Could Nepal's Disaster Have Been Foreseen?

'This is no longer a single-hazard problem; it is a cascading, multi-hazard problem. A landslide never occurs in isolation.
Whenever there is a landslide, the river gets blocked, a natural dam forms, and after days or sometimes months, that dam breaches and a flood is released downstream.
This entire chain is the cascading effect of what looks, at first, like a single disaster.'

By the time rescue crews reached the Bhotekoshi valley in Neal on Wednesday morning, there was very little left to rescue.

A wall of ice, rock and mud had come down from a glacier high above the Nepal-China border and, within minutes, swallowed entire stretches of Rasuwa and Nuwakot districts.

As of Thursday morning, more than 160 people were confirmed dead, and Nepali officials said close to 600 tourists remained unaccounted for, among them dozens of Indians, Americans, Britons and Australians, many believed to have been travelling to or from Kailash Mansarovar.

Scientists at the US Geological Survey have since clarified that the tremor first picked up in the area was not an earthquake at all, but the seismic signature of the glacier collapsing on itself.

It is a chillingly familiar story. Almost exactly a year earlier, the same stretch of river flooded after a glacial lake across the border in Tibet spilt over; nine people died then, and a bridge linking the two countries was washed out.

Barely a fortnight ago, a similar cascade struck Uttarakhand's Tamak Nala. Before that, there was Dharali, and before that, the glacial lake outburst that tore through Sikkim in 2023, taking a hydropower project down with it.

For Dr Vikram Gupta, an engineering geologist at Sikkim University who has spent years mapping landslide risk across the Himalaya, none of this counts as surprise any more.

In this interview with Prasanna D Zore/Rediff, he explains why the trouble almost always begins in places nobody is watching, why satellites can only tell you what has already happened rather than what is about to, and why, in his reckoning, the Himalaya's biggest disasters no longer have much to do with heavy rain or cloudbursts at all.

'It was a glacier avalanche that triggered a seismic signal resembling one'

The disaster in Nepal appears to have begun high in the mountains, with a massive block of ice and rock breaking away, before turning into a devastating flood downstream.
From a geological standpoint, how does a single event on a mountain slope trigger such an extraordinary chain reaction across an entire river valley?

This kind of event is becoming very common now. You have seen it in Chamoli, you have seen it in Dharali, and you have seen it in the Sikkim glacial lake outburst.

Geologically, the upper slopes in these regions are extremely unstable -- there is loose material all along the glaciated terrain.

When this material gets mobilised, for various reasons, but increasingly because of global warming and climate change, conditions become highly unstable.

The cascading effect we are now seeing is very common, and it is well documented and well understood.

Is this a pattern across the entire Himalayan range, or specific to certain stretches?

It is common across the High Himalaya, yes.

There was initial speculation about an earthquake near the Nepal-Tibet border, but the evidence has since pointed to a glacial collapse. What does that tell us about how difficult it is to identify the trigger of a Himalayan disaster in real time?

When we analysed the data, it became clear this was not an earthquake -- it was a glacier avalanche that triggered a seismic signal resembling one. That has now been confirmed.

As for whether we have the infrastructure to identify such triggers in real time along a mountainous river -- the honest answer is no. But while we may not be able to predict these events as they happen, we can certainly reduce the risk they pose to communities downstream.

How can the risk be reduced if we cannot know, in real time, what is happening in the upper reaches of the Himalaya?

We already know these events are becoming common because of climate change, so the first step is to avoid building habitations right along the rivers. That is non-negotiable.

Second, wherever habitations already exist, we need sensor networks along the rivers that can track rising or falling water levels -- a sudden rise itself becomes an early alarm.

In India, the Central Water Commission has, I believe, made a recommendation along these lines to NITI Aayog, though I am not certain what guidelines the government has actually put in place.

How important is it to identify the precise trigger -- in this case, a glacial avalanche that generated a seismic signal -- before we can prepare for the next disaster?

It is very important, but also very difficult, because most of these disasters originate in upstream regions where there are no habitations and access is extremely hard. It took more than twenty-four hours just to establish what had actually happened in Nepal, and even now there is some uncertainty about the exact sequence.

But broadly, the sequence is settled now -- a glacial avalanche caused a seismic event, which led to the formation of a temporary dam upstream, and that dam then burst?

