Brief communication: A ~50 Mm³ ice-rock avalanche on 22 March 2021 in the Sedongpu valley, southeastern Tibetan Plateau

On 22 March 2021, a ~50 M m3 ice-rock avalanche occurred from 6500 m asl in the Sedongpu basin, southeastern Tibet. The avalanche transformed into a highly mobile flow which temporarily blocked the Yarlung Tsangpo river. The avalanche flow lasted ~5 minutes and produced substantial geomorphological reworking. This event, and previous ones from the basin, occurred concurrently with, or shortly after recorded positive air temperature anomalies. The occurrence of future large mass flows from the basin cannot be ruled out, and their impacts must be carefully considered given implications for sustainable hydropower and associated socioeconomic development in the region.

Numerical simulations of wet snow avalanches: interplay between cohesion and friction

<p>Wet snow avalanches present distinctive features such as unusual trajectories, peculiar deposit shapes, and a rheological behavior displaying a combination of granular and pasty features depending on the actual snow liquid water content. Complex transitions between dry (cold) and wet (hot) flow regimes can also occur during a single avalanche flow. In an attempt to account for this complexity, we report on numerical simulations of avalanches using a frictional-cohesive rheology implemented in a depth-averaged shallow-flow model. Through extensive sensitivity studies on synthetic and real topographies, we show that cohesion plays a key role to enrich the physics of the simulated flows, and to represent realistic avalanche behaviors. First, when coupled to a proper treatment of the yielding criterion, cohesion provides a way to define objective stopping criteria for the flow, independently of the issues incurred by artificial diffusion of the numerical scheme. Second, and more importantly, the interplay between cohesion and friction gives raise to a variety of nontrivial physical effects affecting the dynamics of the avalanches and the morphology of the deposits. The relative weights of frictional and cohesive contributions to the overall stress are investigated as a function of space and time during the propagation, and related to the formation of specific features such as lateral levées, hydraulic jumps, etc. This study represents a first step towards robust avalanches simulations, spanning the wide range of possible flow regimes, through shallow-flow approaches. Future improvements involving more refined cohesion parameterizations will be discussed.</p>

From avalanche-obstacle interaction processes to physics-based impact pressure calculations – an insight from DEM simulations

<p>Calculating snow avalanche impact pressure is an essential task for safe construction and hazard mapping in mountainous regions. Although the avalanche-obstacle interaction crucially depends on the flow regime, practitioners mostly assume that the impact pressure is similar to the dynamic pressure in inviscid fluids, that is, it is proportional to the square velocity weighted by an empirical drag coefficient. Field measurements indicate that the drag coefficients cover more than one order of magnitude. In the absence of a physics-based framework, setting the right drag coefficient requires good working knowledge and experience from practitioners. Indeed, even for trained engineers it may be unclear how the impact pressure depends on the expected flow regime, on obstacle width, or on terrain configuration. To address these questions, we simulate the avalanche impact pressure on obstacles of varying geometry for four distinct avalanche flow regimes using the Discrete Element Method and a cohesive contact model. The results allow us to quantify the influence of the obstacle width and shape on the average impact pressure, as well as the detailed pressure distribution on the obstacle surface. Furthermore, we propose a novel method for estimating the drag coefficient based on simple geometrical considerations and key characteristics of avalanche flow. Our results are validated using experimental data from the Vallée de La Sionne test site, and make a step forward in the derivation of a physics-based framework for computing snow avalanche impact pressures for varied flow regimes depending on obstacle shape and dimensions.</p>

Perspectives on Snow Avalanche Dynamics Research

As an introduction for non-specialists to the Special Issue on snow avalanche dynamics, this paper first outlines how understanding the dynamics of snow avalanches can contribute to reducing risk for settlements and infrastructure. The main knowledge gaps in this field of research concern (i) the properties of the flow regimes and the transitions between them, and (ii) the dynamics of mass change due to erosion and deposition. These two aspects are intertwined and determine not only the reach of an avalanche, but also its velocity, course and impact pressure. Experimental studies described in this Special Issue comprise a wide range of scales from small rotating drums to real snow avalanches. In addition, several papers describe post-event field surveys of specific avalanches and analyze them using different methods and techniques, demonstrating how valuable qualitative insight can be gained in this way. The theoretical developments range from exploratory studies of fluid–particle interactions to a comprehensive review of half a century of avalanche flow modeling in Russia.

