Dr. Michael Lukas Kyburz

Powder snow avalanches (PSAs) radiate infrasound energy, yet the source mechanism remains unclear, limiting hazard monitoring and mitigation with infrasound-based technologies. Here, we analyze a unique data set from a large PSA to improve the understanding of the source mechanism. Through comparison of cluster activity within the airborne layers of the PSA and the recorded infrasound signal in the frequency domain, we demonstrate that infrasound is mainly generated from particle clusters suspended by turbulent eddies or ejected from the denser basal layer. Further correlating infrasound amplitudes with radar-derived spatial distributions of these clusters, we reveal a distributed source extending hundreds of meters behind the avalanche front. Additionally, we establish a relationship between infrasound and kinetic energy of suspended particles. These findings deepen our understanding of the complex dynamics of infrasound generation, offering valuable insights for avalanche detection and early warning strategies, and fundamental comprehension of PSA dynamics.

In mountainous regions, snow avalanches pose significant threats to both populations and infrastructure. As the flow progresses, it can accumulate additional mass by entraining bed material along its path, thereby amplifying its destructive potential. Volume increases of over tenfold the initial volume have been documented from avalanche observation and measurements. Entrainment is usually accounted for in depth-averaged models through a source term in the mass conservation equation and simplified entrainment criteria. However, quantitative assessment of erosion/entrainment rates with various flow types and bed material properties has seldomly been explored under well-controlled conditions. In this work, we investigate avalanche dynamics as well as erosion/entrainment mechanisms on the basis of depth-resolved particle-based simulations. Through such a modeling approach, processes like frontal ploughing, basal abrasion as well as erosion-deposition waves naturally emerge. Our findings highlight that, the interaction of the flow with the initially static bed material may have multiple and diverse effects on the flow behavior. First, the bed material can be eroded, but only partially entrained in the flow. Second, while the flow mobility is generally reduced by the presence of the bed, it may be enhanced in some cases due to the mechanical weakening of the bed. Finally, we propose a relationship linking the entrainment rate to a dimensionless parameter that compares the flow-induced stress to the bed shear resistance. This work contributes to a better understanding of the dynamics of snow avalanches and entrainment process and can lead to a refinement of depth-averaged formulations for practical purposes. Future research should consider factors such as rate-dependent snow rheology, air pore pressure and bed fluidization mechanisms to further refine our understanding and predictive capabilities in avalanche modeling.

With advancements in computational power and recent developments in particle-based numerical modeling, it is now possible to perform fully three-dimensional (3D) and real-scale snow avalanche simulations using methods such as the Material Point Method (MPM). These 3D MPM simulations explicitly model flow processes and velocity components in the slope-normal direction, allowing for accurate capture of phenomena such as flow detachment from the terrain in response to abrupt topography variations. In a recent case study where we applied MPM to simulate a snow avalanche at slope scale in 3D from release to run-out, we observed significant slope-normal velocity components, particularly in regions with pronounced variations in terrain curvature. This observation raised the question of the practical significance of slope-normal flow velocity components, especially in terms of their interaction with objects susceptible to uprooting, protection galleries, or other structures where slope-normal velocities might exert enough force to dislodge or damage roofs. To investigate this further, we use the flow conditions from a slope-scale MPM simulation of the 2019 “Salez” avalanche in Davos, Switzerland, which overflows a road gallery, as boundary conditions for Discrete Element Method (DEM) simulations. In these DEM simulations, we focused on the avalanche flow on the gallery roof, analyzing the resulting normal and shear forces as well as the velocity components normal and parallel to the roof. A comparison between the stresses obtained from DEM simulations and the calculations based on the Swiss Guidelines reveals that the guidelines provide conservative yet relatively accurate stress estimates. By distinguishing between the normal and parallel velocity components relative to the gallery roof, we find that the normal component is closely correlated with the resulting stresses, highlighting the potential significance of normal velocity components in determining the load on galleries.

