the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Supershear crack propagation in snow slab avalanche release: new insights from numerical simulations and field measurements
Abstract. The release process of dry-snow slab avalanches begins with a localized failure within a porous, weak snow layer that lies beneath a cohesive slab. Subsequently, rapid crack propagation may occur within the weak layer, eventually leading to a tensile fracture across the slab, resulting, if the slope is steep enough, to its detachment and sliding. The dynamics of crack propagation is believed to influence the size of the release area. However, the relationship between crack propagation dynamics and avalanche size remains incompletely understood. Notably, crack propagation speeds estimated from avalanche video analysis are almost one order of magnitude larger than speeds typically measured in field experiments. To shed more light on this discrepancy and avalanche release processes, we used discrete (DEM: discrete element method) and continuum (MPM: material point method) numerical methods to simulate the so-called propagation saw test (PST). On low angle terrain, our models showed that the weak layer failed mainly due to a compressive stress peak at the crack tip induced by weak layer collapse and the resulting slab bending. On steep slopes, we observed the emergence of a supershear crack propagation regime: the crack speed becomes higher than the slab shear wave speed. This transition occurs if the crack propagates over a distance larger than the super-critical crack length (approximately 5 m). Above the super-critical crack length, the fracture is mainly driven by the slope-parallel gravitational pull of the slab (tension) and, thus, shear stresses in the weak layer. These findings represent an essential additional piece in the dry-snow slab avalanche formation puzzle.
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Status: open (until 15 Aug 2024)
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RC1: 'Comment on nhess-2024-70', Anonymous Referee #1, 16 Jul 2024
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The authors used two numerical models to investigate the crack propagation speed in snow slab avalanches. They were able to explain the observed difference in crack propagation speed between field experiments and avalanches. Both models consistently showed the existence of two propagation regimes: an initial collapse-driven slower propagation is followed by a supershear crack propagation driven by shear stresses once the crack size is large enough. The combination of numerical results and experimental data confirms the model-based conclusions of previous publications.
I found the article to be well-written, clear and concise. The numerical models' findings are consistent with the experimental data, and the presented results support the conclusions.
I have only a few comments listed below.Line 99: This sentence is a repetition of the sentence on line 95
Line 151: I think you mean Figure 2c only at this place
Figure 2: Why do you use different bin sizes, resulting in different column thickness for each dataset? Is there a specific reason or meaning?
Figure 4: The symbols used for the crack propagation speed (v/cs) in the subplot are not consistent with the other figures. I would suggest adding an explanation in the caption since it is difficult to understand what the plots represent. Moreover, I couldn’t find what Φ* represents. I assume that it is the threshold slope angle, but it should be specified.
Line 212-213: It is not clear to me what mechanism the authors are describing. Can you explain in more detail? There is no evidence in the presented data of this observation. Can you provide some reference?
Citation: https://doi.org/10.5194/nhess-2024-70-RC1
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