Avalanche flow regime transitions in a changing climate

<p>In the near future, climate change will impact the snow cover in Alpine regions. Higher precipitations and warmer temperatures are expected at lower altitude, leading to larger gradients of snow temperature, snow water content and snow depth between the top and the bottom of slopes. As a consequence, climate change will also indirectly influence the behavior of snow avalanches.</p><p>The present work aims to investigate how changes in snow cover properties will affect snow avalanches dynamics. Experimental studies allowed to characterize different avalanche flow regimes which result from particular combinations of snow physical and mechanical properties. In particular, expected variations of snow temperatures with elevation will cause more frequent and more extreme flow regime transitions inside the same avalanche. For example, a fast avalanche characterized by cold and low-cohesive snow in the upper part of the avalanche track will transform more frequently into a slow flow made of wet and heavy snow when the avalanche will entrain warm snow along the slope. A better understanding of these flow regime transitions, which have already been largely reported, is crucial, because it affects both daily danger assessment (e.g. forecasting services, road controls) and hazard mapping of avalanches.</p><p>To date, most avalanche modeling methods are not considering temperature effects and are then unable to simulate flow regime transitions and unprecedented scenarios. Our goal is then to develop a model capable of simulating reported flow regimes, flow transitions and the interactions between the snow cover and the flow (erosion, entrainment). Since these elements are not yet fully understood, we firstly model these mesoscopic processes using a 2D Discrete Element Model (DEM) in which varying particle cohesion and friction mimic the effect of changes in snow temperature. Flow regimes are simulated by granular assemblies put into motion by gravity on an inclined slope, which interact with a granular and erodible bed surface. Simulations are calibrated using experimental data coming from the avalanche test site located in Vallée de la Sionne, which record avalanches since more than 20 years. This modeling will then be used as an input to improve slope-scale models and make them more appropriate for avalanche risk management in the context of climate change.</p>

How obstacle geometry and snow properties influence avalanche impact pressure

<p>In order to estimate avalanche loads on buildings and structures of various sizes and geometries,  practitioners are interested in recommendations or experimental data for a wide variety of obstacle geometries and sizes. Full-scale avalanche measurements are performed across the world since the late 1970s to increase knowledge about avalanche flow behaviour, including impact on structures. These structures are usually equipped with sensors to measure impact pressure, avalanche velocity and/or snow density. Modifying the structure profile is hardly possible because of high construction costs. To date, it has thus been possible to test and calibrate empirical relationships used in engineering only on a limited number of structures for which experimental data exist. We therefore aim to calibrate the drag coefficient and amplification factor for a broader range of obstacle shapes and sizes. In this context the drag coefficient generalizes the drag coefficient used in Newtonian fluid mechanics when computing the flow past an obstacle. The amplification factor reflects the snow load’s deviation from a hydrostatic-like pressure. To estimate these two parameters, we simulate how an avalanche interacts with differently sized and shaped obstacles using the Discrete Element Method (DEM). First, we test the DEM model’s capacity to reproduce full-scale pressure measurements performed on two different obstacles at the Vallée de la Sionne test site by comparing simulated and measured impact pressures. Second, we run new simulations involving other geometries and dimensions, for which no experimental data exist. Our results show that the pressure distribution depends not only on the obstacle geometry, but also on avalanche flow regime and snow properties. We eventually examine the pressure distribution for different generic geometries and avalanche scenarios. This analysis should ultimately help to improve extant engineering guidelines.</p>

Seismic localization and dynamical characterization of snow avalanches and slush flows of Mt. Fuji, Japan

<p>Snow avalanches and slush flows are often released at the stratovolcano of Mt. Fuji, which is the highest mountain of Japan (3776 m a.s.l.). These flows represent a major natural hazard as they may attain run-out distances up to 4 km, destroy parts of the forest, and sometimes damage infrastructure. We detected large dimension flows released in the winter seasons of 2014, 2016 and 2018 using the local seismic network installed to monitor the volcanic activity of Mt. Fuji. The maximum detection distance of the seismic network is approximately 15 km for the largest avalanche size class 4–5 (Canadian avalanche classification).  Using data from several seismic sensors, we applied the automated approach of amplitude source location (ASL) based on the decay of the seismic amplitudes with distance to localize and track the avalanche flow paths. We also conducted numerical simulations with Titan2D to reconstruct the avalanche trajectories and thus to assess the precision of the seismic tracking as a function of time, showing mean location errors ranging between 85 and 271 m. The average front speeds estimated from the seismic tracking, which ranged from 27 to 51 m s<sup>−1</sup>, are consistent with the numerically predicted speeds. In addition, we correlated the source amplitudes and the estimated seismic energies with the approximate run-out distances of the avalanches deduced from the ASL method. The obtained scaling relationships can be useful to empirically classify the flow size. An important task in the near future will be to develop highly effective methods for automatically detecting and tracking avalanche events in the seismic data in near-real time. One approach for the automatization of avalanche detection is the discrimination of seismic sources in the continuous recordings by applying machine learning classification methods. We expect that the precision of the flow tracking could be improved through adaptive weighting of the signals from different stations according to the source–receiver distances and angles.</p><p> </p><p> </p>

Comments on “On a Continuum Model for Avalanche Flow and Its Simplified Variants” by S. S. Grigorian and A. V. Ostroumov

This note first summarizes the history of the manuscript “On a Continuum Model for Avalanche Flow and Its Simplified Variants” by Grigorian and Ostroumov—published in this Special Issue—since the early 1990s and explains the guiding principles in editing it for publication. The changes are then detailed and some explanatory notes given for the benefit of readers who are not familiar with the early Russian work on snow avalanche dynamics. Finally, the editor’s personal views as to why he still considers this paper of relevance for avalanche dynamics research today are presented in brief essays on key aspects of the paper, namely the role of simple and complex models in avalanche research and mitigation work, the status and possible applications of Grigorian’s stress-limited friction law, and non-monotonicity of the dynamics of the Grigorian–Ostroumov model in the friction coefficient. A comparison of the erosion model proposed by those authors with two other models suggests to enhance it with an additional equation for the balance of tangential momentum across the shock front. A preliminary analysis indicates that continuous scouring entrainment is possible only in a restricted parameter range and that there is a second erosion regime with delayed entrainment.