Secondary Geomorphic Processes and Their Influence on Alluvial Fan Morphology, Channel Behavior and Flood Hazards

Alluvial fans form through primary and secondary geomorphic processes. Primary processes comprise major geomorphic events that build the fan by transporting sediment from the watershed to the fan; these events occur at decadal time scales. Secondary processes include the smaller flood events which recruit little sediment from the watershed but can re-mobilize and rework sediment already delivered to the fan. The experiments described herein study the effects of secondary processes on alluvial fan morphology and channel behavior. We conducted four experiments, in which alluvial fans were built by alternating between primary events, with high flows and high sediment supply, and secondary events, with moderate flows and no sediment supply. In our experiments, primary events are best described as debris floods and secondary events correspond to smaller floods with much lower sediment concentrations. The duration of the secondary events varied between experiments, but the number and duration of the primary events was held constant keeping the total volume of sediment delivered to the fan the same. We monitored fan gradient, area, channel patterns and avulsions. Experiments with longer secondary events generated larger fans with gentler gradients. In addition, longer secondary events led to increased flow channelization and centralization between primary events. These morphologic changes resulted in fewer avulsions during primary events, which occurred later during the event. These results indicate that changes to the relative duration of primary and secondary events caused by climate change can affect fan morphology, wet fraction, and avulsion frequency, with implications for potential flood hazard. Plain Language Summary Alluvial fans are triangular shaped deposits of water-borne sediment at the mouth of steep creeks. As development in mountainous areas increases, infrastructure built on alluvial fans continues to be at risk from flooding. Alluvial fans are formed through competing processes which act to both add sediment to the fan surface and remove and rework existing sediment on the fan surface. Major fan-building floods add sediment to the fan; we refer to these flood events as "primary events". They encompass relatively rare floods with unusually high sediment concentrations. Secondary events are minor floods that rework the sediment on fans. In this paper, we examine how secondary events can alter the shape and size of the fan, the position of channels on the fan surface, and the behavior of the fan during subsequent fan-building floods. Our results indicate that the duration of secondary events has important effects on both the fan dimensions and their response to flood events. Fan channels formed under longer secondary events better contain flow during subsequent primary events, thereby reducing flood hazard. Climate change will alter the timing and frequency of primary and secondary events, which could have substantial impacts on how some fans evolve in the future.