Kolkata, June 28, 2021

On February 7, 2021, a flash flood event occurred in the Rishiganga and Dhauliganga valleys of Chamoli district, Uttarakhand causing heavy toll on human lives and massive destruction of two hydro power projects at Raini and Tapovan. A 13.2 MW Rishiganga Hydel Project near Raini has completely been wiped out and the barrage site at the under-construction Tapovan-Vishnugarh Hydel Project (520 MW) of NTPC Ltd. at Tapovan was severely damaged, by the torrent debris-laden flood water. However, no appreciable damage has been noticed further downstream of Joshimath, but the fury of the flash flood reached down to the Srinagar reservoir site in Rudraprayag district of Uttarakhand. Geological Survey of India, the 171-year-old premier geo-scientific organisation, has sent a team to find out the causes leading to the Disaster.
According to Saibal Ghosh, Director GHRM Centre, GSI Central Headquarters, “The event was initiated by a snow/ ice/ rock mass avalanche along with portion of jointed rock mass having approximate dimension of 400 m x 700 x 150 m from the left valley wall of Raunthi Garh (a left bank tributary of Rishiganga River). The dislodged rock/ ice constituting from the avalanche crashed to the Raunthi Garh valley floor after travelling about 2900 m with a vertical fall of around ~1800 m. The energy of impact at a very high speed has pulverized the rock /snow /ice, besides generating melt water and the entire dislodged material of snow /ice /rock has rapidly flown in the downstream of Raunthi Garh and further down slope along the Rishiganga valley, causing this huge deluge and impacts to the elements-at-risks exposed across its path including formation of a temporary lake on Rishiganga River just immediately upstream of the confluence with Raunthi Gad.”
The second reason for the disaster as pointed out by Shri Ghosh is because of the “Observed change in the hydro-meteorological conditions between February 4 & 6, 2021 (heavy snowfall followed by sudden warmer climate) possibly triggered this huge snow and rock avalanche/ landslide causing sudden domino effect of flash flood in the downstream.”
According to Shri Satya Prakash Shukla, DDG, International Affairs Division, GSI Central Headquarters and the leader of the team of Geologist who did the first-hand study of the disaster site, “Lessons have also been learnt from this event of February 7, 2021 which is an excellent example of multi-hazard phenomena that occurred during the winter time, when this type of phenomenon leading to such disastrous deluge is least expected in the Himalayas.
This deluge event was initiated by a large ice/ snow/ rock avalanche that occurred in the high-altitude area within Raunthi Garh, primarily because of a wedge failure in widely-spaced jointed rock mass that generated a huge debris flow, including formation of a lake in Rishiganga River and caused severe inundation in the downstream. Also, immediately before the event, there was an influx of warm weather, after a two-day long high amount of snowfall in the area, which could be instrumental in triggering this mass wasting and subsequent deluge. Apparently, no ground evidence so far has observed the occurrence of any Glacial Lake Outburst Flood (GLOF) causing this event. It was ascertained that the momentum of flowing debris was extraordinarily high and the same allowed backflow of debris near the confluence between Rishiganga River and Raunthi Garh, and also near the confluence with Dhauliganga River. An artificial dam was formed near the confluence of Raunthi Garh and Rishiganga River, by the flowing debris which blocked the flow of Rishiganga River and formed a small lake temporarily posing another domino effect.
A study by NRSC, ISRO indicates that the time taken from the initiation of the avalanche and its disastrous impact up to Tapovan barrage site near Joshimath was barely ~50 minutes, which indicates availability of a very low lead time for raising any warning for the downstream areas. Vulnerability of the open joints in rock mass in the high-altitude areas where freeze and thaw actions are prevalent, and the areas having risks posed by smaller mountain glaciers are also becoming important locations where such type of hazard can be initiated. The steep higher order streams and the narrow river valleys in the high-altitude areas of Himalayas remain extremely hazard prone, which are not only vulnerable to GLOF/ Landslide Lake Outburst Flood (LLOF) hazards but have also became locale for a major disaster caused due to the domino effect of large/ mega landslide and avalanche.``

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