Abstract
This paper presents a numerical investigation of the behaviour of dry granular flows generated by the collapse of prismatic columns via 3D Distinct Element Method (DEM) simulations in plane strain conditions. Firstly, by means of dimensional analysis, the governing parameters of the problem are identified, and variables are clustered into dimensionless independent and dependent groups.Secondly, the results of the DEM simulations are illustrated. Different regimes of granular motion were observed depending on the initial column aspect ratio. The profiles observed at different times for columns of various aspect ratios show to be in good agreement with available experimental results.Thirdly, a detailed analysis of the way energy is dissipated by the granular flows was performed. It emerges that most of the energy of the columns is dissipated by inter-particle friction, with frictional dissipation increasing with the column aspect ratio. Also, the translational and rotational components of the kinetic energy of the flows, associated to particle rotational and translational motions respectively, were monitored during the run-out process. It is found that the rotational component is negligible in comparison with the translational one; hence in order to calculate the destructive power of a granular flow slide, only the translational contribution of the kinetic energy is relevant.Finally, a methodology is presented to calculate the flux of kinetic energy over time carried by the granular flow through any vertical section of interest. This can be related to the energy released by landslide induced granular flows impacting against engineering structures under the simplifying assumption of neglecting all structure-flow interactions. This represents the first step towards achieving a computational tool quantitatively predicting the destructive power of a given flow at any location of interest along its path. This can be useful for the design of engineering works for natural hazard mitigation. To this end, also the distribution of the linear momentum of the flow over depth was calculated. It emerges that the distribution is initially bilinear, due to the presence of an uppermost layer of particles in an agitated loose state, but after some time becomes linear.This type of analysis showcases the potential of the Distinct Element Method to investigate the phenomenology of dry granular flows and to gather unique information currently unachievable by experimentation.
Highlights
Long run-out debris flows can travel distances several times larger than the initial size of the source topography, sweeping away populated areas located far away from the landslide source (Carrara et al 2008; Crosta et al 2005)
In the case of rock avalanches and granular flows, the assessment of the final run-out distance is of primary importance, as it determines the extent of regions affected by the avalanche or landslide
Four typical distinct regimes can be identified: an initial transient acceleration (A), a constant velocity flow (B), a gradual deceleration (C) and a final static deposition (D). It emerges that in terms of Distinct Element Method (DEM) simulations, only 3D analyses accounting for the effect of particle shape, provide results that are in agreement with experiments
Summary
Long run-out debris flows (e.g. rock and debris avalanches) can travel distances several times larger than the initial size of the source topography, sweeping away populated areas located far away from the landslide source (Carrara et al 2008; Crosta et al 2005). Reference source not found.(a)), is made of particles of the same PSD as the granular column that were kept fixed at all times to simulate a non-erodible base of the same roughness as the flowing material. Li is the normalized run-out distance; [ H ] = H f Li is the normalized maximum final deposit height; [V ] = v f chosen in a variety of ways, one of which is the velocity of propagation of the flow front, i.e. v f = dL dt ); T = t initial column aspect ratio; ε = ρs gHi ( Kn D ) is a characteristic compressive strain of the granular column and [ S ] = Hi D is the model-to-particle size ratio.
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