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<br />l <br /> <br />the lower fan. Since most fans exhibit significant local <br />topographie variations. the actual flooding pattern on much <br />of the fan is erratic and may alternate between a confined <br />braided channel and sheet flow conditions. Ultimately, the <br />flood flows drain off the fan onto an apron or into a river <br />channel. <br /> <br />The flood patterns described above were observed in both the <br />field studies and the laboratory models of alluvial fans. <br />figure 3.5 shows the progression from channelized to braided <br />flow during the 1979 flood event on the Rancho Mirage fan. <br />Similarly, figure 4.1 shows the progression from channelized <br />to braided sheet flow ~bserved in the idealized model of a <br />SY. fan. <br /> <br />4.2 'Variations ill floC1d Behavie>r <br /> <br />fan flooding processes are far from steady. Observed <br />channels and flow paths on a fan mayor may not carry water <br />during future events. depending on the stability of the <br />channel. the magnitude of the discharge, and the amount of <br />sediment carried out of the watershed and onto the fan. If <br />an existing channel is either overtopped or filled with <br />sediment. an avulsion will occur and a new channel will be <br />formed elsewhere. The process of alternate formation and <br />abandonment of flow paths is the essence of alluvial fan <br />construction. As the channels move over geologic time, the <br />fan aggrades in a roughly uniform manner to produce the <br />typical conical fan shape (see figure 3.1). <br /> <br />Two time scales for processes on fans can be identified: <br />geologic time and human time scales. Over geologic time, <br />fans aggrade uniformly and show no bias or stable <br />entrenchments. No area of a fan is any more li~ely to <br />experience flooding than any other area. The one exceptIon <br /> <br />39 <br />