditional probability equation is to be applied to alluvial fans, however, there is need for a more <br />flexible and realistic approach to the definition of flow path uncertainty, best obtained perhaps, by <br />field evidence for the nature and spatial distribution of processes. A more realistic, process-based <br />approach to flood hazard delineation on alluvial fans remains a challenge to the technical <br />community. However, a mathematical framework for moving from process to the delineation of <br />risk zones has already been correctly set forth by FEMA. Further discussion of methods to <br />delineate flood hazards on alluvial fans, a secondary charge of this committee, is discussed in <br />Chapter 3. <br />If it is determined that a particular flooding source does not match with the default <br />assumptions of FEMA's alluvial fan method (for example, when the potential for channel <br />movement is not random) the default risk delineation method is inapplicable. However, this does <br />not mean that the area is necessarily free from alluvial fan flooding as discussed next, <br /> <br />The Problem of Defining Alluvial Fan Flooding <br /> <br />The following definition is published in the NFIP regulation (section 59.1): <br /> <br />Alluvial fan flooding means flooding occurring on the surface of an alluvial fan or <br />similar landform which originates at the apex and is characterized by high-velocity <br />flows; active processes of erosion, sediment transport, and deposition; and <br />unpredictable flow paths. <br /> <br />The primary purpose of this definition was to identity the existence of and extend <br />jurisdiction over flooding situations that may have been excluded or improperly dealt with under <br />prior regulations. According to the NFIP rules, alluvial fan flooding is a type of flooding that is <br />recognized by characteristics that distinguish it from ordinary flooding. Almost everyone that the <br />committee heard from held the view that there is a strong correlation, or even an exclusive <br />relationship, between the term alluvial fan flooding and flooding that occurs on an alluvial fan. <br />By this reasoning, if one determines that an area is not an alluvial fan, then it is not subject to <br />alluvial fan flooding. The current definition, however, states that alluvial fan flooding can be <br />present not only on alluvial fans but also on the rather ambiguous category of "similar <br />landform[s]." Such reasoning leads to conflicts. For example, people who believe that alluvial fan <br />flooding is flooding that occurs only on alluvial fans and who obtain technical advice that their <br />community contains no alluvial fans will therefore conclude that there are no areas subject to <br /> <br />~ <br /> <br />, <br />~.:\ <br /> <br />