High-speed photography of fractures in weak snowpack layers

During the winters of 2002–2003 and 2003–2004, fractures in weak snowpack layers were recorded with a portable digital high-speed camera in the Columbia Mountains of British Columbia, Canada. Fractures were photographed at 250 frames per second in 21 compression tests, four rutschblock tests, three cantilever beam tests as well as on five skier-tested slopes. Theoretical slab avalanche release models generally assume propagation of a brittle shear fracture in an incompressible weak snowpack layer. However, displacement measurements of markers placed in the snow above the weak layer indicated that slope normal displacement (due to crushing of the weak layer) was directly caused by the fracture in the weak layer and independent of slope angle, whereas the slope parallel displacement following fracture was probably dependent on slope angle. Furthermore, displacement measurements from rows of markers placed in the snow above weak layers resulted in fracture speed measurements ranging from 17 to 26 m/s, in good agreement with the only other published fracture speed measurement known to the authors: 20 m/s.