Avalanche Triggering Research Articles

The unacknowledged risk of Himalayan avalanches triggering

A “universal” model for avalanche triggering, as well as for collapse of suspended seracs, is presented based on Quantized Fracture Mechanics, considering fracture, friction, adhesion and cohesion. It unifies and extends the classical previous approaches reported in the literature, including the role of the slope curvature. A new size-effect, that on mountain height rather than the classical one on snow slab thickness, is also discussed and demonstrated thanks to glaciers data analysis from the World Glacier Inventory (http://nsidc.org/data/glacier_inventory/browse.html, 2014). The related most noteworthy result is that snow precipitation needed to trigger avalanches at 8,000 m could be up to 4 times, with a realistic value of 1.7 times, smaller than at 4,000 m. This super-strong size-effect may suggest that the risk of Himalayan avalanches is today still unacknowledged. A discussion on the recent Manaslu tragedy concludes the paper.

Snow Pressure on a Semiflexible Retaining Structure

AbstractSnow avalanche hazard is one of the most critical aspects in the urbanization of mountainous areas. The risk from these natural phenomena can be reduced if proper countermeasures for reducing the danger or preventing avalanche triggering are taken. For instance, in the avalanche initiation zone, devices able to withstand snowpack gliding and prevent crack propagation can be installed. These can be classified as rigid (snow bridges) or flexible (snow net) structures, depending on the mechanisms involved under snow loading. In recent years, a new kind of structure called a snow umbrella, based on a net panel supported by rigid beams, has been designed and installed in the northwestern Italian Alps. As the real behavior of these structures is far from being simple to understand because of the complex interaction with snowpack movements, an umbrella was instrumented and monitored during the winter season of 2010–2011. The recorded data are used for testing two possible pressure distributions and evalu...

DTM Generation and Avalanche Hazard Mapping using Large Format Digital Photogrammetric Data and Geomatics Technique

Abstract The main objective of the study is Digital Terrain Model (DTM) generation from aerial photogrammetric data and identify and map the potential avalanche prone zones in Manali region. Avalanche is a dynamic hazardous phenomenon in the snow-bound mountainous terrain. Mapping of avalanche prone terrain is crucial to minimize the avalanche hazard. Nowadays, airborne data capturing technology, such as large-format Photogrammetry, has opened new vistas for the mapping of complex and inaccessible mountainous areas. In the present study, large format digital Photogrammetry data of 20 cm ground sample distance (GSD) have been used to generate high-resolution and accurate Digital Elevation Model and ortho-images. Digital terrain model along with its derivative terrain products and land cover map generated from land cover classification of derived ortho-photo is analyzed to locate the probable avalanche zone. The terrain characteristics, snow-pack condition and prevailing meteorological conditions are the groups of variables that influence the occurrence of avalanche. Amongst these, the terrain characteristics is the most influencing factor, and easier to map due to its stable nature along the time. Therefore advanced geo-informatics techniques have been used by mixing terrain property, Digital Elevation Model (DEM) and satellite imagery to determine the different geographical factors that affect the avalanche triggering. Also the derived information was combined in Analytic Hierarchy Process to extract a map of the avalanche prone zones in the study area standard mapping techniques as coarse-resolution data are not very appropriate for such studies.

Learning from the past: statistical performance measures for avalanche warning services

Avalanche warning services (AWS) are operated to protect communities and traffic lines in avalanche-prone regions of the Alps and other mountain ranges. In times of high avalanche danger, these services may decide to close roads or to evacuate settlements. Closing decisions are based on field observations, avalanche release statistics, and snow forecasts issued by weather services. Because of the spatial variability in the snowpack and the insufficient understanding of avalanche triggering mechanisms, closing decisions are characterized by large uncertainties and the information based on which AWS have to decide is always incomplete. In this paper, we illustrate how signal detection theory can be applied to make better use of the information at hand. The proposed framework allows the evaluation of past road closures and points to how the decision performance of AWS could be improved. To illustrate the proposed framework, we evaluate the decision performance of two AWS in Switzerland and discuss the advantages of such a formalized decision-making approach.

