Chapter 23 - Dynamics of Snow Avalanches

Abstract

After a general introduction to snow avalanches and their consequences, type classification is discussed, and classification schemes based on morphology, genesis and magnitude are described briefly. The first technical section deals with deformation and displacement of snow slopes prior to avalanche release, with the failure process, and with the propagation of initial failure. Representative values are given for stress levels, stress gradients, volumetric and deviatoric strain rates, interface glide velocity, and failure propagation speeds. The following section describes various types of avalanche motion after release, including sliding of a rigid body, low-speed sliding of loose snow, high-speed motion of dense snow, high-speed motion of diffused “dust clouds”, flowing of slush, and rolling and bounding of blocks and clods. Representative values are suggested for slope angles, initial accelerations, flow density, driving stresses, and travel velocities. The third technical section considers idealized theoretical analyses of avalanche motion. Topics dealt with include driving force, motion resistance, equations of motion, energy and power considerations, retardation and runout distance, flow regimes, and physical models. The final technical section covers the dynamic forces imposed by snow avalanches and their associated “winds”. Measured values of impact stresses are summarized, and direct impact stresses for “wide” avalanches are deduced from simple theory. Dust avalanches are regarded as analogous to incompressible fluids at low subsonic velocities, so that peak impact stress is a stagnation pressure for appropriate effective density. Various interpretations of surface avalanches are mentioned, and special attention is given to calculation of impact stress from the Rankine-Hugoniot conditions for elastic and plastic waves in the snow. Forces induced by interfacial shear and avalanche deflection are considered briefly, and forces created by avalanche winds, or “air blast”, are discussed. In the conclusion there is a simplified tabulation of representative values for stress ranges, typical strain rates and typical velocities in the various avalanche processes. The overall intent in this review is to provide a quantitative introduction to the dynamics of snow avalanches in accordance with well-established mechanical principles, avoiding unnecessary complications, controversy, and specialized jargon as far as possible.