The Heat of the Flow: Thermal Equilibrium in Gravitational Mass Flows

Flow behavior and mobility of gravitational mass flows such as rapid-moving landslides, ice-rock avalanches, or snow avalanches strongly depend on the material temperature. Flow temperature dependence is particularly pronounced for materials with high homologous temperatures, such as snow or ice under natural conditions. The interplay between mechanisms driving the temperature evolution in flowing geomaterials remain largely unknown. Here we present laboratory experiments in a rotating drum, measuring the temperature evolution of steadily flowing snow at ambient temperatures below freezing. After initial heating the flow reaches thermal equilibrium. To describe the thermal energy balance, we derive an analytical model, taking into account frictional energy dissipation and heat exchange with the ambient medium. The model accurately captures the measured temperature evolution and predicts the observed thermal equilibrium, where ambient cooling compensates frictional heating. It allows to determine heat transfer coefficients and total shear stresses of the flowing material based on measured temperatures. Plain Language Summary Snow avalanches, landslides, or ice-rock avalanches produce heat as they flow down the mountain. We quantify this heat production in laboratory experiments with flowing snow. Alongside we discovered a new phenomenon, namely, the thermal equilibrium in gravitational mass flows. We reproduce this phenomenon experimentally and derive an analytical model to predict the temperature evolution of the flowing geomaterial. Exciting aspects of the model are its simplicity and the possibility to determine material properties, such as heat transfer coefficients and total shear stresses, in an entirely new way. This type of simple model and experiment helps to uncover the physics behind flow type transitions and opens a new way to investigate the frictional behavior for various kinds of mass flows. The thermal equilibrium appears due to the natural compensation of frictional energy dissipation by ambient cooling. For a wide range of gravitational mass flows and in particular snow avalanches, the temperature evolution dictates the resulting mobility and run out. Therefore, this research is an important piece in the puzzle to develop methods to predict the destructive potential of natural hazards.