The catastrophic flood that swept the border region between Nepal and China's Tibet appears increasingly likely to have been a "cascading disaster," in which glaciers and rock in the high mountains collapsed, dammed a river, and then unleashed a surge when the impounded water burst through. Scientists in and outside Korea say the disaster did not end with the glacier's collapse; rather, as water, mud, and massive boulders mixed and rushed downstream, the scale of destruction grew rapidly.
A flash flood that struck the Rasuwa area of northern Nepal and parts of Tibet on the 26th left at least around 160 people dead and hundreds missing as of the 27th. Nine South Koreans have also lost contact.
Scientists currently see a large rock-ice avalanche in the Langtang Lirung area of Nepal as the most likely cause of the flood. A rock-ice avalanche is when glacial ice and mountain rock collapse together and pour down into a valley.
Daniel Shugar, a professor at the University of Calgary in Canada, told the New York Times (NYT) that "satellite images before and after the incident suggest a section of glacier about 600 meters wide detached and plunged into a valley roughly 1,200 meters below." Kristen Cook, a researcher at Université Grenoble Alpes in France, said, "Seismic signals indicate that hundreds of millions of tons of ice, rock, and other material may have moved."
A cascading disaster then appears to have unfolded as a massive volume of ice and rock fell into a narrow valley. Choi Myung-gi, a professor with the Republic of Korea Industrial Field Professors, said, "Ice and rock dammed the river to form a kind of 'temporary dam,' and when the dam failed, the trapped water, soil, and stones surged downstream all at once."
According to the International Centre for Integrated Mountain Development (ICIMOD), the water level at Galchi on the Trishuli River rose as much as 9 meters in about 30 minutes. At Malekhu downstream, the level increased about 7 meters over a similar period. The torrent was so strong that some hydrological gauging stations on the Trishuli were damaged or swept away.
Liz Stephens, a professor at the University of Reading in the United Kingdom, said, "Most people think flash floods are caused by heavy rain, but in high mountain regions like Nepal and Tibet, landslides, avalanches, and glacier-related hazards can occur in sequence and trigger floods."
In this process, debris flows—water heavily mixed with mud, gravel, and boulders—scour the riverbed and banks, entraining more material. Fatima M. Pillosu, a researcher at the University of Reading, said, "A debris flow is like 'liquid concrete,' heavier than water," adding, "The large boulders moving with it can destroy buildings and bridges, and even after the flow stops, mud and rock harden in thick layers, complicating rescue operations."
Scientists are also investigating the initial cause of the glacier and rock collapse. The leading factors under discussion are glacial retreat and permafrost thaw underway in the Himalayas. Glaciers, while filling valleys, also physically buttress adjacent slopes and rock walls; as glaciers thin and recede, once ice-supported rock faces become exposed and hillslopes can become unstable.
Permafrost refers to ground or bedrock that has remained frozen for at least two years. In high mountains, ice frozen within rock fractures acts as an adhesive holding blocks together. Thus, when temperatures rise and the ice melts, the bonds among rocks can weaken, making rockfalls or landslides more likely.
Ahn Jin-ho, head of the Cryosphere Science Education Research Center at the SNU Institute for Future Innovation and a professor in the Department of Earth and Environmental Sciences, explained, "Global warming driven by rising greenhouse gases is accelerating the melt of glaciers and snowpack, which is the major backdrop for disasters in high mountain regions like this one."
Meanwhile, experts stress that early warning is crucial because it is difficult to predict precisely when cascading hazards will occur. Jeff Da Costa, a researcher at the University of Reading, said, "Even if we cannot confirm exactly what hazard occurred upstream, a sudden change in river level or discharge alone can trigger a warning downstream," noting, "In places like Nepal where headwaters lie across borders, sharing observational data among countries is key to gaining warning time."
Professor Choi said water levels, ground movement, and vibration upstream should be monitored in real time and combined with satellite and drone data. He said, "No matter how sophisticated the monitoring and forecasting system is, it is ineffective without an emergency communication system that immediately turns those signals into action," adding, "We need a system in which warnings actually lead to evacuations."