Avalanche Test Site Research Articles

Cold-to-warm flow regime transition in snow avalanches

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.

Four decades of observations from NGI's full-scale avalanche test site Ryggfonn—Summary of experimental results

The Norwegian Geotechnical Institute (NGI) has run full-scale avalanche experiments at the Ryggfonn test-site in Western-Norway for close to 40years. The construction of an avalanche catching dam in 1981 laid the cornerstone of the so-called “Ryggfonn project”. Over these years of operation, various kinds of instrumentation and structures have been placed along the avalanche path to gain in-depth understanding of avalanche dynamics and the interaction of avalanches with structures. Avalanche measurements provide benchmarks for the development and calibration of numerical avalanche models. Increasingly, these models are used for hazard zoning to estimate runout distances or impact pressures with varying return periods for assessing endangered areas. However, these models are imperfect and require a high degree of expert judgment for specifying the required model parameters. At this point, it is valuable to have reference events to evaluate simulation results.In this paper, we summarize runout, velocity, and impact pressure observations from the Ryggfonn test site as well as some derived quantities such as the retarding acceleration or density estimates. We try to relate the measurements to ambient (in-situ) conditions during the events. For Ryggfonn, the runout observations suggest a 10 to 15% increase in runout distance comes along with a decrease of a factor 10in probability. The expected front velocity of an avalanche as it enters the runout area at Ryggfonn is about 27 ± 10ms−1, but may reach 50ms−1. The impact pressures at the beginning of the runout area are typically of the order of 100kPa, but may reach several hundred kPa. Observations from a power line assembly give some insight in the vertical pressure distribution and its dependency of the avalanche velocity. The combination of these measurements provides some implications for the avalanche density suggesting a range of flowing densities. Our observations affirm a dependency of dynamical parameters on the ambient conditions, which were made at other locations.

The full-scale avalanche test-site at Lautaret Pass (French Alps)

The full-scale avalanche test site at Lautaret Pass in the southern French Alps has been used by IRSTEA-Cemagref Research Institute since 1972. Over recent years, two avalanche paths have been used routinely to release avalanches and study avalanche dynamics and interactions between avalanches and obstacles. Avalanche flows are generally dense and dry, sometimes with a powder cloud on top. Main avalanche path no. 2 is dedicated to studies on avalanche dynamics. Within the flow of the avalanche, flow height and vertical profiles of pressure and velocity are measured along a 3.5m tripod. The snow volume released in the release zone is quantified by differential analysis of laser scanning measurements performed before and after triggering. High-speed positioning of the avalanche front along the track is carried out by terrestrial oblique photogrammetry. Above the dense layer, the upper layer of the avalanche is characterized by particle and air flux measurements. Avalanche path no. 1 is smaller in size and particularly well-suited to experiments on structures exposed to small to medium-size avalanches (<1000m3). A macroscopic sensor structure consisting of a one square-meter plate supported by a 3.5m high steel cantilever beam is fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the beam deformations and avalanche velocity is measured by optical sensors. For these experimental devices dedicated to improving our understanding of avalanche physics, a national and international partnership has been developed over the years, including INSA de Lyon, CNRS and Université Joseph Fourier (France), Aalto University (Finland), Nagoya University (Japan), Boku University (Austria) and IGEMA (Bolivia).

Two-dimensional radar imaging of flowing avalanches

Radar has emerged as an important tool in avalanche research. However, existing radar sensors suffer from coarse range resolution capabilities. This limits the usefulness of the data they collect in validating models of avalanche dynamics. This paper details the development of a frequency modulated continuous wave, phased array radar, and its associated signal processing, for non-invasive measurements of entire avalanche events. The radar outperforms existing avalanche radar sensors in terms of range resolution, and it provides cross-range resolution using a phased array receiver. The radar has been operating at the Vallée de la Sionne avalanche test site in Switzerland since the 2010 winter season. It has successfully gathered measurements of entire natural avalanche events. In this paper we show two-dimensional radar images of a naturally occurring avalanche, the first of their kind, which reveal movements of layers or particles of the flowing avalanche in unparalleled detail. Furthermore, the potential of the measured data is shown with tracking of avalanche fronts in two spatial dimensions. This marks an important step towards providing a library of high-quality avalanche measurements to improve our knowledge of avalanche dynamics.

