Nepal is facing the risk of a secondary flash flood after debris from an ice-rock avalanche that may have triggered deadly flooding accumulated at a river confluence, creating a temporary dam, a senior cryosphere specialist at the International Centre for Integrated Mountain Development (ICIMOD) has warned.
The term cryosphere refers to parts of the Earth's surface covered by frozen water.
As of Friday, more than 470 people had died in flash floods in Nepal and Tibet, with nearly 1,000 still reported missing as search and rescue operations continued for a third day.
Researchers from ICIMOD, based near Kathmandu, are investigating whether an ice-rock avalanche originating at a high-altitude location may have triggered the floods.
"This is basically an ice and rock avalanche. So, as per current understanding, there is a detachment of the bedrock, which means that the bedrock from below the glacier is detached from its position and came down to the valley that created this disaster," said Mohd Farooq Azam, a senior cryosphere specialist at ICIMOD.
"This debris that came down with this flash flood is now also accumulating at the river confluence, where there is temporary damming at the moment. This dam is collecting water, and it is expected that over the next three days, there will be more water collected in this temporary pond or lake. So, there are chances that there may be some secondary flash flood in this valley," he said.
Azam noted that it was also possible that the lake could breach gradually, allowing water to be released without causing another flash flood.
Drawing similarities with flash floods that hit Uttarakhand's Chamoli in February 2021, Azam said, "I would say that this is the same kind of situation when in Chamoli, some mass came down and blocked the Rishi Ganga river for some time."
"Fortunately, the secondary lake that developed breached slowly without any flash flood. So, let us hope that there is no secondary flash flood (in Nepal) because of (a) sudden breach of this temporary lake," the cryosphere specialist said.
Nepal has issued a fresh flood warning after the water level at the site of the flash floods increased. The National Disaster Risk Reduction and Management Authority (NDRRMA) said a rise in water levels at the same site increased the risk of another flood in the area.
Even a small secondary flash flood could be more devastating as the valley is "already much more vulnerable and very sensitive", Azam cautioned.
Asked whether such disasters could be prevented, Azam said, "It is difficult to say that such disasters can be prevented, but there are several disasters where we understand in advance that there may be a disaster. Specifically, when we talk about glacial lake outburst floods, because these lakes are very much visible and they are expanding."
An assessment of a glacial lake can indicate whether it is potentially dangerous, he explained. "In that case, if we are basically monitoring the lake and we are sharing the data, then it helps to develop some strategy at regional scale," Azam said.
Azam highlighted that flash floods or glacial lake outburst floods occurring upstream do not recognise borders and can travel downriver, affecting other countries.
"In this particular case, we see that the disaster happened in the Nepal Himalaya and downstream, it immediately entered into the Tibetan boundary, where it hit the infrastructure there and then again it came down towards Nepal," the cryosphere specialist said.
"So, this is why we need to have a regional cooperation and data sharing policy to better understand these disasters and perhaps to be ready for such disasters in advance," he said.
(With inputs from PTI)