Unravaling the cascading mechanisms of rock-ice avalanche triggering hyper-mobility glacial debris flow in southeast Tibet

Under the background of global warming, the risk of geo-hazard in the cryosphere has increased with the retreat of glaciers. Several similar large-scale glacial debris flows with high mobility occurred in the southeast Tibet Plateau during the summer season which has drawn the attention of scientists. One typical event occurred on 10 September 2020 near Namcha Barwa Peak. The initial landslide finally changed into a glacial debris flow with high water content and high mobility under the condition of little precipitation. To solve the questions: 1) why is the glacial debris flow in southwest Tibet more prone in the warm season? 2) How is the initiation mechanism of this glacial debris flow with little rainfall? 3) What is the major source of water for this large debris flow? and 4) Which factors dominate the high mobility characteristic of this debris flow event? By conducting field investigation and comparing the satellite images before and after the event, we have revealed a rock-ice avalanche on the ridge above the landslide area to be contemporary with the event. This finding produced the hypothesis on the initiation process: rock-ice avalanche – moraine deposit failure – glacial debris flow, which has been inferred for many other similar events but not quantitatively proved. To test the hypothesis, we conducted thermal-hydraulic-mechanical coupled numerical modeling with the impact of freeze-thaw cycles and rock-ice avalanche on the stability of the moraine deposit. The results demonstrate that the avalanche event triggered the moraine landslide, with freeze-thaw cycles as the control factor. Generally, long-term freeze-thaw cycles alone are insufficient to set off the hazard chain. At the same time, seasonal temperature variation that controls ice-water phase change dominates the stability of moraine deposits under rock-ice avalanche in different seasons. In warm seasons, rock-ice avalanches would trigger moraine deposit failure more easily due to abundant water content that facilitates pore pressure increase, and liquefaction of moraine. By conducting multi-phase modeling of glacial debris flow, we have proven that the initial water content and entrainment of water during the development of the debris flow are the main water sources of this debris flow event. Moreover, the high water content in the initial landslide together with the entrainment process should also account for the high mobility characteristic of glacial debris flow. This work answered the long-lasting scientific questions about the initiation mechanism and dynamics of hyper-mobility glacial debris flow disaster chain under the background of climate change.