Avulsions and the spatio-temporal evolution of debris-flow fans

Debris flows are water-laden masses of sediment that move rapidly through channel networks and over alluvial fans, where they can devastate people and property. Episodic shifts in the position of a debris-flow channel, termed avulsions, are critical for debris-flow fan evolution and for understanding flow hazards because avulsions distribute debris-flow deposits through space and time. However, both the mechanisms of flow avulsion and their effects on the long-term evolution of debris-flow fans are poorly understood. Here, we document and analyze the spatial and temporal patterns of debris-flow activity obtained by repeat topographic analyses, dendrogeomorphic and lichenometric reconstructions, and cosmogenic radionuclide dating on 16 fans from Japan, USA, Switzerland, France, and Kyrgyzstan. Where possible, we analyze the observed spatio-temporal patterns of debris-flow activity in conjunction with high-resolution topographic data to identify the main controls on avulsion. We identify two main processes that control avulsions on debris-flow fans, operating over distinct time scales. First, during individual flows or flow surges, deposition of sediment plugs locally blocks channels and forces subsequent flows to avulse into alternative flow paths. Plug deposition is a stochastic process but appears to depend in part on the sequence of flow volumes, the geometry of the channel, and the composition of the flows. Second, over time scales of tens of events, the average locus of debris-flow deposition gradually shifts toward the topographically lower parts of a fan, highlighting the importance of topographic compensation for fan evolution. Our documented debris-flow avulsions often, but not always, follow a pattern of channel plugging, backstepping of deposition toward the fan apex over one or more flows, avulsion and establishment of a new active channel. Large flows can have contrasting impacts, depending on whether or not they follow smaller flows that have deposited channel plugs. These results suggest that avulsions and spatiotemporal patterns of debris-flow fan formation strongly depend on both the magnitude-frequency distribution and the sequence of the flows feeding a fan. While individual avulsions are generally abrupt and difficult to predict, the presence of debris-flow plugs and patterns of backstepping may be useful as indicators of impending avulsions. Over longer time scales, the compensational tendency of flows to avulse into topographic depressions on the fan may also be used to identify sectors of the fan that are at risk of future inundation