Importance of water and water producing processes in cascading events in mountainous regions

Over the last years, several multiphase avalanches have been observed, some of them leading to a cascade of events, such as in Chamoli, India, 2021, where a mixture of ice and rock fell down Ronti Peak, and transitioned to a debris flow with large amounts of water being involved. Another example is the event that occurred at Pizzo Cengalo, Switzerland, in 2017, where the rock face collapsed on the underlying glacier, entraining part of it, and also transitioning to a debris flow. When such a mass movement occurs, and leads to a cascade of events, the runout distances are much longer, and the consequences, both for humans and infrastructure, are much more important. 

When a multiphase avalanche turns into a cascade of events, the amount of water present in the flow seems to be a determining factor for the runout distance. The sources of water, for both of the events aforementioned remain debated, and the amounts of water that can be generated by the melting of the ice in the flow or by entrainment are poorly constrained. Indeed, from the moment that ice and snow are involved in a multi-material gravitational flow, they have the potential to melt due to friction between the different components of the flow and with the ground, and hence generate water. Material entrainment on the way also has the potential to either directly incorporate water in the flow, or bring in material with a high water content (i.e. hydrated sediments) or ice, that has the ability to melt while the flow propagates. An accurate modelling the thermal aspect of the flow as well as its ability to entrain material on the way is necessary to quantify the amount of water present in the flow.

Here, using a multiphase depth-average model specifically designed to handle gravitational flows made of rocks/ice/water/snow or any single components of these, we want to assess 1) the impact of heat transfers between the materials and 2) entrainment of multiphase ground material on the flow behaviour and more specifically on the water content in the flow and the consequences it has in term of runout distances and potential for cascading events. 

First results show that both entrainment and heat transfer within the flow play a major role in water production. Our experiments suggest that heat transfer between rocks and ice leads to the most efficient water production. Material entrainment also plays a major role in incorporating water in the flow, or producing it by melting entrained ice. Better constrains regarding material thermal properties, ground composition and potential for entrainment are however necessary to accurately quantify the amounts of water that can join the flow and influence the runout distances.