CR5.3

Snow cover models have mostly been developed to support avalanche forecasting. Recently developed snow instability metrics can help interpreting modeled snow cover data. However, presently snow cover models cannot forecast the relevant avalanche problem types – an essential element to describe avalanche danger. We present an approach to detect, track and assess weak layers in snow cover model output data to eventually assess the related avalanche problem type. We demonstrate the applicability of this approach with both, SNOWPACK and CROCUS snow cover model output for one winter season at Weissfluhjoch. We introduced a classification scheme for four commonly used avalanche problem types including new snow, wind slabs, persistent weak layers and wet snow, so different avalanche situations during a winter season can be classified based on weak layer type and meteorological conditions. According to the modeled avalanche problem types and snow instability metrics both models produced weaknesses in the modeled stratigraphy during similar periods. For instance, in late December 2014 the models picked up a non-persistent as well as a persistent weak layer that were both observed in the field and caused widespread instability in the area. Times when avalanches released naturally were recorded with two seismic avalanche detection systems, and coincided reasonably well with periods of low modeled stability. Moreover, the presented approach provides the avalanche problem types that relate to the observed natural instability which makes the interpretation of modeled snow instability metrics easier. As the presented approach is process-based, it is applicable to any model in any snow avalanche climate. It could be used to anticipate changes in avalanche problem type due to changing climate. Moreover, the presented approach is suited to support the interpretation of snow stratigraphy data for operational forecasting.

How to cite: Reuter, B., Viallon-Galinier, L., Mayer, S., Hagenmuller, P., and Morin, S.: A tracking algorithm to identify slab and weak layer combinations for assessing snow instability and avalanche problem type, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1578, https://doi.org/10.5194/egusphere-egu21-1578, 2021.

The progressive failure of a snow layer deposited on a stiff substrate is at the base of the comprehension of several physical processes that can be found both in natural and artificial conditions. For instance, glide avalanches often originate from the reduction of the basal friction between the snowpack and the underlying ground due to the presence of liquid water film or depth hoar at the snow-ground interface. Moreover, the interaction between snow and construction materials relates to many other applications such as the study of new and more efficient snow removal techniques, the safety of travelers along snow covered roads, the snow redistribution from roofs and buildings, etc. 

Despite this large number of application fields, laboratory investigations are still limited. We performed cold room tests on artificially made snow-mortar interface specimens through a direct shear test device. The effects of confinement pressure, temperature and dry snow hardness (due to sintering times) were taken into account. The tests were carried out in displacement-controlled conditions in order to study the entire failure process at the interface and the following irreversible sliding. The results show some interesting and encouraging aspects for understanding the shear strength of the interface. From a micromechanical point of view we recorded the tests with a high-definition video camera and analyzed the data with the Particle Image Velocimetry technique to obtain the motion fields on the external side of the specimens. Here, we present and discuss some preliminary results of the experimental activity and suggest some future implementations and further developments of the studied topic.       

How to cite: Vallero, G., Barbero, M., Barpi, F., Borri-Brunetto, M., and De Biagi, V.: A new experimental set-up to study the shear strength of snow-mortar interfaces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4056, https://doi.org/10.5194/egusphere-egu21-4056, 2021.

An existing orthotropic elasto-plastic model, which takes the microstructure fabric of trabecular bone into consideration, is extended for snow and used to study the stress response in a layered snowpack. The mean intercept length, determined from X-ray tomography, represents the microstructure fabric in the macroscopic constitutive law. The yield surface for snow accounts for strength asymmetry of snow in tension and compression and shows isotropic hardening till ultimate strength is reached and then softens till complete failure. Tomographic image dataset of various snow types in conjunction with 3D μ- FE analysis of these snow types was used to evaluate the elastic and failure criteria constants in the model. The macroscopic law is implemented as a user subroutine FE code to predict the stress-strain response of snow samples and shows good agreement with the μ-FE based data.

The stress-strain law is used to study stresses in a snowpack of length 5m and thickness between 0.11 to 0.81m with a strong layer of round grain and a weak layer of faceted grains. A plane strain finite element analysis is performed. The density of the strong and weak layers is approximately 210kg/m³, and 118kg/m³, respectively. The snowpack was subjected to gravity, and a skier loads (80kg) and stresses were investigated for slope angles of 0^o, 30^o, and 90^o. The variation of compressive stress normal to slope and shear stress along the snowpack's length for different thicknesses of the strong layer is computed. The maximum normal compressive stress and shear stresses are observed at the centre of the weak layer. The normal compressive stress pattern obtained is in agreement with the previous studies.

How to cite: Singh, A. K., Mahajan, P., and Srivastava, P. K.: Fabric based strength criterion and its application on a layered snowpack, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5498, https://doi.org/10.5194/egusphere-egu21-5498, 2021.

