The avalanche monitoring system av mount Pizzac

The monitoring system of Mount Pizzac has been created in order to study the avalanche dynamics and the effect of its impact on the structures. In its extreme dimensions the monitored avalanche gets off at 2200 meters and stops at 1745 meters, thus following a trajectory of 836 meters with an average gradient of 310. The necessary structures for monitoring have been installed in the track of the gully between the above limit of the flowing zone and the central track of the accumulation zone, for a whole length of 418 meters. They allow to observe the development of the event, thus recording continuously: pressures, speed and geometric variations of the body of the avalanche. In particular six steel poles, each one fitted out with n. 8 pressure measuring devices (each one with an area of 7850 mm") allow to determine the profile of the pressures continually with a resolution of 50 em up to a maximum height of 5 m. Moreover, five of the six poles placed along the flowing zone of the avalanche are fitted out with measuring devices able to check the flow height, allowing the recreation both in time and space. A small-sized wedge-shaped obstacle (area 1 m-) allows to estimate the influence of the form and area over the power of the avalanche impact. Furthermore, knowing the time when the avalanche flow passes by means of sections placed at known distances, it is possible to determine the average speed of the front in 14 tracks of the flow line. The monitoring system has three cameras which permit to record the event automatically. 15 events have been recorded since 1993, when the monitoring system was first installed. The avalanches which occured most frequently were wet snow flowing avalanches in springtime and dry snow flowing avalanches in wintertime, with average volumes of 2000 m", Particularly for average speeds of the avalanche front ranging from 2.5 to 23 m/s, the pressures have recorded variable readings going from 5 to 175 kPa

Taking Into Account Wet Avalanche Load for the Design of Pylon-like Structures.

Wet snow avalanches interact with infrastructures around the world but their significance on the structure design is frequently neglected due to the low velocity, which characterize the flow and thus the expected low impact pressures. Recent pressure measurements performed at the Swiss Vallée de la Sionne full-scale test site show that wet avalanche pressures, measured on a 20 m high tower, are considerably higher than those predicted by conventional avalanche engineering guidelines, thus potentially becoming relevant for the design of tower-like structures. In order to understand under which circumstances wet avalanches can become more relevant than their dry counter part and in order to establish simple rules to evaluate the pressure the avalanche exerts on a tower-like object, we analyse pressure and velocity data collected at the Vallée the la Sionne on obstacles of different shape and dimension

Granulation of snow: From tumbler experiments to discrete element simulations

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

Modelling erosion, entrainment and deposition in cohesive granular flows: Application to dense snow avalanches

Gravitational mass movements may erode and/or entrain a significant amount of bed material that can strongly affect the flow dynamics until the moving mass eventually deposits and comes to rest. Snow avalanches generally release on slopes covered by a metastable and thus potentially erodible snow cover that can have a wide range of strength – or cohesion – depending on the type of snow and its physical properties. As the avalanche flows, the snow cover is fully or partially entrained at the front and at the base of the flow, increasing the mass of the avalanche. Conversely, at the tail, snow may be deposited along the track, reducing the overall flowing mass. The balance between entrainment and deposition therefore determines the growing or decaying of the avalanche in terms of mass. To date, it remains unclear how cohesion influences these processes and what consequences it has for avalanche dynamics and run-out. Here, we perform simulations based on the Discrete Element Method (DEM) to analyze the influence of cohesion and slope angle on the erosion, entrainment, mixing and deposition processes. This method makes it possible to follow the dynamics of the particles within the flow very precisely, something that cannot be done in real experiments. In the model, the cohesion is represented as the combined effects of a fragmentation potential associated with the strength of the bonds, and an aggregation potential associated with the stickiness of the particles. For various combinations of input parameters and material properties, we release a heap of particles over an erodible bed and simulate the entrainment and deposition mechanisms. Our results show on the one hand that a low strength ( 3 kPa) favors basal abrasion as the flow front is not able to destabilize the erodible bed at once. In this case, the entrainment velocity decreases typically below 1 m/s. This has important consequences on mixing: the front in granular flows with low strength and adhesion is typically made of freshly entrained material coming from the whole depth of the bed, while remains of the released material can be found at the front of highly cohesive avalanches. Finally, the deposition process is analyzed by evaluating the relationship between the deposit thickness hstop and slope angle θ which extends the framework of the model of hstopθ from cohesionless to cohesive granular flows. We find that a higher bond strength of the flowing material increases the deposition height. Our work improves our understanding of the mechanics of cohesive granular flows and may contribute to improving parameterizations in depth-averaged models used to simulate geophysical mass flows such as rock, ice, snow avalanches, debris flows and landslides.ISSN:0165-232XISSN:1872-744

Numerical investigation of the effect of cohesion and ground friction on snow avalanches flow regimes

