China-Nepal floods: first research shows that no one could have outrun debris flow

Not even the world’s fastest sprinters could have outrun the devastating floodwaters that struck a border crossing between China and Nepal last week, according to one of the first peer-reviewed assessments of the disaster.
Researchers from the Chinese Academy of Sciences (CAS) calculated that the surface velocity of the debris flow was 19 metres per second (62 feet per second) when it struck the checkpoint at Gyirong Port in Tibet.
This is almost twice Usain Bolt’s average speed when he set the 100 metres world record of 9.58 seconds in 2009 and 50 per cent higher than his top speed.
In a paper posted online on Tuesday by the Chinese-language journal Yangtze River, the team used digital models and frame-by-frame analysis to reconstruct the event, which was triggered by the collapse of a high-altitude glacier in Nepal.
“This disaster involved a continuous transition from ice avalanche to debris flow, then to mudslide and finally to flood,” wrote the team from CAS Institute of Mountain Hazards and Environment.
“The time from the occurrence of the ice avalanche to its impact on the Gyirong Port was extremely short, and the effective response time was limited, which put forward higher requirements for rapid identification, information transmission and emergency decision-making.”
The collapse of the glacier in Langtang Lirung on the Nepalese side of the Himalayan border created an ice-rock avalanche and debris flow that quickly reached Gyirong Port, almost 22km (14 miles) away, and then continued to cascade downstream.
The floods killed at least 16 people in China, while another 546 are still missing. In Nepal the death toll has risen to 1,058 with 4,606 people still missing.

The researchers calculated that the floods still had a surface velocity of around 11.7 metres per second by the time they reached a bridge on Nepal’s Trishuli River 52km away from the border crossing.
This meant that it took between 46 and 75 minutes for the flow to reach the bridge, “providing a potential time window for downstream disaster response”.
But the team said this was not the same as an actual warning time because factors such as monitoring, communications and the ability to respond all came into play.
The Nepalese authorities have criticised the Chinese government for failing to provide timely warnings once the flow had crossed the border.
The floods and other similar events have shown how difficult it is to detect and monitor high-altitude ice avalanches.
The Chinese researchers concluded that a feasible response to such incidents could include tracking the potential route of the resulting ice flows and ensuring warnings are issued as soon as possible.
This could be done by moving monitoring points further upstream and by using multiple sources such as remote sensing, water level monitoring and seismic data.
The team also said such avalanches could create temporary dams in narrow valleys – and although these created further risks, they could also buy more time for evacuations and emergency response efforts.
Some media reports have said that Chinese researchers had warned of a flood risk in this region a year ago, pointing to research published online in July last year just days after a bridge at the border crossing was destroyed when the natural dam holding back a glacial lake collapsed.

Scientists from the Chengdu University of Technology looked at how glacial lakes were rising as a result of climate change and said there was a risk of further incidents upstream from the bridge.
Although the research called for better early warning systems for upstream floods, it did not examine the potential for a glacier collapse.
The CAS researchers said that while climate change would have an effect on the stability of glaciers and other ice formations, it was not the only factor.
The team also said further research was needed on ways to improve disaster detection and early warnings.
They also warned there was some uncertainty about their research because it had relied on limited data and estimates rather than direct measurements and field verification.
In a second study published on Tuesday in the CAS-run journal Science Bulletin, another team described the catastrophe as a sudden failure driven by long-term glacier dynamics rather than an immediate trigger such as heavy rainfall.
The researchers from institutes in mainland China and Hong Kong also found that seismic precursor signals hours before the collapse coincided with smaller-scale snow and ice activity witnessed on-site.