Analyzing Sequence of Events and Slope Failure: Case Study of Dhauliganga Flash Floods, Chamoli, Uttarakhand, India

The aim of the present study is to understand the chronological sequence of incidents that happened in Dhauliganga Flash Floods. A large part of a north-facing overhung wedge collapsed on 7th February 2021, having a composite mass of rock and ice. This wedge failure led to massive debris flow into Raunthi Gad, which blocked the Dhauliganga River's flow near Raini village. This natural dam failed around 10:30 h and brought a devastating flash flood in the Dhauliganga River. In this flash flood, the flow of Rishiganga River was also blocked, and an elongated lake was created, which later vanished with time. This event damaged two ongoing hydropower projects; Rishiganga Hydro (13.2 MW) and Tapovan Vishnugad (520 MW), and many people were killed. The whole sequence of events was analyzed using high-resolution images of Pleiades and Google Earth. The data derived from NASA suggests that the region received a significant amount of rainfall with a sudden rise in relative humidity and a drop in solar flux on 4th February. To model the debris flow to calculate the motion of the movement from initiation to runout simulation of the event was done. Further, the slope stability analysis of the triangular wedge, which failed, was performed using the Limit Equilibrium Method, suggesting the slope is susceptible to stress-controlled failure.

Based on Spatio-temporal satellite images, a similar type of ice-avalanche break-off and deposition of sediments event was observed between 19th September-9th October 2016 in the same area. Also, developments of cracks were visible in the satellite images from October 2017, certainly indicating impending possibilities of slope failures which happened on 07th February 2021. Thus, regular monitoring of such vulnerable areas should be done to avoid future slope failures and human loss.