Two days after a glacier collapse sent a catastrophic flood roaring through parts of Nepal and Tibet on Wednesday, a newly formed river has broken the banks, and a renewed flood threat is looming.
The river breach on Friday halted rescue operations in the area after flash floods and mudslides caused by glacial collapse from the Himalayas led to 469 confirmed deaths. More than 1,000 people remain missing across Nepal and China’s Tibet.
A lake has formed by the buildup of debris at the border of Nepal and China at the flood site, and as it continues to rise, it increases the flood risk.
What happened?
On Friday, officials in Nepal’s Rasuwa district said they had been notified that the barrier lake had begun to overflow.
“We can see that the level of water in the river is rising,” Narendra Pariyar, Rasuwa’s most senior official, said.
He added that it is currently unclear how high the water level is. “We do not know how big it is and how high is its water level. We can see that the level of water in the river is rising.”
Scientists say this disaster has created not one but two new lakes. One has formed high up near the glacier, where meltwater is now trapped.
The other, much larger and more dangerous lake sits on a river in Nepal, where a huge landslide blocked the valley and dammed the river.
What was the threat?
Nepali and Chinese authorities warned on Thursday that the lower lake had formed due to the buildup of debris at the site of the flood.
The lake is rising in level, which means it can burst, Nepal’s National Disaster Risk Reduction and Management Authority (NDRRMA) warned.
Chinese authorities said they have mobilised an engineering team of eight members to carry out aerial surveys and 3D modelling of the barrier lake, Chinese state broadcaster CCTV reported on Friday.
However, there are challenges when it comes to accessing the site.
“After the glacier collapse created the lake, many isolated cliffs and steep cliffs appeared along it,” one of the engineers, Chen Hanxiao, told CCTV on Thursday evening.
Where is this lake?
The lake is near the confluence of the Chhochen Khola and Purepu Tsangpo rivers on the Lhende river system in Nepal.
It is flowing upstream of the Bhotekoshi and Trishuli rivers that carry water south towards Kathmandu, The Kathmandu Post reported.
How was this lake formed?
The glacial collapse which led to the Wednesday flash floods also caused mudslides, pushing rock, ice, mud and debris into river systems.
“The new lake appears to be an avalanche-induced barrier lake. The ice-rock avalanche deposited a huge volume of rock, ice and debris in the river valley, blocking or severely constricting the drainage,” Pawan Bhattarai, assistant professor at the civil engineering department of Nepal’s Kathmandu-based Tribhuvan University, told Al Jazeera.
“Water then accumulated behind this natural barrier, forming the lake. Satellite imagery and even some Chinese media reports has confirmed the barrier lake.”
These immense piles of debris blocked river channels, creating a natural dam and hence causing this lake to form.
“The satellite images and drone videos taken show two lakes formed after the event. One is formed in the upper part and other in the Lehende river,” Nitesh Khadka, a Nepali researcher and physical geographer who specialises in glacial floods, told Al Jazeera.
Khadka explained that the upper lake – the one in China – formed when the melted glacial water pooled and prevented natural drainage.
“The lake on the Lehende is formed when land/mudslide blocked the flow and water got impounded. It’s a natural process and can’t be prevented,” Khadka added.
“If it poses a danger, safe and control breaching could be done but it also seems infeasible due to complex terrain.”
Meanwhile, the terrain where the mudslide barrier lake – on the Nepal-China border – has formed is remote and steep, where it is also very difficult to bring crews or heavy machinery.
“The avalanche and formation of the natural dam itself were probably not preventable,” Bhattarai explained.
“However, the consequences could potentially have been reduced through better satellite monitoring, rapid detection of river blockage and lake formation, real-time river monitoring, timely evacuation, and strong Nepal-China transboundary early-warning systems.”
Why is it such a big risk?
When sudden lakes form, the surrounding area is not in a position to hold it. Additionally, when a barrier lake is held behind only by loosely packed rocks and mud, even moderate overflow can erode the barrier and trigger a sudden, destructive outburst flood downstream.
“The danger is not just the lake itself, but the possibility of sudden dam failure. In a steep valley like this, even a short-lived breach can generate a fast-moving debris-laden flood with very little warning for downstream communities,” Bhattarai said.
China’s Ministry of Water Resources estimated on Thursday that the lake is holding about 1.5 to 2 million cubic metres of water and warned that about 3 million cubic metres more could flow in over three days.
This is the equivalent of about 1,200 Olympic-sized swimming pools.
“It is a threat, because if a formed lake suddenly breaches it may cause another flood; it may mobilise the sediments that will hamper search and rescue operation and will also put dangers to those surviving in the hydro tunnels,” Khadka said.
This means that if the lake causes a flood, the flood will pick up mud and rocks, making the water even more destructive and making it harder for teams to search for the missing.
Khadka added that this also poses a threat to workers or engineers in hydropower tunnels downstream, or the displaced flood survivors in the vicinity. If a sudden flood rushes down, it could slam into these tunnels and power stations, trapping or injuring anyone who is still there.
“We may not be able to stop the natural process completely, but we can reduce the flood risk through continuous monitoring, safe lowering of the lake where feasible, early warning and timely evacuation downstream,” Bhattarai said.
What’s the status of rescue efforts?
Rescue efforts have been impeded due to the floods and mudslides.
In the hard-hit Rasuwa district, they were paused on Friday since the barrier lake broke its banks. However, after the brief pause, the efforts were resumed. China’s state broadcaster CCTV said on Friday that the risks from the overflowing barrier lake were “manageable”.
According to a World Health Organization (WHO) official on Friday, six health facilities have been damaged by the floods, and eight health workers are missing.
Nepal has said it does not need help from abroad.
“The offer for help is natural. Our security agencies are doing the search and rescue operation. For now we don’t require such help,” Lok Bahadur Chhetri, Foreign Ministry spokesperson, said on Friday.
Has this happened before?
While this exact barrier river has not formed before, glacial collapse and glacial lake outburst floods have happened before in the Himalayas.
“Similar cascading events have happened before. Nepal’s 2012 Seti flood is an important example, where rockfall, temporary river blockage and a rock-ice avalanche combined to generate a destructive flood,” Bhattarai said.
“The exact mechanism in Rasuwa is still being investigated, but the broader process is not unprecedented.”
Simply put, a glacial collapse refers to when a large mass of glacier ice detaches from its bed.
This sometimes involves millions of cubic metres of ice, rock and water, which can transform into a highly mobile ice-rich avalanche or debris flow.
Glacial lake outburst floods (GLOF) refer to the sudden and catastrophic release of vast volumes of water from melting glacier-fed lakes.
In July 2016, the lower part of a valley glacier in Tibet’s Aru Range detached, sending about 68 million cubic metres of ice down the mountain and killing nine herders and hundreds of animals. This was located in far western Tibet, near the Pangong Tso region along the India-China border. Two months later, a nearby glacier collapsed, releasing about 83 million cubic metres of ice.
In November 2022, about 40 million cubic metres of glacier ice detached from the Bukadaban Feng glacier in the Kunlun Mountains on the northern edge of the Tibetan Plateau, in China’s Qinghai province. An ice avalanche crashed into a lake below, creating a 13-meter (42ft) high wave, according to analysis by the European Space Agency.
GLOFs have also hit northern Pakistan. In May 2022, a flood hit the Hassanabad area of Hunza Valley after a sudden breach of an ice-dammed lake formed by the surging Shishper Glacier.
