The Role of Frictional Melting in Rock/Ice Avalanche Dynamics: Thermomechanical Modelling of the Piz Cengalo and Chamoli Events

The rock/ice avalanches that occurred in Switzerland (Piz Cengalo, 23.8.2017) and the Indian Himalaya (Chamoli, 7.2.2021) and their subsequent debris flows have highlighted the question of how much water can be generated by frictional melting of glacier ice and entrained snow. The Piz Cengalo event initiated with the release of some 3.2 mio m³ of rock and entrainment of approximately 0.6 mio m³ of glacier ice.  The source of water necessary to trigger the debris flow activity has been attributed in part to frictional melting of ice.  First calculations by Shugar et al. (2017) indicate that the Chamoli event initiated with a total of 27 mio m3 of rock (80% by volume) and glacier ice (20%). Additional entrainment of snow increased the total ice content of the flow.  Meltwater generated by frictional melting has likewise been suggested as the cause of the catastrophic flood wave.

In order to analyze the the Piz Cengalo event, the SLF developed a mathematical model to simulate rock/ice avalanches, including snow, water and soil entrainment (Margreth and Bartelt, 2018).  The model calculates the internal heat energies of the rock, ice and water phases and includes the melting of ice.  Frictional work drives the melting process, but is additionally supplemented by heat exchanges/contact between the rock-ice-water components. The model correctly provides the overall temperature rise of an event, which can be found from simple energy arguments. The numerical model, however, predicts the rate of melting, which depends on when ice and snow are entrained into the flow, as well as terrain features, which influence frictional work rates.   An important result of the model calculations is that meltwater is not spread homogeneously over the depositions, but is concentrated at specific locations in the deposits, facilitating the formation of secondary mass movements.

Using this mathematical model to simulate the Chamoli event we find a total maximum meltwater production of 2.5mio m³ to 3.0 mio m³.  This estimate should not be characterized as small:  during the flow in the upper reaches of the Raunthi Gadhera frictional heating generated 50’000 tons of water every second.  This rate decreased significantly as the flow reached the Dhauliganga river valley.    In the end, only half of the initial glacier ice melted.  From downstream gauge station measurements, Indian government officials have estimated the total volume of the water surge to be approximately 6 mio m³ over a period of one hour (Rautela et al., 2021).  In agreement with the field observations of Rautela and co-authors we find the remaining 3mio m³ of water to come from entrainment of ponded water in the Raunthi Gadhera valley and the subsequent blockage and dam break at the Dhauliganga river inlet.  That is, secondary sources of water are necessary to reproduce the observations.  A similar result is found for the Piz Cengalo case study.  Under extreme assumptions, we find that of the initial 0.6 mio m³ of glacier ice, between 0.08 mio m³ and 0.12 mio m³ melted, leaving significant amounts of ice in the depositions.