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

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 Mohr-Coulomb 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 approximate to 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.