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The Swiss Plasma Center is one of the world’s leading fusion research laboratories. Through a wide range of research programs, all connected to education and training at different levels, we work to advance our understanding of the physics of plasmas and develop fusion as an energy source.

Swiss Plasma Center News

Simons Foundation funds collaboration on stellarators

Research

The Swiss Plasma Center participates in an international collaboration, funded by the Simons Foundation, to unveil the optimal magnetic configurations of stellarators so that their performances are similar to those of tokamaks - while re

EPFL Physics Thesis Distinction to Anna Teplukhina

Research

Anna Theplukhina recently received the EPFL Physics Thesis Distinction for her thesis on 'Realistic multi-machine tokamak profile simulations and numerical ramp-down optimization using the RAPTOR code'.

The last 3 papers of the Swiss Plasma Center

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Deep inside the deadly avalanche that climate change built

J. Gaume

2019-02-06.

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Modeling of crack propagation in weak snowpack layers using the discrete element method

J. Gaume; A. van Herwijnen; G. Chambon; K. W. Birkeland; J. Schweizer

The Cryosphere. 2018-10-08. Vol. 9, num. 5, p. 1915-1932.

DOI : 10.5194/tc-9-1915-2015.

Dry-snow slab avalanches are generally caused by a sequence of fracture processes including (1) failure initiation in a weak snow layer underlying a cohesive slab, (2) crack propagation within the weak layer and (3) tensile fracture through the slab which leads to its detachment. During the past decades, theoretical and experimental work has gradually led to a better understanding of the fracture process in snow involving the collapse of the structure in the weak layer during fracture. This now allows us to better model failure initiation and the onset of crack propagation, i.e., to estimate the critical length required for crack propagation. On the other hand, our understanding of dynamic crack propagation and fracture arrest propensity is still very limited. To shed more light on this issue, we performed numerical propagation saw test (PST) experiments applying the discrete element (DE) method and compared the numerical results with field measurements based on particle tracking. The goal is to investigate the influence of weak layer failure and the mechanical properties of the slab on crack propagation and fracture arrest propensity. Crack propagation speeds and distances before fracture arrest were derived from the DE simulations for different snowpack configurations and mechanical properties. Then, in order to compare the numerical and experimental results, the slab mechanical properties (Young's modulus and strength) which are not measured in the field were derived from density. The simulations nicely reproduced the process of crack propagation observed in field PSTs. Finally, the mechanical processes at play were analyzed in depth which led to suggestions for minimum column length in field PSTs.

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

J. Gaume; G. Chambon; A. van Herwijnen; J. Schweizer

Geophysical Research Letters. 2018-08-28. Vol. 45, num. 16, p. 8363-8369.

DOI : 10.1029/2018GL078900.

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 tau(max) induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that tau(max) increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, tau(max) decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length a(c) 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.

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Lawinenforscher arbeiteten für «Frozen»-Film

J. Gaume

2018-08-07.

Abkühlung zum LesenLawinen können nun mitdem Computer exaktsimuliert werden. Das hilftnicht nur Disney.

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L’avalanche de plaque prédite en 3D

J. Gaume

2018-08-06.

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Avalanche prediction model a boon for rescuers and filmmakers

J. Gaume

2018-08-04.

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The subtle mechanics of an avalanche - as seen in 3D

J. Gaume

2018-08-03.

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Unified modeling of avalanche release and flow using MPM

J. Gaume

OSUG Snow workshop, Centre d'Etude de la Neige, MétéoFrance, Grenoble, France.

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Unified modeling of snow and avalanche mechanics using the material point method

J. Gaume

EPFL IIE Seminar Series, EPFL, Lausanne, Switzerland.

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A mechanically-based model of snow slab and weak layer fracture in the Propagation Saw Test

L. Benedetti; J. Gaume; J.-T. Fischer

International Journal of Solids and Structures. 2018-02-09.

DOI : 10.1016/j.ijsolstr.2017.12.033.

