Floods on alluvial fans: implications for fan hazards, morphology and reworking

Abstract:

Flood events are the agents of change on alluvial fans. However, most alluvial fan experiments have used constant flow rates to model fans and the channels upon them. Here, we present results from a series of alluvial fan experiments with recurring floods. We trialled a different flood hydrograph in each experiment: 1) constant flow, 2) stepped floods, 3) a low flood peak that decayed slowly, and 4) a high flood peak that decayed rapidly. Importantly, the same volume of water was delivered in each flood, and in each experiment. Despite this similarity, the different flood types significantly altered fan slope. In addition, the higher discharges generated in the peaked floods led to larger avulsions, increased lateral migration rates, and increased geomorphic activity (erosion and deposition). These results challenge the notion that a single representative discharge can be used to approximate the range of flows in a natural stream. Moreover, the results suggest that temporal changes in flow during a flood are crucial for stream channel responses. Our findings show how altered basin hydrology (for instance, through changes to land cover) could affect flood responses and hazards in streams on alluvial fans.