The effective mechanical behavior of snow can be deduced from microstructural homogenization through numerical simulations. Although such numerical upscaling of elasticity and strength of snow microstructures is standard (using FEM), numerical schemes to study generic features of the transition from small to large strain situations that involve yielding and failure are scarce. This prevents the development of accurate homogenized constitutive models valid for the post-failure and large deformation regimes. It has been shown that treating this transition is feasible using DEM under the assumption of particulate microstructures. However, this requires snow microstructures to be segmented into a granular collection of (usually spherical) cohesive elements. Here, we suggest generating random porous microstructures by level-cutting Gaussian random fields and using the material point method to numerically simulate them under mechanical loading. This allows investigating both small and large deformation characteristics of irregular porous media, such as snow, where a segmentation into grains and bonds can be ambiguous. We demonstrate our approach by examining elasticity and failure as a function of a wide range of solid volume fractions, from 20% (low-density snow) to 80% (high-density firn), as the most important control on the mechanical behavior. Observing that onset of failure can be well described through the second order work, we show that the failure strength follows a power law similar to that of the elastic moduli. Moreover, we propose that the failure envelope can be approximated by a porosity-dependent quadratic curve in the space of the two first stress invariants. Furthermore, we observe that plastic deformation appears to be governed by an associative plastic flow rule. Finally, these results combined with a viscoplastic Perzyna model and a sintering (hardening) model should allow us to develop a universal homogenized snow constitutive model.

How to cite: Blatny, L., Löwe, H., Wang, S., and Gaume, J.: A unified framework for computational microstructure-based snow mechanics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6108, https://doi.org/10.5194/egusphere-egu21-6108, 2021.

How to cite: Dick, O., Viallon-Galinier, L., Hagenmuller, P., Fructus, M., Lafaysse, M., and Dumont, M.: Can Saharan dust deposition impact snow stability in the French Alps?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6600, https://doi.org/10.5194/egusphere-egu21-6600, 2021.

For a slab avalanche to release, a weak layer buried below a cohesive snow slab is required, and the system of weak layer and slab must support crack propagation over large distances. This process, called “dynamic crack propagation”, is highly relevant for avalanche release, and computational models are nowadays able to model crack propagation over increasingly larger scales. Field measurements on dynamic crack propagation are however very scarce, although these are required to validate models. We therefore performed a series of flat field PST experiments up to ten meters long over a period of 10 weeks. During this time, PST results evolved from crack arrest to full propagation and back to crack arrest – reflecting the life cycle of the weak layer. All PST experiments were analyzed using digital image correlation to derive high-resolution displacement fields to compute dynamic crack propagation metrics, including crack length and speed as well as touchdown distance, the distance from the crack tip to the trailing point where the slab comes into contact with the substratum. Comparing the displacement fields during sawing to a mechanical model, we estimated the effective elastic modulus of slab and weak layer as well as the specific fracture energy of the weak layer. Our results show how dynamic crack propagation characteristics change over the life cycle of a weak layer and how these measures relate to snowpack properties such as load and effective elastic modulus of the slab. We found that crack speed was highest for PSTs resulting in full propagation and that the touchdown length increased with increasing elastic modulus of the slab. Our dataset provides unique insight into the dynamics of crack propagation, and provides valuable data to validate models used to study sustained crack propagation.

How to cite: Bergfeld, B., van Herwijnen, A., Bobillier, G., and Schweizer, J.: Characteristics of dynamic crack propagation in a weak snowpack layer over its entire life cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7772, https://doi.org/10.5194/egusphere-egu21-7772, 2021.

Knowledge of the snow avalanche release area is key information in snow avalanche studies. However, it is not easy to obtain from a remote location. The study of the seismic vibrations produced in the initial stages of the snow avalanche, makes possible to identify their origin and to link them to the starting area of the snow avalanche. We developed a methodology for this purpose, applied to seismic data acquired from a 3D seismic station (2Hz eigenfrequency) placed at Cavern A in Vallée de la Sionne experimental site (VDLS, WSL-SLF), deployed in 2013 by UB-RISKNAT. This is the closest position to the snow avalanche release areas, at 700 m to the farthest point. We focus on spontaneous triggered snow avalanches to achieve better signal-to-noise ratio and to be more realistic on its application.

For the isolation of the Signal Onset (SON) section of seismic data, which corresponds to those vibrations produced by the initial stage of the snow avalanche, we use the STA/LTA ratios and seismic signal amplitude, common methodologies in seismology. The STA/LTA is used for the identification of the first vibrations produced by the movement of the snow mass and the seismic signal amplitude thresholds for the identification of the end of the SON section -when the snow avalanche front reaches the seismic sensor position-. The 3D seismic data [ZNE components] of the SON section were processed in time windows. The study of polarization of the particle motion to obtain the direction of the back-azimuth of the signal (Vidale, 1986; Jurckevicks, 1988) was carried out for each time window of the seismic signal. The accumulation of back-azimuth directions for the entire SON section is related to the origin of the vibrations and, by extension, to the snow avalanche release area.

