Effect of fluidization in increasing the run-out distance of granular flows generated from different aspect ratio collapsing columns

Abstract

Abstract We investigate the propagation dynamics of fluidized granular flows in a horizontal channel in order to evaluate the factors controlling the efficiency of fluidization in increasing the run-out distance of natural granular mixtures such as pyroclastic density currents. For this, we use a two-phase numerical model able to simulate dam-break experiments, which permits us to describe depth-dependent variations of flow properties and the effect of pore pressure on the rheology of the granular material. We show that the interplay between column collapse timescale and flow front velocity plays a primary role in determining the effective influence of fluidization on run-out distance. For high aspect ratio columns, collapse velocity decreases abruptly after reaching its peak, a significant portion of the collapse occurs when the flow front has travelled a long distance from the reservoir and, importantly, the decrease of basal pore pressure with time in the reservoir translates into a reduced velocity of the granular material entering into the propagation channel during final phases of collapse. Thus, at some point, the collapsing material is not able to affect significantly the flow front dynamics, in contrast to low aspect ratio collapsing columns. These results are consistent with complementary analogue experiments, which show that the granular material at the front of the deposit originates from lower levels of the collapsed column. Comparison with new experimental data also reveals that the effective pore pressure diffusion coefficient is an increasing function of column height, and can be considered as proportional to a weighted average of flow thickness during propagation. This is consistent with experiments on static defluidization columns, but had not been tested in dam-break experiments until this study. Considering this type of dependency, under our experimental and simulation conditions, the non-dimensional run-out distance presents a relative maximum for an aspect ratio between \(1\) and \(2\), and then it decreases abruptly. Our observations suggest that the effect of fluidization in increasing run-out distance is limited under conditions of sudden collapse of a volume of fluidized material with no initial velocity, such as collapsing domes. This has implications for the long-lasting debate on the influence of fluidization in the transport dynamics of natural granular flows.