Jürg Schweizer

Last Name

Schweizer

First Name

Jürg

ORCID

0000-0001-5076-2968

Organisational unit

01109 - Lehre Bau, Umwelt und Geomatik

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Publications 1 - 10 of 30

Stress Concentrations in Weak Snowpack Layers and Conditions for Slab Avalanche Release
Gaume, Johan; Chambon, Guillaume; van Herwijnen, Alec; et al. (2018)
Geophysical Research Letters
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 a꜀ 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.
The source, quantity, and spatial distribution of interfacial water during glide-snow avalanche release: experimental evidence from field monitoring
Fees, Amelie; Lombardo, Michael; van Herwijnen, Alec; et al. (2025)
The Cryosphere
Glide-snow avalanches release at the soil-snow interface due to a loss friction which is suspected to be linked to interfacial water. Presently, the formation and distribution of the interfacial water are not well understood and glide-snow avalanches are considered unpredictable. We investigated the source, quantity, and spatial distribution of interfacial water before and during avalanche release through spatio-temporal field monitoring. The measurement setup consists of a sensor grid covering a slope with frequent glide-snow avalanche activity. The 24 grid sensors measured the soil temperature and liquid water content (LWC) throughout seasons 2021/22 to 2023/24. Snow and interfacial temperature and LWC were monitored locally with a vertical sensor profile ranging from the soil into the snow. Seven glide-snow avalanches released over the sensor grid and their investigation showed the following: (i) interfacial water originated from geothermal heat, rain, and meltwater percolation; (ii) the quantity of snow LWC was lower for glide-snow avalanches that released in early winter than in spring; (iii) soil temperatures in the release area were higher than in the remaining slope if interfacial water originated from geothermal heat; (iv) if interfacial water originated from rain and/or melt, we observed (locally) higher soil LWC in the release area; and (v) for the majority of observed avalanches the spatial variability in soil LWC across the slope reached a local minimum at the time of avalanche release. In the future, with continued monitoring, the spatio-temporal investigation of the soil LWC and temperature will help to quantify the drivers of glide-snow avalanche release at the slope scale. This will contribute to improved glide-snow avalanche forecasting and mitigation.
Capillary suction across the soil-snow interface as a mechanism for the formation of wet basal layers under gliding snowpacks
Lombardo, Michael; Fees, Amelie; Udke, Annegret; et al. (2025)
Journal of Glaciology
Capillary suction across the soil-snow interface is a possible mechanism for the formation of wet basal snow layers, which are necessary for snow gliding and glide-snow avalanches. However, little is known about the conditions under which this process occurs. We investigated capillary suction across the soil-snow interface considering realistic snow and soil properties. Snow properties were determined from snow profiles and soil properties were determined from field measurements of liquid water content, matric potential, soil texture and bulk density for 40 alpine soils in Davos, Switzerland, as well as a field site in the region (Seewer Berg) with glide-snow avalanche activity. For the alpine soils investigated here, the results show that capillary flow from the soil to the snow is possible for realistic snow properties but requires a soil saturation of similar to 90% or higher at the soil surface. When comparing the 90% saturation threshold to field measurements, the results suggest that capillary suction across the soil-snow interface is unlikely to contribute significantly to the formation of wet basal layers on Seewer Berg. These results are also relevant for soil and snow hydrology, where water transport across the soil-snow interface is important and understudied.
Dynamic crack propagation in weak snowpack layers: insights from high-resolution, high-speed photography
Bergfeld, Bastian; Van Herwijnen, Alec; Reuter, Benjamin; et al. (2021)
The Cryosphere
Dynamic crack propagation in snow is of key importance for avalanche release. Nevertheless, it has received very little experimental attention. With the introduction of the propagation saw test (PST) in the mid-2000s, a number of studies have used particle tracking analysis of high-speed video recordings of PST experiments to study crack propagation processes in snow. However, due to methodological limitations, these studies have provided limited insight into dynamical processes such as the evolution of crack speed within a PST or the touchdown distance, i.e. the length from the crack tip to the trailing point where the slab comes to rest on the crushed weak layer. To study such dynamical effects, we recorded PST experiments using a portable high-speed camera with a horizontal resolution of 1280 pixels at rates of up to 20 000 frames s(-1). We then used digital image correlation (DIC) to derive high-resolution displacement and strain fields in the slab, weak layer and substrate. The high frame rates enabled us to calculate time derivatives to obtain velocity and acceleration fields. We demonstrate the versatility and accuracy of the DIC method by showing measurements from three PST experiments, resulting in slab fracture, crack arrest and full propagation. We also present a methodology to determine relevant characteristics of crack propagation, namely the crack speed (20-30 m s(-1)), its temporal evolution along the column and touchdown distance (2.7 m) within a PST, and the specific fracture energy of the weak layer (0.31.7 J m(-2)). To estimate the effective elastic modulus of the slab and weak layer as well as the weak layer specific fracture energy, we used a recently proposed mechanical model. A comparison to already-established methods showed good agreement. Furthermore, our methodology provides insight into the three different propagation results found with the PST and reveals intricate dynamics that are otherwise not accessible.
Field measurements of snowpack response to explosive loading
Simioni, Stephan; Sidler, Rolf; Dual, Jürg; et al. (2015)
Cold Regions Science and Technology
