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 (a) constant flow, (b) alternating high and low flows, (c) a moderate flood peak that decayed slowly, alternating with a constant low flow, and (d) a high flood peak that decayed rapidly, alternating with a constant low flow. We found that different hydrographs generated fans with different slopes, even though all experiments had the same mean flow and sediment supply. 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, our findings indicate that hydrograph shape can govern the geomorphic impact of a flood event. This means that altered basin hydrology (for instance, through land cover change) likely exerts an important impact on geomorphic change and natural hazards on alluvial fans. Plain Language Summary: The steep streams that flow down alluvial fans experience a wide range of high and low flows. Here, we use a series of experiments with a small‐scale model of a fan to explore the importance of this flow variability. We show how the type of flow variability influences hazards such as stream bank erosion, or the rapid inundation of areas that were previously dry. Our results suggest that when high flows occur in these steep streams, their size and duration control their impact on the stream channel. Anything that changes the size and duration of high flows (for instance, a change to the landscape upstream) could alter the severity of future flood impacts. Key Points: Experiments with the same mean flow but different hydrograph shapes generated alluvial fans with different slopesLateral migration and geomorphic change increased non‐linearly with flow, so that small changes to hydrographs impacted flood responseA single, constant flow is inappropriate to represent the wide range of flows on natural fans [ABSTRACT FROM AUTHOR]