Dry-snow slab avalanche release is the result of failure initiation in a weak snowpack layer buried below a cohesive snow slab, which is then followed by rapid crack propagation. The Propagation Saw Test (PST) is a field experiment which allows to evaluate the critical crack length for the onset of crack propagation and the propagation distance. Although a widely used method, the results from this field test are difficult to interpret in practice because (i) the fracture process in multilayer systems is very complex and only partially explored and (ii) field data is typically insufficient to establish direct causal links between test results and snowpack characteristics. Furthermore, although several studies have focused on the critical crack length assuming linear elasticity for the slab, it still remains unclear how the complex interplay between the weak layer failure and slab fracture impacts the outcome of the PST. To address this knowledge gap, an analytical model of the PST was developed, based on the Euler-Bernoulli beam theory, in order to compute both the critical crack length and the propagation distance as a function of snowpack properties and beam geometry (e.g. beam length and slab height). This work aims to create a link between the two main outcomes of the PST, namely full propagation (END) and slab fracture (SF), and the quantitative results of critical crack length and propagation distance. Moreover, introducing empirical relationships based on laboratory experiments (Scapozza, 2004; Sigrist, 2006) between the elastic modulus, the tensile strength and slab density, it is possible to describe the onset of slab fracture for a given geometry of the PST using only the slab density. As a result, the model allows to reproduce the increasing trend of the propagation distance with increasing slab density, as observed in field experiments. For slabs characterized by low density, slab fracture occurs before reaching the critical crack length (SFb); for intermediate density values, slab fracture occurs after the onset of crack propagation in the weak layer (SFa); then, large densities lead to full propagation in the weak layer without slab fracture (END).

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Dense flow avalanche pressure on obstacle studied with DEM

M. Kyburz; B. Sovilla; J. Gaume; C. Ancey

EGU, Vienna, Austria, April 09-13, 2018.

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Micromechanical modeling of crack propagation for snow slab avalanche release. Workshop: Accounting for phase transitions in granular media: from micromechanics to macroscopic unified modeling

G. Bobillier; J. Gaume; A. van Herwijnen; J. Dual; J. Schweizer

Milano, Italy, September 6-7, 2018.

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Modeling crack propagation for snow slab avalanche release with discrete elements

G. Bobillier; J. Gaume; A. van Herwijnen; J. Dual; J. Schweizer

ECCM ECFD, Glasgow, UK , 2018.

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Dense flow avalanche pressure on obstacle studied with DEM

M. Kyburz; B. Sovilla; J. Gaume; C. Ancey

ECCM ECFD, Glasgow, UK, 2018.

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Avalanche pressures at the Vallée de la Sionne test site: interaction of avalanches and narrow strictures studied with DEM.

M. Kyburz; B. Sovilla; J. Gaume; C. Ancey

2018. Proceedings of the International Snow Science Workshop 2018 , Innsbruck, Austria , 2018.

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Modeling the Propagation Saw Test with Discrete Elements

G. Bobillier; J. Gaume; A. van Herwijnen; J. Dual; J. Schweizer

2018. Proceedings of the International Snow Science Workshop 2018 , Innsbruck, Austria , 2018.

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Unified modeling of the release and flow of snow avalanches using MPM.

J. Gaume; T. Gast; J. Teran; A. van Herwijnen; C. Jiang

2018. Proceedings of the International Snow Science Workshop 2018 , Innsbruck, Austria , 2018.

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Dynamics of propagating anticracks in snow slab avalanches

J. Gaume

EGU, Vienna, Austria, April 09-13, 2018.

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Unified modeling of the release and flow of snow avalanches using MPM

J. Gaume

EGU, Vienna, Austria, April 09-13, 2018.

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Unified modeling of the release and flow of snow avalanches using the material point method

J. Gaume; T. Gast; J. Teran; A. van Herwijnen; C. Jiang

ECCM ECFD, Glascow, UK, 2018.

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Unified modeling of slab avalanche release and flow using the material point method

J. Gaume

SLF Seminar, Davos, Switzerland.

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A review of finite-element modelling in snow mechanics

E. Podolskiy; G. Chambon; M. Naaim; J. Gaume

Journal of Glaciology. 2017-07-10. Vol. 59, num. 218, p. 1189-1201.

DOI : 10.3189/2013JoG13J121.

