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<br />. <br />I: <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />Ie <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />I. <br />. <br />'. <br />I. <br />I. <br />. <br />. <br />. <br />. <br /> <br />4 <br /> <br />02/23/00 <br /> <br />2 APPROACH <br /> <br />The approach for alluvial fan flooding identification and mapping first seeks to identifY whether the <br />area under study is an alluvial fan and second, which portions, if any, of this area are characterized <br />by or subject to active alluvial fan flooding, After these steps, various methods unique to different <br />situations can be employed to analyze and defme the IOO-year flood within the areas of alluvialfan <br />flooding identified on the alluvial fan. Thus, the approach for the identification and mapping of <br />alluvial fan flooding can be divided into three stages. Each of these stages must be addressed and <br />thoroughly documented during the study process, <br /> <br />I. Recognizing and characterizing alluvial fan landfonns <br />2, Defining the nature of the alluvial fan environment and identifYing active and inactive <br />areas of the fan <br />3. Defining and characterizing the IOO-year flood within the defined areas <br /> <br />Each stage is described in detail in later sections. Additional information can also be found in the <br />National Research Council report, Alluvial Fan Flooding (1996). Coordination with the Regional <br />Project Officer (PO) and Headquarters is required from the onset of the study, because each stage <br />builds upon the previous stage and because of the complexity of many alluvial fans. Figure 2 shows <br />the progression of the process. <br /> <br />Progression through each of the stages results in a procedure that narrows or divides the problem to <br />smaller and smaller areas. In Stage I, the landform on which the flooding occurs must be <br />characterized. If the location of study is an alluvial fan, the analyst proceeds to Stage 2, in which <br />the parts of the alluvial fan that are still active or inactive are identified. Finally, in Stage 3 the <br />analyst utilizes various methods to define and analyze the IOO-year flood within each identified area <br />of alluvial fan flooding. Progression through these stages requires a variety of maps and <br />photographs, as well as a significant amount of fieldwork and analysis to fully understand the flood- <br />hazard, <br />