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<br />local depressions exist and alternating construction and <br /> <br /> <br />breaching of sediment dams causes pulses of sediment and <br /> <br /> <br />water to move down the watershed channels. Small landslides <br /> <br /> <br />may occur in the steepest slopes of the watershed. adding to <br /> <br /> <br />the sediment moving downslope. As the flood wave nears the <br /> <br /> <br />fan apex. it becomes confined by a single channel, where <br /> <br /> <br />velocities and depths of flow become very large. The <br /> <br /> <br />sediment transport capacity of the stream enables it to <br /> <br /> <br />carry very large volumes of sediment onto the fan. in some <br /> <br /> <br />cases forming mud flows. <br /> <br />When the floodwave enters the apex of an active <br />Cunentrenchedl fan, it has an .immediate tendency to spread <br />laterally. thus losing both depth and velocity and <br />infiltrating into the porous fan surface. The resulting <br />loss of competence causes deposition of sediment in the apex <br />area;' channel migration and braiding become significant as <br />flow momentum is lost and sediment deposition causes <br />alteration of the channel geometry. If sediment deposition <br />is rapid. as is the case for low slope fans or highly <br />erodible watersheds, channel avulsions near the apex will <br />occur. Conversely. if initial sediment load at the apex is <br />less than the transport capacity, erosion will occur at the <br />apex and an incised channel will form and progressively <br />deepen. <br /> <br />As the flood moves down the fan, an unsteady progression of <br />hydraulic conditions takes place. The flow initially is in <br />a single channel and follows the direction of the upstream <br />watershed channel. Braiding begins at a point downstream <br />where deposition and bed form changes cause flow <br />instability; the wldth of the flooded area increases rapidly <br />and depths of flow decrease rapidly once braiding begins. <br />If the fan surface is smooth and no topographic constraints <br />on the width of the flooded area exist, the braiding process <br />will create shallow, sheet flooding conditions over much of <br /> <br />38 <br /> <br />