Cellular-Automata Model for Dense-Snow Avalanches

This paper introduces a three-dimensional model for simulating dense-snow avalanches, based on the numerical method of cellular automata. This method allows one to study the complex behavior of the avalanche by dividing it into small elements, whose interaction is described by simple laws, obtaining a reduction of the computational power needed to perform a three-dimensional simulation. Similar models by several authors have been used to model rock avalanches, mud and lava flows, and debris avalanches. A peculiar aspect of avalanche dynamics, i.e., the mechanisms of erosion of the snowpack and deposition of material from the avalanche is taken into account in the model. The capability of the proposed approach has been illustrated by modeling three documented avalanches that occurred in Susa Valley (Western Italian Alps). Despite the qualitative observations used for calibration, the proposed method is able to reproduce the correct three-dimensional avalanche path, using a digital terrain model, and the order of magnitude of the avalanche deposit volume.     

How disorder can diminish avalanche risks: effect of size distribution

We study the behavior of disk assemblies with a variable disorder distribution. The packing is first consolidated and then continuously tilted very slowly. The amount of displaced disks for each tilted angle is recorded. Large displacements of the disks can occur due to some local or global mechanical instabilities. The definition of neighboring disks is based on radical (extension of Voronoï) tessellation rules to decompose, in a unique and perfectly defined manner, the two-dimensional space for polydisperse disks. In this way, by comparing the characteristics of stability for one disk to the neighboring ones for local ordered cluster, we can predict the global amount of displaced disks. Some tilting cycles have been performed to check the correlation between the instability of the packing structure (collective displacements) with micro and macro order parameters.     

Avalanching flow of cohesive powders

The flow dynamics of cohesive powders is investigated in rotating cylinders with an L : R ratio of 3 : 1 using experiments and DEM simulations. Flow onset and steady-state behavior are compared for free-flowing (cohesionless) dry glass beads, wet glass beads, and “dry” cohesive powders (lactose, microcrystalline cellulose). The avalanching dynamics of powders is substantially different from those observed for free-flowing or wet-cohesive glass beads. Dry cohesive powders exhibit history-dependent flow dynamics, significant dilation, aperiodic avalanche frequencies, and variable avalanche size. These behaviors also provide a route for effective characterization of cohesive forces under dilated conditions characteristic of unconfined flows.     

High-speed video recording of basal shear layers in snow chute flows

Run-out distances and flow velocities of snow avalanches are mainly determined by frictional processes originating from the interaction with the ground. At the SLF snow chute at the Weissfluhjoch near Davos, a setup was developed which allowed us to record high-speed movies of the basal shear layer of small-scale avalanches with a frame rate of 1000 frames per second. Shear processes could be observed in high-resolution slow motion. Downstream velocity profiles were extracted by a pattern matching algorithm. The comparison of computed profiles with velocity profiles obtained from optical sensors showed good agreement. However, the temporal and spatial resolutions are much higher for the high-speed video data. Because the optical velocity sensors are one-dimensional, we found that they overestimate the velocities when a flow-normal velocity component exists as well. All measured velocity profiles exhibited very high shear rates near the ground. The maximum shear rates were up to 600/s for dry snow and 200/s for wet snow avalanches. The observations of the video images suggested a turbulent motion of the snow in the basal shear layer.     

Line creep in paper peeling

The dynamics of a “peeling front” or an elastic line is studied under creep (constant load) conditions. Our experiments show in most cases an exponential dependence of the creep velocity on the inverse force (mass) applied. In particular, the dynamical correlations of the avalanche activity are discussed here. We compare various avalanche statistics to those of a line with non-local elasticity, and study various measures of the experimental avalanche-avalanche and temporal correlations such as the autocorrelation function of the released energy and aftershock activity. From all these we conclude, that internal avalanche dynamics seems to follow “line depinning”-like behavior, in rough agreement with the depinning model. Meanwhile, the correlations reveal subtle complications not implied by depinning theory. Moreover, we also show how these results can be understood from a geophysical point of view.     

Fuzzy modelling of powder snow avalanches

This paper examines powder snow avalanches by introducing a predetermined degree of variation, or fuzziness, in model parameters. Given a value of vagueness in the parameters, fuzzy set theory makes it possible to evaluate the vagueness in the results. The use of a more complex stochastic analysis can be avoided. Six parameters of the model are taken to be affected by a certain amount of uncertainty; the response of the numerical model is calculated by solving the fuzzy equations. In this way, it is possible to evaluate how the results are affected by a given change in the model parameters.The paper first presents a well-known avalanche model and its solution considering the influence of friction. A brief introduction of the fuzzy set is given with regard to the avalanche model mentioned. Later, the fuzzy solution of the model in terms of velocity and average pressure is calculated for three different levels of imprecision in the data. At the end, the results are presented and commented.     

