'It appears to be a combination: A glacier breaking away, and a dam-like formation downstream reaching capacity and releasing water suddenly.'
Four days after a wall of ice, rock and water tore through the Trishuli valley and across the Nepal-Tibet border, the scale of what happened on the morning of August 26 is only beginning to sink in.
Entire settlements along a 70-odd kilometre stretch of the Trishuli were flattened within minutes; bridges, hydropower stations and the busy Gyirong border crossing were simply erased. Bodies have since been recovered as far away as India, some 150 kilometres downstream.
What began as a story about an unusually violent flash flood has, in the days since, turned into something more unsettling: mounting evidence that a chunk of glacier broke away high in the Himalaya, possibly near Langtang Lirung, and came crashing down with such force that seismometers as far as the United States picked up the tremor.
Scientists piecing together satellite images now believe the collapsing ice dammed a river before bursting through, sending a surge estimated at well over a hundred feet high hurtling downstream.
It is against this backdrop -- a still-unfolding disaster, an exhausted rescue effort, and a region that has seen flash floods of this kind before, in Sikkim, Uttarakhand and Nepal's Khumbu valley -- that Professor Deepankar Choudhury of IIT Bombay, a specialist in geotechnical and earthquake engineering, spoke about what such a collapse can tell Prasanna D Zore/Rediff, why it behaves like a seismic event even without an earthquake, and why the Himalaya, as one of the world's youngest mountain ranges, may simply not forgive the way we build upon it any longer.
The first of an illuminating two-part interview.
The Nepal disaster was initially reported as a flash flood, but the evidence now points to a glacier collapse that triggered an avalanche of ice, rock and debris. From a geotechnical standpoint, what appears to have happened?
Before I answer that, I extend my deepest grief and sadness for this recent flash flood disaster in Nepal. I was on an official visit to Sikkim during this disaster in Nepal, but that visit was for an entirely separate assignment -- a government of Sikkim project, to assess foundation and geotechnical related studies for a proposed sky-deck through ropeway at around 3,300 metres altitude, meant to boost tourism there.
So, I didn't get much detailed information about the Nepal disaster. I was at such high altitude with hardly any mobile signal for days. So, I want to be upfront: I haven't had the chance to study this event in depth yet, and I'm cautious about commenting authoritatively without proper research.
As I've said before, a technical expert shouldn't speak like a layperson simply because a camera is pointed at them -- that does a disservice to the profession. That said, from what little I've followed -- mostly through news reports and updates from my postdoctoral scholar and PhD students of IIT Bombay, who've been tracking it closely -- it doesn't look like there's one single, clean cause.
It appears to be a combination: A glacier breaking away, and a dam-like formation downstream reaching capacity and releasing water suddenly.
I'm also told there have been flood warnings as far as Bihar, which fits with what's being reported about the Trishuli river again showing signs of rising.
When a natural dam formed by debris fills up and gives way, it can trigger flooding much like a structural dam failure would. In situations like this, the sensible thing is to evacuate people downstream well in advance.
The collapse reportedly generated forces powerful enough to register as a seismic event. How does the failure of a glacier and mountainside produce that kind of energy, and what does it tell us about the scale of this disaster?
Actually, this is quite well understood in our field. Fault movements broadly fall into three types -- divergent, convergent, and strike-slip or transform movement.
When a glacier suddenly collapses, it can drop one section of crustal plate/ground relative to its surroundings. That abrupt vertical shift between two points can itself generate seismic energy, enough to register as a small earthquake.
This isn't new or unusual. There's plenty of documented literature on it. In the North Sea, for instance, there have been tsunami-like events triggered not by earthquakes but by icebergs -- large ships and submarines have struck them, and when a section of an iceberg breaks off and slams into the seabed, it produces the same kind of plate/ground movement that generates seismic-scale energy.
Small, localised tremors from a glacier collapse are entirely consistent with that. It's a well-established phenomenon in earthquake engineering -- nothing out of the ordinary, scientifically speaking.
Your research covers slope stability and how rainfall and seismic activity combine to trigger landslides. Are we now seeing a dangerous convergence of warming temperatures, melting ice, unstable slopes and extreme weather across the Himalaya?
Definitely -- that's a real and growing concern. With climate change, what worries us more than a steady rise in temperature is the delta-T -- the swing between extremes.
A region that experiences sharp fluctuations rather than a stable high or low is far more vulnerable. When that kind of volatility sets in, you start seeing glacier movement, sudden failures, cloudbursts and similar events far more often.
So, while I wouldn't reduce this disaster to a single cause without proper study, climate variability is almost certainly one of several contributing factors.
Is the melting of glaciers and thawing permafrost effectively weakening the structures that have held these mountain slopes together for centuries?
SEE: Flash floods hit Nepal-China border. Video: ANI
Yes, absolutely. As I said a moment ago, the wide swings in temperature that come with climate change are quite capable of destabilising terrain across the Himalayan belt -- glaciers collapsing, sliding, and then striking the river systems below.
Here's the part that concerns me as an engineer: Most dams downstream are designed for a specific discharge capacity based on historical river flow behaviour, and for seismic loading, as they should be. But very few are designed for a catastrophic, sudden extreme event like a glacier breaking away entirely.
Whether today's structures adequately account for this additional climate-driven risk, I'm honestly not certain. This is precisely the moment to revisit our designs -- to retrofit existing structures and rethink the assumed service life of dams in this region with that possibility of extreme events built in.