Chinese authorities have raised the flood alert near the Nepal border after a lake formed by debris from a glacier collapse began overflowing, increasing the risk of flooding and landslides downstream.
The development has forced Chinese rescue teams to withdraw from the area and move to safer locations as authorities assess the possibility of the lake breaching its debris barrier and triggering further flooding.
The lake formed after a debris barrier developed upstream of the Gyirong border port, where a flash flood on Wednesday caused widespread destruction and killed hundreds of people.
State broadcaster CCTV reported on Friday that the lake’s water volume had risen to more than 2.5 million cubic metres, up from an estimated 1.5 to 2 million cubic metres recorded by an eight-member Chinese engineering team a day earlier.
Authorities warned that a breach of the debris barrier could send a major surge of water downstream, while unstable mountain terrain could also trigger landslides, rockfalls and avalanches.
The engineering team was deployed on Thursday to assess the lake using aerial surveys and three-dimensional modelling as officials explored ways to reduce pressure building behind the debris barrier.
Engineer Chen Hanxiao said difficult terrain was one of the major challenges facing the team, with steep and isolated cliffs forming around parts of the lake following the glacier collapse.
Chen said one possible way to control the lake would be to create a drainage channel through a lower section of the debris barrier, allowing water to escape gradually.
He said engineers would first have to determine the appropriate width and depth of the channel by calculating the expected water flow and the rate at which the lake could continue filling.
Survey data showed that the lake stretches for about 150 metres and reaches depths of between 40 and 50 metres.
Authorities are also monitoring weather conditions around the Gyirong border port, where light to moderate rainfall has further weakened soil and rocks in the already vulnerable upstream area.
China had earlier warned that an additional three million cubic metres of water could enter the lake within three days, increasing the risk of the debris barrier giving way. Officials expect the highest water flow around September 1.
Aerial footage released by Chinese state broadcaster CGTN showed brown floodwater accumulating inside the basin, submerging vegetation and pressing against a barrier made of rocks and debris.
The disaster has also left hundreds of people unaccounted for on both sides of the border, including up to 540 foreign nationals from 31 countries, according to officials.
Goodness Anunobi
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