The finite-element method (FEM) is one of the main numerical analysis methods in continuum mechanics and mechanics of solids (Huebner and others, 2001). Through mesh discretization of a given continuous domain into a finite number of sub-domains, or elements, the method finds approximate solutions to sets of simultaneous partial differential equations, which express the behavior of the elements and the entire system. For decades this methodology has played an accelerated role in mechanical engineering, structural analysis and, in particular, snow mechanics. To the best of our knowledge, the application of finite-element analysis in snow mechanics has never been summarized. Therefore, in this correspondence we provide a table with a detailed review of the main FEM studies on snow mechanics performed from 1971 to 2012 (40 papers), for facilitating comparison between different mechanical approaches, outlining numerical recipes and for future reference. We believe that this kind of compact review in a tabulated form will produce a snapshot of the state of the art, and thus become an appropriate, timely and beneficial reference for any relevant follow-up research, including, for example, not only snow avalanche questions, but also modeling of snow microstructure and tire–snow interaction. To that end, this correspondence is organized according to the following structure. Table 1 includes all essential information about previously published FEM studies originally developed to investigate stresses in snow with all corresponding mechanical and numerical parameters. Columns in Table 1 provide references to particular studies, placed in chronological order. Rows correspond to the main model parameters and other details of each considered case.

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Les tests de stabilité à la loupe

J. Gaume; A. van Herwijnen

Neige et avalanches. 2017-04-04. num. 157, p. 15-16.

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Johan Gaume a mis au point un nouveau modèle pour évaluer les conditions de déclenchement des avalanches

J. Gaume

2017-03-03.

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Un modèle numérique pour prévenir les avalanches

J. Gaume

2017-02-05.

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Un modèle numérique pour prévenir les avalanches

J. Gaume

2017-02-05.

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Du nouveau dans la prévision du risque d’avalanche

J. Gaume

2017-02-01.

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Du nouveau dans la prévision du risque d'avalanche

J. Gaume

2017-02-01.

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Towards a better forecast of slab avalanches

J. Gaume

2017-01-31.

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Assessing snow instability in skier-triggered snow slab avalanches by combining failure initiation and crack propagation

J. Gaume; B. Reuter

Cold Regions Science And Technology. 2017. Vol. 144, p. 6-15.

DOI : 10.1016/j.coldregions.2017.05.011.

Dry-snow slab avalanches start with a local failure in a weak snowpack layer buried below cohesive snow slab layers. If the size of the failed zone exceeds a critical size, rapid crack propagation occurs possibly followed by slab release if the slope is steep enough. The probability of skier-triggering a slab avalanche is generally characterized by classical stability indices that do not account for crack propagation. In this study, we propose a new model to evaluate the conditions for the onset of crack propagation in skier-triggered slab avalanches. For a given weak layer, the critical crack length characterizing crack propagation propensity was compared to the size of the area where the skier-induced stress exceeds the shear strength of the weak layer. The ratio between both length scales yields a stability criterion combining the processes of failure initiation and crack propagation. The critical crack length was calculated from a recently developed model based on numerical simulations. The skier-induced stress was computed from analytical solutions and finite element simulations to account for slab layering. A detailed sensitivity analysis was performed for simplified snow profiles to characterize the influence of snowpack properties and slab layering on crack propagation propensity. Finally, we applied our approach to manually observed snow profiles and compared our new criterion to Rutschblock scores.

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Scaling laws for the mechanics of loose and cohesive granular materials based on Baxter's sticky hard spheres

J. Gaume; H. Lowe; S. Tan; L. Tsang

Physical Review E. 2017. Vol. 96, num. 3, p. 032914.

DOI : 10.1103/PhysRevE.96.032914.

We have conducted discrete element simulations (PFC3D) of very loose, cohesive, granular assemblies with initial configurations which are drawn from Baxter's sticky hard sphere (SHS) ensemble. The SHS model is employed as a promising auxiliary means to independently control the coordination number zc of cohesive contacts and particle volume fraction f of the initial states. We focus on discerning the role of zc and f for the elastic modulus, failure strength, and the plastic consolidation line under quasistatic, uniaxial compression. We find scaling behavior of the modulus and the strength, which both scale with the cohesive contact density.c = zcf of the initial state according to a power law. In contrast, the behavior of the plastic consolidation curve is shown to be independent of the initial conditions. Our results show the primary control of the initial contact density on the mechanics of cohesive granular materials for small deformations, which can be conveniently, but not exclusively explored within the SHS-based assembling procedure.

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Fragmentation of wind-blown snow crystals

F. Comola; J. F. Kok; J. Gaume; E. Paterna; M. Lehning

Geophysical Research Letters. 2017. Vol. 44, num. 9, p. 4195-4203.

DOI : 10.1002/2017GL073039.

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.

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

J. Gaume; A. van Herwijnen; G. Chambon; N. Wever; J. Schweizer

The Cryosphere. 2017.

