Scientists are examining a new theory to determine what triggered the devastating flash floods along the Nepal-Tibet region, after two initial explanations were ruled out. Nepal Centre for Disaster Management president Bishal Nath Upreti said the disaster was initially believed to be linked to a glacial lake outburst flood (GLOF), but that theory was later abandoned after authorities found no evidence of a glacial lake bursting along the affected route in Tibet. The disaster has raised fresh questions about the exact chain of events that unleashed such a massive flood and debris flow. According to Upreti, researchers are now studying satellite imagery to understand what happened at the remote site, which has not yet been reached by scientists.
GLOF theory discarded after no lake burst found
In the immediate aftermath of the disaster, the sudden release of a huge volume of water from a glacial lake was considered one of the possible explanations. Such events, known as glacial lake outburst floods, can send enormous quantities of water rushing through mountain valleys within a short period.
However, information gathered later suggested that there had been no glacial lake burst in Tibet along the route affected by the disaster. This led authorities and scientists to move away from the GLOF explanation.
“It has already been five or six days. Initially, many thought it was a glacial lake outburst flood, or GLOF. Later, it became clear that there had been no lake burst in Tibet along that line, so that theory was discarded,” Upreti said.
Scientists also rule out earthquake as the original trigger
Another explanation emerged during the initial assessment, suggesting that an earthquake had caused a huge mass of ice and rock to crash into the valley, triggering the devastating sequence.
However, Upreti said information from the US Geological Survey indicated that the sequence may have been exactly the opposite. According to him, the massive fall of rock and ice generated the earthquake rather than an earthquake causing the rock and ice fall. The tremor was initially measured at magnitude 4.2 but was subsequently revised to 5.2, he said. This distinction is significant because it changes the understanding of what set the disaster in motion.
” It was then thought that an earthquake had caused an ice and rock fall. But the USGS later clarified that it was the other way around- a massive rock and ice fall into the valley generated the earthquake. It was initially recorded at 4.2 magnitude but later revised to 5.2. So, the disaster was not caused by an earthquake; rather, the falling ice and rock generated the earthquake,” Upreti added.
More than 5,000 dead or missing; toll could rise further
Upreti also pointed to the extraordinary scale of the destruction. He said more than 5,000 people have already been reported dead or missing following the floods, while warning that the figure could eventually reach 7,000 or even 10,000. He also described another landslide that occurred several hours after the first major event. According to him, the second landslide formed a temporary dam, which subsequently breached and sent another, comparatively smaller flood downstream.
“About three hours and eight minutes later, another landslide occurred, creating a dam that breached and caused another, smaller flood. Fortunately, it flowed slowly and did not result in a major disaster. We have not seen many such incidents in recent times. A similar situation occurred in Chamoli, Uttarakhand, where ice and rock fell from a height of nearly two kilometres, creating a similar debris flow. There were around 200 deaths then. Here, however, more than 5,000 deaths or missing persons have already been recorded, and the figure could rise to 7,000 or even 10,000,” he remarked.
What makes this disaster different from a conventional GLOF?
A conventional GLOF occurs when water stored behind or within a glacial lake is suddenly released, sending a powerful surge downstream. The resulting flood can carry rocks, soil and other debris and cause extensive destruction along mountain valleys. The Nepal-Tibet disaster, however, appears to have involved a different mechanism. Upreti said the available evidence points towards a massive movement of ice and rock rather than the sudden release of water from a glacial lake.
“We have always known about glacial lake outburst floods, where millions of cubic metres of water can rush down a valley, almost like a tsunami from the sky. But this was different; it was simply ice and rock falling down. Scientists are still trying to determine exactly what happened. Since nobody has reached the site, the research is based mainly on satellite imagery. One emerging hypothesis is that it was actually a massive rockslide, with the ice riding over the falling rock, rather than the glacier itself sliding down. As a scientist, I cannot confirm this yet; these are still developing hypotheses,” he further stated.
New rockslide theory emerges, but scientists remain cautious
One of the emerging possibilities is that a huge rockslide may have occurred in the mountainous terrain, with a layer of ice moving along with the collapsing rock. This would be different from a glacier itself sliding downhill and could help explain the combination of rock, ice and debris involved in the event.
However, Upreti stressed that this remains a hypothesis and not an established scientific conclusion. Since researchers have not yet physically reached the affected location, they are relying heavily on satellite observations to reconstruct the sequence of events.
Why satellite imagery is crucial to the investigation
The remoteness of the disaster site has made a direct scientific assessment difficult. In the absence of ground-level access, satellite images can provide researchers with valuable clues about changes in the terrain, the location of landslides and the movement of ice and rock.
Scientists will need to establish the precise sequence of events before determining whether the rockslide hypothesis can explain the scale of the resulting floods and debris flows. The ongoing investigation could also provide important insights into similar hazards in the Himalayan region.
A disaster whose exact trigger is still being established
The evolving explanations underline how difficult it can be to identify the immediate trigger of a major Himalayan disaster in its early stages. What initially appeared to be a GLOF was later questioned, while the earthquake theory also changed after the reported USGS assessment of the sequence. For now, scientists are examining whether a massive rock and ice fall, potentially involving a rockslide with ice moving over it, could have initiated the destructive chain of events. Upreti has made it clear that the theory is still developing and requires further scientific investigation before it can be confirmed.
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