Monitoring of snow avalanches using a seismic array: Location, speed estimation, and relationships to meteorological variables

Monitoring snow avalanches is necessary in order to better understand their triggering mechanisms and ultimately improve forecast performance. Seismic monitoring has been developed by several groups over the last 20 years and holds great potential to detect, locate, and characterize snow avalanches. During the 2009–2010 winter, a seismic antenna was installed in the French Alps close to the village of Saint‐Christophe‐en‐Oisans (1700 m above sea level). The array of seven sensors operated during 50 days in October and November 2009 under snow‐free conditions and during 40 days in January and February 2010 in presence of snow. It recorded different types of seismic events including snow avalanches, rockfalls, shots, and regional and local microearthquakes. Eighty avalanche signals were visually identified. Using a beam‐forming method, we were able to locate snow avalanches on slopes of various orientations in a radius of about 3 km and track their propagation. The location technique allowed for the estimation of avalanches' front speed, which ranged between 12 and 32 m s−1. The method can also distinguish dry and wet snow avalanches. Durations of avalanches can be as long as 380 s because of the length of the slopes in the area. Seismic monitoring provides a catalog of avalanches with precise times, which can be used to analyze the impact of meteorological forcings on the avalanche triggering. Snowfall is found to be the dominant forcing of avalanche activity during this period, as revealed by the strongest correlation. For the period of study, our results suggest that the impact of precipitation on the snowpack instability lasts for about 6 days.

Active Area Uniformity of InGaAs/InP Single-Photon Avalanche Diodes

We present a detailed characterization of the active area uniformity of InGaAs/InP Single-Photon Avalanche Diodes (SPADs) from two different design iterations. Nonuniformity of the electric field within the device active area has been measured through 2-D scans of detection efficiency and timing response to a pulsed laser. Additionally, we measured the near-infrared luminescence emitted by hot carriers during the avalanche. The nonuniformity is stronger at lower excess bias, with much higher and nonuniform electric field at the edge of the active area than expected, and it is fainter at higher excess bias, due to the saturation of the avalanche triggering efficiency. The main drawbacks are that the detection efficiency is position dependent when the SPAD is not fiber pigtailed and that the temporal response is worse, because of the nonuniform delay in the avalanche build-up across the SPAD active area.

Numerical study of fracture arrest on snow cover

Under the hypothesis of a perfectly brittle phenomenon, avalanche triggering can be investigated numerically by means of Linear Elastic Fracture Mechanics (LEFM). Since, however, the real phenomenon is intrinsically dynamical, another aspect to investigate is represented by dynamic fracture propagation. In this paper, we model dynamic crack propagation into a dry snow slab and we investigate the possibility to arrest the crack propagation through the presence of weak zones distributed along the extension of the snow slope. Assuming that the weak layer is almost collapsed, we simulate the efficiency of artificial voids in the slab to arrest fracture propagation, into the framework of Dynamical Fracture Mechanics. We put forward here a new philosophy for the use of artificial discontinuities (void) into the snowpack able to perform as crack arresters distributed along the snow slope area: the target is to split a large avalanche slab into smaller slabs, causing small avalanches to propagate with less catastrophic effects.

Rock‐and‐soil avalanches: Theory and simulation

We present a 2‐D Contact Dynamics discrete element model for simulating initiation and motion of rock avalanches, integrating hillslope geometry, Mohr‐Coulomb rock behavior, pore pressure before avalanche triggering, and avalanche trigger. Avalanche motion is modeled as a dense granular flow of dry frictional and cohesive particles. On the basis of granular physics and shear experiments, we review some of the theories for the unexpectedly long runout of rock avalanches. Different causes are evoked, according to the strength (strong or weak) of the slip surface relative to the bulk. “Mechanical fluidization” and “acoustic fluidization” theories state that agitation of rock particles reduces frictional strength, increasing runout. Conversely, granular mechanics suggests that, as “shear‐strain” rate increases, granular material becomes more agitated, more dissipative, and more resistant. Another theory states that dynamic fragmentation of clasts creates an isotropic pressure that drives longer runout. In contrast, granular mechanics suggests that fragmentation may induce fluidization and strengthening of the granular material, while particle size reduction (among others) induces weakening of the granular flow and enhances long runout. Runout is also enhanced for column‐like rock masses collapsing from steep hillslopes. Long runout may also be linked to thermal weakening mechanisms at the slip surface (e.g., thermal pressurization, and shear melting), which may lower drastically the shear strength. The model is illustrated with a hypothetical example of a rain‐triggered avalanche, mobilizing shallowly dipping layers. Several phases are identified, including slope failure, avalanche triggering resulting from slip weakening, and avalanche motion in which rocks are folded and sheared.

