[1] Snow entrainment alters the speed and hence the run-out distance of avalanches, yet little is known about this significant process. We studied entrainment in snow avalanches using observations from (1) the Swiss Vallée de la Sionne test site, (2) the Italian Pizzac site, (3) catastrophic avalanches that occurred during the winter 1998-1999 in Switzerland, and (4) a medium-sized spontaneous avalanche that occurred in 2000 in Davos, Switzerland. We determined mass and energy balances for 18 avalanche events. On average, the mass increased by a factor of 4. The primary mode of entrainment appeared to be frontal ploughing, although entrainment behind the avalanche front was also observed.Step entrainment, where a snow cover layer fractures and is entirely consumed by the avalanche, also occurred. Basal erosion was negligible. Mass availability and snow cover structure were the limiting factors governing entrainment. Other factors such as track topography and avalanche dimension played a secondary role. Using the experimental results, we introduced an entrainment model into a Saint-Venant type flow model where the internal shear deformation of the avalanche is governed by a Bagnold law and the shear stress at the basal layer is treated as a Voellmy fluid. The model with entrainment not only improves the prediction of the velocities and flow heights in comparison to measurements, but also reproduces the variations in run-out distances, which characterize avalanches with similar terminal velocities but different masses.Citation: Sovilla, B., P. Burlando, and P. Bartelt (2006), Field experiments and numerical modeling of mass entrainment in snow avalanches,

[1] 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.Citation: Sovilla, B., M. Schaer, M. Kern, and P. Bartelt (2008), Impact pressures and flow regimes in dense snow avalanches observed at the Vallée de la Sionne test site,

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 depth; at the tail, plug flows overriding highly sheared layers near the bottom of the flow are observed. The measured velocities change strongly with height above the ground and fluctuations around the measured mean velocity can be identified. We find that the dense flows are laminar, undergoing a transition from supercritical to subcritical flow behaviour from the head to the tail. Furthermore, we provide real-scale experimental evidence that the mean shear rate and the magnitude of velocity fluctuations increase with the mean discharge.

Five avalanches were artificially released at the Vallée de la Sionne test site in the west of Switzerland on 3 February 2015 and recorded by the GEOphysical flow dynamics using pulsed Doppler radAR Mark 3 radar system. The radar beam penetrates the dilute powder cloud and measures reflections from the underlying denser avalanche features allowing the tracking of the flow at 111 Hz with 0.75 m downslope resolution. The data show that the avalanches contain many internal surges. The large or “major” surges originate from the secondary release of slabs. These slabs can each contain more mass than the initial release, and thus can greatly affect the flow dynamics, by unevenly distributing the mass. The small or “minor” surges appear to be a roll wave‐like instability, and these can greatly influence the front dynamics as they can repeatedly overtake the leading edge. We analyzed the friction acting on the fronts of minor surges using a Voellmy‐like, simple one‐dimensional model with frictional resistance and velocity‐squared drag. This model fits the data of the overall velocity, but it cannot capture the dynamics and especially the slowing of the minor surges, which requires dramatically varying effective friction. Our findings suggest that current avalanche models based on Voellmy‐like friction laws do not accurately describe the physics of the intermittent frontal region of large mixed avalanches. We suggest that these data can only be explained by changes in the snow surface, such as the entrainment of the upper snow layers and the smoothing by earlier flow fronts.

GEOphysical flow dynamics using pulsed Doppler radAR (GEODAR), a custom radar system, images avalanches over the entire slope with high spatial and temporal resolution at the experimental test site Vallée de la Sionne in Switzerland. Between winter seasons 2009/2010 and 2014/2015, data have been acquired from 77 avalanches. These data sets describe a wide variety of avalanches, which we classify in terms of seven flow regimes and combinations thereof. These flow regimes expand on previous classifications, with four identifiable dense flow regimes (where interaction between granules and with the flow bed dominates dynamics) and two different dilute flow regimes (where interaction between snow particles and the air becomes dominant). There is a further regime identified where snow balls simply roll down the mountain. A cold dense regime and a warm shear regime behave like noncohesive granular flows with velocity shear throughout the flow. A sliding slab regime and a warm plug regime occur when cohesion dominates and causes the flow units to act as solid‐like objects sliding on a thin shear zone. An intermittent regime connects the cold dense regime with the suspension regime and is characterized by highly fluctuating density and surging activity. GEODAR enables localization of these flow regimes and transitions between them in time and space. We discuss flow regime transitions in terms of snow properties, topography, speed, and size of the avalanches. This paper also serves as a reference for the data set which is made publicly available and should prove to be an invaluable resource for the development of physically based avalanche models.

[1] The snow surface height was precisely measured, with a laser scanner, before and after the passage of two dry-mixed avalanches in Vallée de la Sionne during the winter of [2005][2006]. The measurements were used to calculate the depth of the deposited snow along each entire avalanche path with a height resolution of 100 mm and a horizontal resolution of 500 mm. These data are much more accurate than any previous measurements from large avalanches and show that the deposit depth is strongly negatively correlated with the slope angle. That is, on steep slopes the deposit is shallow, and on gentle slopes the deposit is deep. The time evolution of the snow depth, showing the initial erosion and final deposition as the avalanche passed, was also observed at one position using a frequency-modulated continuous wave radar. Measurements at a nearby position gave flow speed profiles and showed that the avalanche tail consists of a steady state subcritical flow that lasts for about 100 s. Eventually, the tail slowly decelerates as the depth slightly decreases, and then it comes to rest. We show that the dependency between the slope angle and the deposition depth can be explained by both a cohesive friction model and the Pouliquen h stop model.

No abstract

Since 1993 the Avalanche Centre of Arabba has managed a test site to determine avalanche-dynamics parameters (Sommavilla and others, 1997; Sommavilla and Sovilla, 1998). The system is located on an avalanche track which is representative for the Dolomites, northern Italy It monitors avalanche pressures, speed, flow height and variations of the avalanche shape and extent. In winter 1997/98, together with the standard measurements, a series of new field measurements and observations of the snow cover on the avalanche path were conducted for the first time in order to accurately determine the avalanche mass balance. The information collected is typical for dense flow avalanches which have small dimension and develop mainly along a channelled path. In winter 1997/98, four events were studied. For each event, in several sections from the starting zone to the deposition zone, manual measurements were carried out in order to investigate mass entrainment and deposition processes. The mass evolution of the avalanche has been determined. It is shown that the avalanche mass increases by up to 720% with respect to the initial release mass. This entrainment process is related to the speed reached by the avalanche front. In addition, it has been determined that during the acceleration phase of the avalanche front the underlying snow cover is mostly eroded and there is no deposition of snow. In the deceleration phase, by contrast, erosion decreases progressively, reaching the value zero, while deposition becomes progressively larger. These results underscore the importance of the mass balance as a fundamental component in avalanche-dynamics research.