Measured shear rates in large dry and wet snow avalanches

Abstract

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.

Wet-snow avalanche interaction with a deflecting dam: field observations and numerical simulations in a case study

“…This assumption is in agreement with measured shear rates in wet-snow avalanche flow, which indicate that wet-snow avalanches are characterized by slow, plug flow which overrides a highly sheared basal layer (Kern et al, 2009;Sovilla et al, 2008). There are two major concerns related to the performance of the VS model.…”

“…The dynamics of wet-snow avalanches differ from those of dry-snow avalanches (Sovilla et al, 2008;Kern et al, 2009). They are slower than dry dense avalanches, but typically have larger flow depths and higher flow densities.…”

“…Recent investigations have shown that, in this flow regime, they can exert enormous forces in spite of their small velocities (Sovilla et al, 2010a(Sovilla et al, , 2008Baroudi et al, 2011). Furthermore, their flow is strongly influenced by rugged topography; they may spread out laterally forming finger and following small terrain irregularities regardless of the original flow direction, and may have a long run-out in spite of their low velocities (Sovilla et al, 2008;Kern et al, 2009). …”

Dense avalanche friction coefficients: influence of physical properties of snow

“…We may therefore expect that the frictional properties change along the covered distance. In addition, the avalanches involve mechanical and thermodynamical transformations and energy dissipation that may change the mechanical and thermodynamic properties of flowing snow (Buser and Bartelt, 2009). Accounting for all these complex processes in propagation models is currently out of reach.…”

“…To account for flow depth variations in time and space during an avalanche event, the hydraulic framework has been adopted and a second generation of models, based on depth-averaged hydraulic equations, has emerged (Kulikovskii and Eglit, 1973;Savage, 1979;Brugnot and Pochat, 1981;Bartelt and others, 1999;Barbolini and others, 2000;Naaim and others, 2004;Buser and Bartelt, 2009). These models usually include a Voellmy-like, two-parameter friction model, as well as some additional parameters accounting for curvature, erosion and deposition processes and for the anisotropy of normal stresses.…”