A new experimental snow avalanche test site at Seehore peak in Aosta Valley (NW Italian Alps) ? Part II: Engineering aspects

The estimate of the effects produced by the impact of a snow avalanche against an obstacle is of the utmost importance in designing safe mountain constructions. For this purpose, an ad-hoc instrumented obstacle was designed and built in order to measure impact forces of small and medium snow avalanches at Seehore peak (NW Italian Alps). The structural design had to consider several specific and unusual demands dictated by the difficult environment. In this article, the new test facility is described from the engineering point of view, discussing the most important aspects of the analyzed problems which were solved before and after the construction. The performance of the instrumented obstacle in the first two operating seasons, and some proposals for future upgrading are eventually illustrated.

A new experimental snow avalanche test site at Seehore peak in Aosta Valley (NW Italian Alps)—part I: Conception and logistics

Abstract Experimental test sites are important to understand the physical flow processes occurring within an avalanche. They can be at different scale: laboratory or real scale. In Europe, a dozen of real-scale test sites have been realized, though at present only a few of them are still operative. The primary goal is to measure avalanche dynamical variables under controlled conditions. Not all the observed phenomena have been fully understood yet and more work is needed especially regarding small to medium avalanches. The aim of this paper is to present a new experimental test site at a real scale in order to study specific topics: the dynamics of small and medium avalanches, the avalanche release processes and the interaction between avalanche flows and obstacles. The test site, called Seehore, is located in Aosta Valley in the North-western Italian Alps. The slope, with an elevation difference of about 300 m (from 2300 to 2570 m asl), has a mean slope angle of about 28°. Avalanches are artificially released on a routine basis to secure the ski-runs, as the site is located within a ski resort (Monterosa Ski); they are usually dense slab avalanches of small or medium size, but also a powder cloud may occasionally form. The site is instrumented with a steel obstacle, described in details in a companion paper, which measures the effects of avalanches impacting on it. Before, during and after each artificial release physical properties of the snow in the avalanche release, track and deposition zones are recorded; front velocity, erosion and deposition mass are estimated by field surveys or by videogrammetry and terrestrial laser-scan measurements.

Actions of snow avalanches on a protection gallery

Better knowledge of the actions of snow avalanches on slightly inclined protection galleries such as snow sheds is vital for improving the design of such structures. Experimental measurements on a plate equipped with force sensors were carried out on a full-scale avalanche test site at Col du Lautaret (French Alps). For each artificial release of an avalanche, the impact effect and the resulting temporal variations were measured. This real-scale experiment was completed by laboratory-scale experimental tests using granular flows in an inclined channel that identified the relation between load and slope change. These experiments allowed us to measure the granular temperature of the flow. Finally, to assess the dynamic effect of the avalanche loading on the structure, numerical simulations of a real gallery were conducted with a simplified multifiber model, taking into account the tempo-spatial evolutions of the impact pressure observed during experiments.

Sensitivity of snow avalanche simulations to digital elevation model quality and resolution

AbstractDigital elevation models (DEMs), represent the three-dimensional terrain and are the basic input for numerical snow avalanche dynamics simulations. DEMs can be acquired using topographic maps or remote-sensing technologies, such as photogrammetry or lidar. Depending on the acquisition technique, different spatial resolutions and qualities are achieved. However, there is a lack of studies that investigate the sensitivity of snow avalanche simulation algorithms to the quality and resolution of DEMs. Here, we perform calculations using the numerical avalance dynamics model RAMMS, varying the quality and spatial resolution of the underlying DEMs, while holding the simulation parameters constant. We study both channelized and open-terrain avalanche tracks with variable roughness. To quantify the variance of these simulations, we use well-documented large-scale avalanche events from Davos, Switzerland (winter 2007/08), and from our large-scale avalanche test site, Vallée de la Sionne (winter 2005/06).We find that the DEM resolution and quality is critical for modeled flow paths, run-out distances, deposits, velocities and impact pressures. Although a spatial resolution of ∼25m is sufficient for large-scale avalanche modeling, the DEM datasets must be checked carefully for anomalies and artifacts before using them for dynamics calculations.

Velocity profile inversion in dense avalanche flow

AbstractWe present a velocity time series of a dry-mixed real-scale avalanche that was recorded at different heights on a mast at the Swiss Vallée de la Sionne avalanche test site. The dataset is of extraordinary quality in terms of completeness and spatio-temporal resolution, and allows the evolution of distinct flow phases of the avalanche to be traced. In particular, we found velocity profile inversions at the transition from the avalanche front to the avalanche body. We provide a tentative empirical description of the profile inversions and briefly discuss our observations in the light of theoretical considerations.

