Fast Versus Slow Avalanche Impact Dynamics: Insights from Measurements at Lautaret Pass Avalanche Test-Site, France (original) (raw)
Related papers
Journal of Geophysical Research, 2008
A fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures. This task is complicated because snow avalanches flow in a variety of regimes, primarily depending on snow temperature and moisture content. In this paper we address this problem by analyzing measured impact pressures, flow velocities, and flow depths of five Vallée de la Sionne avalanches. The measurements are made on a 20 m high tubular pylon instrumented with high-frequency pressure transducers and optoelectronic velocity sensors. In the observed avalanches, we find both subcritical and supercritical flow regimes. Typical Froude numbers were smaller than 6. The subcritical regime (Fr < 1) is characterized by a flow plug riding above a highly sheared basal layer. The measured pressures are large and velocity-independent in contradiction to calculation procedures. Pressure fluctuations increase with flow depth, indicating a kinematic stick-slip phenomena which is largest at the basal layer. Supercritical flow regimes (1 < Fr < 6) are characterized by a sheared flow all over the avalanche depth. In this regime the impact pressure is velocity-dependent. We derive relationships governing impact pressure as a function of the Froude number, and therefore flow regime, encompassing all the observed avalanches.
Effects of flow regime and sensor geometry on snow avalanche impact-pressure measurements
Journal of Glaciology, 2011
Impact pressures of snow avalanches have been measured at the Swiss Vallée de la Sionne experimental test site using two kinds of sensor placed at different locations in the avalanche flow. Pressures measured in a fast dry-snow avalanche and a slow wet-snow avalanche are compared and discussed. The pressures recorded using the two types of sensor in the dense flow of a dry-snow avalanche agree well, showing negligible dependence on the measurement device. On the other hand, significantly different pressures are measured in the slow dense flow of a wet-snow avalanche. This is attributed to the slow drag and bulk flow of this type of avalanche, leading to the formation and collapse of force-chain structures against the different surfaces of the sensors. At a macroscopic scale, limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor, according to a Mohr–Coulomb failure criterion....
Experimental study of dense snow avalanches: velocity profiles in steady and fully developed flows
Annals of Glaciology, 2004
In order to study channelled snow flows over rough surfaces, a laboratory-scale experiment was installed at the “Col du Lac Blanc”, a 2800 m high pass in the French Alps, near the Alpe d’Huez ski resort. It consists of a 0.2 mwide, 10 m long channel fed with snow by a motorized hopper. Both the slope of the channel and the feeding rate of the hopper can be modified. Sensors in the channel provide measurements of the velocity profile, the flow height and the shear and normal stresses at the bottom of the flow. Velocity profiles for different slopes are presented in this paper. Results indicate the presence of a highly active layer at the bottom. This layer is mainly responsible for the avalanche velocity, while the upper layer has a much smaller velocity gradient. A first interpretation of both layers is given.
On snow avalanche flow regimes: Inferences from observations and measurements
Mixed dry-snow avalanches are commonly thought to consist of a dense core and a dilute suspension layer, even though observations and measurements from Canada and Russia have long indicated the presence of an intermediate-density layer ("light flow" or "saltation layer"). We summarize field observations and measurements from Norway and Switzerland, both from spontaneous events and from avalanches released at the test sites Ryggfonn and Valí ee de la Sionne. Deposition patterns, high-frequency impact pressure records and radar measurements show that a substantial mass fraction of mixed dry-snow avalanches is flowing in this "fluidized" regime, par-ticularly the head. Based on mechanical considerations, we suggest close correspon-dence with the grain-inertia regime observed in granular flows; however, the role that the interstitial air plays in avalanches is not clarified at present. Distinguishing between three avalanche flow regimes instead of only two ...
