Decoupling the role of inertia, friction and cohesion in dense granular avalanche pressure build‐up on obstacles

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 build‐up 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 macro‐scale, 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 cohesion‐less flows. These findings enhance our understanding of the interaction of cohesive granular flows, such as snow avalanches, and structures at the micro and macro‐scale.

Dense flow avalanche pressure on obstacle studied with DEM.

Christophe Ancey, Johan Alexandre Philippe Gaume, Michael Lukas Kyburz, Betty Sovilla

Snow avalanches are a major hazard in mountainous areas and have a significant impact on infrastructures, economy and tourism of such regions. Obtaining a thorough understanding on the pressure exerted by avalanches on infrastructures is crucial for the development of design criteria so that they can withstand avalanche impact. Avalanches are characterized by two main pressure regimes depending on flow dynamics and snow properties: in the inertial regime the pressure is proportional to velocity, while in the gravitational regime it is proportional to flow depth. Today, the knowledge on avalanche impact relies mostly on empirical equations and it is not clear yet how to describe the coefficients of proportionality, namely the drag coefficient and the amplification factor in the inertial and gravitational regimes, respectively. In order to investigate the origin of these coefficients, we developed a Discrete Element Model (DEM) capable of resolving the three-dimensional avalanche flow field around a generic infrastructure.

2018

Granulation of snow: From tumbler experiments to discrete element simulations

Johan Alexandre Philippe Gaume, Michael Lehning, Betty Sovilla, Walter Steinkogler

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.

2015

Physics-based estimates of drag coefficients for the impact pressure calculation of dense snow avalanches.

Christophe Ancey, Johan Alexandre Philippe Gaume, Michael Lukas Kyburz

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.

2022

Physics-based estimates of drag coefficients for the impact pressure calculation of dense snow avalanches.

Christophe Ancey, Johan Alexandre Philippe Gaume, Michael Lukas Kyburz

2022