With ongoing global warming, snow avalanche dynamics may change as snow cohesion and friction strongly depend on temperature. In the field, a diversity of avalanche flow regimes has been reported including fast, sheared flows and slow plugs. While the significant role of cohesion and friction has been recognized, it is unclear how these mechanical properties affect avalanche flow regimes. Here, we model granular avalanches on a periodic inclined plane, using the distinct element method to better understand and quantify how inter-particle cohesion and ground friction influences avalanche velocity profiles. The cohesion between particles is modeled through bonds that can subsequently break and form, thus representing fragmentation and aggregation potentials, respectively. The implemented model shows a good ability to reproduce the various flow regimes and transitions as observed in nature: for low cohesion, highly sheared and fast flows are obtained while slow plugs form above a critical cohesion value and for lower ground frictions. Simulated velocity profiles are successfully compared to experimental measurements from the real-scale test site of Vallée de la Sionne in Switzerland. Even though the model represents a strong simplification of the reality, it offers a solid basis for further investigation of relevant processes happening in snow avalanches, such as segregation, erosion and entrainment, with strong impacts on avalanche dynamics research, especially in a climate change context.ISSN:1932-620

Influence of snow cover properties on avalanche dynamics

The destructive power of an avalanche depends, among other things, on the overall mass and the snow conditions in the avalanche path. So far, the knowledge on the effect of snow conditions on avalanche behavior is limited and largely qualitative. We investigate the effects of snow cover properties on avalanche dynamics, such as run-out distance and front velocity. Therefore, five avalanches with similar initial mass and topography but different flow dynamics were selected from the Vallee de la Sionne test site (Western Swiss Alps) database. For each of these avalanches, the snow conditions were reconstructed using the three-dimensional surface process model Alpine3D and the snow cover model SNOWPACK. For the investigated avalanches the data shows that the total mass, mainly controlled by entrained mass, defines run-out distance but does not correlate with front velocity. A direct effect of snow temperature on front velocity, development of the powder cloud and deposition structures could be observed. A snow temperature warmer than approximately 2 C was identified as critical value for changes in flow dynamics. No direct correlations of flow dynamical parameters with snow density and type of entrained snow were observed. (C) 2013 Elsevier B.V. All rights reserved

Influence of snow depth distribution on surface roughness in alpine terrain: A multi-scale approach

In alpine terrain, the snow-covered winter surface deviates from its underlying summer terrain due to the progressive smoothing caused by snow accumulation. Terrain smoothing is believed to be an important factor in avalanche formation and avalanche dynamics, and it affects surface heat transfer, energy balance as well as snow depth distribution. To assess the effect of snow on terrain, we use an adequate roughness definition. We developed a method to quantify terrain smoothing by combining roughness calculations of snow surfaces and their corresponding underlying terrain with snow depth measurements. To this end, elevation models of winter and summer terrain in three selected alpine basins in the Swiss Alps characterized by low, medium and high terrain roughness were derived from high-resolution measurements performed by airborne and terrestrial lidar. The preliminary results in the selected basins reveal that, at basin scale, terrain smoothing depends not only on mean snow depth in the basin but also on its variability. The multi-temporal analysis over three winter seasons in one basin suggests that terrain smoothing can be modelled as a function of mean snow depth and its standard deviation using a power law. However, a relationship between terrain smoothing and snow depth was not found at pixel scale. Further, we show that snow surface roughness is to some extent persistent, even in-between winter seasons. Those persistent patterns might be very useful to improve the representation of a winter terrain without modelling of the snow cover distribution. This can for example improve avalanche release area definition and, in the long term, natural hazard management strategies

Slab avalanche release area estimation: a new GIS tool

Location and extent of avalanche starting zones are of crucial importance to correctly estimate the potential danger that avalanches pose to roads, railways or other infrastructure. Presently, release area assessment is based on terrain analysis combined with expert judgment. Tools for the automatic definition of release areas are scarce and exclusively based on parameters derived from summer topography, such as slope and curvature. This leads to several limitations concerning the performance of such algorithms. Foremost, they neglect the smoothing effect of the snow cover on terrain morphology. Winter terrain often considerably deviates from its underlying summer terrain, thus changing potential release area size and location of surface slab avalanches. Hence, we present a new GIS based tool which estimates potential release areas by association of traditional contributory variables, such as slope and forest cover with variables particularly related to snow cover influence on topography. We introduce a scale dependent roughness parameter and a wind shelter parameter accounting for varying winter topography and snow deposition patterns with increasing snow depth. Further, uncertainty in the definition of the parameters is accounted for by using a fuzzy logic classification approach. This approach is especially useful for defining release area scenarios e.g. depending on snow depth, which is not possible with existing tools