DOI : 10.5194/tc-11-217-2017.

The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting.

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Temporal evolution of crack propagation propensity in snow in relation to slab and weak layer properties

J. Schweizer; B. Reuter; A. van Herwijnen; B. Richter; J. Gaume

The Cryosphere. 2016-11-11. Vol. 10, num. 6, p. 2637-2653.

DOI : 10.5194/tc-10-2637-2016.

If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests (PSTs) on 7 sampling days during a 2-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, increased overall during the measurement period. The increase was not steady and the lowest values of critical cut length were observed around the middle of the measurement period. The relevant mechanical properties, the slab effective elastic modulus and the weak layer specific fracture, overall increased as well. However, the changes with time differed, suggesting that the critical cut length cannot be assessed by simply monitoring a single mechanical property such as slab load, slab modulus or weak layer specific fracture energy. Instead, crack propagation propensity is the result of a complex interplay between the mechanical properties of the slab and the weak layer. We then compared our field observations to newly developed metrics of snow instability related to either failure initiation or crack propagation propensity. The metrics were either derived from the SMP signal or calculated from simulated snow stratigraphy (SNOWPACK). They partially reproduced the observed temporal evolution of critical cut length and instability test scores. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.

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Estimating the effective elastic modulus and specific fracture energy of snowpack layers from field experiments

A. VAN HERWIJNEN; J. GAUME; E. H. BAIR; B. REUTER; K. W. BIRKELAND et al.

Journal of Glaciology. 2016-07-25. Vol. 62, num. 236, p. 997-1007.

DOI : 10.1017/jog.2016.90.

Measurements of the mechanical properties of snow are essential for improving our understanding and the prediction of snow failure and hence avalanche release. We performed fracture mechanical experiments in which a crack was initiated by a saw in a weak snow layer underlying cohesive snow slab layers. Using particle tracking velocimetry (PTV), the displacement field of the slab was determined and used to derive the mechanical energy of the system as a function of crack length. By fitting the estimates of mechanical energy to an analytical expression, we determined the slab effective elastic modulus and weak layer specific fracture energy for 80 different snowpack combinations, including persistent and nonpersistent weak snow layers. The effective elastic modulus of the slab ranged from 0.08 to 34 MPa and increased with mean slab density following a power-law relationship. The weak layer specific fracture energy ranged from 0.08 to 2.7 J m−2 and increased with overburden. While the values obtained for the effective elastic modulus of the slab agree with previously published low-frequency laboratory measurements over the entire density range, the values of the weak layer specific fracture energy are in some cases unrealistically high as they exceeded those of ice. We attribute this discrepancy to the fact that our linear elastic approach does not account for energy dissipation due to non-linear parts of the deformation in the slab and/or weak layer, which would undoubtedly decrease the amount of strain energy available for crack propagation.

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Snow instability evaluation: calculating the skier-induced stress in a multi-layered snowpack

F. Monti; J. Gaume; A. van Herwijnen; J. Schweizer

Natural Hazards and Earth System Sciences. 2016-03-18. Vol. 16, num. 3, p. 775-788.

DOI : 10.5194/nhess-16-775-2016.

The process of dry-snow slab avalanche formation can be divided into two phases: failure initiation and crack propagation. Several approaches tried to quantify slab avalanche release probability in terms of failure initiation based on shear stress and strength. Though it is known that both the properties of the weak layer and the slab play a major role in avalanche release, most previous approaches only considered slab properties in terms of slab depth, average density and skier penetration. For example, for the skier stability index, the additional stress (e.g. due to a skier) at the depth of the weak layer is calculated by assuming that the snow cover can be considered a semi-infinite, elastic, half-space. We suggest a new approach based on a simplification of the multi-layered elasticity theory in order to easily compute the additional stress due to a skier at the depth of the weak layer, taking into account the layering of the snow slab and the substratum. We first tested the proposed approach on simplified snow profiles, then on manually observed snow profiles including a stability test and, finally, on simulated snow profiles. Our simple approach reproduced the additional stress obtained by finite element simulations for the simplified profiles well – except that the sequence of layering in the slab cannot be replicated. Once implemented into the classical skier stability index and applied to manually observed snow profiles classified into different stability classes, the classification accuracy improved with the new approach. Finally, we implemented the refined skier stability index into the 1–D snow cover model SNOWPACK. The two study cases presented in this paper showed promising results even though further verification is still needed. In the future, we intend to implement the proposed approach for describing skier-induced stress within a multi-layered snowpack into more complex models which take into account not only failure initiation but also crack propagation.

