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

Flood events are the agents of change on alluvial fans. However, most alluvial fan experiments have used constant flows to model fans and the channels upon them. Here, we present results from a series of alluvial fan experiments with different patterns of flow variation (i.e. different hydrograph shapes). We conducted experiments with 1) constant flow, 2) alternating high and low flows, 3) a moderate flood peak that decayed slowly, alternating with a constant low flow, and 4) a high flood peak that decayed rapidly, alternating with a constant low flow. Importantly, all experiments had the same mean flow and sediment supply, but the different hydrographs generated fans with different slopes. In addition, higher peak flows led to increased lateral migration rates and increased erosion and deposition. These results challenge the notion that a single representative flow can be used to approximate the geomorphic effects of a range of flows in a natural stream. Moreover, the data suggest that hydrograph shape can govern the geomorphic impact of a flood event. Our findings indicate how altered basin hydrology (for instance, through changes to land cover) could influence geomorphic change and natural hazards on alluvial fans.