Bidispersity unlikely as a factor for the long runout of large mass
flows: scale bias in analogue granular flow experiments
Abstract
The bidispersity observed in the grain-size distribution of rock
avalanches and volcanic debris avalanches (rock/debris avalanches) has
been proposed as a property contributing to their long runout. This has
been supported by small-scale analogue experimental studies which
propose that a small proportions of fine particles, mixed with coarser,
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 evaluate the potential of bidispersity in
enhancing runout. Structure-from-motion photogrammetry is used to assess
centre of mass mobility. The findings suggest that the processes
generating increased runout in small-scale avalanches are
scale-dependent and not representative of rock/debris avalanche
dynamics. In small-scale experiments, the granular mass is
size-segregated with fine particles migrating to the base through
kinetic sieving. At the base, they reduce frictional areas between
coarse particles and the substrate, and encourage rolling. The reduced
frictional energy dissipation increases kinetic energy conversion, and
avalanche mobility. However, kinetic sieving does not occur in
rock/debris avalanches due to a dissimilar granular flow regime. The
proposition of this hypothesis overlooks that scale-dependent behaviours
of natural events are omitted in small-scale experiments. At the small
scale, a collisional regime enables the necessary agitation for kinetic
sieving. However, rock/debris avalanches are unlikely to acquire a
purely collisional regime, and rather propagate under a frictional
regime, lacking widespread agitation. Therefore, bidispersity is
unlikely to enhance the mobility of rock/debris avalanches by enabling
more efficient shearing at their base.