Discrete element simulation of the Jiufengershan rock‐and‐soil avalanche triggered by the 1999 Chi‐Chi earthquake, Taiwan

We present Contact Dynamics discrete element simulations of the earthquake‐triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping ∼22°SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple “slip‐weakening” criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop‐up structure at the toe of the dip slope. As the avalanche propagates, the pop‐up deforms into an overturned fold, which overrides the surface of separation along a décollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (μSS = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake‐ and rain‐triggered avalanches are similar.

Application of Gradient Theory and Quantized Fracture Mechanics in Snow Avalanches

Avalanche initiation in snow slabs of high enough length-to thickness ratio by failure of the interface between the show slab and the underlying bedrock is considered. In this direction Quantized Fracture Mechanics (QFM), an extension of the theory of Linear Elastic Fracture Mechanics (LEFM), as well as gradient theory are employed. Mode-II fracture is assumed in both cases. The two models, although having a completely different origin, lead to similar expressions for the height of the fallen snow, which is critical for slab avalanche triggering.

Diffusion dominated dynamics of avalanching systems

A surprisingly large number of systems in nature are thought to be governed by internal dynamics which causes avalanches of various sizes. In such systems energy, which is delivered from outside, is redistributed as a result of the occurrence of localized avalanches. Starting an avalanche requires that some threshold condition be satisfied. Random driving (energy input) brings the system into a strongly inhomogeneous state, so that the probability of triggering an avalanche in a large part of the system is small. In most physical systems energy redistribution may occur due to diffusive processes without avalanches. Diffusion also makes the system more uniform, making large avalanche triggering more probable. The observed behavior of a such system may crucially depend on the competition between diffusion and driving. In this paper, the effects of diffusive processes are investigated using a dissipative, isotropic one-dimensional model, in which avalanches can propagate in both directions. It is shown that the system behavior changes progressively as the diffusion rate increases. In the absence of diffusion, many small avalanches are triggered. Increasing the diffusion rate gradually suppresses these small avalanches and leads to the development of large, quasi-periodic bursts.

Triggering of dry snow slab avalanches: stress versus fracture mechanical approach

Abstract This paper analyses the conditions for triggering of dry snow slab avalanches. As suggested by several Authors, we assume as a basic mechanism for avalanche triggering the mode II fracture of the weak layer lying beneath the stiff snow slab, i.e. we assume the presence of super-weak zones in the basal layer. By means of a linear elastic analysis, the shear stresses in the weak layer as well as the strain energy release rate caused by an increment of the super-weak zone are evaluated. Hence we introduce a stress failure criterion as well as an energy one. It is shown that the latter criterion can be seen as an extension of the criterion firstly proposed by McClung [McClung, D.M., 1979. Shear fracture precipitated by strain softening as a mechanism of dry slab avalanche release. Journal of Geophysical Research, 84(B7), 3519-3526.] for dry snow slab avalanche release. Finally we couple the two criteria, showing that the weak layer can fail only in a min–max band of thickness.

Strain localization in a continuum as an instability event

The paper presents a novel physical model, based on perturbation theory, to describe localization pattern formation in a solid material as a result of system instabilities. Such kind of approach has been inspired by the theory of population dynamics. In particular, the sinergetic phenomenon of strain localization into a stressed continuum, and its subsequent evolution to cohesive cracking, is obtained through the competition of an external source of energy (e.g., strain energy) and of the internal behavior of the material. The hypothesis of mobile energy entities within material bulk is put forward. These entities, which under low strain conditions are evenly distributed throughout the body, can be considered as strain quanta. The quantization of mechanical quantities is not new in continuum and fracture mechanics, [see, e.g., Novozhilov (1969, Prik Mat Mek 33:212–222)]. With increasing strain, a certain critical point is reached when the homogeneous situation becomes unstable and the strain quanta begin to aggregate into bands, leading to periodic strain localization patterns. The model, which is only theoretical at this stage, can be applied to the particular case of dry snow avalanches. In these cases, snow avalanche triggering is due toinstability (onset of sliding onto a weak plane) and is controlled by external loading (e.g., weight of the slope, load by skiers) and by internal factors (e.g., temperature changes, snow phase transformations etc.).