The Extended Column Test: Test effectiveness, spatial variability, and comparison with the Propagation Saw Test

The Extended Column Test (ECT) is a new stability test that aims to assess the fracture propagation potential across a 0.90 m wide isolated column. This paper: 1) describes the test procedure and presents new recording standards for the test, 2) uses two independent datasets (each consisting of over 300 tests) to assess the effectiveness of the test, 3) looks at the spatial variability of ECT results from several test grids, and 4) compares adjacent results between the ECT and the Propagation Saw Test (PST) on stable and unstable slopes. Our results indicate that the ECT is an effective stability test, with a false-stability rate less than other standard snow stability tests. Results are sometimes quite spatially uniform, though occasionally slopes may exhibit variable ECT results. In comparison to the PST, our data suggest that the ECT has a lower false-stability rate, but a higher false instability rate. Overall, the ECT is better at discriminating between stable and unstable slopes in our dataset. No test is perfect and all tests must be used in conjunction with additional data, but our results show that the ECT is valuable additional tool for assessing snow stability.     

Segregation and stability of a binary granular heap

We measure stability of two-dimensional granular mixtures in a rotating drum and relate grain configurations to stability. We use two types of grains which differ in both size and shape, with the larger grains reaching a larger average angle before an avalanche. In our mixtures, the smaller grains cluster near the center of the drum, while the larger grains remain near the outer edge, a pattern suggesting that grain size rather than avalanche angle determines the segregation behavior. One consequence of the size segregation is that the smaller grains heavily influence the stability of the heap. We find that the maximum angle of stability is a non-linear function of composition, changing particularly rapidly when small grains are first added to a homogeneous pile of large grains. We conclude that the grain configuration within the central portion of the heap plays a prominent role in stability. This work was supported in part by the National Science Foundation’s Research Experience for Undergraduates Program under PHY-0243904.     

WeMOI and NAOi influence on major avalanche activity in the Eastern Pyrenees

Major avalanche episodes registered in the Eastern Pyrenees (north-eastern Iberian Peninsula) are correlated with both the North Atlantic Oscillation (NAO) and the Western Mediterranean Oscillation (WeMO). The NAO is one of the most important and recurrent patterns of atmospheric circulation variability in the Northern Hemisphere and affects the behaviour of surface temperatures and precipitation in Western Europe. The Western Mediterranean Oscillation index (WeMOi) controls intense precipitation in the eastern part of the Iberian Peninsula. The link between both indices and major avalanche activity is undertaken by applying a cumulative index. A link between avalanche activity and the NAO has been found in Iceland, but it has not been detected in areas closer to the Pyrenees, such as the French Alps. This paper examines the linkage to the NAO in an area further from the centres of the dipole than is the case for Iceland. Results show a significant negative correlation between major avalanche activity in the Eastern Pyrenees and both NAO index (NAOi) and WeMOi, which is even higher with the cumulative NAOi (CNI) and the cumulative WeMOi (CWI). The cumulative index reflects the cumulative effect of snow accumulation in the avalanche starting zones as precipitation is affected by the variations of NAOi and WeMOi. Consequently, the correlation is negative since it has been demonstrated that increased precipitation in the Eastern Pyrenees is linked to negative values of both low frequency patterns. Nevertheless, results from the diverse avalanche forecasting regions in the Eastern Pyrenees suggest a different response of avalanche activity to both indices, confirming the Pyrenees as a complex snow-climate boundary. Lastly, possible changes in avalanche activity in the coming decades may be suggested by the NAO evolution associated with global warming scenarios that are foreseen and the present interannual trend of the WeMOi.     

Life after computer simulations: towards establishing bulk evolution rules based on discrete granular dynamics

Today, parallel processing supercomputers allow simulations of up to 10⁶ discrete elements. and advances in processing power should allow 10⁹-10¹⁰ discrete elements. The availability of such large data sets offers real scope for statistically accurate calculations to describe the long-range evolution of the bulk flow and stress fields during industrial handling and processing of material in granular and powder states.We show that careful post-processing analysis of granular dynamics simulation results in terms of time series/signal processing can lead to a greater understanding of the micro-dynamics of granular systems and the defining events in the evolution of such systems. In particular, we apply the Fourier transform to investigate periodicity throughout the duration of the simulation and the wavelet transform to investigate disturbances which are localised in time and which are much more difficult to model with Fourier methods. Whilst we are essentially pointing to post-simulation/experiment techniques which are generic, we illustrate their importance for the investigation of segregation phenomena in poured binary granular heaps, with a particular focus on the diffusive mechanism of kinetic sieving.