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<br />4. FLOOD DYNAMICS AliQ FLOOD HAZARDS <br /> <br />4.1 Flood Processes <br /> <br />The behavior of flood flows on alluvial fans is the product <br />of a number of processes which are operable in either the <br />upstream watershed or on the fan surface. Pro~esses in the <br />watershed include overland flow, temporary water storage, <br />infiltration, erosion. sediment transport. and temporary <br />sediment storage. Processes on the fan surface include <br />channel formation through erosion, deposition of sediment <br />caUSing channel braiding, infiltration. and lateral channel <br />migration. Watershed processes transform storm rainfall <br />into discharges of water and sediment at the fan apex. The <br />vari~tions in water and sediment discharge with time (peak <br />discharge. hydrograp~ shape. sediment concentration) are <br />dependent on watershed characteristics which affect these <br />processes. Likewise. the depth. velocity. sediment <br />concentration. and location of flood flows on the fan <br />surface depend on the watershed discharge and the morphology <br />of the fan itself. <br /> <br />A typical flash flood scenario on a fan will be described in <br />order to illustrate the interactions whiCh influence flood <br />behavior. Then, in Section 4.3, the influence of fan and <br />watershed characteristics on flood dynamics and hazards will <br />be assessed. Rainfall associated with flash floods is <br />usually of high intensity and short duration. Raindrops <br />striking exposed. non-vegetated ground surfaces dislodge <br />soil particles. As the drops combine, flow down the slope <br />begins moving sediment toward the ravines. steep slopes <br />cause rapid coalescence of the runoff into small channels <br />where high velocities cause further erosion. Detention of <br />both water and sediment occurs where vegetation is thick or <br /> <br />37 <br />