The detachment mechanism of the rockslide causing the Chamoli February 7^th, 2021 debris flow disaster

On February 7^th, 2021, a rockslide of about 20 Mio m³ detached in a height of 5600 m asl. from the northern flank of Mount Ronti (Chamoli district, Uttarakhand state, India), turned into a rock mass fall and produced a debris flow. When the rock mass hit the Ronti Gad valley after a fall height of 1800 m the rock mass mixed with melting dead ice together with snow and ice avalanche material of previous debris flows. The debris flow destroyed hydroelectric infrastructure between 10 - 20 km down the valley killing 204 people either working at or visiting the power plants. By combining remote sensing, structural geology and kinematics/mechanical analysis of the rockslide, we demonstrate that a 600 m wide and almost 800 m long block of quartzite, bordered laterally by two joints and a newly formed tension crack on the top detached from an underlying layer of biotite-rich paragneisses. Assuming full hydrostatic heads in both joints and in the tension crack as well as 75% of the full hydrostatic head in the lower boundary surface between quartzites and paragneisses, the rock block analysis yields a friction angle of 32° for both joints, which is a plausible value of the friction angle of joints in quartzites. The detachment of the block has been the result of the widening of the tension crack on top, of a progressive propagation of the lateral joints together with a catastrophic failure of the detachment plane at the border between quartzites and paragneisses. At the time of the failure, all discontinuities must have been almost completely filled with water raising the question, if the frequency of rockslides in the Himalayas is increasing as temperatures rise and permafrost is thawing due to climate change.