Modelling the system behaviour of wet snow avalanches using an expert system approach for risk management on high alpine traffic roads

Abstract. The presented approach describes a model for a rule-based expert system calculating the temporal variability of the release of wet snow avalanches, using the assumption of avalanche triggering without the loading of new snow. The knowledge base of the model is created by using investigations on the system behaviour of wet snow avalanches in the Italian Ortles Alps, and is represented by a fuzzy logic rule-base. Input parameters of the expert system are numerical and linguistic variables, measurable meteorological and topographical factors and observable characteristics of the snow cover. Output of the inference method is the quantified release disposition for wet snow avalanches. Combining topographical parameters and the spatial interpolation of the calculated release disposition a hazard index map is dynamically generated. Furthermore, the spatial and temporal variability of damage potential on roads exposed to wet snow avalanches can be quantified, expressed by the number of persons at risk. The application of the rule base to the available data in the study area generated plausible results. The study demonstrates the potential for the application of expert systems and fuzzy logic in the field of natural hazard monitoring and risk management.

Dislocation avalanche correlations

Recently, mechanical tests on ice as well as dislocation dynamics simulations have revealed that plastic flow in materials exhibiting a largely dominating slip system displays a scale-free intermittent dynamics characterized by dislocation avalanches with a power law distribution of amplitudes. To further explore the complexity of dislocation dynamics during plastic flow, we present a statistical analysis of dislocation avalanche correlations and avalanche triggering. It is shown that the rate of avalanche triggering immediately after any avalanche is larger than the background activity due to uncorrelated events. This self-induced triggering increases in intensity, and remains over the background rate for longer times, as the amplitude of the mainshock increases. This analysis suggests that stress redistributions and the associated collective dislocation rearrangements may be responsible for aftershock triggering in the complex process of plastic deformation.

Photogrammetric avalanche volume measurements at Vallée de la Sionne, Switzerland

AbstractDuring winter 1999 three large avalanche events were triggered by explosives at SLF’s avalanche test site, Vallée de la Sionne, canton Valais, Switzerland. One important goal of these large-scale field experiments was to measure the release and deposition volumes of avalanches by photogrammetric methods. In this paper, the photogrammetric measurements of all three avalanches are summarized. For one avalanche event it was possible to realize the whole measuring procedure as planned, and to obtain volume measurements before and after the avalanche triggering In the other two avalanche events, the photographs before the triggering of the avalanche failed. Nevertheless the photographs taken after the avalanche provide valuable information on the fracture depth at the fracture line. The mean fracture depth of the largest avalanche was about 2.10 m, varying between 1 and 3.5 m over a width of > 1000 m. The total volume of the deposition of all three avalanche events was about 1300 000 m3. The deposits are distributed over a length of > 1000 m with depths up to 30 m. The difference between the released and deposited volumes proved that avalanches entrain a large amount of snow along the avalanche track. Furthermore, the snow distribution in the deposition zone provides important information about the behaviour of a dense flowing avalanche in the runout zone.

Influence of traps on avalanche triggering during breakdown of gallium phosphide p-n junctions

Deep centers are found to strongly influence the statistical delay of microplasma breakdown in gallium phosphide red LEDs. It is postulated that this effect is caused by Shockley electrons triggering an avalanche or by subthreshold mechanisms of impact ionization. It is shown that the statistical delay in the microplasma breakdown can be used to determine the parameters of the deep centers if the charge state of the deep centers is varied by reducing the p-n junction voltage.

The role of ionization coefficient in the operation of avalanche diodes above breakdown

The behavior of the avalanche triggering probability in pn junction diodes biased above breakdown is a sensitive function of the form of the field dependence of αe and αh. In this paper we present theoretical predictions of electron and hole initiated avalanche triggering probabilities in planar, cylindrical, and spherical junctions. These probabilities are calculated as a function of excess bias above breakdown for various forms of αe and αh versus E. We also show recent experimental data for electron and hole triggering in planar diodes. The absolute electron triggering probability is measured to an accuracy of about ± 40%. The form of the electron and hole triggering probabilities versus bias are more accurately determined. Both the absolute and relative triggering probabilities are in good agreement with the theoretical prediction based on αe and αh forms of either Grant or Van Overstraeten and De Man.