Two days after a glacier collapse sent a devastating mudslide-driven flash flood through parts of Nepal and Tibet last Wednesday, a fresh risk of another severe flood has emerged in the same area.
A new lake formed in the area after the glacier collapse. Water from that lake has been spilling over since last Friday. This has spread intense panic in the region.
Rescue operations had to be temporarily suspended over fears that another deadly disaster could strike.
The flash flood and landslides caused by the glacier collapse have so far killed 623 people, according to confirmed reports. More than 2,400 people remain missing.
What happened
Officials in Nepal’s Rasuwa district said last Friday they had received reports that water from a lake formed by debris was beginning to overflow.
Rasuwa district’s top official, Narendra Pariyar, said: “We can see the river’s water level rising.”
Pariyar added: “It isn’t clear exactly how much the water level has risen right now. We don’t know how large it has become or what its water level is. But we can see the river’s water level rising.”
Scientists say the disaster created not one but two new lakes. One formed at a higher elevation near the glacier, where glacial meltwater is now trapped.
The other is much larger and more dangerous. This lake formed on a river in Nepal, where a large landslide blocked the valley’s path. This blocked the river’s flow, creating the lake.
What’s the risk
Nepali and Chinese authorities warned last Thursday that the downstream lake had formed from accumulated debris in the flood-affected area.
Nepal’s National Disaster Risk Reduction and Management Authority warned that the lake’s water level is rising. As a result, it could overflow or its dam could break at any time, triggering a massive flash flood.
China’s state broadcaster CCTV reported last Friday that Chinese authorities had deployed an eight-member engineering team. The team is conducting an aerial survey of the debris-formed lake to build a three-dimensional model of it.
Reaching the site where the lake formed will require overcoming considerable challenges.
Chen Hanxiao, a member of the Chinese engineering team, told CCTV the day before, Thursday, that after the lake formed from the glacier collapse, many isolated steep mountains and inaccessible slopes had formed around it.
Where the lake formed
The larger, more dangerous lake formed near the confluence of the Chochen Khola and Purepu Sangpo rivers, part of Nepal’s Lende river system.
The Kathmandu Post reported that the lake is located upstream of the Bhotekoshi and Trishuli rivers. Water from these two rivers flows south toward Kathmandu.
How the lake formed
Alongside the flash flood caused by the glacier collapse last Wednesday, a landslide also occurred. This caused rock, ice, mud and debris to accumulate in the river’s path.
Pawan Bhattarai, assistant professor in the civil engineering department at Kathmandu-based Tribhuvan University, told Al Jazeera that the new lake is likely a dam lake formed by an avalanche. A massive collapse of ice and rock sent huge volumes of rock, ice and debris into the river valley, blocking or severely narrowing the drainage path.
Pawan further said that water then began accumulating behind this naturally formed dam, creating the lake. Satellite images and reports from several Chinese media outlets have also confirmed the lake’s existence.
Nitesh Khadka, a Nepali researcher and physical geographer who studies glacier-related flooding, told Al Jazeera: “After the disaster, satellite images and drone footage showed that two lakes had formed — one upstream, and the other on the Lende river.”
Explaining further, Khadka said the lake formed upstream on Chinese territory resulted from trapped glacial meltwater. That lake has blocked the normal drainage path of the water.
Khadka also said the lake on the Lende river formed after a landslide or mudslide blocked the river’s flow, trapping the water. This is a natural process that cannot be prevented. But he said that if it becomes dangerous, the dam could potentially be breached in a safe, controlled way to release the water. However, given the remote and complex terrain, he said that too seems unlikely to be feasible.
The dam-like lake formed by the mudslide at the Nepal-China border is also located in remote, steep terrain. Getting rescue workers or heavy equipment there is extremely difficult.
Pawan explained that neither the glacier collapse nor the resulting natural dam could likely have been prevented.
However, he said better satellite monitoring, quickly identifying blocked river paths and newly formed lakes, repeated monitoring of river flow, timely evacuation of people to safe areas, and a strong early-warning system across the Nepal-China border could have helped reduce the damage.
Why this is such a major risk
When a lake forms suddenly, the surrounding terrain usually isn’t equipped to hold that much water. At the same time, when a lake’s water is held back by a natural dam made of loosely packed rock and mud, even a moderate overflow can erode that dam. This can cause the dam to suddenly break, sending a massive, destructive flood downstream.
Footage captured by security cameras on the Chinese side of the border shows a powerful surge of mud, water and debris crashing into a multi-story building. The building is submerged within moments. Buses and vehicles are swept away like toys in the flood.
Pawan said the danger isn’t just about the lake itself, but the risk of the natural dam suddenly breaking. In such steep valleys, even a temporary dam breach can trigger a fast-moving, debris-laden flood or flash flood. As a result, downstream residents get very little time to take warning.
China’s Ministry of Water Resources calculated Thursday that the lake currently holds 1.5 million to 2 million cubic meters of water. It warned that roughly another 3 million cubic meters of water could flow into the lake over the next three days. That volume of water is roughly equivalent to about 1,200 Olympic-sized swimming pools.
Khadka said this is a major threat, because if the newly formed lake’s dam suddenly breaks, another large-scale flood could occur. This could sweep away silt and debris accumulated on the riverbed, hampering rescue efforts, and could also endanger people still alive in hydropower project tunnels.
This means that if the lake overflows into a flood, the flood’s current could carry mud and rock along with it, further increasing the water’s destructive power and making the work of rescue teams searching for missing people even harder.
Khadka also said the danger extends to workers and engineers in downstream hydropower project tunnels, as well as flood-displaced people sheltering nearby. A sudden, large-scale flood could strike these tunnels and power stations with force, trapping or injuring anyone inside.
State of the rescue operation
Rasuwa district was the hardest-hit by Wednesday’s flood. Because of the newly formed lake’s overflow, rescue operations in Rasuwa district were temporarily suspended on Friday. However, after a brief pause, rescue efforts have resumed.
China’s state broadcaster CCTV said Friday that the risk from the new lake’s overflow is “manageable.”
A World Health Organization (WHO) official said Friday that six of their health care centers were damaged in the flood, and eight health workers are missing.