The likely causes of the 4 October 2023 Sikkim flood disaster – GWC Mag

The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.

Amongst a deluge of tragic news stories, the appalling flood in Sikkim in NE India on 4 October 2023 has received less attention than it deserved. The basic information about the event is now clear – a significant Glacial Lake Outburst Flood (GLOF) was released from South Lhonak, sweeping down the Teesta River to the Teesta III hydroelectric dam at Chungthang. The dam was quickly overwhelmed, and collapsed, releasing a catastrophic flood that has caused very major impacts downstream, including at least 40 fatalities.

Yesterday, Praful Rao posted a set of images on the ever-wonderful Save the Hills blog that show the scale of the impacts of the flooding. These images, for example, displays the depth to which properties have been buried in Rangpo:-

Inevitably there is now considerable discussion about the likely causes of the GLOF, which occurred during a reported period of heavy rainfall. Over the weekend, scientists have been working on this issue. Max Van Wyk de Vries of the University of Oxford has posted a very interesting thread on Twitter based on his work using satellite imagery to assess slope movement:-

Some perspectives on the #sikkimflood disaster:
1. A landslide of waterlogged lateral moraine triggered the GLOF.
2. As at Chamoli, the landslide moved tens of m prior to collapse.
3. Anthropogenic activity influenced both the consequences and the triggering of this disaster.
1/8 pic.twitter.com/gqfTGjbraQ

— Max Van Wyk de Vries (@Max_VWDV) October 8, 2023

There are eight posts in this thread, so I strongly recommend taking a more detailed look. But, in essence, Max is able to show that an area of moraine above the lake was mobile even before the failure last week. Post-event satellite imagery suggests that it has failed.

Interestingly, NDTV has accessed Maxar satellite imagery that appears to show a landslide:-

(thread) 1. @ndtv analysis of day-old satellite images show a landslide, exposed shoreline and broken ice floes on Sikkim’s South Lhonak Lake with water continuing to flow out more than 3 days since the lake breached its banks to trigger deadly floods downstream. 📸:@Maxar pic.twitter.com/BWRKvk1dRh

— Vishnu Som (@VishnuNDTV) October 7, 2023

It is perhaps surprising that a landslide on this scale was sufficient to trigger a breach event, but it does appear to be the best candidate.

That this lake was a potential cause of a GLOF was well-established, and indeed there have been attempts to lower the lake level with siphons in recent years. There are research papers highlighting the risks at this site – for example, Sattar et al. (2021) wrote:-

“Our results show that the GLOF susceptibility will increase due to the expansion of the lake towards steep slopes, which are considered potential starting zones of avalanches. These avalanches can create an impulse-wave when hitting the lake and are considered the most likely GLOF trigger for the South Lhonak Lake.”

This seems remarkably prophetic. That the Chungthang dam in Sikkim was vulnerable is also not news – for example, the Pulitzer Center has an article from 2014 about the project, which noted that:

“In Chungthang, a sudden influx of glacial debris is likely to compromise the flush gates and the discharge capability of the dam.”

In the aftermath of the 2021 Chamoli disaster in India I highlighted that dam designs in the Himalayas (and elsewhere) are often not adequately considering the hazards associated with extreme events, especially in a changing climate. This accident reinforces the point.

Today is 60th anniversary of what is, to date, the ultimate landslide-induced dam disaster, at Vajont in Italy. It is remarkable that lessons have still not been learnt.

Reference

Sattar, A., Goswami, D., Kulkarni, A., Emmer, A. et al. 2021. Future Glacial Lake Outburst Flood (GLOF) hazard of the South Lhonak Lake, Sikkim Himalaya. Geomorphology,388, https://doi.org/10.1016/j.geomorph.2021.107783.

Text © 2023. The authors. CC BY-NC-ND 3.0
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