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I I I I I I I I I I I I I I I I I I I 7 07/16/99 3 STAGE I: RECOGNIZING AND CHARACTERIZING ALLUVIAL FAN LANDFORMS Determining whether or not a landform is an alluvial fan. As defined in this appendix, alluvial fan flooding occurs only on alluvial fans. Therefore, the first stage ofthe process is to determine if the landform in question is an alluvial fan. If, after following the guidelines in this section, it is concluded that the landform is not an alluvial fan, then the methods described in this appendix are not intended for or necessarily applicable to the landform in question. An alluvial fan is a sedimentary deposit located at a topographic break such as the base of a mountain front, escarpment, or valley side, that is composed of streamflow and/or debris flow sediments and which has the shape of a fan, either fUlly or partially extended These characteristics can be categorized by composition, morphology, and location as follows. 3A Composition Alluvial fans are landforms constructed from deposits of alluvial sediments or debris flow materials. These deposits, "alluvium", are an accumulation of loose, unconsolidated to weakly consolidated sediments. Alluvium refers to sediments transported by either streamflow or debris flows. Geologic maps and field reconnaissance can be used to determine if the landform is composed of alluvium. 3B Morphology Alluvial fans are landforms that have the shape of a fan, either partly or fully extended. Flow paths may radiate outward to the perimeter of the fan, however drainage may exhibit a range of patterns such as dendritic, anastomosing, and distributary. Topographic maps and aerial photos can be used to assess this criterion. 3C Location Alluvial fan landforms are located at a topographic break where long-tenn channel migration and sediment accumulation become markedly less confined than upstream of the break. This locus of increased channel migration and sedimentation is referred to as the alluvial fan apex. The topographic apcx is at the extrcme upstream extent of the alluvial fan landform. The hydrographic apex is the highest point on the alluvial fan where there exists physical evidence of channel bifurcation and/or significant flow outside the defined channel; its location may be either coincidental with or at a point downstream of the topographic apex as seen in Figure I. The hydrographic apex may be discharge dependent and may vary with the magnitude of the flooding event.