Avalanche control by explosives is among the key temporary preventive measures. Hitherto, little is known about wave propagation in a snowpack caused by an explosion. During the winter 2013–2014 we performed field experiments on a flat study site. We triggered slurry explosive charges at different heights above the snow surface. At three different distances from the point of explosion we measured surface air pressure and accelerations of the snowpack at various depths. Cameras were placed in the snow pits for recording weak layer failure and crack propagation. We report empirical relations for the decay of near-surface air pressure, accelerations, displacement velocities and displacement with distance from the explosion and depth within the snowpack. Waves within the snowpack arrived earlier at the sensors than the corresponding air pressure waves at the microphones. Air pressure decayed stronger than accelerations within the snowpack. Weak layer failure mainly happened in the top part of the snowpack. We observed two types of weak layer failure, one caused by the direct impact of the air pressure wave close to the point of observation, the other by failure induced by the air pressure wave closer to the point of explosion and subsequent crack propagation. Our measurements increase the understanding of acoustic wave propagation in snow and can be used for comparison with numerical simulations.
Speed and attenuation of acoustic waves in snow: Laboratory experiments and modeling with Biot's theory
Capelli, Achille; Kapil, Jagdish C.; Reiweger, Ingrid; et al. (2016)
Cold Regions Science and Technology
Monitoring acoustic emissions (AE) prior to imminent failure is considered a promising technique for assessing snow slope instability. Gaps in elastic wave propagation characteristics in snow hinder quantitative interpretation of AE signals. Our study focuses on characterizing the propagation of acoustic reference signals in the ultrasonic range across cylindrical snow samples with varying density (240–450 kg m− 3). We deduced the acoustic attenuation coefficient within snow by performing experiments with different column lengths to eliminate possible influences of the snow-sensor coupling. The attenuation coefficient was measured for the entire burst signal and for single frequency components in the range of 8 to 35 kHz. The acoustic wave propagation speed, calculated from the travel time of the acoustic signal, varied between 300 m s− 1 and 950 m s− 1, depending on the density and hardness of snow. From the sound speed we also estimated the Young's modulus of our snow samples; the values of the modulus ranged from 30 to 340 MPa for densities between 240 and 450 kg m− 3. In addition, we modeled the sound propagation for our experimental setup using Biot's model for wave propagation in a porous medium. The model results were in good agreement with our experimental results and suggest that our acoustic signals consisted of Biot's slow and fast waves. Our results can be used to improve the identification and localization of acoustic emission sources within snow in view of assessing snow slope instability.
Crack propagation speeds in weak snowpack layers
Bergfeld, Bastian; van Herwijnen, Alec; Bobillier, Grégoire; et al. (2022)
Journal of Glaciology
For the release of a slab avalanche, crack propagation within a weak snowpack layer below a cohesive snow slab is required. As crack speed measurements can give insight into underlying processes, we analysed three crack propagation events that occurred in similar snowpacks and covered all scales relevant for avalanche release. For the largest scale, up to 400 m, we estimated crack speed from an avalanche movie; for scales between 5 and 25 m, we used accelerometers placed on the snow surface and for scales below 5 m, we performed a propagation saw test. The mean crack speeds ranged from 36 ± 6 to 49 ± 5 m s−1, and did not exhibit scale dependence. Using the discrete element method and the material point method, we reproduced the measured crack speeds reasonably well, in particular the terminal crack speed observed at smaller scales. Finally, we used a finite element model to assess the speed of different elastic waves in a layered snowpack. Results suggest that the observed cracks propagated as mixed mode closing cracks and that the flexural wave of the slab is responsible for the energy transfer to the crack tip.
Glide-snow avalanches: a mechanical, threshold-based release area model
Fees, Amelie; van Herwijnen, Alec; Lombardo, Michael; et al. (2024)
Natural Hazards and Earth System Sciences
Glide-snow avalanches release at the ground-snow interface due to a loss in basal friction. They pose a threat to infrastructure because of the combination of unreliable mitigation measures, limited forecasting capabilities, and a lack of understanding of the release process. The aim of this study was to investigate the influence of spatial variability in basal friction and snowpack properties on the avalanche release area distribution and the release location. We developed a pseudo-3D, mechanical, threshold-based model that consists of many interacting snow columns on a uniform slope. Parameterizations in the model are based on our current understanding of glide-snow avalanche release. The model can reproduce the power law glide-snow avalanche release area distribution as observed on Dorfberg (Davos, Switzerland). A sensitivity analysis of the input parameters showed that the avalanche release area distribution was mostly influenced by the homogeneity (correlation length and variance) of the basal friction and whether the basal friction was reduced suddenly or in small increments. Larger release areas were modeled for a sudden decrease and a more homogeneous basal friction. The spatial variability of the snowpack parameters had little influence on the release area distribution. Extending the model to a real-world slope showed that the modeled location of avalanche releases qualitatively matched the observed locations. The model can help narrow down the length scales and timescales for field investigations. Simultaneously, it can grow in complexity with our increasing understanding of glide-snow avalanche release processes. Input parameters such as the basal friction or snowpack parameters could potentially all be connected to the liquid water content. This would allow for the use of meteorological measurements to drive the model. The model has the potential to help identify potentially dangerous conditions for large or numerous avalanches which would help improve glide-snow avalanche forecasting.
Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area
Gaume, Johan; Chambon, Guillaume; Eckert, Nicolas; et al. (2015)
The Cryosphere
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 slabWL 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.
Modeling of crack propagation in weak snowpack layers using the discrete element method
Gaume, Johan; van Herwijnen, Alec; Chambon, Guillaume; et al. (2015)
The Cryosphere
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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Jürg Schweizer

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