A Himalayan tragedy

Why in the News

Scientists investigating the flash flood on the Bhotekoshi river in Nepal have attributed it to a sturzstrom, a high speed rock and ice avalanche near Langtang Lirung on the Nepal China border, rather than to a glacial lake outburst flood (GLOF). The reattribution matters because a flood on the same river in July of the previous year, which also occurred without rainfall, was concluded to be a glacial lake outburst flood, and that conclusion shaped what was being watched for. The flood struck at around 8.40 a.m. and has left more than 600 dead and close to 2,500 missing, including foreign nationals, mostly from India. What is now contested is whether a monitoring and warning system built around glacial lakes can see a hazard that begins as a bedrock and ice collapse.

What is a sturzstrom?

1. The definition: A massive rock avalanche in which a large volume of rock suddenly collapses and travels at extremely high speed over a long distance.
2. Its high mountain form: Where the collapsing rock is mixed with ice, snow, firn and frozen soil, the same event is described as a rock and ice avalanche.
3. Why the runout is so long: Pressure and friction inside the moving mass generate meltwater, which lubricates the mass and its base, reduces resistance and carries the debris much further than a dry rockslide would.
4. How it ends: As it descends it takes in more ice, snow, rock, soil and water, and can transform into a massive debris flow before depositing its material.

Why was the glacial lake explanation set aside?

1. The precedent pointed the other way: A rainless flood on the Bhotekoshi in July of the previous year was concluded by scientists to be associated with a glacial lake outburst flood, so that was the first hypothesis tested.
2. The velocity did not fit: The speed and ferocity of this flood, which washed away villages across three districts, Rasuwa, Nuwakot and Dhading, prompted experts to examine it from a different angle.
3. The onset signature differs: A lake breach releases a stored volume over a measurable period, while this flood arrived with no such build up recorded at downstream gauges.
4. It has no Nepali precedent: No comparable disaster involving a sturzstrom had been recorded in Nepal before this event.

What does the transboundary geography add to the problem?

1. The source area straddles the border: Nepalese and Chinese authorities have concluded that the source lay in the Langtang Lirung area, and identifying the exact collapse point and which side of the boundary it sat on may not be possible.
2. The trigger was a compound event: Satellite imagery shared by the Chinese side pointed to a combination of an ice avalanche, permafrost movement and a rockslide, with the rockslide on the Chinese side and its impact extending into Nepal.
3. The avalanche began in Nepal: A senior divisional hydrologist at Nepal’s Department of Hydrology and Meteorology states the avalanche originated on Nepalese territory very close to the boundary and induced flooding that crossed into China, damaging both sides.
4. The channel is itself transboundary: The location identified is the Lhende Khola, a high altitude river rising in Gyirong County in Tibet and flowing south into Nepal as a tributary of the Bhotekoshi and Trishuli systems.
5. The corridor carries trade and pilgrims: Rasuwagadhi in Rasuwa is the checkpoint through which most of Nepal’s trade with China passes, and from May to September hundreds of pilgrims cross there into Tibet for the Kailash Mansarovar pilgrimage, with Indians the largest share.

How far did early warning and climate attribution actually get?

1. The systems worked and still failed: Experts state that early warning systems were activated as intended but were rendered ineffective by the speed of the flood.
2. The structural difficulty is the terrain: The head of the United Nations Office for Disaster Risk Reduction (UNDRR) has said the event again showed how fragile mountain ecosystems are and how hard it is to build early warning in mountain environments.
3. Attribution stops short of a direct link: Scientists have not ruled out a role for climate change and point to rising temperatures warming the Himalayas and accelerating glacier melt, without directly linking this disaster to global warming.
4. Access is limiting the assessment itself: The International Federation of Red Cross and Red Crescent Societies (IFRC) reports that remoteness combined with damaged infrastructure is obstructing both damage assessment and the delivery of assistance, with around 93,000 people estimated to be affected.

What does the 2021 Chamoli comparison establish?

