scholarly journals Forest damage and snow avalanche flow regime

2015 ◽  
Vol 15 (6) ◽  
pp. 1275-1288 ◽  
Author(s):  
T. Feistl ◽  
P. Bebi ◽  
M. Christen ◽  
S. Margreth ◽  
L. Diefenbach ◽  
...  
Keyword(s):  
Flow Regime ◽  
Dynamic Pressure ◽  
Forest Damage ◽  
Snow Avalanches ◽  
Particle Distributions ◽  
Wet Snow ◽  
Homogeneous Particle ◽  
Impact Area ◽  
Avalanche Flow ◽  
The Impact

Abstract. Snow avalanches break, uproot and overturn trees causing damage to forests. The extent of forest damage provides useful information on avalanche frequency and intensity. However, impact forces depend on avalanche flow regime. In this paper, we define avalanche loading cases representing four different avalanche flow regimes: powder, intermittent, dry and wet. Using a numerical model that simulates both powder and wet snow avalanches, we study documented events with forest damage. First we show that in the powder regime, although the applied impact pressures can be small, large bending moments in the tree stem can be produced due to the torque action of the blast. The impact area of the blast extends over the entire tree crown. We find that, powder clouds with velocities over 20 m s-1 can break tree stems. Second we demonstrate that intermittent granular loadings are equivalent to low-density uniform dry snow loadings under the assumption of homogeneous particle distributions. The intermittent regime seldom controls tree breakage. Third we calculate quasi-static pressures of wet snow avalanches and show that they can be much higher than pressures calculated using dynamic pressure formulas. Wet snow pressure depends both on avalanche volume and terrain features upstream of the tree.

scholarly journals Forest damage and snow avalanche flow regime

2015 ◽  
Vol 3 (1) ◽  
pp. 535-574
Author(s):  
T. Feistl ◽  
P. Bebi ◽  
M. Christen ◽  
S. Margreth ◽  
L. Diefenbach ◽  
...  
Keyword(s):  
Flow Regime ◽  
Dynamic Pressure ◽  
Forest Damage ◽  
Snow Avalanches ◽  
Particle Distributions ◽  
Wet Snow ◽  
Homogeneous Particle ◽  
Impact Area ◽  
Avalanche Flow ◽  
The Impact

Abstract. Snow avalanches break, uproot and overturn trees causing damage to forests. The extent of forest damage provides useful information on avalanche frequency and intensity. However, impact forces depend on avalanche flow regime. In this paper, we define avalanche loading cases representing four different avalanche flow regimes: powder, intermittent, dry and wet. In the powder regime, the blast of the cloud can produce large bending moments in the tree stem because of the impact area extending over the entire tree crown. We demonstrate that intermittent granular loadings are equivalent to low-density uniform dry snow loadings under the assumption of homogeneous particle distributions. In the wet snow case, avalanche pressure is calculated using a quasi-static model accounting for the motion of plug-like wet snow flows. Wet snow pressure depends both on avalanche volume and terrain features upstream of the tree. Using a numerical model that simulates both powder and wet snow avalanches, we study documented events with forest damage. We find (1) powder clouds with velocities over 20 m s−1 can break tree stems, (2) the intermittent regime seldom controls tree breakage and (3) quasi-static pressures of wet snow avalanches can be much higher than pressures calculated using dynamic pressure formulas.

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

2021 ◽  
Author(s):  
Michael Lukas Kyburz ◽  
Betty Sovilla ◽  
Johan Gaume ◽  
Christophe Ancey
Keyword(s):  
Drag Coefficient ◽  
Flow Regime ◽  
Dynamic Pressure ◽  
Flow Regimes ◽  
Impact Pressure ◽  
Snow Avalanche ◽  
Hazard Mapping ◽  
Mountainous Regions ◽  
Avalanche Flow ◽  
The Impact

<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>

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

2021 ◽  
Author(s):  
Guillaume Chambon ◽  
Thierry Faug ◽  
Mohamed Naaim
Keyword(s):  
Numerical Simulations ◽  
Flow Regimes ◽  
Liquid Water Content ◽  
Snow Avalanches ◽  
Shallow Flow ◽  
Distinctive Features ◽  
Wide Range ◽  
Wet Snow ◽  
Sensitivity Studies ◽  
Avalanche Flow

