Avalanche Head Research Articles

Potential and challenges of depth-resolved three-dimensional MPM simulations: a case study of the 2019 ‘Salezer’ snow avalanche in Davos

Abstract Avalanche modeling is an essential tool to assess snow avalanche hazard. Today, most popular numerical approaches adopt depth-averaged equations. These methods are computationally efficient but limited in capturing processes occurring in the flow depth direction, e.g. erosion or deposition, which are often considered using ad hoc parameterizations or neglected completely. However, processes such as snow erosion, can crucially influence flow dynamics and run-out and are often not negligible. We address these issues by using a new three-dimensional (3-D) model, based on the material point method and finite strain elastoplasticity. To assess the possibilities and challenges associated with these highly detailed but computationally expensive calculations, we simulated the ‘Salezer’ snow avalanche that released in Davos, Switzerland in 2019. To reproduce the event in our simulations, we use the release areas mapped in a photogrammetric drone survey and estimate the snow conditions on the day of the event. We compare macroscopic features, such as flow outline and snow deposition of the simulated avalanche to field observations. An in-depth analysis of transient 3-D flow structures at the avalanche head not only demonstrates the degree of physical detail in the model, but also highlights challenges which still need to be addressed.

Runaway Electrons Emitted by Electron Avalanches in Nanosecond Discharges in Air

Nanosecond electric discharges in atmospheric air at high overvoltages are considered. Plasma channels and diffuse plasma glow simultaneously exist in these discharges. Channels are created by subnanosecond streamers formed in the cathode region. Diffuse glow is caused by runaway electrons emitted by electron avalanche heads. The necessary electric field enhancement is created due to the plasma configuration in the avalanche head and body.

Monte-Carlo simulation of the ionization processes for discharges in the left branch of the Paschen curve

The paper deals with the development of the simulation methods applied to the low-pressure discharges in which the reduced electric field E/p is extremely high and there exists a problem for the correct description of the ionization processes in the classical electron avalanches. Here we use the PIC/MC code xoopic for calculation of the impact ionization coefficient α, the electron drift velocity and the electron diffusion coefficient D in the electron avalanches in nitrogen. The range of calculations covers the ratio E/p ≤ 1000 V·(cm·Torr)−1. The simulation method was tested on a simple analytical model of an electron avalanche and showed good agreement in low and moderate E/p. It is shown that in conditions of high E/p the spatial distribution of electrons in the avalanche head based on the concepts of electron mobility during their drift motion and diffusion coefficient is not quite correct. The work confirmed that at the very low pressures pre-breakdown phenomena and the transition to the next stage of high-current breakdown cannot be based on ideas about classical electron avalanches.

Measurement of snow particle size and velocity in avalanche powder clouds

ABSTRACTParticle size, particle speed and airflow speed have been measured in the powder snow clouds of avalanches to investigate the suspension and transportation processes of snow particles. The avalanches were artificially triggered at the Lautaret full-scale avalanche test-site (French Alps) where an ultrasonic anemometer and a snow particle counter were setup in an avalanche track for measurements. Relatively large particles were observed during passage of the avalanche head and then the size of the particles slightly decreased as the core of the avalanche passed the measurement station. The particle size distribution was well fitted by a gamma distribution function. A condition for suspension of particles within the cloud based on the ratio of vertical velocity fluctuation to particle settling velocity suggests that the large particles near the avalanche head are not lifted up by turbulent diffusion, but rather ejected by a process involving collisions between the avalanche flow and the rough snow surface. Particle speeds were lower than the airflow speed when large particles were present in the powder cloud.

Uniformity of dielectric barrier discharges using mesh electrodes

In this paper, the uniformities of dielectric barrier discharges (DBDs) produced by different mesh electrodes are studied. If the aperture of the mesh electrode is long enough, the discharge spots are periodic. If the aperture is smaller than 0.5 mm, the discharge spots will be distributed randomly and even more densely than when produced by a planar electrode. Because the evaluation of the uniformity of the DBD, which consists of a large number of microdischarges, is different from that of a glow DBD or a Townsend DBD, the coefficient of variation (CV) of the gray level distribution of the discharge light emission is used to describe the uniformity of the DBD. The smaller the value of CV, the more uniform is the discharge. The experimental results show that the uniformity of the DBD produced by some mesh electrodes is even better than that by a planar electrode if the aperture of the mesh electrode is small enough. On the other hand, a new dimensionless scale invariant (h), which includes the combined effect of the aperture and the normalized change rate of the field strength of the avalanche head, is presented to analyze the physical mechanisms leading to a relatively uniform DBD according to the condition of the slow development of many avalanches at a low electric field. The calculated results show that the variation trend of h for different mesh electrodes is almost the same as that of the CV. This means that the dimensionless scale invariant h can be used to study the discharge uniformity.

