Effects of debris-flow magnitude–frequency distribution on avulsions and fan development

Shifts in the active channel on a debris-flow fan, termed avulsions, pose a large threat because new channels can bypass mitigation measures and cause damage to settlements and infrastructure. Recent, but limited, field evidence suggests that avulsion processes and tendency may depend on the flow-size distribution, which is difficult to constrain in the field. Here, we investigate how the flow magnitude–frequency distribution and the associated flow–magnitude sequences affect avulsion on debris-flow fans. We created three experimental fans with contrasting flow-size distributions: (1) a uniform distribution; (2) a steep double-Pareto distribution with many flows around the mean and a limited number of large flows; and (3) a shallow double-Pareto distribution with fewer flows around the mean and more abundant large flows. The fan formed by uniform flows developed through regular sequences of stepwise channelization, backstepping of deposition toward the fan apex, and avulsion over multiple flows. In contrast, the wide range of sizes in the double-Pareto distributions led to distinct avulsion mechanisms and fan evolution. Here, large flows could overtop channels, creating levee breaches that could initiate avulsion immediately or in subsequent events. Moreover, sequences of small- to moderate-sized flows could deposit channel plugs, triggering avulsion in the next large flow. This mechanism was most common on the fan formed by a steep double-Pareto distribution but was rare on the fan formed by a shallow double-Pareto distribution, where large flows were more frequent. We infer that some flow-size distributions are more likely to cause avulsions – especially those that produce abundant sequences of small flows followed by a large flow. Critically, avulsions in our experiments could occur by either large single events or over multiple flows. This observation has important implications for hazard assessment on debris-flow fans, suggesting that attention should be paid to flow history as well as flow size.