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<br />Introduction
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<br />The Federal Emergency Management Agency (FEMA), which administers the National
<br />Flood Insurance Program (NFIP), is authorized to identifY natural hazards throughout the United
<br />States and its territories. The geographical diversity of the nation provides a wide range of natural
<br />hazards, but one of FEMA' s key responsibilities is to map areas that are subject to a 1 percent
<br />probability of being flooded in any year (the" 1 OO-year flood"). The purpose of this charge is to
<br />meet the NFIP's requirement that the burden of paying for flood damage be shifted from the
<br />general public to those living at risk. In most riverine environments, where channels change their
<br />locations only gradually and where catastrophic alterations in their form and flood conveyance
<br />capacity during a single event are rare, the procedures for mapping the depth and velocity of
<br />floods are generally agreed on. The technical and regulatory community has developed certain
<br />language, procedures, and a way of depicting reality (i.e" a paradigm) that allows the
<br />identification, delineation, and mitigation of flood hazards (see, e.g" Hydrologic Engineering
<br />Center, 1976, volume 6; and Bedient and Huber, 1992, Chapter 7), Although all floods behave, in
<br />detail, differently from the paradigm, once an estimate of the 1 percent peak flood discharge is
<br />agreed on, institutionalized procedures make the calculation of the extent, depth, and velocity of
<br />the flood hazard relatively straightforward and reproducible by different analysts. This report uses
<br />the term riverine flooding to represent those cases where application of this standard paradigm
<br />allows one to successfully assess and manage flood risk
<br />However, where catastrophic changes in river channel form and position can occur during
<br />a single flood, the traditional paradigm and associated hydraulic procedures cannot be relied on,
<br />For example, if a flood deposits large quantities of sediment on the channel bed in a reach, the
<br />conveyance capacity of the channel could be reduced drastically and the flow forced overbank at a
<br />lower discharge than would be predicted from prestorm surveys of the channel geometry. If
<br />overbank flooding causes erosion of a new channel or the reoccupation of an old channel, flood
<br />risk assessments based on the historical flow path would misrepresent the location and intensity of
<br />flooding downstream of the change. Both of these types of channel changes (form and position)
<br />can occur with great frequency and intensity on a type of landform called an alluvial fan. An
<br />alluvial fan, as defined by this committee, is "a sedimentary deposit toc(1ted at a topographic
<br />break, such as the base of a mountain front, escarpment, or valley side, that is composed of
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<br />6
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