'High risk' of lake bursting and causing second flood, warns China

China has warned there is a "high risk" that a lake which formed after the recent landslide near the Nepal-Tibet border may breach its banks, causing a second flood downstream.

The warning comes after Wednesday's deadly flash flood in the same area which scientists say was triggered by the collapse of a mountain glacier into a river .

As of Friday, an estimated 2.5 million cubic metres of water had accumulated, according to broadcaster CCTV. Officials said that three million cubic metres of water is expected to flow into the lake between Thursday and the weekend, posing a "high risk of breach".

But experts have told the BBC that it is possible that the lake could drain out naturally over time.

China's Ministry of Water resources said that the barrier lake - which is a body of water formed when debris dams up a river - had formed at the confluence of the Chhochen Khola and Purepu Tsangpo rivers.

It is near the Langtang Lirung peak where the collapsed glacier is believed to have originated, and several kilometres upstream from Gyirong Port, the major land crossing linking Nepal and China which was hit by Wednesday's massive flood.

The barrier lake had started to overflow on Thursday. Then on Friday morning, a Chinese military drone captured footage of an avalanche on a snow-capped mountain which deposited an estimated 50,000 cubic metres of ice debris into the barrier lake.

Authorities issued warnings and orders to prepare for emergency evacuations. Rescue work was suspended as most of the Chinese emergency services stopped at a village about 7km (4 mi) from Gyirong Port.

This was because the barrier lake at that time was "overflowing non-stop" which could lead to fresh floods or debris flows, according to a CCTV report.

But by Friday afternoon, state media reported that the "risk from the upstream barrier lake had come under control" and that rescue operations had resumed.

Authorities have since sent engineering teams to monitor the lake's water levels and the atmospheric conditions, and drones have been flown in to drop off equipment and supplies.

Jakob Steiner, a geoscientist with the University of Graz who has researched lake bursts in Nepal, told the BBC there was still a possibility that the lake could breach its banks.

"Not all the material on the mountain could have come down, it is still unstable, so there'a a chance that something will drop," he said, adding that if the material dropped into the lake or even further upstream, "then we have a problem".

Chinese state media have also reported there will be heavy rainfall and storms in the region in the coming days, and there are concerns that this may cause the lake to swell.

If the lake breaches its banks, there will not likely be a massive deluge of water as seen on Wednesday as the lake has been formed recently and not very deep, said Dr Steiner.

However, the water discharged could further undercut the valley's slopes and cause more erosion - which could cause more landslides.

"Authorities need to be very careful for the rescue and prepared for any secondary wave and post-disaster landslides as the valley walls have been scoured and much more vulnerable," said Mohd Farooq Azam, a glacio-hydrologist with scientific institution the International Centre for Integrated Mountain Development.

Steiner said it was unlikely that the Chinese could stop the lake from overflowing.

He pointed out that while it is technically possible to artificially drain the lake using excavators, there are no proper roads which would allow them to transport heavy machinery to the lake's location.

But both experts said it is possible that the problem could eventually resolve by itself.

Azam points to recent flash floods in India's Chamoli and Dharali where artificial lakes "drained slowly without much impact downstream".

Similarly, "the only thing [China] can do is hope that the water will slowly drain out naturally" out of the barrier lake in Tibet, said Steiner.

"And it will slowly drain – the melting season will be over by October so things are going to get calmer. There's going to be less water, and then by winter [what's left] will be all frozen."

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Separately, Nepali authorities have warned of the risk of blockages causing further floods along the Bhote Koshi and Trishuli rivers.

They have warned people to stay away from the rivers until further notice.

Scientists have said that the flood on Wednesday was likely to have been caused by part of a glacier near Langtang Lirung collapsing into the river below, which in turn caused a massive surge downstream carrying water and debris.

Several hundred people have died in Nepal and with hundreds more still missing after the flash flood. On the Tibet side of the border alone, more than 550 are missing - nearly half of whom are foreigners.

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