A3 Avalanches and Mass Flows

Topic

Snow avalanches are rapid gravitational flows descending a slope; they are triggered when the shear stress exerted by the weight of the snowpack on a weak layer exceeds the shear strength of that weak layer or the critical interface between layers. Two main types of snow avalanches are distinguished: slab avalanches, in which a cohesive snow layer fractures suddenly above the underlying weak layer, and powder avalanches, consisting of clouds of airborne snow particles that behave like a dense fluid as they descend.

The fundamental physics of a snow avalanche combines fracture mechanics—which explains how the avalanche initiates—with the dynamics of dense granular flows, which explains how it propagates once started. During the initiation stage, the shear strength of the weak layer depends on two of its properties: cohesion (the strength with which the snow particles within the weak layer hold together) and the internal friction angle (a measure of the particles' resistance to sliding over one another under a given load). The greater the cohesion and internal friction angle of the weak layer, the higher its shear strength, and the more shear stress is required to trigger the avalanche.

Once the avalanche flow has begun, a model of dense granular flow dynamics divides the resistance encountered by the flow into two components. The first component is basal friction, which depends on the extent to which the avalanche rubs against the underlying terrain as it descends. The second component is a turbulent one, which depends on the speed of the snow avalanche itself and becomes increasingly significant as the avalanche flows faster; consequently, basal friction dominates at low speeds, whereas the turbulent component dominates at high speeds. By combining these two resistance components, this model of dense granular flow dynamics captures the full behavior of the snow avalanche, from its initiation zone to its final deposition site.

Glacial debris flows combine meltwater with sediment and, unlike the previously described snow avalanches, exhibit viscoplastic behavior: a glacial debris flow acts as a rigid solid as long as the applied stress remains below a certain threshold, but begins to flow like a viscous liquid once that stress exceeds that threshold. This viscoplastic behavior distinguishes glacial debris flows from both slab avalanches and powder snow avalanches, even though all three phenomena share the same gravitational origin mentioned earlier.

The expected behavior of these mass flows includes peak velocities ranging from ten to one hundred meters per second for powder snow avalanches, and impact pressures of ten to one thousand kilopascals against any obstacles in their path. The runout distance of a snow avalanche or glacial debris flow scales with its total volume: the greater the total volume of snow, ice, or sediment involved, the further the mass flow travels before coming to a halt, with the initial shear stress and shear resistance described earlier ultimately determining how much mass is set in motion and how far it travels.

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