Snow fracture in relation to slab avalanche release: critical state for the onset of crack propagation

Kok

et al. 2019

The wind‐driven saltation of granular material plays a key role in various geophysical processes on Earth, Mars, Venus, and Titan. Although interparticle cohesion is known to limit the number of grains lifted from the surface through aerodynamic entrainment and granular splash, the role of cohesion in the development of saltation from onset to steady state is still poorly understood. Using a numerical model based on the discrete element method, we show that saltation over cohesive beds sustains itself at wind speeds 1 order of magnitude smaller than those necessary to initiate it, giving rise to hysteresis in which the occurrence of transport depends on the history of the wind. Our results further suggest that saltation over cohesive beds requires much longer distances to saturate, thereby increasing the size of the smallest stable bed forms. These findings have implications for dune formation, dust emission, and snow sublimation over cohesive beds.

Section: Saltation Modelmentioning

confidence: 99%

Cohesion‐Induced Enhancement of Aeolian Saltation

Comola

Kok

et al. 2019

“…Maximum shear stress τ max as a function of (a) slab thickness D , (b) slab elastic modulus E , (c) weak layer elastic modulus E wl , (d) crack length a , (e) slab density ρ , and (f) slope angle ψ . Symbols:

τ_{\max}^{FEM}

; continuous black line:

τ_{\max}^{mod}

(equation ; dashed gray line:

τ_{\max}^{SCM}

(equation (Gaume et al, ). System properties (if not varied): a = 0.3 m, D = 0.2 m, ρ = 250 kg/m 3 , E = 10 MPa, E wl =1 MPa for ψ = 0 ∘ (full symbols), and ψ = 40 ∘ (open symbols).…”

Section: Resultsmentioning

confidence: 99%

“…To explain the observed trends, we derived an analytical expression for the maximum stress based on FEM results. Previous work showed that the maximum stress at the crack tip was primarily a function of the normal stress

σ = ρgD \cos ψ

, the shear stress

τ = ρgD \sin ψ

and the ratios a /Λ and ( a /Λ) 2 (Gaume et al, ), where g is the gravitational acceleration. Here

normalΛ = {false(E^{'} D D_{wl} false/ G_{wl} false)}^{1 false/ 2}

is a characteristic length of the system associated with the elastic mismatch between the slab and the weak layer,

E^{'} = E false/ false(1 - ν^{2} false)

and with the shear modulus of the weak layer G wl = E wl /(2(1 + ν )); where ν is the Poisson's ratio.…”

Section: Resultsmentioning

confidence: 99%

“…Here

normalΛ = {false(E^{'} D D_{wl} false/ G_{wl} false)}^{1 false/ 2}

is a characteristic length of the system associated with the elastic mismatch between the slab and the weak layer,

E^{'} = E false/ false(1 - ν^{2} false)

and with the shear modulus of the weak layer G wl = E wl /(2(1 + ν )); where ν is the Poisson's ratio. Gaume et al () suggested that additional terms might improve the accuracy of their analytical model; hence, we fitted the following analytical expression to FEM results, including a coupled term a 2 /(Λ D ):

τ_{\max}^{mod} = τ ([], 1 + α \frac{a}{Λ}) + σ ([], β \frac{a}{Λ} + γ \frac{a^{2}}{normalΛ D} + δ {((), \frac{a}{Λ})}^{2}) .

…”

Section: Resultsmentioning

confidence: 99%

“…They showed that the critical crack length is almost independent of slope angle by assuming a rigid weak layer and a slope independent failure criterion. In contrast, Gaume et al () developed the shear collapse model (SCM) and showed that the critical crack length a c decreased with increasing slope angle by accounting for a more realistic behavior of the weak layer. The maximum shear stress

τ_{\max}^{SCM}

derived by Gaume et al () based on numerical simulations is as follows:

τ_{\max}^{SCM} = τ ((), 1 + \frac{a}{Λ}) + \frac{1}{2} σ {((), \frac{a}{Λ})}^{2},

where τ and σ are the shear and normal stresses acting on the weak layer far from the crack, respectively, a is the crack length, and Λ is a characteristic length of the system (see below).…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Stress Concentrations in Weak Snowpack Layers and Conditions for Slab Avalanche Release

Chambon

Herwijnen³

et al. 2018

Self Cite

Dry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. The onset of rapid crack propagation occurs if stresses at the crack tip in the weak layer overcome its strength. In this study, we use the finite element method to evaluate the maximum shear stress τmax induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that τmax increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, τmax decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length ac for the onset of crack propagation. The model allows for remote triggering from flat (or low angle) terrain. Yet it shows that the critical crack length decreases with increasing slope angle.

Fragmentation of wind‐blown snow crystals

Comola

Kok

et al. 2017

Self Cite

Understanding the dynamics driving the transformation of snowfall crystals into blowing snow particles is critical to correctly account for the energy and mass balances in polar and alpine regions. Here we propose a fragmentation theory of fractal snow crystals that explicitly links the size distribution of blowing snow particles to that of falling snow crystals. We use discrete element modeling of the fragmentation process to support the assumptions made in our theory. By combining this fragmentation model with a statistical mechanics model of blowing snow, we are able to reproduce the characteristic features of blowing snow size distributions measured in the field and in a wind tunnel. In particular, both model and measurements show the emergence of a self‐similar scaling for large particle sizes and a systematic deviation from this scaling for small particle sizes.