Formation of levees and en-echelon shear planes during snow avalanche run-out

AbstractSnow avalanches often form levees and en-echelon shear planes in the run-out zone. We describe the formation of these depositional structures using a simple model that accounts for the role of granular fluctuations in avalanche motion. A mathematical feature of this model is the existence of a bifurcation saddle point, describing how granular fluctuations control the avalanche velocity in the runout zone. The saddle point discriminates between a flowing and stopping regime and defines the physical boundary between the flow and non-flow regions of the avalanche, i.e. the location of shear planes in the avalanche deposits. The formation of a shear plane depends on the interplay between terrain slope and avalanche mass flux, which varies from avalanche head to tail. Levees can form immediately at the avalanche front or, for steep slopes and low mass fluxes, at the avalanche tail. At ravine and gully shoulders the mass flux is restricted, thus initiating levee formation. We find that the levee lines are parallel to the flow direction when the mass flux is constant; en-echelon shear lines occur when the mass flux is decreasing. We test the model using several case studies where we have accurate laser scans of avalanche deposits. Our results suggest that avalanche flow parameters can be determined from simple levee measurements or, conversely, formation of levees and flow fingers can be predicted once the parameters governing the granular fluctuations are known.

Role of pore pressure gradients in sustaining frontal particle entrainment in eruption currents: The case of powder snow avalanches

[1] We present a model that underscores the role played by the porous snow cover in sustaining large, rapid, dilute powder avalanches over weakly cohesive snow. The model attributes massive localized material injection into the avalanche head to synergistic pressure gradients established within the porous cover by the very static pressure field that this influx induces along the pack surface. Treating massive frontal snow entrainment as a source of fluid, we show that static pressure time-histories recorded at the Vallee de la Sionne (Switzerland) conform to the classical two-dimensional Rankine half-body flow field. We calculate pore pressure within the snow cover and, from the resulting pressure gradient, find stresses on a vertical failure plane. After inferring an upper bound for snow cohesion from pressure records, we derive a sufficient condition for steady failure that sets the depth through which the cover changes from porous solid to fluidized suspension. Fluidization of the top surface imposes another relation among maximum density, internal friction and cohesion of the pack, maximum cloud size and minimum avalanche speed. Altogether, these conditions dictate which snow covers can produce powder snow avalanches. We suggest how similar “eruption currents” sustained by massive frontal entrainment may be relevant to other fluid-particle suspensions.

Controlling branching in streamer discharge by laser background ionization

Irradiation with a KrF laser controlled the positive streamer branching in atmospheric argon gas. This laser irradiation changed the amount of background ionization before the streamer discharge. Measuring the ionization current allowed us to evaluate the initial electron density formed by the KrF laser. We observed characteristic feather-like branching structure and found that it was only suppressed in the irradiated region. The threshold of ionization density which can influence the branching was evaluated to be 5 × 105 cm−3. The relationship between the size of avalanche head and mean distance between initial electrons explained this suppression behaviour. These experimental results support that the feather-like structure originates from the branching model of Loeb–Meek, a probabilistic merging of individual avalanches.

Snow avalanche flow-regime transitions induced by mass and random kinetic energy fluxes

AbstractWe perform a linear-stability analysis of a system of ordinary differential equations describing the motion of dense flowing avalanches. The depth-averaged equations relate the mean translational velocity of the avalanche to the production of turbulent kinetic energy associated with the random motion of the snow granules. Flow friction is described by an extended Voellmy model, where both the Coulomb and turbulent resistance are related to the random kinetic energy. We identify two snow avalanche flow regimes: (1) an unstable frictional flow regime characterized by low random kinetic energy production and (2) a stable, collisional flow regime at the avalanche front. Flow-regime transitions are governed by the production of random kinetic energy, which is controlled by mass. The unstable regime is characterized by a saddle point which the avalanche encounters at the onset of motion and deposition. Depending on the release mass and/or mass growth, the avalanche head will either fluidize, leading to far-reaching avalanches, or starve and die out. At the tail of the avalanche this saddle point also controls the commencement of deposition and therefore the stopping behaviour of avalanches. We discuss the conditions for flow-regime transitions and the enhanced mobility of snow avalanches.

Frictional relaxation in avalanches

AbstractWe use velocity profile measurements captured at the Vallée de la Sionne test site, Switzerland, to find experimental evidence for the value of extreme, Voellmy-type runout parameters for snow avalanche flow. We apply a constitutive relation that adjusts the internal shear stress as a function of the kinetic energy associated with random motion of the snow granules, R. We then show how the Voellmy dry-Coulomb and velocity-squared friction parameters change (relax) as a function of an increase in R. Since the avalanche head is characterized by high random energy levels, friction decreases significantly, leading to rapidly moving and far-reaching avalanches. The relaxed friction parameters are near to values recommended by the Swiss avalanche dynamics guidelines. As the random kinetic energy decreases towards the tail, friction increases, causing avalanches to deposit mass and stop even on steep slopes. Our results suggest that the Voellmy friction model can be effectively applied to predict maximum avalanche velocities and maximum runout distances. However, it cannot be applied to model the full range of avalanche behaviour, especially to find the distribution of mass in the runout zone. We answer a series of questions concerning the role of R in avalanche dynamics.

