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<br />INTRODUCTION <br /> <br />J3 <br /> <br />alluvial fan flooding in their community. FEMA, from its different perspective, might respond that <br />within the community there are landforms similar to alluvial fans and that these experience alluvial <br />fan flooding as explained in the regulatory definition. The community may counter that the similar <br />landforms are inactive pediments and not alluvial fans; to which the response might be, "Just <br />because you call something a different name, does not mean it is not subject to the severe hazards <br />associated with alluvial fan flooding." The community's reply may then be that the amount of <br />peril and the degree of uncertainty posed by flooding in its case are less than in other regions of <br />the country that clearly have alluvial fans on which processes envisaged by FEMA are active and <br />extensive. This hypothetical interchange typifies many of the discussions heard by the committee <br />during the open, consultative portions of its meetings. There appeared to be a willful lack of <br />reflection on the meaning of words, a general confusion about the diversity of flooding and <br />sedimentation processes that occur on alluvial fans in a range of alluvial fan environments, and a <br />lack of knowledge about information that could be quickly and inexpensively obtained through <br />field examination of particular sites before hazard identification and delineation are carried out. <br /> <br />THE COMMITTEE'S RESPONSE <br /> <br /> <br />As a result of the committee's site inspections, the members' own field experience, <br />consultations with many experienced individuals, including FEMA staff and its consultants, <br />surveys of the literature, and extensive discussions at four meetings, the committee produced the <br />following five products, which are presented in this report: <br /> <br />) <br /> <br />]. A revised definition of alluvial fan flooding. including criteria that can be applied by <br />FEMA to identity those parts of alluvial fans that require special regulatory oversight to deal <br />adequately with the uncertainty in flood processes. The committee also provides discussion of the <br />constraints within which the alluvial fan flooding concept could be extended to other landforms. <br />This definition is presented and analyzed in the latter part of Chapter 1. Although alluvial fan <br />flooding is a general term that can involve flooding over an entire fan surface, the FEMA mandate <br />is to determine the extent of flooding associated with a flood having a 100-year recurrence <br />interval (i.e., a I percent probability in a given year). Hence, the term alluvial fan flooding is used <br />in two ways. In the geomorphic sense, it can be any flood on an alluvial fan. But in the FEMA <br />sense, it is the distribution of 100-year flood water on the fan. The reader is cautioned that the <br />term is used in both ways, including in this report. <br />2. A description of the flood and sedimentation processes that build alluvial fans and <br />contribute to the existence of alluvial fan flooding (Chapter 2). The chapter also illustrates why <br />the recognition of how floods occur is significant for risk assessment. An understanding of <br />process issues is necessary for realistic and flexible regulatory practice and flood risk assessment <br />in a range of environmental conditions. For example, the process approach to understanding what <br />is distinctive about alluvial fan flooding at a particular location and the conclusion that both the <br />process and the channel form on alluvial fans respond to the environmental history of the site <br />allow a quick and simple mapping of process zones on alluvial fans in the manner demonstrated in <br />Figure 1-2. Such a map, which can be made quickly and inexpensively using methods described in <br />Chapter 3, outlines areas that require various forms of attention from the point of view of flood <br /> <br />'. <br />f <br /> <br />-:t. <br />