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<br /> <br />SUMMARY <br /> <br />1 <br /> <br />INTRODUCTION <br />Origin of the Problem, 8 <br />The Committee's Response, 13 <br />The NFIP Definition of Alluvial Fan Flooding, 14 <br />Implications of Alluvial Fan Flooding as a Distinct Type of Flooding, 18 <br />Implications of Alluvial Fan Flooding for Floodplain Management, 23 <br />The Committee's Definition of Alluvial Fan Flooding, 25 <br />BOX <br />Selecting the Site Visit Locations, 9 <br /> <br />6 <br /> <br />2 <br /> <br />FLOODING PROCESSES AND ENVIRONMENTS <br />ON ALLUVIAL FANS <br />Formation and Nature of Alluvial Fans, 29 <br />Flooding Processes on Alluvial Fans, 39 <br />Change Over Time, 43 <br />Reports of Flooding on Alluvial Fans, 45 <br /> <br />29 <br /> <br />3 <br /> <br />INDICATORS FOR CHARACTERIZING ALLUVIAL <br />FANS AND ALLUVIAL FAN FLOODING <br />Stage I: Recognizing and Characterizing Alluvial Fan Landforms, 54 <br />Stage 2: Defining the Nature of the Alluvial Fan Environment and <br />Identifying the Location of Active Erosion and Deposition, 60 <br />Stage 3: Defining and Characterizing Areas of I 00- Year Alluvial Fan Flooding, 72 <br />Available Methods of Analysis, 75 <br />Summary, 80 <br />BOX <br />"Time" in the Context of Alluvial Fan Flooding, 53 <br /> <br />51 <br /> <br />ix <br /> <br />. - <br />velocity flows; active processes of erosion, sediment transport and deposition; and unpredictable <br />flow paths." In addition, although aI/uvial fan flooding is a general term that can involve <br />flooding over an entire surface, the FEMA mandate is to determine the extent of hazard <br />associated with a flood with a 100-year recurrence interval (i.e., a I percent probability in a given <br />year), Hence, the term alluvial fan flooding is used in two ways. In the geomorphic sense, it can <br />be any flood on an alluvial fan. But in the NFIP sense, it is the distribution of 100-year floodwater <br />on the fan, The reader is cautioned that the term is used in both ways, including in this report. <br />The problem with the current definition is that it is very broad, and often is applied to <br />many landforms that are not alluvial fans, such as alluvial plains, pediments, deltas, and braided <br />streams, One approach to reduce this confusion is to define alluvial fan flooding so that it applies <br />strictly to alluvial fans, and to use different language, such as uncertain flow path flooding, when <br />dealing with" similar landforms." But such a change is not as simple as it may sound-it requires <br /> <br />1 <br />