A comparison of powder-snow avalanches at Vallée de la Sionne, Switzerland, with plume theories

AbstractPowder-snow avalanches are natural hazards which affect the way populations live in mountainous areas. Field measurements from avalanches remain one of the most significant and useful sources of information about their dynamics and behaviour. In this paper, we consider all the video data from the Swiss Vallée de la Sionne test site from the years 2003–05. General scaling laws are sought for the avalanche front velocity based on plume theories. Avalanche Froude numbers are found, comparing three different length scales: the cube root of the fracture volume; the avalanche height; and the depth of entrained snow cover. We discuss the difficulties in defining the volume of a powder-snow avalanche: should we include just the head or also the turbulent wake that extends back to the starting zone? This relates to whether we use a compact model for the avalanche, such as the KSB model (Ancey, 2006; Turnbull and others, 2006) or a plume model (Turner, 1973). Observations are made regarding the lateral spreading behaviour of the avalanches. We show that the slow lateral spreading can be explained by large internal velocities and anisotropic turbulence generated by the large-scale motion in the avalanche head.

Calculation on runaway process of high-energy fast electrons under nanosecond-pulse

The runaway breakdown model induced by fast electrons is promising in explaining the nanosecond-pulse breakdown. In this paper, runaway process and collision ionization of fast electrons are discussed. Based on the relations between the electron energy and effective retarding force, the evolution of injected electron energy as a function of distance away from the avalanche head was simulated. The higher applied electric filed strength is, the lower the runaway energy threshold is and the more fast electrons can runaway, and gas pressures affect the runaway process of fast electrons greatly. Moreover, the runaway breakdown process under the high-voltage nanosecond pulse is described qualitatively.

The presence of a local minimum in the dependence of breakdown delay time on magnetic field strength

The experimentally observed nonmonotonic dependence of the breakdown delay time on the magnetic field strength is analyzed theoretically. Calculations with allowance for both an increase in the electric field in the avalanche head because of the change in the diffusion coefficients and an apparent increase in the pressure in the presence of a magnetic field demonstrate that such a dependence is indeed possible.

Ping-pong ball avalanche experiments

AbstractPing-pong ball avalanche experiments have been carried out for the last 3 years at the Miyanomori ski jump in Sapporo, Japan, to study three-dimensional granular flows. Up to 550 000 balls were released near the top of the landing slope. The balls then flowed past video cameras positioned close to the flow, which measured individual ball velocities in three dimensions, and air-pressure tubes at different heights. The flows developed a complicated three-dimensional structure with a distinct head and tail, lobes and “eyes”. “Eyes” have been observed in laboratory granular flow experiments, and the other features are similar not only to snow avalanches” but also to other large-scale geophysical flows. The velocities attained showed a remarkable increase with the number of released balls. A power law for this relation is derived by similarity arguments. The air-pressure data are used to deduce the structure of the airflow around the avalanche and, in conjunction with the kinetic theory of granular matter, to estimate the balance of forces in the avalanche head.

The effect of air-bearing surface roughness on avalanche test

The traditional method of glide height measurement is used for avalanche height evaluation. The air-bearing surface (ABS) of the avalanche head is assumed to be perfectly flat. This assumption, however, is not appropriate for today’s drive. In addition, a glide avalanche head normally has a different surface finish than that of a real recording head. So what an avalanche head is “seeing” on the disk surface may not be what a real head “sees” in the drive. Therefore, it becomes extremely important that one can correctly report the true avalanche height. In this study, we proposed one possible way of doing so by correctly calibrating the flying height of a glide head and the defining glide avalanche contact as “0.02% bearing depth.”