<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>

Observations of the dynamic structure of snow avalanches

1993 ◽  
Vol 18 ◽  
pp. 313-316 ◽  
Author(s):  
K. Nishimura ◽  
N. Maeno ◽  
F. Sandersen ◽  
K. Kristensen ◽  
H. Norem ◽  
...  
Keyword(s):  
Dynamic Structure ◽  
Velocity Shear ◽  
Sensor Data ◽  
Video Tape ◽  
Snow Avalanches ◽  
Western Norway ◽  
Wet Snow ◽  
Avalanche Flow ◽  
Dynamic Structures ◽  
Internal Velocity

During two winters, 1990–92, the dynamic structures of snow avalanches were studied in western Norway. Artificially released wet-snow avalanches ran down the avalanche chute and stopped in front of the retaining dam. Running velocity distributions were obtained not only by video tape recorder, but also by various other recording instruments. Internal velocity was derived for the last avalanche by frequency analysis of impact pressure and optical sensor data. The vertical velocity shear of the avalanche flow has been estimated to be in the range 1–10 s−1.

scholarly journals Release temperature, snow-cover entrainment and the thermal flow regime of snow avalanches

2015 ◽  
Vol 61 (225) ◽  
pp. 173-184 ◽  
Author(s):  
Cesar Vera Valero ◽  
Katreen Wikstroem Jones ◽  
Yves Bühler ◽  
Perry Bartelt
Keyword(s):  
Boundary Conditions ◽  
Snow Cover ◽  
Flow Regime ◽  
Internal Heat ◽  
Heat Energy ◽  
Random Fluctuation ◽  
Irreversible Processes ◽  
Snow Avalanches ◽  
Avalanche Flow ◽  
Initial And Boundary Conditions

AbstractTo demonstrate how snow-cover release and entrainment temperature influence avalanche runout we develop an avalanche dynamics model that accounts for the thermal heat energy of flowing snow. Temperature defines the mechanical properties of snow and therefore the avalanche flow regime. We show that the avalanche flow regime depends primarily on the temperature of the snow mass in the starting zone, as well as the density and temperature of the entrained snow cover, which define the influx of heat energy. Avalanche temperature, however, not only depends on the initial and boundary conditions, but also on the path-dependent frictional processes that increase internal heat energy. We account for two processes: (1) frictional shearing in the slope-parallel flow direction and (2) dissipation of random fluctuation energy by inelastic granular interactions. In avalanche flow, nonlinear irreversible processes are coupled with variable initial and boundary conditions that lead to transitions in flow regime. Snow avalanches thus exhibit a wide variety of flow behaviour with variation in snow-cover temperature.

scholarly journals Cold-to-warm flow regime transition in snow avalanches

2018 ◽  
Author(s):  
Anselm Köhler ◽  
Jan-Thomas Fischer ◽  
Riccardo Scandroglio ◽  
Mathias Bavay ◽  
Jim McElwaine ◽  
...  
Keyword(s):  
Snow Cover ◽  
Flow Regime ◽  
Flow Regimes ◽  
Test Site ◽  
Regime Transition ◽  
Snow Avalanches ◽  
Protection Measures ◽  
Flow Regime Transition ◽  
Snow Conditions ◽  
The Impact

Abstract. Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T < −1 °C) at high elevations near the surface, but warm (T > −1 °C) at lower elevations or deeper in the snow pack. The intake of thermal energy in the form of warm snow is believed to cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete, when the deposit at runout is characterized only by warm snow, or as partial, if there is a warm flow regime but the furthest deposit is characterized by cold snow. We introduce a transition factor Ft, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude Hs, which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that Ft is related to the snow cover properties, i.e. approximately proportional to Hs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find, that, if Hs is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