1. The mechanism has an Indian precedent: The 2021 Chamoli disaster in Uttarakhand began with a bedrock failure beneath a glacier, which triggered a rock and ice avalanche that transformed into a debris flow and flood.
2. It is described the same way in the literature: Chamoli has been scientifically described as a rock and ice avalanche and, in some literature, explicitly as a sturzstrom.
3. The scale of material involved: Research found that more than 25 million cubic metres of rock and ice broke loose high in the Uttarakhand Himalaya before moving rapidly downstream.
4. The comparison narrows the watch list: Two events of the same mechanism a few years apart in the same mountain system establish an unstable rock and ice slope, not only a moraine dammed lake, as a recurring source of catastrophic flooding.

Challenges to early warning in high mountain catchments

1. The monitored hazard is the wrong hazard: Warning networks in the Himalaya are built around glacial lake inventories and lake level telemetry, which observe nothing at an unstable rock and ice face. Eg. Nepal’s national hazard inventories catalogue glacial lakes by area and moraine condition, with no equivalent register of unstable rock and ice faces. Fix. Add slope stability and permafrost monitoring at identified rock and ice faces to the existing glacial lake inventories.
2. Lead time collapses to minutes: A sturzstrom accelerates and reaches settlements in the time a lake outburst would take to build, so a warning chain designed for hours has nothing to work with. Eg. Riverside towns such as Betrawati in Nuwakot were buried on the same morning the collapse occurred. Fix. Move from advisory dissemination to automatic siren triggering at gauge stations, removing the human decision step from the chain.
3. The hazard does not respect the border: The collapse zone straddles the Nepal China boundary, so neither state’s monitoring network alone observes the full source area. Eg. The trigger was established only from satellite imagery shared by the Chinese side. Fix. Put the Lhende Khola and comparable transboundary catchments under a standing data sharing arrangement with agreed real time thresholds.
4. Exposure is concentrated in the valley floor: Trade routes, hydropower works, bazaars and pilgrimage traffic all occupy the same narrow corridor a debris flow uses. Eg. The Rasuwagadhi corridor carries most of Nepal’s China trade and the seasonal Kailash Mansarovar pilgrimage. Fix. Apply a debris flow runout based building line along these corridors rather than a flood return period line drawn from rainfall records.
5. Assessment capacity fails exactly when it is needed: Remote terrain and destroyed access mean the size of the disaster is unknown for weeks, which delays both relief and reconstruction decisions. Eg. Officials state the full scale of damage may take weeks or months to assess. Fix. Pre position aerial survey and satellite tasking agreements so a post event damage assessment does not depend on ground access.

Conclusion

The scientific account of this flood has moved from a glacial lake outburst to a sturzstrom originating on an unstable rock and ice slope near the Nepal China border. That shift changes what has to be monitored, since a lake inventory does not observe a bedrock face, and it places the source area inside a boundary zone neither country monitors alone. Search and rescue continues with more than 15,000 personnel deployed. The next milestone is the joint Nepalese and Chinese determination of the exact collapse point, which will decide whether transboundary monitoring of that catchment is put in place.

Back2Basics: Glacial Lake Outburst Flood (GLOF)

1. What it is: The sudden release of water held in a glacial lake, which sends a large volume downstream in a short period.
2. How the lake forms: Meltwater collects behind a moraine ridge or an ice dam left by a retreating glacier, so the impounding structure is loose debris or ice rather than rock.
3. What breaches it: Overtopping by an avalanche or landslide into the lake, seepage that erodes the moraine from within, or failure of the ice dam itself.
4. Why it is tracked in the Himalaya: Glacier retreat is adding to both the number and the volume of these lakes, which is why national inventories monitor them and why they were the first explanation tested for this flood.

Matching Previous Year Question

“[2021, GS1, 10 marks] Differentiate the causes of landslides in the Himalayan region and Western Ghats. (150 words) हिमालय क्षेत्र तथा पश्चिमी घाटों में भू-स्खलनों के विभिन्न कारणों का अंतर स्पष्ट कीजिए। (150 शब्दों में उत्तर दीजिए)”