Mudslide reveals gaps in early warning systems

The sheer scale and speed of the mudslide that struck the China-Nepal border on Aug 26 exposed critical gaps in mountain hazard warning systems, experts say.

The tsunami-like surge of water, mud and debris has killed more than 1,410 people and left 5,651 missing.

Kristen Cook, a research scientist at the French National Research Institute for Sustainable Development and the University of Grenoble Alpes, said the event was unusually destructive primarily because of its sheer scale.

"The initial rock failure is one of the biggest landslides in the world in recent decades," Cook said.

The overall process was not fundamentally different from other major mountain disasters, she said, but the size of the collapse set it apart.

"The very large volume of rock and ice meant that the debris had a huge amount of energy from the collapse, and also that there was a lot of ice available to melt and provide water for the flow," she said.

Cook compared the event with the 2021 Chamoli disaster in India and the 2023 Shovi event in Georgia. The Chamoli landslide was about 10 times smaller, while the Shovi one was about 100 times smaller, she said.

Jakob Steiner, a geoscientist at the University of Graz in Austria, said one of the most striking features was the effect downstream.

"Definitely the difference between it being a relatively small source area for such a huge, large wave," Steiner said.

"By now, we understand that this has been the collapse of a mountain rather than a glacier," he said. "The glacier was just sitting on top of the mountain, so the glacier collapsed together with the mountain.

"The whole thing came down and, of course, it was a lot of material — it was a lot of rock and a lot of ice," he added. "That this big amount of material turned into a flood so quickly is surprising."

Cook agreed, raising questions about how the avalanche turned into such a fast-moving surge of water.

"We know that it happens, but exactly what is happening inside the flow as it transforms is very difficult to know for certain," she said.

"Some of the collapsed ice would melt as the flow moved down the valley, adding water, while the flow would also pick up water from the river. Mud and rocks further increased its volume," she said.

"The key part is that the valley is steep enough and narrow enough that the flow is able to keep moving as this transition happens."

The disaster has also raised questions about climate change.

Permafrost changes

Steiner said glacier retreat and permafrost changes may have altered conditions around the mountain before its collapse.

The glacier's retreat had left previously ice-covered rock more exposed to erosion, while melting permafrost could weaken the structure of the rocks, he said.

Steiner also pointed to an important long-term concern.

"For some hazards, landslides are increasing," he said, adding, "But the interaction between different hazards, the occurrence of multihazard events, there seems to be an indication that it's increasing."

The speed of the disaster also exposed detection capabilities, Cook said. "This event is a tragic lesson about the limitations in current early warning systems," she said. Current systems often rely on river monitoring, but instruments can be destroyed by a major flood before they can transmit useful data.

One potential alternative in the future is seismic monitoring, Cook said. "Seismometers record the ground shaking caused by landslides and floods, even at long distances," she said. "They can 'see' the event within seconds and could potentially raise an alert within minutes.

"Such systems don't exist yet, but scientists are working on it," she said.

Steiner said the disaster also underscored the particular difficulty of issuing warnings across an international border.

A warning chain that requires an official to check sensor data and then contact authorities in another country could take too long, he said.

"The hazards are so fast, we need to be faster," he said.

Countries should make greater use of satellite data that are already accessible to both sides, he said. "Nature is changing, and it's changing so rapidly. We really have to speed up because otherwise this is going to happen more often," he added.