Hanging glacier avalanche (Raunthigad - Rishiganga) and Debris flow disaster of 7th February 2021, Uttarakhand, India, A Preliminary assessment

Abstract

A catastrophic debris flow in the Rishiganga and Dhauli ganga river in Uttarakhand, India on 7th February 2021 left a trail of disaster. Around 200 people lost their lives, two hydro-power project 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 detachment of 0.59 km2 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 35o 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 oC in 2012 to a peak of 0.4 oC 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 suggest warm permafrost below 50 m and conditions favorable for intense frost cracking at 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 modeling study suggest 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 cryospheric areas of the Himalaya to capture the early warning signs for better preparedness.

Flood Management Issues in Hilly Regions of Uttarakhand (India) under Changing Climatic Conditions

“…In 2021, the Chamoli region experienced a flash-flood event resulting from an avalanche near the Ronti Glacier (Figure 10) at an elevation of 4064 m. At the confluence of the Rishi and Dhauliganga rivers, the height is 1908 m, while Nanda Devi, at 7817 m, is the highest point in the region. The floods, while travelling downstream, demolished the bridge near Raini village, damaged the Rishiganga and the National Thermal Power Corporation (NTPC) Tapovan Hydel plant, and killed over 200 people [45]. The velocity and flood discharge at 89 m upstream of the Rishiganga Dam is estimated to be 24 m/s and 12,448 cumecs.…”

“…The length of the rockslide surface is about 1182 m, having a maximum width of 755 m and a mean slope of 62 at 7817 m, is the highest point in the region. The floods, while travelling downstream, demolished the bridge near Raini village, damaged the Rishiganga and the National Thermal Power Corporation (NTPC) Tapovan Hydel plant, and killed over 200 people [45]. The velocity and flood discharge at 89 m upstream of the Rishiganga Dam is estimated to be 24 m/s and 12,448 cumecs.…”

7 February Chamoli (Uttarakhand, India) Rock-Ice Avalanche Disaster: Model-Simulated Prevailing Meteorological Conditions

“…The event took place in the Tapovan area of Joshimath in the Chamoli District of Uttarakhand, India on 7 February 2021 at 4:51 UTC [51][52][53]55] (Figure 1A,B). During this event, a massive rock and ice avalanche happened approximately 22 km upstream of Tapovan Hydropower Project, below Ronti peak in the Nanda Devi massif [51,53,55], which led to a sequence of events that resulted in the loss of many lives and the infrastructural damage of two major hydropower projects in the Rishiganga and Dhauliganaga valleys (Figure 1C).…”

“…We have utilized the WRF model to numerically simulate meteorological conditions during an extreme event generally referred to as the Chamoli rock-ice avalanche event [51,52] or Chamoli disaster, Uttarakhand, India (30.37 • N, 79.73 • E). The triggering factors behind this rock-ice avalanche event could be unusual local meteorological conditions (temperatures, precipitation, winds speed and pattern, wind stress, radiative and turbulent fluxes, etc.…”

“…The triggering factors behind this rock-ice avalanche event could be unusual local meteorological conditions (temperatures, precipitation, winds speed and pattern, wind stress, radiative and turbulent fluxes, etc. ), climatic conditions (long-term changes in surface and air temperatures and precipitation patterns), and geological factors (extremely steep slope, destabilization of the rock due to the decreased ice cover, seismic activity) [28,[51][52][53][54].…”