Avalanche flow regime transitions in a changing climate

2020 ◽  
Author(s):  
Camille Ligneau ◽  
Betty Sovilla ◽  
Johan Gaume
Keyword(s):  
Climate Change ◽  
Snow Cover ◽  
Flow Regime ◽  
Flow Regimes ◽  
Future Climate Change ◽  
Hazard Mapping ◽  
Snow Avalanches ◽  
Avalanche Flow ◽  
Regime Transitions ◽  
Snow Temperature

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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 Vall&amp;#233;e 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.&lt;/p&gt;

scholarly journals Observations of the dynamic structure of snow avalanches

1993 ◽  
Vol 18 ◽  
pp. 313-316 ◽  
Author(s):  
K. Nishimura ◽  
N. Maeno ◽  
F. Sandersen ◽  
K. Kristensen ◽  
H. Norem ◽  
...  
Keyword(s):  
Dynamic Structure ◽  
Velocity Shear ◽  
Sensor Data ◽  
Video Tape ◽  
Snow Avalanches ◽  
Western Norway ◽  
Wet Snow ◽  
Avalanche Flow ◽  
Dynamic Structures ◽  
Internal Velocity

During two winters, 1990–92, the dynamic structures of snow avalanches were studied in western Norway. Artificially released wet-snow avalanches ran down the avalanche chute and stopped in front of the retaining dam. Running velocity distributions were obtained not only by video tape recorder, but also by various other recording instruments. Internal velocity was derived for the last avalanche by frequency analysis of impact pressure and optical sensor data. The vertical velocity shear of the avalanche flow has been estimated to be in the range 1–10 s−1.

scholarly journals Cold-to-warm flow regime transition in snow avalanches

2018 ◽  
Vol 12 (12) ◽  
pp. 3759-3774 ◽  
Author(s):  
Anselm Köhler ◽  
Jan-Thomas Fischer ◽  
Riccardo Scandroglio ◽  
Mathias Bavay ◽  
Jim McElwaine ◽  
...  
Keyword(s):  
Snow Cover ◽  
Flow Regime ◽  
Flow Regimes ◽  
Test Site ◽  
Regime Transition ◽  
Snow Avalanches ◽  
Protection Measures ◽  
Flow Regime Transition ◽  
Snow Conditions ◽  
The Impact

Abstract. Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T‾<-1 ∘C) at high elevations near the surface, but warm (T‾>-1 ∘C) at lower elevations or deeper in the snowpack. The intake of warm snow is believed to be of major importance to increase the temperature of the snow composition in the avalanche and eventually cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete when the deposit at runout is characterized only by warm snow or as partial if there is a warm flow regime, but the farthest deposit is characterized by cold snow. We introduce a transition index Ft, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude Hs, which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that Ft is related to the snow cover properties, i.e. approximately proportional to Hs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find that, if Hs is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

scholarly journals Slow drag in wet-snow avalanche flow

2010 ◽  
Vol 56 (198) ◽  
pp. 587-592 ◽  
Author(s):  
B. Sovilla ◽  
M. Kern ◽  
M. Schaer
Keyword(s):  
Mean Value ◽  
Immersion Depth ◽  
Snow Avalanches ◽  
Pressure Time ◽  
Large Fluctuations ◽  
Load Cells ◽  
Snow Pressure ◽  
Wet Snow ◽  
Avalanche Flow ◽  
The Mean

AbstractWe report impact pressures exerted by three wet-snow avalanches on a pylon equipped with piezoelectric load cells. These pressures were considerably higher than those predicted by conventional avalanche engineering guidelines. The time-averaged pressure linearly increased with the immersion depth of the load cells and it was about eight times larger than the hydrostatic snow pressure. At the same immersion depth, the pressures were very similar for all three avalanches and no dependency between avalanche velocity and pressure was apparent. The pressure time series were characterized by large fluctuations. For all immersion depths and for all avalanches, the standard deviations of the fluctuations were, on average, about 20% of the mean value. We compare our observations with results of slow-drag granular experiments, where similar behavior has been explained by formation and destruction of chain structures due to jamming of granular material around the pylon, and we propose the same mechanism as a possible microscale interpretation of our observations.

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