A preliminary assessment of the Chamoli rock and ice avalanche in the Indian Himalayas by remote sensing

Abstract

Introduction Rock and ice avalanches are one of the most hazardous slope failures in high mountains (Schneider et al. 2011). They may cause serious damage to settlements and infrastructure. Vulnerability to such events increases as major roads, hydropower plants, trunk powerlines, and skiing resorts are constructed closer to glaciated areas. For instance, in August 1965, the collapse of a glacier destroyed the construction site of the Mattmark hydropower plant in Switzerland. The site at an elevation of 2,100 m above sea level (m a.s.l.) was buried by ~ 2 × 10 m of ice detached from the Allalin glacier claiming 88 casualties (Faillettaz et al. 2012). The impact of rock and ice avalanches is often limited locally but they may also endanger more distant places by a long runout (Schneider et al. 2011). This is especially the case of ice and rock avalanches that change into debris flows or flash floods. One of the most destructive events of this type was the collapse of rock and ice at Mt. Huascaran in the Cordillera Blanca in Peru in 1970, in which the debris flow reached the Pacific Ocean traveling a distance of 180 km. It destroyed the town of Yungay leaving behind more than 6,000 casualties (Evans et al. 2009). By volume of transported material, the Kolka-Karmadon rock and ice avalanche in the High Caucasus in 2002 was even larger with a total volume of approximately 100 × 10 m (Huggel et al. 2005). Rock and ice avalanches are often recurrent. For instance, the Bisgletscher hanging glacier at Weisshorn in the Pennine Alps has a record of more than 20 events since 1636 (Raymond et al. 2003). The ice avalanche at the Mattmark dam reoccurred in 2000, this time without damage. The 1970 Mt. Huascaran event had a predecessor in 1962 and an even much older prehistoric avalanche or series of events (Klimeš et al. 2009). The recent event experienced in the Garhwal Himalayas in India called the Chamoli Disaster is remarkable due to its size, vast impact on hydropower projects, and speculations about its source. The event occurred in the morning of 7 February 2021. On that day and on several of the following days, the disaster gained the wide attention of the media covering the dramatic rescue operations of construction workers blocked in the tunnel of the Tapovan hydropower project. Attention was raised by videos showing the flood wave rolling over its dam leaving behind a damaged concrete structure and the valley covered with mud. The first assessment of the event was provided by Shrestha et al. (2021) who identified the source of the flood at the slope of Mt. Ronti. The report of Rana et al. (2021) includes photographs of the valley taken several days after the event. Further study by Pandey et al. (2021) reports the exact timing of the event based on measurements by seismic stations. The event occurred in a rugged high elevation part of the Central Himalaya (Fig. 1). The area belongs to the Higher Himalayan Crystalline and the slope deformation evolved in Vaikrita Group metamorphics, namely in the lowermost Joshimath Formation (schist, gneiss, and leucogranite) (Kanyan et al. (2021). Roti Gad Valley is traversed by Vaikrita Thrust in the N-S direction. The Munsiari Thrust which corresponds to the Main Central Thrust (MCT) (Kanyan et al. 2021) and which marks the transition between the Lesser and Higher Himalaya is in the proximity towards SW. The rocks of the zone are heavily stressed due to the convergence in the zone of the MCT which reaches about ∼5 to 10 mm/year (Jade et al. 2014) in Garhwal and Kumaon region. The rocks are highly jointed and fractured which adversely affects their strength (Pandey et al. 2021). It is interesting to note in this respect that open fractures in the depth of 990 m below the surface led to a disastrous groundwater ingress into Tapovan headrace tunnel during its construction in 2009 (Nawani 2015). The relief above the altitude of about 3500 m a.s.l. is formed by debris covered valley glaciers while the lower part is typical with V-shaped valleys formed by a fast incision due to the uplift and high monsoon precipitation. Remote sensing provides a wide range of techniques for the assessment of mass wasting in mountains, including snow cover mapping, landform recognition, estimation of flow velocity, and volume change (e.g., Huggel et al. 2005; Kääb et al. 2005; Kropáček et al. 2015). The advent of very high-resolution satellite systems, i.e., with a resolution close to 1 m, in 2000 allowed detailed analysis of surface features such as cracks, crevasses, and small water bodies. The temporal aspect of data acquisition was largely addressed by the introduction of Planets satellite systems operated by PlanetLab providing quasi-daily imagery in the VHR mode on a global scale. In this study, we would like to show how remote sensing can provide timely an unbiased information on the mechanism, extent, and course of such events.