Impact energy of an avalanche on a structure

AbstractThe impact energy of an avalanche on the flat surface of an instrumented structure has been quantified by full-scale experiments performed at the Lautaret avalanche test site, France. The deformation and acceleration of the structure were measured during the avalanche. The impact energy of the avalanche was calculated from an energy balance via the mechanical energy theorem. First the elastic potential energy and kinetic energy of the structure were calculated from the measured deformations and an adequate mechanical model of the structure. Internal energy dissipation due to material damping and potential plastic deformation was calculated. Finally the mechanical work done on the structure by the avalanche forces was deduced from the energy balance. Results show that the elastic energy is the main component of the energy injected in the structure. The kinetic and viscous components are negligible because of the very low displacement rate of the structure during the impact. Another important result is that the maximum of the mechanical power (work rate) is done when the head of the avalanche impacts the structure. This occurs before the pressure reaches its nominal value, approximately when the product of the pressure by the pressure rate is maximum.

An instrumented structure to measure avalanche impact pressure: Error analysis from Monte Carlo simulations

Avalanche impact pressure on a flat surface has been quantified by full-scale experiments performed at the Lautaret avalanche test site (France). An inverse analysis method is used to reconstruct the pressure applied on this instrumented structure from the deformations recorded during the impact. The deformation-to-pressure transfer function of this large sensor is determined by analytical modelling and is used to quantify the pressure of an artificially released avalanche on the structure. Advanced sensitivity and error analyses are used to determine the uncertainty of the reconstructed pressure. First the uncertainties of the different parameters and processes involved are identified and estimated in the deformation-to-pressure transfer function through probability distributions. Then the sensitivity of the reconstructed pressures to these parameters is investigated to identify the most important parameters. Finally, the distribution of the uncertainty in terms of the probability density of the total error in the pressure is estimated using Monte Carlo simulations. Results are discussed on the basis of the drag coefficient to Froude number dependence and compared with previous avalanche data.

Avalanche impact pressure on an instrumented structure

Abstract Full-scale experiments have been conducted on the Lautaret avalanche test site (France, Hautes-Alpes) to quantify avalanche impact pressure. A structure with a flat surface is used as a large sensor and an inverse analysis procedure is developed to reconstruct the pressure applied on this obstacle. This method is validated by numerical simulations and in-situ impact hammer tests are performed to acquire the frequency transfer function of the sensor. Experimental measurements are carried out with the device during an avalanche and are processed to quantify the impact pressure. Results show that the loading rate can reach up to 100 kPa/s with a nominal pressure of 35 kPa for an avalanche with a front velocity of around 17 m/s and a density of around 100 kg/m3. The nominal impact pressure of this avalanche, with a Froude number of around 5, is roughly consistent with the kinetic pressure estimation. Interactions between the flow and the structure form a dihedral snow deposit upstream of the obstacle (stagnation zone), modifying the shape of the obstacle and reducing the hydrodynamic drag pressure, which is nevertheless mainly depending on the velocity of the flow. Results are discussed on the basis of the drag coefficient, Cr, to the Froude number, Fr, relationship which can be expressed as Cr = 10.8 Fr− 1.3 from the data.

Measurements and analysis of full-scale avalanche impact pressure at the Vallée de la Sionne test site

Impact pressures of eight snow avalanches measured at the Swiss avalanche test site Vallée de la Sionne are reported. Avalanche typologies varied between dense and powder. Measurements were performed on obstacles of different shape and dimension. High-frequency pressure sensors were mounted on a 20 m high tubular pylon and on a 5 m high steel wedge. To interpret the influence of sensor dimension on impact pressure measurements, the total pressure exerted on the steel wedge was recorded using two bi-axial sensors and compared to the pressure recorded by the high-frequency sensors. On a small concrete wall, a 1 m 2 pressure plate mounted with 4 load-gauge bolts measured normal and tangential forces. Along the tubular pylon (between 1–6 m above ground) optoelectronic sensors recorded the avalanche flow velocity. Flow depths were measured by mechanical sensors. Analysis of high resolution impact forces in combination with velocity measurements allowed us to reconstruct the flow structure. We combined impact pressure with observation on avalanche structure to obtain load distribution and size effects for different avalanche typologies. Measured pressures are compared to the existing Swiss calculation procedure. It is shown that actual calculation formulas are not able to properly reproduce the measured pressure values and the load distribution.

Comparison of avalanche-velocity measurements by means of pulsed Doppler radar, continuous wave radar and optical methods

Velocities of mixed dense-flow/powder-snow avalanches have been measured by means of pulsed Doppler radar and by continuous wave radar at the full scale avalanche test site Vallee de la Sionne. From the radar data, we derive velocities of the saltation layer and of the powder part of the avalanche. The results obtained by the two different radar measurement techniques are compared and also are checked against the velocity data obtained by opto-electronic velocity sensors installed at different heights on the 20 m high mast in the avalanche track and against videogrammetry velocity data. We demonstrate that the measurements are consistent and discuss how information about the avalanche structure can be derived from the measurements.

