Laboratoire de simulation Snow Avalanche

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Transition from sub-Rayleigh anticrack to supershear crack propagation in snow avalanches

Johan Alexandre Philippe Gaume, Bertil Trottet, Alec van Herwijnen

Avalanches can occur when a porous snow layer lies beneath a dense cohesive snow slab. Field experiments and simulations now reveal different crack-propagation regimes in slab avalanches, similar to rupture propagation following an earthquake. Snow slab avalanches, characterized by a distinct, broad fracture line, are released following anticrack propagation in highly porous weak snow layers buried below cohesive slabs. The anticrack mechanism is driven by the volumetric collapse of the weak layer, which leads to the closure of crack faces and to the onset of frictional contact. Here, on the basis of snow fracture experiments, full-scale avalanche measurements and numerical simulations, we report the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation. This transition follows the Burridge-Andrews mechanism, in which a supershear daughter crack nucleates ahead of the main fracture front and eventually propagates faster than the shear wave speed. Furthermore, we show that the supershear propagation regime can exist even if the shear-to-normal stress ratio is lower than the static friction coefficient as a result of the loss of frictional resistance during collapse. This finding shows that snow slab avalanches have fundamental similarities with strike-slip earthquakes.

NATURE PORTFOLIO2022

Different erosion and entrainment mechanisms in snow avalanches

Johan Alexandre Philippe Gaume, Xingyue Li, Betty Sovilla

Erosion significantly affects the dynamics of gravity-driven mass flows. In snow avalanches, the snow cover can be substantially eroded but only partially entrained, however, there are very limited investigations to substantiate this difference. Here, we study various erosion and entrainment behaviors in snow avalanches using the material point method (MPM), finite strain elastoplasticity and critical state soil mechanics. With varied snow properties, distinct erosion patterns are obtained and analyzed with the mass change rate. When there is significant eroded and entrained mass, properties of released snow and erodible bed snow have clear correlations with the eroded mass, but not with the entrained mass, disclosing the difference in erosion and entrainment. Both enhanced and inhibited avalanche mobilities due to erosion and entrainment are captured under different conditions of snow properties and lengths of release and erodible zones. These new insights may stimulate the development of advanced erosion and entrainment models for large-scale avalanches.

PERGAMON-ELSEVIER SCIENCE LTD2022

A new insight into the dynamic impact between geophysical flow and rigid barrier

Xingyue Li

Understanding the interaction between complicated geophysical flows and barriers remains a critical challenge for hazards countermeasures in engineering design. In this study, the impact of geophysical flows on a rigid barrier is studied numerically using coupled computational fluid dynamics and discrete element method (CFDDEM). Three types of fluid: Bingham, Herschel-Bulkley, and water are chosen as the fluid phase to mix with particles in the simulation, corresponding to Case B-P, Case HB-P, Case W-P, respectively. The results of twophase simulations are then compared with initial volume controlled dry granular flow (Case P) under the same Froude condition. Two impact mechanisms, namely pileup and runup mechanisms, are recognised in flowbarrier interaction. The effects of fluid properties including viscosity and density on the impact force against the flume base and barrier and inter-particle contacts are also systematically examined. In addition, the dynamic pressure coefficient alpha is also predicted by a newly proposed analytical model, which has been validated by the data from experimental and numerical tests. Findings indicate that the new model may provide a useful reference to calculate the impact force for the design of a rigid barrier in intercepting hazardous geophysical flows.

ELSEVIER SCI LTD2022