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A new crack propagation criterion for skier-triggered snow slab avalanches

J. Gaume; B. Reuter; A. van Herwijnen; J. Schweizer

2016. International Snow Science Workshop , Breckenridge, Colorado, USA .

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Critical length for the onset of crack propagation in snow: reconciling shear and collapse

J. Gaume; A. van Herwijnen; G. Chambon; N. Wever; J. Schweizer

2016. International Snow Science Workshop , Breckenridge, Colorado, USA .

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Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area

J. Gaume; G. Chambon; N. Eckert; M. Naaim; J. Schweizer

The Cryosphere. 2015-04-27. Vol. 9, num. 2, p. 795-804.

DOI : 10.5194/tc-9-795-2015.

Dry-snow slab avalanches are generally caused by a sequence of fracture processes, including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually increased our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by slab elasticity and possible tensile failure of the slab. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and trigger a tensile failure within the slab; (2) partial-slope release if fracture arrest and slab tensile failure occur due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as often claimed in the literature.

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A new mixed-mode failure criterion for weak snowpack layers

I. Reiweger; J. Gaume; J. Schweizer

Geophysical Research Letters. 2015-03-27. Vol. 42, num. 5, p. 1427-1432.

DOI : 10.1002/2014GL062780.

The failure of a weak snow layer is the first in a series of processes involved in dry‐snow slab avalanche release. The nature of the initial failure within the weak layer is not yet fully understood but widely debated. The knowledge of the failure criterion is essential for developing avalanche release models and hence for avalanche hazard assessment. Yet different release models assume contradictory criteria as input parameters. We analyzed loading experiments on snow failure performed in a cold laboratory with samples containing a persistent weak snow layer of either faceted crystal, depth hoar, or buried surface hoar. The failure behavior of these layers can be described well with a modified Mohr‐Coulomb model accounting for the possible compressive failure of snow. We consequently propose a new mixed‐mode shear‐compression failure criterion that can be used in avalanche release models.

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Evaluating snow weak-layer failure parameters through inverse finite element modelling of shaking-platform experiments

E. A. Podolskiy; G. Chambon; M. Naaim; J. Gaume

Natural Hazards and Earth System Sciences. 2015-01-15. Vol. 15, num. 1, p. 119-134.

DOI : 10.5194/nhess-15-119-2015.

Snowpack weak layers may fail due to excess stresses of various natures, caused by snowfall, skiers, explosions or strong ground motion due to earthquakes, and lead to snow avalanches. This research presents a numerical model describing the failure of "sandwich" snow samples subjected to shaking. The finite element model treats weak layers as interfaces with variable mechanical parameters. This approach is validated by reproducing cyclic loading snow fracture experiments. The model evaluation revealed that the Mohr–Coulomb failure criterion, governed by cohesion and friction angle, was adequate to describe the experiments. The model showed the complex, non-homogeneous stress evolution within the snow samples and especially the importance of tension on fracture initiation at the edges of the weak layer, caused by dynamic stresses due to shaking. Accordingly, a simplified analytical solution, ignoring the inhomogeneity of tangential and normal stresses along the failure plane, may incorrectly estimate the shear strength of the weak layers. The values for "best fit" cohesion and friction angle were ≈1.6 kPa and 22.5–60°. These may constitute valuable first approximations in mechanical models used for avalanche forecasting.

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Granulation of snow: From tumbler experiments to discrete element simulations

W. Steinkogler; J. Gaume; H. Loewe; B. Sovilla; M. Lehning

Journal Of Geophysical Research-Earth Surface. 2015. Vol. 120, num. 6, p. 1107-1126.

DOI : 10.1002/2014Jf003294.