Yes, more or less that much is established. What we still lack is quantification -- how much volume of glacier displaced, and how much water actually came down. That data is yet to be worked out.

'We no longer need a cloudburst-type event to trigger these disasters'

We have hundreds of satellites in orbit. Could one of them have picked up warning signs and issued an alert before this happened?

A satellite can only show you a picture after something has already happened. It cannot predict an event.

Can it at least detect the formation of an artificial lake once it has formed, and trigger an alert from there?

Yes, it can detect a lake once it has formed. But the real problem is that we do not know how long that lake had already been sitting there before it was even noticed -- it could be a matter of minutes, hours or days. That gap is still unresolved.

You have studied landslide-prone Himalayan terrain extensively. Are we now looking at a genuinely new and dangerous phenomenon -- where rising temperatures, melting ice and weakening slopes combine to cause disasters that no longer need a cloudburst or heavy rainfall to begin with?

Yes, that is exactly what I am saying. Over the last two decades, largely because of global warming, we no longer need a cloudburst-type event to trigger these disasters.

A sudden rise in temperature and the release of an entire volume of melt-water is now, by itself, the trigger in most cases.

Sikkim had its own devastating disaster in 2023. Looking at Nepal now, would you say a clear pattern has emerged -- that the real danger begins far upstream while the worst destruction happens many kilometres downstream -- and that this pattern is now well established enough to demand serious study?

Yes, that is exactly what our data shows. Look at what happened in Uttarakhand. The first such event we recorded was in Himachal Pradesh in 2003, when the Parichu Lake, on the Tibetan side along the Sutlej river, burst. Then it happened again in 2005.

Since then there has been a whole series of these events -- the Kedarnath disaster in Mandakini, though that had a somewhat different character, then Dharali, and just a couple of days before Nepal, the Tamak Nala disaster in Uttarakhand. And now this.

'We have been learning this same lesson for the last ten to fifteen years: We need to stop encroaching on our rivers and start respecting their land.'

And Sikkim had a similar event -- a hydropower station washed away?

SEE: Flash flooods hit Nepal-China border. Video: ANI

Yes, that was 2023, from the South Lonak Lake outburst.

Does this mean our traditional way of assessing disaster risk has become too narrow, and that we now need to study the entire mountain-to-river system above -- glaciers, slopes and lakes together -- rather than looking at individual hazards such as earthquakes in isolation?

Yes, exactly. This is no longer a single-hazard problem; it is a cascading, multi-hazard problem. A landslide never occurs in isolation.

Whenever there is a landslide, the river gets blocked, a natural dam forms, and after days or sometimes months, that dam breaches and a flood is released downstream. This entire chain is the cascading effect of what looks, at first, like a single disaster.

So should our whole model for assessing disaster risk change? What new kind of study should scientists and researchers now be undertaking?

Yes, the time has come for multi-hazard disaster studies. Looking at hazards in isolation is simply not acceptable any more. We also need proper guidelines on where infrastructure can and cannot be built. For now, we need to give the rivers their space -- it is their natural ecosystem, and we ought to respect that instead of encroaching on it, which is exactly what we have been doing by allowing more and more settlements along riverbanks.

Can scientists identify warning signs on an unstable Himalayan slope or glacier before it actually collapses -- could this particular avalanche have been picked up in advance?

In principle, yes. There are satellite-based techniques such as InSAR -- interferometric synthetic aperture radar -- that can measure slope and ice movement down to the millimetre scale. That gives us some sense of which slopes are moving. But the Himalaya is enormous, and identifying every unstable slope across that scale remains genuinely difficult.

For Himalayan India -- Sikkim, Arunachal Pradesh, Himachal Pradesh, Uttarakhand -- and also for the plains downstream, where there is already talk of these floodwaters reaching the Kosi and the Gandak, what is the single biggest lesson from Nepal?

We have, in truth, been learning this same lesson for the last ten to fifteen years: We need to stop encroaching on our rivers and start respecting their land.

Does that mean we need to fundamentally rethink where we build roads, hydropower projects, hotels and settlements in Himalayan river valleys, given how common these disasters have become?

Yes, absolutely. It is becoming extremely common now, and that rethink is overdue.