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Bidispersity unlikely as a factor for the long runout of large mass flows: scale bias in analogue granular flow experiments
  • Symeon Makris,
  • Irene Manzella,
  • Alessandro Sgarabotto
Symeon Makris
School of Geography, Earth and Environmental Science, University of Plymouth

Corresponding Author:[email protected]

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Irene Manzella
Department of Applied Earth Sciences, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente
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Alessandro Sgarabotto
University of Plymouth
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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.
03 Oct 2023Submitted to ESS Open Archive
17 Oct 2023Published in ESS Open Archive