Kulikovskiy–Sveshnikova–Beghin model of powder snow avalanches: Development and application

A simple theoretical model, the Kulikovskiy–Sveshnikova–Beghin (KSB) model, is outlined, describing the motion of a particle cloud moving down an incline. This model includes both the entrainment of surrounding ambient fluid and the entrainment of particles from the slope and is equally valid for Boussinesq and non‐Boussinesq flows. However, this model can predict physically impossible densities when there is significant particle entrainment. Modifications to the model are proposed which eliminate this problem by including the entrained snow volume. With the modified model, physically realistic mean densities are predicted which have a significant impact on the Richardson number–dependent ambient entrainment. The improvements are illustrated by comparing analytical solutions to the original and the modified KSB equations for the case of a particle cloud traveling on a slope of constant angle, with constant ambient fluid and particle entrainment. Solving the modified model numerically, predictions are compared with data from several large powder snow avalanches at the Swiss Vallée de la Sionne avalanche test site. The modified KSB model appears to capture the dynamics of the avalanche front well; however, problems remain with relating the theoretical geometry to a real avalanche geometry. The success of this model in capturing the front dynamics shows that with careful assumptions that reflect the physics, it is possible to describe aspects of complex flows such as powder snow avalanches with simple models.

Air pressure data from the Vallée de la Sionne avalanches of 2004

Despite their clear danger to humans, snow avalanches are hard to document. They occur in inaccessible and dangerous locations, often at times of bad weather. Observation instruments frequently malfunction in the harsh conditions or are destroyed. Measurements of powder snow avalanches are particularly difficult, as these occur less frequently and are usually very large. To understand the air flow in front of and inside powder snow avalanches, we have designed an air pressure sensor to survive the harsh conditions. It consists of a differential pressure transducer, with a high‐frequency response, built into a specially designed housing unit. We mounted this 10 m from the ground on a measurement mast in the Vallée de la Sionne avalanche test site in Switzerland. Data from five powder snow avalanches over the winter of 2004 were recorded. Three of these were natural releases, and two were artificially triggered with explosives. We present an analysis of the sensor response and an interpretation of the signals in terms of simple flow fields. We show how these data can be used to deduce information about the speed, size, and location of the avalanches using a dipole approximation. Our sensor has two major limitations: The length of the internal tubing produces low‐frequency resonances, and there is only one transducer, so a complete flow model is needed to deduce the three velocity components and pressure. We discuss these limitations and give a design for a new sensor to overcome them.

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 &gt; 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 &gt; 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.

Particle densities, velocities and size distributions in large avalanches from impact-sensor measurements

AbstractIn winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1–19 m above ground were carried out in the upper run-out zone of the avalanche test site at Vallée de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and &gt; 3 m. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow),particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peak detection and correction for grazing impacts, pressure measurements will become premier tool for testing granular flow models.

IT contingency planning: A public sector perspective, part 2

The full-scale avalanche test site at Lautaret Pass in the southern French Alps has been used by IRSTEA-Cemagref Research Institute since 1972. Over recent years, two avalanche paths have been used routinely to release avalanches and study avalanche dynamics and interactions between avalanches and obstacles. Avalanche flows are generally dense and dry, sometimes with a powder cloud on top. Main avalanche path no. 2 is dedicated to studies on avalanche dynamics. Within the flow of the avalanche, flow height and vertical profiles of pressure and velocity are measured along a 3.5 m tripod. The snow volume released in the release zone is quantified by differential analysis of laser scanning measurements performed before and after triggering. High-speed positioning of the avalanche front along the track is carried out by terrestrial oblique photogrammetry. Above the dense layer, the upper layer of the avalanche is characterized by particle and air flux measurements. Avalanche path no. 1 is smaller in size and particularly well-suited to experiments on structures exposed to small to medium-size avalanches (< 1000 m3). A macroscopic sensor structure consisting of a one square-meter plate supported by a 3.5 m high steel cantilever beam is fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the beam deformations and avalanche velocity is measured by optical sensors. For these experimental devices dedicated to improving our understanding of avalanche physics, a national and international partnership has been developed over the years, including INSA de Lyon, CNRS and Université Joseph Fourier (France), Aalto University (Finland), Nagoya University (Japan), Boku University (Austria) and IGEMA (Bolivia).