Avalanche modeling is an essential tool to assess snow avalanche hazard. Today, most popular numerical approaches adopt depth-Averaged equations. These methods are computationally efficient but limited in cap turing processes occurring in the flow depth direction, e.g., erosion or deposi tion, which are often considered using ad hoc parameterizations or neglected completely. However, processes such as snow erosion, can crucially influence the flow dynamics and run-out and are often not negligible. We address these issues by using a new three-dimensional model, based on the Material Point Method (MPM) and finite strain elastoplasticity. To assess the possibilities and challenges associated with these highly detailed but computationally ex pensive calculations, we simulated the "Salezer" snow avalanche that released in Davos, Switzerland, in 2019. To reproduce the event in our simulations, we use the release areas mapped in a photogrammetric drone survey and es timate the snow conditions on the day of the event. We compare macroscopic features, such as flow outline and snow deposition, of the simulated avalanche to field observations. An in-depth analysis of transient 3D flow structures at the avalanche head demonstrates the degree of physical detail in the model, but also highlight challenges which still need to be addressed.

In avalanche engineering and hazard mapping, computing impact pressures exerted by avalanches on rigid structures has long been a difficult task that requires combining empirical equations, rules of thumb, engineering judgment and experience. Until the 1990s, well-documented avalanches were seldom, and the main source of information included back-analysis of damage to structures and scarce field measurements. By the 1990s, several field sites were equipped across Europe, and since then they have provided new insights into the physics of impact. The main problem has been the difficulty in interpreting and generalizing the results to propose sound methods for estimating impact pressure. Testing a wide range of flow conditions has also been difficult in the field. To go a step forward in the elaboration of new guidelines for computing avalanche forces, we developed a numerical code based on the Discrete Element Method (DEM), which made it possible to simulate how an avalanche interacts with a rigid obstacle and to study how impact pressure depends on obstacle shape and size, as well as the avalanche flow regime. We extracted pressure and velocity data from the Vallée de la Sionne database to validate the DEM code, calibrate the model parameters, and elaborate avalanche scenarios. We studied four avalanches scenarios related to distinct flow regimes of the avalanche's dense core. In these scenarios, snow cohesion and velocity were imposed at the upstream boundary of the computational domain. Building on earlier work, we generalized an empirical equation for computing impact pressure as a function of snow cohesion, velocity, flow regime, and structure shape and size. Various coefficients were defined and calibrated from our DEM data. Within the range of tested values, we found good agreement between estimated pressure and field data.

Abstract: The calculation of the impact pressure on obstacles in granular flows is a fundamental issue of practical relevance, e.g. for snow avalanches impacting obstacles. Previous research shows that the load on the obstacle builds up, due to the formation of force chains originating from the obstacle and extending into the granular material. This leads to the formation of a mobilized domain, wherein the flow is influenced by the presence of the obstacle. To identify the link between the physical mobilized domain properties and the pressure exerted on obstacles, we simulate subcritical cohesionless and cohesive avalanches of soft particles past obstacles with circular, rectangular or triangular cross-section using the Discrete Element Method. Our results show that the impact pressure decreases non-linearly with increasing obstacle width, regardless of the obstacle’s cross-section. While the mobilized domain size is proportional to the obstacle width, the pressure decrease with increasing width originates from the jammed material inside the mobilized domain. We provide evidence that the compression inside the mobilized domain governs the pressure build-up for cohesionless subcritical granular flows. In the cohesive case, the stress transmission in the compressed mobilized domain is further enhanced, causing a pressure increase compared with the cohesionless case. Considering a kinetic and a gravitational contribution, we are able to calculate the impact pressure based on the properties of the mobilized domain. The equations used for the pressure calculation in this article may be useful in future predictive pressure calculations based on mobilized domain properties.