It is well known that snow avalanches exhibit granulation phenomena, i.e., the formation of large and apparently stable snow granules during the flow. The size distribution of the granules has an influence on flow behavior which, in turn, affects runout distances and avalanche velocities. The underlying mechanisms of granule formation are notoriously difficult to investigate within large-scale field experiments, due to limitations in the scope for measuring temperatures, velocities, and size distributions. To address this issue we present experiments with a concrete tumbler, which provide an appropriate means to investigate granule formation of snow. In a set of experiments at constant rotation velocity with varying temperatures and water content, we demonstrate that temperature has a major impact on the formation of granules. The experiments showed that granules only formed when the snow temperature exceeded -1(degrees)C. No evolution in the granule size was observed at colder temperatures. Depending on the conditions, different granulation regimes are obtained, which are qualitatively classified according to their persistence and size distribution. The potential of granulation of snow in a tumbler is further demonstrated by showing that generic features of the experiments can be reproduced by cohesive discrete element simulations. The proposed discrete element model mimics the competition between cohesive forces, which promote aggregation, and impact forces, which induce fragmentation, and supports the interpretation of the granule regime classification obtained from the tumbler experiments. Generalizations, implications for flow dynamics, and experimental and model limitations as well as suggestions for future work are discussed.

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Evaluation of slope stability with respect to snowpack spatial variability

J. Gaume; J. Schweizer; A. van Herwijnen; G. Chambon; B. Reuter et al.

Journal of Geophysical Research: Earth Surface. 2014-08-21. Vol. 119, num. 9, p. 1783-1799.

DOI : 10.1002/2014JF003193.

The evaluation of avalanche release conditions constitutes a great challenge for risk assessment in mountainous areas. The spatial variability of snowpack properties has an important impact on snow slope stability and thus on avalanche formation, since it strongly influences failure initiation and crack propagation in weak snow layers. Hence, the determination of the link between these spatial variations and slope stability is very important, in particular, for avalanche public forecasting. In this study, a statistical‐mechanical model of the slab‐weak layer (WL) system relying on stochastic finite element simulations is used to investigate snowpack stability and avalanche release probability for spontaneously releasing avalanches. This model accounts, in particular, for the spatial variations of WL shear strength and stress redistribution by elasticity of the slab. We show how avalanche release probability can be computed from release depth distributions, which allows us to study the influence of WL spatial variations and slab properties on slope stability. The importance of smoothing effects by slab elasticity is verified and the crucial impact of spatial variation characteristics on the so‐called knock‐down effect on slope stability is revisited using this model. Finally, critical length values are computed from the simulations as a function of the various model parameters and are compared to field data obtained with propagation saw tests.

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Prédétermination des hauteurs de départ d'avalanches : une approche par extrêmes spatiaux

J. Gaume; N. Eckert; G. Chambon; M. Naaim

La Houille Blanche. 2013-10-05. num. 5, p. 30-36.

DOI : 10.1051/lhb/2013040.

Numerical models of snow avalanches propagation have acquired a central role among decision tools for avalanche protection engineering. Nevertheless, the systematic implementation of these models still faces a number of difficulties including the precise evaluation of avalanche release depths. In our work, the predetermination of snow depths in any potential release zone is achieved by spatial interpolation of the daily precipitation data acquired in 40 meteorological sites in the French Alps since 1966. Given the rarity of avalanches, extreme precipitation data (annual maxima) are considered. We use the formal framework of max-stable processes which are the generalization of univariate extreme value theory to the spatial multivariate case. Using different models for the spatial evolution of the parameters of the GEV distribution, we are able to establish snow precipitation maps for different return periods. The results show that, for a given return period and at fixed elevation, snowfalls are higher in the NE of the French Alps. However variance maxima are located in the SE which corresponds to the Mediterranean influence that tends to bring more variability. Finally, we show that the spatial dependence of extreme snow precipitations depends on the orientation of the local Alpin axis. Sensitivity of the model to the extremal dependence and to the spatial evolution of the GEV parameters is discussed. Cross-validation is used to demonstrate the robustness of the retained model.

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Mapping extreme snowfalls in the French Alps using max-stable processes

J. Gaume; N. Eckert; G. Chambon; M. Naaim; L. Bel

Water Resources Research. 2013-02-28. Vol. 49, num. 2, p. 1079-1098.

DOI : 10.1002/wrcr.20083.