Laboratory experiments on the dynamics of powder-snow avalanches in the run-out zone

AbstractWe report on laboratory experiments on the motion of powder-snow avalanches along a bent chute. The avalanches are simulated as turbulent boundary-layer flows of polystyrene particles in still water along a chute consisting of a straight inclined part, a curved part and a second, possibly inclined, run-out zone. An ultrasonic measuring technique is used to determine mean particle speeds (via the Doppler shift of the reflected signal) and the particle concentration (via the attenuation of the echoes). By ensemble averaging, individual turbulence effects are eliminated. As measuring procedures,profileswere determined for particle velocity and density across the boundary layer; these were taken (i) for the avalanche tail along the entire track, i.e. in the steep part and the run-out zone, and (ii) for the avalanche head in the run-out zone below the kink in the terrain. Moreover,time sectionsof velocity and density (i.e. time series at fixed positions along a line through the boundary layer) were recorded and the particle mass deposited in the various zones of the track was measured.The analysis of the data reveals the following results: a concave change in the terrain topography acts as a very efficient mechanism for particle sedimentation, thus affecting particle concentration and velocity, and considerable reduction close to the ground, whereas the density is reduced throughout the depth, thus leading to a considerable reduction of dynamic pressure close to the ground and leaving it somewhat greater at higher altitudes. We conclude with practical considerations for the field glaciologist.

Laboratory experiments on the dynamics of powder-snow avalanches in the run-out zone

AbstractWe report on laboratory experiments on the motion of powder-snow avalanches along a bent chute. The avalanches are simulated as turbulent boundary-layer flows of polystyrene particles in still water along a chute consisting of a straight inclined part, a curved part and a second, possibly inclined, run-out zone. An ultrasonic measuring technique is used to determine mean particle speeds (via the Doppler shift of the reflected signal) and the particle concentration (via the attenuation of the echoes). By ensemble averaging, individual turbulence effects are eliminated. As measuring procedures, profiles were determined for particle velocity and density across the boundary layer; these were taken (i) for the avalanche tail along the entire track, i.e. in the steep part and the run-out zone, and (ii) for the avalanche head in the run-out zone below the kink in the terrain. Moreover, time sections of velocity and density (i.e. time series at fixed positions along a line through the boundary layer) were recorded and the particle mass deposited in the various zones of the track was measured.The analysis of the data reveals the following results: a concave change in the terrain topography acts as a very efficient mechanism for particle sedimentation, thus affecting particle concentration and velocity, and considerable reduction close to the ground, whereas the density is reduced throughout the depth, thus leading to a considerable reduction of dynamic pressure close to the ground and leaving it somewhat greater at higher altitudes. We conclude with practical considerations for the field glaciologist.

Computation of corona onset using the ring-charge method

A new method of computing the corona-onset voltage, in wire-plane systems, is described. It is based on the principle that the positive ions, formed during the growth of the avalanche, are concentrated in circular rings located at places where ionisation collisions are believed to occur, and the number of electrons at the avalanche head is doubled. Onset level is reached when the resultant electric field at the anode tip drops below a certain value. The method has been successfully applied to wire-plane configurations subjected to positive d.c. stress in atmospheric air. Results are in excellent agreement with those found in the literature.

Development of an electron avalanche in a gas in the nanosecond range

1. A mathematical model has been constructed for the development of an avalanche in a uniform field; in contrast with previous models, this model takes into account electrostatic repulsion, the nonlinear field dependence of the collisional-ionization coefficient, and the radial component of the field. 2. The experimental minimum [9, 10] of the dependence of the effective collisional-ionization coefficient on the number of electrons in the avalanche has been confirmed theoretically. 3. In air, with E/p = 132 V/cm · torr, p = 760 torr, E = 105 V/cm, and with considerable multiplication, the avalanche head has a velocity 1.5 times the initial drift velocity. 4. In a developing electron cloud there is a region in which the collisional ionization and electron drift velocity are relatively high (the head of the cloud), and there is one in which they are relatively low (the tail). 5. The maximum expansion velocity is ∼10 times as great as the diffusive-expansion velocity, because of electrostatic repulsion of avalanche electrons. However, the expansion velocity is roughly half the drift velocity, or one-third the velocity at which the head of the electron cloud moves toward the anode.

Investigation of Light Emission by Electron Avalanches in Binary Mixtures Where Xenon is the Parent Constituent. Attempted Interpretation of the Visible Continuum Emitted by Xenon

The field of use of gas detectors has been considerably extended through the addition of a third electrode in a parallel plane geometry. Apart from the possibility of obtaining spectrometric performances, pictures of objects emitting an X radiation may be obtained. Detection efficiency can be important when the filling gas is xenon which has a high atomic number. A polyatomic vapour must be added to ensure great electronic amplification. The localization data is provided either by a spark or by an electron avalanche. In this study we were interested in the light accompanying the electron avalanche. In this proportional mode, two kinds of information are available (a) the energy information given by the number of electrons of the avalanche head and (b) the spatial information from the light emitted during the avalanche. A coincidence measurement was made between the electric information constituted by the electrons of the avalanche head and the light information coming from the output slit of a grating monochromator. Coincidence information, which varies numerically in proportion to the number of avalanches per second, is sent to a recorder whose speed is co-related with the rotation speed of the grating. Minute by minute automatic zero reset of the recording enables a picture to be obtained by means of wave length intervals whose width is governed by the rotation speed of the grating.