Numerical Calculations & Scenario Reconstruction of the 7th Feb'21, Chamoli Event-In Terms of Velocity-Energy and Sediment-Water Amount Changes

Rock falls, rock slides and rock avalanches occurring in glaciated environments and permafrost regions are characterized by their sudden and complex character, high magnitude-mobility and cascading secondary hazards. The flow mobility is enhanced by the presence of ice and snow by up to 25% to 30%, with respect to rock avalanches of comparable magnitude evolving in non-glacial settings. Their dynamics are controlled by interaction between the detached rock and the icy component during all phases of motion, from initiation to the final deposition (Sosio et al., 2015).  The 7^th Feb'21 catastrophe in the Upper part of the Chamoli district of Uttarakhand, India was one such event that impacted the catchments of Ronti Gad, Rishiganga and Dhauliganga valleys by a high magnitude debris flow, triggered by a massive rock-ice slide of 25-27 million cubic meters (ICIMOD, 2021; Pandey et al., 2021; Thaiyan et al., 2021; Shugar et al., 2021). The initial rockslide entrained glacier ice and continued as a rock-ice avalanche which fluidized along the path, evolving into a massive debris flow, traversing 21-22 km downstream in around 16 to 18 minutes (ICIMOD, 2021; Pandey et al., 2021; Thaiyan et al., 2021; Shugar et al., 2021). It destroyed two hydroelectric projects (HEP) enroute, and killed more than 100 workers at the Tapovan HEP. This also led to the formation of the lake at the confluence of Ronti Gad and Rishiganga and a small lake was also observed at the confluence of Rishiganga and Dhauliganga, which was instantaneously breached. This event accentuated the fragility of the Indian Himalayas and its complex periglacial terrain.

In the present work, we try to numerically and conceptually reconstruct the cascade from the initial rockslide to 21 km downstream, till the Tapovan HEP. We segmented the flow path into four major sections based on: i) gradient changes; ii) observed flow physical parameters; iii) channel characteristics; iv) erosion-deposition and entrainment. For each of the four sections, we present the section wise peak velocity and energy calculations based on the fundamental Voellmy-Perla equations and present the result as profile graphs to better understand the velocity-energy changes along the longitudinal profile of the flow path. Next, we estimate the section wise sediment-rock to water amount at the end of each section, using pre-post DEM profile-differencing, satellite images and field data, based on certain logical assumptions. Thus, proposing the plausible stepwise processes and sediment-water interaction as occurred on the morning of 7^th Feb'21. The results, hence obtained were found to be in-line with the available literature and were able to logically justify the so-far-known event parameters. Future work is intended on better validation of the obtained results by using flow models. Thus, aiming to better comprehend and understand such events in the complex Himalayan terrain and being able to predict and mitigate them in the future.

Keywords: Rockslides, rock falls, rock avalanches, debris flows, hydroelectric projects, Indian Himalayas, Glacier, Climate change