Understanding the physical processes involved in snow avalanche-obstacle interaction is essential to be able to estimate the pressure exerted on structures. Although avalanche impact pressure has been measured in field experiments for decades, the underlying physical principles are still elusive. Previous studies suggest that pressure is increased due to the formation of an influenced flow region around the structure, the mobilized domain, which varies in size depending on snow properties such as snow cohesion. Here, we aim to better understand how cohesion, friction, velocity, and their interplay affect avalanche pressure buildup on structures. This is achieved by simulating the avalanche-obstacle interaction with a newly developed numerical model based on the discrete element method, using a cohesive bond contact law. The relevance of the model is tested by comparing simulated impact pressures with field measurements from the Vallée de la Sionne experimental site. Our results show that at the macroscale, impact pressure consists of the inertial, frictional, and cohesive contributions. The inertial and frictional contributions arise due to the existence, shape, and dimension of the mobilized domain. The cohesive contribution increases the particle contact forces inside the domain, leading up to a doubling of the pressure. Based on these physical processes, we propose a novel scaling law to reduce the problem of calculating the pressure induced by cohesive flows, to the calculation of cohesionless flows. These findings enhance our understanding of the interaction of cohesive granular flows, such as snow avalanches, and structures at the microscale and macroscale.

New high-resolution radar measurements performed at the Swiss test site “Vallé de la Sionne” have allowed a reclassification of snow avalanches into 7 different flow regimes. Multiple flow regimes are often simultaneously present in different regions of a single avalanche, and avalanches can change the dominant flow regime as they descend the slope as a result of the entrainment of colder snow at higher altitudes and warmer snow at lower altitudes. 4 of these flow regimes are particularly important for the design of infrastructures impacted by avalanches. These include three dense regimes, namely the cold dense regime characteristic of fast moving dry avalanches, the warm plug and warm shear regimes characteristic of slow moving warm/wet avalanches and one dilute/dense regime, the intermittency regime, characteristic of fully developed powder snow avalanches. Each regime has a distinct impact dynamics, which requires a different modeling approach. The new data suggest that the assumptions underlying current avalanche simulation models and pressure calculation procedures may be too simple. The research community now faces the challenge of developing a better understanding of the physical processes that characterize the individual flow regimes, their transitions, their connection to the snow properties and their interaction with infrastructures.

Recent advances in full-scale avalanche measurements have led to a better understanding of the avalanche flow dynamics. However, the processes involved in pressure build-up on obstacles in the various flow regimes are still elusive. From full-scale experiments it is well established, that in the inertial flow regime, which is mostly typical of fast and cold avalanches, the pressure is proportional to square velocity. The gravitational regime is often observed for warm/wet snow avalanches and features a linear pressure variation with flow depth. It is still unclear how to estimate the coefficients of proportionality, which are needed for the pressure calculation, namely the drag coefficient and the amplification factor in the inertial regime and in the gravitational regime, respectively. In order to investigate the origin of the amplification factor and the drag coefficient, we developed a model based on the Discrete Element Method (DEM), which allows us to simulate the interaction between a cohesive granular flow, such as an avalanche, and a structure. The DEM model is tested by comparing the simulation results to full-scale measurements from our "Vallée de la Sionne" test site. The results of the simulated pressure show that the newly developed model is able to capture both, the flow-depth and velocity-proportional pressure regime. Thus this model will be further improved to take into account more complex physical processes such as the snow compaction and granulation.

The assessment of the impact pressure exerted by avalanches on structures is an important task in avalanche engineering. Impact pressures of more than 50 avalanches were measured at the Vallée de la Sionne experimental site, in Switzerland, in the last 20 years. Measurements performed on a 20m high and 0.6m wide pylon show that the highest long-lasting bending moment is exerted by warm/wet avalanches, characterized by velocity of around 10ms⁻¹, which however can develop very high flow depths, up to 7m in one of our data sets. In spite of the relatively low absolute value of pressures these avalanches exert a constant load for tens of seconds, over large depths. Conversely, the intermittency region, coupled with a dense basal layer, which characterized the front of large powder avalanches exerts the highest short-lasting bending moment. Mesoscale coherent structures which are present in this region can move with velocities up to 60ms⁻¹ and produce peak loads of 800–1000 kPa, which however, only last for a fraction of a second. The dense basal layer of large powder snow avalanches can also exerts a very large long-lasting pressure (up to 1000 kPa), which, however is concentrated in a thin layer close to the base of the flow, and thus contributes marginally to the overall bending moment. Finally, heavy objects transported into the flow, such as rocks, can exert large local pressure peaks that can be larger than 1000 kPa. We compare these measurements to standard calculation procedures and discuss their relevance in term of structure design.