Scale-Dependent Processes and Runout in Bidisperse Granular Flows: Insights From Laboratory Experiments and Implications for Rock/Debris Avalanches

The bidispersity observed in the particle-size distribution of rock avalanches and volcanic debris avalanches (rock/debris avalanches) has been proposed as a factor contributing to their long runout. This has been supported by small-scale analog experimental studies, which observe that a small proportion of fine particles mixed with coarser particles enhances granular avalanche runout. However, the mechanisms enabling this phenomenon and their resemblance to rock/debris avalanches have not been directly evaluated. Here, binary mixture granular avalanche experiments are employed to constrain the processes and conditions under which bidispersity enhances the runout of granular avalanches in experiments. Structure-from-motion photogrammetry is used to measure center of mass displacement and assess energy dissipation. Subsequently, this study evaluates the dynamic scaling and flow regimes in the lab and field to assess whether the runout-enhancing mechanism is applicable to rock/debris avalanches. In small-scale experiments, the granular mass propagates under a collisional regime, enabling kinetic sieving and size segregation. Fine particles migrate to the base where they reduce frictional areas between coarse particles and the substrate and encourage rolling. The reduced energy dissipation increases the kinetic energy conversion and avalanche mobility. However, rock/debris avalanches are unlikely to acquire a purely collisional regime; instead, they propagate under a frictional regime. The size segregation which is essential for the process observed at the lab-scale is prohibited by the frictional regime, as evident by the sedimentology of rock/debris avalanche deposits. The proposal of bidispersity as a runout-enhancing mechanism overlooks that scale-dependent behaviors of natural events are often omitted in small-scale experiments. Large landslides such as rock avalanches and volcanic debris avalanches flow unexpectedly long distances before stopping. The mechanisms generating this phenomenon remain unknown. The fact that they contain large proportions of finer and coarser particles (with relatively smaller quantities of the sizes between them) has been suggested by experiments to be a potential factor for the long distances they cover. In this work, we have carried out experiments to examine the processes enabling this event at the scale of lab experiments in order to assess their potential in real events. We found that this phenomenon is caused by fine particles percolating to the base of the flow. There, they reduce the surface of the coarse particles, which are in contact with the substrate, and also encourage the rolling of the coarser particles. This reduces frictional energy loss, and conserves more energy, which contributes to the flow, leading to longer distances. However, the conditions which allow these processes and the percolation of the fine particles to the base are scale-dependent and are not applicable to large-scale events. When planning granular flow experiments or interpreting their findings, scaling is important to ensure similarity between the experimental conditions and the physical processes targeted. For bidispersity to enhance the runout of analog granular avalanches, a collisional regime and particle segregation are required Dynamic scaling suggests that processes and flow regimes reducing apparent friction in small analog experiments are scale-dependent Lack of signature of agitation and size segregation reported in large field-scale avalanche sedimentology suggests dissimilar processes