The evaluation of extreme snowfalls is an important challenge for hazard management in mountainous regions. In this paper, the extreme snowfall data acquired from 40 meteorological stations in the French Alps since 1966 are analyzed using spatial extreme statistics. They are then modeled within the formal framework of max-stable processes (MSPs) which are the generalization of the univariate extreme value theory to the spatial multivariate case. The three main MSPs now available are compared using composite likelihood maximization, and the most flexible Brown-Resnick one is retained on the basis of the Takeuchi information criterion, taking into account anisotropy by space transformation. Furthermore, different models with smooth trends (linear and splines) for the spatial evolution of the generalized extreme value (GEV) parameters are tested to allow snowfall maps for different return periods to be produced. After altitudinal correction that separates spatial and orographic effects, the different spatial models are fitted to the data within the max-stable framework, allowing inference of the GEV margins and the extremal dependence simultaneously. Finally, a nested model selection procedure is employed to select the best linear and spline models. Results show that the best linear model produces reasonable quantile maps (assessed by cross-validation using other stations), but it is outperformed by the best spline model which better captures the complex evolution of GEV parameters with space. For a given return period and at fixed elevation of 2000 m, extreme 3 day snowfalls are higher in the NE and SE of the French Alps. Maxima of the location parameter of the GEV margins are located in the north and south, while maxima of the scale parameter are located in the SE which corresponds to the Mediterranean influence that tends to bring more variability. Besides, the dependence of extreme snowfalls is shown to be stronger on the local orientation of the Alps, an important result for meteorological variables confirming previous studies. Computations are performed for different accumulation durations which enable obtaining magnitude-frequency curves and show that the intensity of the extremal directional dependence effect is all the more important when the duration is short. Finally, we show how the fitted model can be used to evaluate joint exceedence probabilities and conditional return level maps, which can be useful for operational risk management.

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Influence of weak-layer heterogeneity on snow slab avalanche release: application to the evaluation of avalanche release depths

J. Gaume; G. Chambon; N. Eckert; M. Naaim

Journal of Glaciology. 2013. Vol. 59, num. 215, p. 423-437.

DOI : 10.3189/2013JoG12J161.

The evaluation of avalanche release depths constitutes a great challenge for risk assessment in mountainous areas. This study focuses on slab avalanches, which generally result from the rupture of a weak layer underlying a cohesive slab. We use the finite-element code Cast3M to build a mechanical model of the slab/weak-layer system, taking into account two key ingredients for the description of avalanche release: weak-layer heterogeneity and stress redistribution via slab elasticity. The system is loaded by increasing the slope angle until rupture. We first examine the cases of one single and two interacting weak spots in the weak layer, in order to validate the model. We then study the case of heterogeneous weak layers represented through Gaussian distributions of the cohesion with a spherical spatial covariance. Several simulations for different realizations of weak-layer heterogeneity are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is analyzed. We show, in particular, a heterogeneity smoothing effect caused by slab elasticity. Finally, this mechanically based probabilistic model is coupled with extreme snowfall distributions. A sensitivity analysis of the predicted distributions enables us to determine the values of mechanical parameters that provide the best fit to field data.

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Relative influence of mechanical and meteorological factors on avalanche release depth distributions: An application to French Alps

J. Gaume; G. Chambon; N. Eckert; M. Naaim

Geophysical Research Letters. 2012-06-20. Vol. 39, num. 12, p. n/a-n/a.

DOI : 10.1029/2012GL051917.

The evaluation of avalanche release depth distributions represents a major challenge for hazard management. This paper presents a rigorous formalism in which these distributions are expressed through a coupling of mechanical and meteorological factors. Considering that an avalanche can occur only if the snowfall depth exceeds a critical value corresponding to a stability criterion, release depth distributions obtained from a slab-weak layer mechanical model are coupled with the distribution of 3-day extreme snowfalls. We show that this coupled model is able to reproduce field data from 369 natural slab avalanches in La Plagne (France). Not only the power-law tail of the distribution, corresponding to large slab depths, but also the core of the distribution for shallow slab depths, are well represented. Small to medium-sized avalanches appear to be controlled mainly by mechanics, whereas large avalanches and the associated power-law exponent, are influenced by a strong mechanical-meteorological coupling. Finally, we demonstrate that the obtained distribution is strongly space-dependent, and, using a consistent interpolation formalism, our model is used to obtain release depth maps for given return periods. Citation: Gaume, J., G. Chambon, N. Eckert, and M. Naaim (2012), Relative influence of mechanical and meteorological factors on avalanche release depth distributions: An application to French Alps, Geophys. Res. Lett., 39, L12401, doi:10.1029/2012GL051917.

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Using spatial and spatial-extreme statistics to characterize snow avalanche cycles

N. Eckert; J. Gaume; H. Castebrunet

Procedia Environmental Sciences. 2011. Vol. 7, p. 224-229.

