Discrete modelling of rock avalanches: sensitivity to block and slope geometries

Mollon, Guilhem; Richefeu, Vincent; Villard, Pascal; Daudon, Dominique

doi:10.1007/s10035-015-0586-9

Cited by 30 publications

(11 citation statements)

References 45 publications

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“…On a rough subsurface, the decrease of grain size causes an increase of flow mobility. This is an effect confirmed by other studies (Cagnoli and Romano 2010;Mollon et al 2015;Cagnoli and Piersanti 2017;Lai et al 2017). There is also field evidence that in rock avalanches grain size decreases as travel distance increases (Davies and McSaveney 2009;Zhang et al 2016).…”

Section: Stress Level and Grain Sizesupporting

confidence: 84%

Stress level effect on mobility of dry granular flows of angular rock fragments

Cagnoli

2021

Landslides

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Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows generate stresses that are larger than those generated by small-scale flows, the purpose of these simulations is to understand the effect that the stress level has on flow mobility. The results show that granular flows that slide en mass have a flow mobility that is not influenced by the stress level. On the contrary, the stress level governs flow mobility when granular flow dynamics is affected by clast agitation and collisions. This second case occurs on a relatively rougher subsurface where an increase of the stress level causes an increase of flow mobility. The results show also that as the stress level increases, the effect that an increase of flow volume has on flow mobility switches sign from causing a decrease of mobility at low stress level to causing an increase of mobility at high stress level. This latter volume effect corresponds to the famous Heim’s mobility increase with the increase of the volume of large rock avalanches detected so far only in the field and for this reason considered inexplicable without resorting to extraordinary mechanisms. Granular flow dynamics is described in terms of dimensionless scaling parameters in three different granular flow regimes. This paper illustrates for each regime the functional relationship of flow mobility with stress level, flow volume, grain size, channel width, and basal friction.

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Section: Stress Level and Grain Sizesupporting

confidence: 84%

Stress level effect on mobility of dry granular flows of angular rock fragments

Cagnoli

2021

Landslides

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“…These results are encouraging and suggest the need to further focus experimental work on polydisperse flows (even with spheres) to better constrain the rheophysics of polydisperse mixtures. In particular, capturing the effect of grains angular shapes on energy dissipation rates is important as reported by DEM simulations (Cagnoli & Piersanti, ; Mollon et al, ).…”

Section: Discussionmentioning

confidence: 99%

Continuum Modeling of Pressure‐Balanced and Fluidized Granular Flows in 2‐D: Comparison With Glass Bead Experiments and Implications for Concentrated Pyroclastic Density Currents

2019

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Granular flows are found across multiple geophysical environments and include pyroclastic density currents, debris flows, and avalanches, among others. The key to describing transport of these hazardous flows is the rheology of these complex multiphase mixtures. Here we use the multiphase model MFIX in 2‐D for concentrated currents to examine the implications of rheological assumptions and validate this approach through comparison to experiments of both frictional and fluidized flows made of glass beads (Sauter mean grain‐size of 75 μm). Because the rheology of highly polydisperse, highly angular, polydensity granular mixtures is poorly known, we focus on simplified monodisperse or bidisperse mixtures described by the frictional flow theory of Schaeffer (1987, https://doi.org/10.1016/0022-0396(87)90038-6) and Srivastava‐Sundaresan, often referred to as the Princeton model (Srivastava & Sundaresan, 2003, https://doi.org/10.1016/S0032-5910(02)00132-8). We show that simulations including the latter model replicate well the flow shape, kinematics, and pore fluid pressure that match well‐constrained dam‐break experiments of initially fluidized or pressure‐balanced granular flows. Simulations reveal that pore fluid pressure is intrinsically modulated by dilation and compaction of the flow and hence can be generated in concentrated pyroclastic density currents. We use these simulations to interpret basal pore pressure signals from local flow properties (mixture density, solid velocity, and pore fluid pressure). Rheological changes retard these simulated flows considerably near the predicted runout, but most continuum models cannot inherently predict a zero velocity. We suggest an inertial number parameter that can be used to approximate deposition, and this approach could be a valuable tool used to validate simulations against natural pyroclastic current examples.

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“…Numerical simulations (Mollon et al, 2015) show that also on laterally unconfined planar slopes ending on a depositional horizontal plane with no lateral constraints, an increase in particle size tends to decrease the mobility of the center of mass when a collisional regime is allowed to occur. In debris flows, the presence of intergranular clay and water can alter this trend because of the activation of a different rheology as the fraction of the coarse grains decreases.…”

Section: Grain Size Effect On Flow Mobilitymentioning

confidence: 99%

Combined effects of grain size, flow volume and channel width on geophysical flow mobility: three-dimensional discrete element modeling of dry and dense flows of angular rock fragments

Cagnoli

Piersanti

2017

Solid Earth

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Abstract. We have carried out new three-dimensional numerical simulations by using a discrete element method (DEM) to study the mobility of dry granular flows of angular rock fragments. These simulations are relevant for geophysical flows such as rock avalanches and pyroclastic flows. The model is validated by previous laboratory experiments. We confirm that (1) the finer the grain size, the larger the mobility of the center of mass of granular flows; (2) the smaller the flow volume, the larger the mobility of the center of mass of granular flows and (3) the wider the channel, the larger the mobility of the center of mass of granular flows. The grain size effect is due to the fact that finer grain size flows dissipate intrinsically less energy. This volume effect is the opposite of that experienced by the flow fronts. The original contribution of this paper consists of providing a comparison of the mobility of granular flows in six channels with a different cross section each. This results in a new scaling parameter χ that has the product of grain size and the cubic root of flow volume as the numerator and the product of channel width and flow length as the denominator. The linear correlation between the reciprocal of mobility and parameter χ is statistically highly significant. Parameter χ confirms that the mobility of the center of mass of granular flows is an increasing function of the ratio of the number of fragments per unit of flow mass to the total number of fragments in the flow. These are two characteristic numbers of particles whose effect on mobility is scale invariant.

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Discrete modelling of rock avalanches: sensitivity to block and slope geometries

Cited by 30 publications

References 45 publications

Stress level effect on mobility of dry granular flows of angular rock fragments

Stress level effect on mobility of dry granular flows of angular rock fragments

Continuum Modeling of Pressure‐Balanced and Fluidized Granular Flows in 2‐D: Comparison With Glass Bead Experiments and Implications for Concentrated Pyroclastic Density Currents

Combined effects of grain size, flow volume and channel width on geophysical flow mobility: three-dimensional discrete element modeling of dry and dense flows of angular rock fragments

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