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<br />32 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br /> <br />" <br /> <br />FIGURE 2-3 Slylized view of a b,yada (B) showing alluvial fans (A) merging with an alluvial plain (AP), The <br />bajada is formed by coalescing alluvial fans originating at gullies cut in a dissected piedmont (P) and by <br />debouching on Ihe fan piedmont. Such eroded fan-piedmont remnanls commonly form the slopes above bajadas in <br />the arid soulhwestem United States. SOURCE: Adapted with permission from Peterson (1981). <br /> <br />emphasizes the differences by using the terms streamflow fal/, debris flow fal/, and composite fal/, <br />unless referring to a generic accumulation of any origin as afal/ or alluvialfal/. This distinction is <br />not widely made in the literature, where all fans are usually called alluvial fallS, but it is important <br />because recognition of the nature of flooding and sedimentation processes on a fan and an <br />understanding of the difference in triggering mechanisms and therefore probabilities of debris <br />flows and floods are crucial to the accurate interpretation and prediction of flood risk. <br />The continuum of fan types reflects the range of sediment transport and depositional <br />processes that generate and modifY the landform. Sediment supply and transport mechanisms on <br />fans include debris flow, channelized water flow and sheetflood (extensive, shallow overbank <br />flooding of water or mud), There is no sharp line differentiating channelized flow, and sheetflood. <br />Some fans exhibit all three transport mechanisms, with the frequency and importance of each <br />changing down the fan. <br />There is also a continuum in the intensity of the sedimentation processes and therefore in <br />the activity of the fan-building and fan-modifYing processes. Some fans are accumulating and <br />changing rapidly under current climatic conditions; others are developing only slowly because <br />changes in climate over recent millennia to centuries have caused their channels to deepen and <br />stabilize. Still other fans have been subject to an intensification of flood and sedimentation hazard <br />as a result ofland use or engineering structures in the source area or on the fan itself <br /> <br />< <br />