China warns lake formed by glacier debris near Nepal border starting to overflow

A lake formed by debris ​from a glacier collapse that triggered catastrophic flooding on the China-Nepal ​border has begun to overflow, Chinese authorities said on ‌Friday, ​forcing rescue operations to be halted. Nepal and China both warned on Thursday of fresh risks of flooding in the Himalayas, with a debris dam creating a lake upstream that had been threatening to ‌collapse. China had been racing to reduce the risk of more flooding at the disaster-stricken border crossing linking its Tibet region to Nepal, after hundreds were killed in a flash flood on Wednesday.

Risks of the lake bursting and potential secondary disasters such as mountain collapses and avalanches have prompted all Chinese ‌rescue personnel and vehicles to withdraw to stand by in a safe area, state broadcaster CCTV said on Friday. The lake's reservoir has ‌topped 2.5 million cubic metres of water, up from the estimated 1.5-2 million cubic metres a Chinese eight-member engineering team dispatched to monitor the site recorded a day ago.

Authorities mobilised the engineering team to conduct aerial surveys and 3D modelling of the barrier lake on Thursday, according to CCTV's report. One of the engineers, Chen Hanxiao, said challenges included difficult access ⁠to the ​site.

"After the glacier collapse created ⁠the lake, many isolated cliffs and steep cliffs appeared along its shoreline," Chen told CCTV on Thursday evening. Chen said the general approach to reducing the lake's threat is to excavate ⁠a drainage channel at a lower point of the dam. The width and depth of that channel needed calculations based on how fast the water will flow ​and how quickly the lake will fill.

Data from their survey found the lake was about 150 metres long and 40-50 metres ⁠in depth. The Gyirong border port area has seen light to moderate rainfall which has loosened and destabilised soil and rock surrounding the precarious zone upstream. The rain is forecast to ⁠continue ​over the coming week.

China on Thursday warned that 3 million cubic meters of water — the equivalent of about 1,200 Olympic swimming pools — are expected to flow into the still-dammed lake over the next three days, creating a high risk of a breach, with peak flow expected around ⁠September 1. The formation of a barrier lake was captured in aerial footage by a Chinese rescue team shown by state broadcaster CGTN. It showed ⁠masses of brown water accumulating in ⁠a basin with no visible outlet, that had already submerged bushes and trees and appeared to push against a barrier of debris and stones. Nearly 1,000 remain missing on both sides of the border, including as many as ‌540 foreigners from ‌31 countries, according to officials.