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.