Calculations of Avalanche Friction Coefficients from Field Data

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Abstract
The friction coefficients needed to solve Voellmy’s avalanche-dynamics equations and as input to the numerical, finite-difference computer program AVALNCH are calculated from case studies. The following coefficients of internal frictionvand of surface frictionfworked well for program AVALNCH: for midwinter dry snowv =0.5 to 0.55 m2/s andf= 0.5 to 0.55; for hard slabv =0.7 to 0.8 m2/s andf =0.7 to 0.8; for fresh, soft slabv= 0.4 to 0.5 m2/s and f = 0.4 to 0.5. The program predicted run-out distance well for a variety of conditions but performed less well in cases of sharp, adverse grade in the run-out zone. For the Voellmy approach, large design-size avalanches required turbulent friction coefficients ξ of 1200 to 1600 m/s2and kinetic friction coefficients of 0.15. Two hard-slab avalanches, a slow-moving,wet-slab avalanche, and a soft-slab avalanche that ran through scattered mature timber required ξ of 700 to800 m/s2and μ of 5/V whenVis velocity in m/s. The coefficient of sliding friction for a hard-slab avalanchethat encountered damp snow in the run-out zone was computed directly from movies to be 0.35, 0.43, and 0.32 for three measured sections of the run-out zone.