The entire algorithm has been automated. In its application on all the trigger activations at VDLS since 2015 until 2020, it was achieved a success rate of 78% on snow avalanche release area identification. In addition, we defined an algorithm based on STA/LTA ratio to select the snow avalanches from other seismic events, used with a success rate of 95%.

We present the application of our method in a case study, a large spontaneous snow avalanche released on 16th February 2018 at VDLS. The snow avalanche had two main release areas, clearly identified in photos of the site. The two developed fronts can be recognized in the seismic data. The directions to the release areas from Cavern A position can be identified using the presented method. Also, more interpretations can be done on the downhill snow avalanche path.

How to cite: Roig Lafon, P., Suriñach, E., and Tapia, M.: The use of seismic ground particle motion for snow avalanche release area identification, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9475, https://doi.org/10.5194/egusphere-egu21-9475, 2021.

Dry-snow slab avalanche release is a multi-scale process starting with the formation of localized failure in a highly porous weak snow layer below a cohesive snow slab, which can be followed by rapid crack propagation within the weak layer. Finally, a tensile fracture through the slab leads to its detachment. About 15 years ago, the propagation saw test (PST) was developed. The PST is a fracture mechanical field test that provides information on crack propagation propensity in weak snowpack layers. It has become a valuable research tool to investigate the processes involved in crack propagation. While this has led to a better understanding of the onset of crack propagation, much less is known about the ensuing propagation dynamics. Here, we use the discrete element method to numerically simulate PSTs in 3D and analyze the fracture dynamics using a micro-mechanical approach. Our DEM model reproduced the observed PST behavior extracted from experimental analysis. We developed different indicators to define the crack tip that allowed deriving crack speed. Our results show that crack propagation in level terrain reaches a stationary speed if the snow column is long enough. Moreover, we define stress concentration sections. Their length evolution during crack propagation suggests the development of a steady-state stress regime. Slab and weak layer elastic modulus, as well as weak layer shear strength, are the key input parameters for modeling crack propagation; they affect stress concentrations, crack speed, and the critical length for the onset of crack propagation. The results of our sensitivity study highlight the effect of these mechanical parameters on the emergence of a steady-state propagation regime and consequences for dry-snow slab avalanche release. Our DEM approach opens the possibility for a comprehensive study on the influence of the snowpack mechanical properties on the fundamental processes for avalanche release.

How to cite: Gregoire, B., Bastian, B., Johan, G., Alec, V. H., and Jürg, S.: Micro-mechanical modeling using DEM to study the effect of mechanical properties on crack propagation for snow slab avalanches, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11482, https://doi.org/10.5194/egusphere-egu21-11482, 2021.

Gliding snow avalanches are of growing concern for the management of ski areas, transport corridors and spatial planning. With a warming climate there appear to be increasing reports of gliding snow hazards in alpine regions. The management of gliding snow avalanches can be achieved through either stabilization or artificially triggering a slide. Triggering sliding is attractive because it has the potential to remove the hazard entirely. In this research, we investigate the potential of managing gliding snow avalanches through the early release of snow accumulations using low friction geotextiles.

A series of geotextiles have been installed on slopes between 25 and 35° during the autumn months and the behavior of snow accumulations observed during the winter. Initial findings indicate that reducing the basal friction can be effective in inducing early release of gliding snow avalanches. However, the interaction of the flanking snow pack and stauchwall appear dominant in the behavior of the system. This contribution reports on the initial findings of these experiments and discusses the potential applications to managing gliding snow avalanches.  

How to cite: Glover, J., Althoff, S., Witek, M., Seupel, C., Braun, S., and Lifa, I.: Induced glide-snow avalanches with low friction geotextiles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15260, https://doi.org/10.5194/egusphere-egu21-15260, 2021.

Field observations and measurements show that dry snow slab avalanches initiate by propagating shear fractures within a relatively thin weak layer sandwiched between a planar, stronger, thicker slab above and stronger material below. After initiation, the weak layer fracture can propagate up and down slope for distances which range from about 10 to 100’s of meters to cause tensile fracture through the body of the slab which results in avalanche release.  In this paper, dynamic fracture mechanics is applied to slab tensile fracture after which avalanche release is imminent. Two mechanisms for production of tensile stress are explored employing field measurements of slab properties and lab measurements. The first considers inertial effects related to quasi-brittle fracture near the tip of a propagating weak layer shear fracture. The second is concerned with the tensile stress generation from the stress drop behind the crack front as the fracture propagates in the weak layer. Analysis suggest that both mechanisms can contribute to produce the tensile fracture line which precedes avalanche release. Even though both mechanisms may operate together, they are analyzed separately in this paper. 

How to cite: McClung, D.: Dynamic fracture mechanics in dry snow slab avalanche release, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16565, https://doi.org/10.5194/egusphere-egu21-16565, 2021.