Modelling Wet Snow Avalanches with Thermal Effects, Snowcover Entrainment and Lubricated Sliding

Snow avalanches are dangerous, gravity-driven geophysical flows. They consist of a fast-moving mass of snow clods that are formed when the snowcover is set in motion. Immediately after release, the snowcover breaks up into blocks of diverse sizes which become increasingly rounded by frictional interactions in the core of the avalanche. The primary problem in avalanche dynamics is to physically describe the motion of the churning particle ensemble. Due to the great number of particles, however, the motion of the particle ensemble must still be mathematically described using continuum theories of moving fluids. The ensemble properties of the continuum are defined by the sum of all the frictional interactions between particles and between particles and the ground. The myriad particle interactions are far from simple, but inevitably must be considered in continuum models in order to accurately describe and simulate avalanche flows. This is particularly true for wet snow avalanches where the temperature of the particles is near to the melting point of ice and considerable meltwater can be present in the porous ice-matrix of the clods of flowing snow. The high snow temperature and the presence of meltwater define the mechanical response of the avalanche mass under gravity. Thus, the micro-scale interactions between particles lead to different macroscopic avalanche regimes and therefore different avalanche velocities and runout distances. In this dissertation we develop a continuum model for avalanche flow accounting for the temperature and moisture content of the moving snow. We first extend existing continuum theories to consider the mechanical energy dissipated by moist particle interactions. We must consider two temperatures: the true thermal temperature and the so-called ”granular” temperature, which is associated with random particle movements within the granular ensemble. The model extension therefore requires an internal energy equation to calculate the mean flow temperature as well as an equation to track the kinetic energy of random particle motions. Meltwater produced from frictional heating can be quantified. A third mass conservation equation, however, is necessary to track the meltwater trapped in the pore space of the particles. Knowing this quantity allows us to understand how moist particles damp the granular temperature and to postulate lubrication functions that simulate wet snow sliding. With this approach we can simulate dense, viscous-type flows typically associated with wet snow avalanches. The additional source of heat and meltwater is from snowcover entrainment. Avalanches can start from cold snow slabs but entrain warm snow at lower elevations. Often this snow is wet, either from rain or intense thermal heating. To apply the proposed model to solve practical avalanche problems therefore requires the specification of the snowcover temperature, density and moisture content along the avalanche path. A set of documented case studies is used to test the model by reproducing measured runout distances and area covered by the avalanche deposits. Long avalanche runout is possible depending on the initial and boundary conditions of a specific problem. We performed a sensitivity analysis to demonstrate how runout distances and inundated areas can vary as a function of the initial and boundary conditions such as temperature, moisture content, and density of the snowcover. In a final application we fed the model with simulated snowcover data derived from actual meteorological measurements to forecast wet snow avalanche runout in an operational environment. We demonstrate using sensitivity analysis why the specification of boundary conditions leads to great model uncertainties. This work is a first step towards the application of avalanche dynamics models as operational tools to assess the current wet snow avalanche risk.