DOI : 10.1016/j.proenv.2011.07.039.

In December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Using this case study, this paper illustrates how spatial statistics can be used to analyse such abnormal temporal clusters of snow avalanches. Spatial regression methods are used to quantify aggregation and gradients and highlight the three day snowfall as the main explanatory factor. A max-stable model is developed to evaluate the snowfall return period, so as to compare the studied cycle with previous ones and with empirical return periods for avalanche counts. (C) 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Spatial Statistics 2011

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Cross-comparison of meteorological and avalanche data for characterising avalanche cycles: The example of December 2008 in the eastern part of the French Alps

N. Eckert; C. Coleou; H. Castebrunet; M. Deschatres; G. Giraud et al.

Cold Regions Science and Technology. 2010. Vol. 64, num. 2, p. 119-136.

DOI : 10.1016/j.coldregions.2010.08.009.

In December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Southerly atmospheric fluxes that progressively evolved into an easterly return caused important snowfalls with return periods up to 10 years. Cold temperatures and drifting snow had important aggravating effects. The return period for the number of avalanches was above 50 years in two massifs and some of the avalanche had very long runouts that exceeded historical limits recorded in the French avalanche atlas. Using this case study, this paper illustrates and discusses how avalanche reports, snow and weather data and results from numerical modelling of the snow cover call be combined to analyse abnormal temporal clusters of snow avalanches. For instance, it is shown how statistical techniques developed in other fields can be used to test the significance of different explanatory factors, extract spatio-temporal patterns, compare them with previous cycles and quantify the magnitude/frequency relationship at different scales. (C) 2010 Elsevier B.V. All rights reserved.

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Quasistatic to inertial transition in granular materials and the role of fluctuations

J. Gaume; G. Chambon; M. Naaim

Physical Review. 201-11-11. Vol. E84, num. 5.

DOI : 10.1103/PhysRevE.84.051304.

On the basis of discrete element numerical simulations of a Couette cell, we revisit the rheology of granular materials in the quasistatic and inertial regimes, and discuss the origin of the transition between these two regimes. We show that quasistatic zones are the seat of a creep process whose rate is directly related to the existence and magnitude of velocity fluctuations. The mechanical behavior in the quasistatic regime is characterized by a three-variable constitutive law relating the friction coefficient (normalized stress), the inertial number (normalized shear rate), and the normalized velocity fluctuations. Importantly, this constitutive law appears to remain also valid in the inertial regime, where it can account for the one-to-one relationship observed between the friction coefficient and the inertial number. The abrupt transition between the quasistatic and inertial regimes is then related to the mode of production of the fluctuations within the material, from nonlocal and artificially sustained by the boundary conditions in the quasistatic regime, to purely local and self-sustained in the inertial regime. This quasistatic-to-inertial transition occurs at a critical inertial number or, equivalently, at a critical level of fluctuations.

Swiss Plasma Center Memento

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10:30

11:30

PPB 019

Coherence imaging for plasma spectro-polarimetry

Optical coherence imaging (CI) systems developed at ANU are compact polarization-interferometers that facilitate 2D time-resolved imaging of simple plasma spectroscopic scenes. These systems simultaneously analyse the spectrum and polarization properties of relatively narrow-band spectral scenes (transmitted by an optical interference pre-filter) such as Doppler broadened emission lines or polarized multiplets. The physical parameters determining the emission spectral and polarization properties are recovered from the interferometric phase and amplitude images at one or more appropriately chosen optical path length delays.
CI has been used successfully for Doppler imaging studies of boundary and divertor flows and temperatures in a number of tokamaks around the world, and a system is now being prepared for installation on ITER. By combining with a suitable front-end polarimeter, CI systems have also been used for imaging the motional Stark effect polarization orientation (IMSE) in KSTAR, DIII-D and ASDEX-U. Other applications include synchronous Doppler imaging of plasma instabilities and RMP islands, imaging of spectral line ratios for isotope relative abundances and Thomson scattering.
In this presentation I will describe the optical principles underpinning these interferometric systems and will review Doppler and IMSE results obtained on a number of devices. In particular I will discuss the particular challenges faced by the ITER CI system, including large Zeeman effect and high stray background radiation. I will then consider possible applications for CI systems on the TCV tokamak.

By: Prof. John Howard, Australian National University, Canberra

Only 3 seminars at maximum are shown here. The full list is available on the page SPC Memento.