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The importance of secondary processes on alluvial fan morphology, channel behaviour, and flood hazards Vincent, Lauren


Alluvial fans form through primary and secondary processes. Primary processes act to transport sediment from the drainage basin to the fan and secondary processes re-mobilize and rework sediment previously deposited on the fan. There is still a poor understanding of how primary and secondary processes interact on alluvial fans. The primary objective of this research is to isolate the role of secondary processes in determining alluvial fan behaviour and morphology. Four experiments were conducted, in which alluvial fans were created under different durations of secondary processes. In these experiments secondary processes were run between each primary process, or flood period. The duration of secondary process periods ranged from 5 to 40 minutes across the four experiments. Here primary processes reflect inputs of Q = 100 ml/s and Qb = 10 g/s, and secondary processes reflect inputs of Q = 50 ml/s and Qb = 0 g/s, wherein Q refers to water discharge and Qb refers to sediment feed rate. For each experiment, alternating primary and secondary processes were run until a total of 72 kg of sediment had been added to the fan. Following this, a sequence of nine flood events was run over the fan surface to evaluate fan response to flooding. Experiments with longer durations of secondary processes generated fans with larger areas and gentler gradients. In addition, longer secondary process durations led to increased flow channelization and channel incision between flood periods. The dominant channel pattern throughout the experiments was the formation of a single, centralized channel during secondary process periods, and the bifurcation of flow following the onset of primary processes. The increased incision and flow confinement during prolonged periods of secondary processes resulted in a decreased avulsion frequency during subsequent flood periods. In addition, the onset of avulsion was delayed for experiments with longer secondary process duration. These results indicate that the duration of secondary processes has important effects on both fan morphology and flow behaviour.

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