Measurements of dense snow avalanche basal shear to normal stress ratios (S/N)

Abstract

[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,

Modeling mass‐dependent flow regime transitions to predict the stopping and depositional behavior of snow avalanches

“…We find S / N ratios that vary as a function of location in the avalanche (Figure 15d). This model can now be applied to model snow chute experiments where S , N and H have been measured [ Platzer et al , 2007]. Assuming some R ( t ) we can find frictional constants μ 0 and R 0 which reproduce the measured shear stress and S / N ratios (Figure 14).…”

“…Chute experiments have been conducted to measure the basal shear stress S and normal force N of snow [ Platzer et al , 2007] and granular [ Bartelt et al , 2007; Schaefer et al , 2010] avalanches. In these experiments force plates are positioned at a fixed location on the chute.…”

“…Example results from chute experiments with snow [see Platzer et al , 2007]. (a) Measured shear stress S as a function of measured normal stress N ; (b) Measured S / N as function of time.…”

The relation between dilatancy, effective stress and dispersive pressure in granular avalanches

“…N K can be negative if the center of mass is falling. Basal pressure measurements cannot distinguish between a change in mass M or a change in the location of the center of mass k because the total normal force N is the sum of N g and N K [24]. It is therefore difficult to determine the pressure N K experimentally.…”

“…Thus N K is the dispersive pressure, but defined where it can be identified in experiments, as a reaction at the basal surface; moreover N K ¼ Àp. The mean total normal stress N can be measured in chute experiments using force plates [24]. We emphasize that the volume dilation occurs in a field of constant acceleration (gravity) and therefore the dispersive acceleration can only be captured as a change in acceleration, or a jerk, at the basal boundary.…”