Hanging glacier avalanche (Raunthigad–Rishiganga) and debris flow disaster on 7 February 2021, Uttarakhand, India: a preliminary assessment

Abstract

A catastrophic debris flow in the Rishiganga and Dhauliganga rivers in Uttarakhand, India, on 7 February 2021 left a trail of disaster. Around 100-150 people lost their lives according to Uttarakhand Chief Secretary statement given to ANI news portal, two hydropower projects were badly damaged and a bridge across the Rishiganga River was washed off in the event. Study shows that the debris flow is caused due to detachment of 0.59 km 2 right lobe of a hanging glacier and resultant ice-rock avalanche. This right lobe of the glacier was located over a mountain slope having an average slope of 35° at 4700-5555 m a.s.l. and travelled 12.4 km before hitting the infrastructure projects. Role of precipitation, snow cover, land surface temperature, and permafrost processes were investigated for identifying causes of the event. Since 2012, monsoon precipitation and mean annual land surface temperature (LST) showed significant increasing trend. Snow cover during monsoon months showed increasing trend and September, October and November experienced decreasing trend at glacier elevations. Mean annual LST increased from − 0.3 °C in 2012 to a peak of 0.4 °C in 2016. Central lobe of the glacier advanced during this period and eventually fell off in 2016 suggesting that the LST warming forced reduction of frictional drag at the interface facilitating it advancement and eventual dislodgement. Permafrost modelling suggests warm permafrost below 50 m and conditions favourable for intense frost cracking up to 10-15 m. At ~ 40 m depth, the delayed response of 2012-2016 warming produced peak positive temperature conditions by December and probably facilitated the formation of thin film of water at the deeper layers acting as a lubricant for glacier sliding. It is also suggested that the increase in summer precipitation might have forced thickening of the accumulation area and thereby increasing the shear stress for sliding of the glacier. It is proposed that the recent change in the weather conditions in the region is primarily responsible for this event through geological, glaciological, and permafrost processes. Flood modelling study suggests a flood volume of ~ 10 MCM generating 24.5 m flow depth at the bridge site with 12.7 m/s flow velocity. The event highlighted the need for improved monitoring of the cryosphere areas of the Himalaya to capture the early warning signs for better preparedness.

Assessing potential risk of glacier avalanches to hydropower infrastructure in the Himalayan region

“…Possible factors contributing to the above glacier avalanche event are an amplified freeze-thaw cycle in the headwall and accumulating meltwater in a crack that developed in the preceding months (Shugar et al, 2021). Studies utilizing satellite data suggest that the area experienced ice avalanches in 2000 and 2016 as well (Kropaek et al, 2021;Shugar et al, 2021;Thayyen et al, 2022). Geologically the Tapovan catchment studied here is part of the Joshimath Formation, consisting of metamorphic rocks, including Precambrian upper amphibolites facies, largely psammitic schist and gneiss (Valdiya and Goel, 1983).…”

“…More recently, on 7 th February 2021, at Tapovan in Uttarakhand, India, a debris flow resulted from a failure of the bedrock of a hanging glacier. The flood caused significant destruction downstream, severely damaging two hydropwer projects and causing the death of more than 200 people (Shugar et al, 2021;Thayyen et al, 2022). In November 2021, a relatively small break off of ice and snow in the Mustang Valley, Nepal, turned into a massive debris flow, killing more than 100 yaks and destroying a bridge in the vicinity of an already existing hydropower plant (Acharya et al, 2023).…”