Observations of the dynamic structure of snow avalanches
Annals of Glaciology, 1993
During two winters, 1990–92, the dynamic structures of snow avalanches were studied in western Norway. Artificially released wet-snow avalanches ran down the avalanche chute and stopped in front of the retaining dam. Running velocity distributions were obtained not only by video tape recorder, but also by various other recording instruments. Internal velocity was derived for the last avalanche by frequency analysis of impact pressure and optical sensor data. The vertical velocity shear of the avalanche flow has been estimated to be in the range 1–10 s−1.
Cold Regions Science and Technology, 2008
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 transducers, sampling at 7.5 kHz with circular diameters of 0.05, 0.10 and 0.25 m 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 biaxial sensors and compared to the pressure recorded by the single pressure cells. On a small concrete wall, a 1 m 2 pressure plate mounted with 4 load-gauge bolts measured normal forces. At six locations 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 avalanche structure to obtain load distribution and size effects for different avalanche typologies. Measured pressures are compared to existing guidelines procedures. It is shown that actual calculation formulas are not able to properly reproduce the measured pressure values and the load distribution.
The full-scale avalanche test-site at Lautaret Pass (French Alps)
Cold Regions Science and Technology, 2015
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 (b1000 m 3). 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
Measured shear rates in large dry and wet snow avalanches
Journal of Glaciology, 2009
We present estimates of internal shear rates of real-scale avalanches that are based on velocity measurements. Optical velocity sensors installed on the instrument pylon at the Swiss Vallée de la Sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches. Possible sources of error in the correlation analysis of the time-lagged reflectivity signals measured by optical sensors are identified for real-size avalanches. These include spurious velocities due to noise and elongated peaks. An appropriate choice of the correlation length is essential for obtaining good velocity estimates. Placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data. Coherent signals are found only in the dense flowing cores. We observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche. At the front, large shear rates are found throughout the de...
2018
Recent advances in full-scale avalanche measurements have led to a better understanding of the avalanche flow dynamics. However, the processes involved in pressure build-up on obstacles in the various flow regimes are still elusive. From full-scale experiments it is well established, that in the inertial flow regime, which is mostly typical of fast and cold avalanches, the pressure is proportional to square velocity. The gravitational regime is often observed for warm/wet snow avalanches and features a linear pressure variation with flow depth. It is still unclear how to estimate the coefficients of proportionality, which are needed for the pressure calculation, namely the drag coefficient and the amplification factor in the inertial regime and in the gravitational regime, respectively. In order to investigate the origin of the amplification factor and the drag coefficient, we developed a model based on the Discrete Element Method (DEM), which allows us to simulate the interaction b...
The full-scale avalanche test site, Lautaret, France
The Lautaret full-scale avalanche test site in the southern French Alps has been used by IRSTEA (Cemagref) Research Institute since 1973. Over the recent years two avalanche paths are used to release small to medium avalanches 3 or 4 times each winter. Avalanche flows are generally dense, whether wet or dry, sometimes with a powder part. Main path n°2 (track length 800 m) is dedicated to 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 starting zone is quantified by a differential analysis of laser scanning measurements set before and after triggering. A high rate positioning of the avalanche along the track is determined from terrestrial oblique photogrammetry. Above the dense layer, the saltation layer and the powder part are characterized by particles and air fluxes measurements. In path n°1 smaller in size, medium-size avalanches (track length 500 m) make this track of particular interest for experiments on structures. A macroscopic sensor-structure is set nearly 150 m downhill from the starting zone, that is, in the area where avalanches generally reach their maximum velocity. It consists is a one square-meter plate supported by a 3.5 m high steel cantilever fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the cantilever deformations, while avalanche velocity is measured from optical sensors. Seismic signals generated by avalanches of those 2 paths are recorded by a 3-axial broadband seismometer. Around those experimental devices dedicated to the understanding of avalanche physics, a national and international partnership has been developed from years to years, including INSA de Lyon, CNRS and Université Joseph Fourier (France), Aalto University (Finland), Nagoya University (Japan), Boku University (Austria), IGEMA (Bolivia), OGS (Italy)