A fundamental problem in avalanche science is understanding the interaction between frictional processes taking place at the basal running surface and dissipative mechanisms within the avalanche body. In this paper, we address this question by studying how kinetic energy is dissipated into heat in snow avalanches. In doing so we consider the effect of random granular fluctuations and collisions in depth-averaged snow avalanche models. We show that relationships between the size of the granular fluctuations and the energy dissipated by granular collisions can be obtained by studying the energy input required to maintain steady-state flows. The energy input for granular fluctuations comes from mechanisms operating in the basal layer. The kinetic energy of the flow at the basal layer is converted to granular agitation energy, a random kinetic energy, which in turn is dissipated as heat by both viscous shearing and inelastic collisions at higher levels in the avalanche profile. Thus granular fluctuations play a crucial role in understanding the total dissipation process. We apply our theoretical considerations to develop a constitutive model for dense snow avalanches and are able to accurately model steady-state velocity profiles of both snow-chute experiments and field measurements.

[1] We investigate frictional processes at the basal shear layer of snow flows. A chute is instrumented with basal force plates, velocity and flow height sensors to perform experiments with dry and wet snow. We find that a MohrCoulomb relation of the form S = c + bN accurately describes the relation between normal (N) and shear stress (S). The Coulomb friction coefficient b ranges between 0.22 and 0.55. Several wet snow avalanches exhibited significant cohesion c % 500 Pa. These quantitative measurements of stress, velocity and flow height allow us to probe the relation between basal work, internal dissipation and gravitational potential energy. We find that basal shearing is the primary frictional mechanism retarding snow flows. This mechanism shows no velocity dependence, contrary to many postulated constitutive relations for basal shearing in snow avalanches.Citation: Platzer, K., P. Bartelt, and M. Kern (2007), Measurements of dense snow avalanche basal shear to normal stress ratios (S/N), Geophys. Res. Lett., 34, L07501,

The tail of an avalanche is characterized by diminishing flow heights. The decreasing flow heights are due to increased friction and therefore tails are ideally suited to investigate frictional mechanisms in avalanches. Using chute experiments with granular material, we observe two properties of avalanche tails: (1) Coulomb friction μ increases in proportion to the decrease in the gravitational work rate g; (2) flow heights h are proportional to the square of the basal slip velocity u0; h ∝ u02. Another non‐steady region can be observed at the front of the avalanche. Although this region is shorter than the tail, we were able to detect a hysteresis of the friction μ coefficient as a function of the gravitational work rate. This fact indicates a time dependence of the frictional mechanisms. The results explain why avalanches starve when they are not fed by the intake of additional material at the front.

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Slushflows are gravity mass flows consisting of a mixture of snow and water, which exhibit considerable damage potential for endangered areas. Small scale slushflows with a volume of 10-15 m3 were generated in the 30 m long and 2.5 m wide snow chute of the Swiss Federal Institute of Snow and Avalanche Research at Weissfluhjoch, Davos, Switzerland. Velocity profiles, dynamic pressure, basal and normal shear and flow height data were recorded in order to test suitable instruments for slushflow measurements. From the obtained data, the order of magnitude of the drag factor for slushflows interacting with obstacles could be estimated. We give an overview of the experimental setup and discuss experimental problems arising from the specific characteristics of slushflows. First results are presented, which indicate that the drag factor might be considerably higher than the estimates commonly used for dry flowing avalanches. Compared to snow avalanches, shear and normal stresses are generally higher in slushflows. The analysis of shear stress versus normal stress indicates some viscoplastic behavior. The results imply that slushflows have to be considered when choosing design criteria for avalanche protection measures wherever this kind of flows can occur. An increase in both temperature and winter precipitation could lead to more frequent slushflow events implying the need to redesign countermeasures. The results from the chute experiments are discussed with respect to development of numerical models of slushflows and a future adaptation of the optical velocity measurement devices to slushflows.

Full-scale field tests of dynamic rockfall have been performed on a flexible SPIDER Avalanche system to study the dynamic force distribution along the foundations under dynamic loading. Therefore, an anchor to measure dynamic tensile forces and a pile to measure dynamic compressive forces were each equipped with strain gauges. Furthermore, a static pull loading test with load steps of 1 min duration was performed on the anchor to highlight the difference between dynamic and static loading. Effective kinetic energies applied on the net of the SPIDER Avalanche system range from 25 to 492 kJ with impact velocities between 17 and 25 m/s. The results show that the dynamic forces close to the pile- and anchor head are higher and that they are decreasing with increasing distance of pile and anchor. However, the dynamic tensile force distribution is nonlinear over the length of the anchor, whereas the dynamic compressive force distribution is linear along the pile length. The comparison of static and dynamic tensile forces shows that dynamic tensile forces are depleted within a shorter distance of the anchor compared to the static tensile forces. Dynamic tensile forces present 25% less in value than the static tensile forces.

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