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I I I I I I I I I I I I I I I I I I I 5 07/16/99 The approach for alluvialfanflooding identification and mapping first seeks to identify if the area under study is an alluvial fan, and second, which portions, if any, of this area are characterized by or subject to active alluvial fan flooding. After these steps, varying methods unique to different situations can be employed to analyze and define the 100-year flood within the areas of alluvial fan flooding identified on the alluvial fan. Thus, the approach for the identification and mapping of alluvial fan flooding can be divided into three stages. Each of these stages must be addressed and thoroughly documented during the study process. 1. Recognizing and characterizing alluvial fan landforms. 2. Defining the naturc of thc alluvial fan environment and identifying active and inactive areas of the fan. 3. Defining and characterizing the IOO-year flood within the defined areas. Each stage is described in detail in later sections. Additional information can also be found in the National Research Council report, Alluvial Fan Flooding (1996). Coordination with the Regional Project Officer (PO) and Headquarters is required from the onset of the study since each stage builds upon the previous stage and because of the complexity of many alluvial fans. Figure 2 shows the progression of the process. Progression through each of the stages results in a procedure that narrows or divides the problem to smaller and smaller areas. In Stage 1, the landform on which the flooding occurs must be characterized. If the location of study is an alluvial fan, the analyst proceeds to Stage 2, in which the parts of the alluvial fan that are still active or inactive are identified. Finally, in Stage 3 the analyst utilizes various methods to define and analyze the 100-year flood within each identified area of alluvial fan flooding. Progression through these stages requires a variety of maps and photographs, as well as a significant amount of fieldwork and analysis to fully understand the flood- hazard.