How a high-altitude ice collapse evolved into a mudslide: expert

At around 10:30 am on Aug. 26, a disaster originating on the Nepalese side caused major casualties and left people missing at Gyirong Port in Gyirong county, Xigaze, southwest China's Xizang Autonomous Region.

The disaster may have unfolded in just seven minutes -- from its sudden onset in a glacier-covered area more than 5,200 meters above sea level, to its rapid descent and impact on the Port.

What caused the disaster? Was it linked to a glacial lake outburst? Why was it so devastatingly fast? People's Daily spoke with Guo Zhaocheng, a senior remote-sensing engineer at the China Aero Geophysical Survey and Remote Sensing Center under the Ministry of Natural Resources, to answer these questions.

According to Guo, this was not a glacial lake outburst, but rather a chain of disasters triggered by an ice collapse.

In the aftermath, some speculated that a glacial lake outburst might have been responsible.

"Glacial lake outbursts are indeed highly dangerous, but this disaster was not caused by one," Guo said.

High-resolution remote-sensing images taken before and after the disaster show that a high-elevation ice avalanche occurred in the upper reaches of the Cuojian River, upstream of Gyirong Port. The Cuojian River is a tributary gully of the Guobaxiaqu River, which itself flows into the Donglin Zangbo River.

After the ice mass broke away from the high slope, it moved rapidly down a steep channel, scouring and entraining loose materials that had accumulated there over the years, including glacial debris. This formed a high-speed debris flow. As it continued downstream, the debris flow gradually evolved into a mudslide before eventually reaching Gyirong Port.

"More precisely, this was a chain disaster involving the successive evolution of a high-altitude ice collapse into a high-speed debris flow and then a mudslide," Guo said. "The disaster did not suddenly form near the port. It originated in a high-altitude glacier area upstream and propagated and intensified downstream along the Cuojian River, the Guobaxiaqu River and the Donglin Zangbo River."

Why did the disaster keep growing as it moved downstream?

Guo explained that the collapsing ice mass itself had substantial gravitational potential energy. As it raced downhill, it scoured the riverbed and eroded the banks, continuously incorporating loose rocks, soil and glacial debris along its path. As the amount of material involved increased, so did its destructive power. The steep terrain acted like a natural acceleration channel, enabling the debris to travel long distances at high speed.

How do ice collapses differ from glacial lake outbursts?

Both ice collapses and glacial lake outbursts are associated with the high-mountain cryosphere, but their formation mechanisms, moving materials and modes of destruction differ.

A glacial lake outburst occurs when water stored in a glacial lake is suddenly released after the dam containing it becomes unstable, generating a flood. A high-altitude ice collapse, by contrast, occurs when ice suddenly breaks away from a steep glacier or mountainside, potentially generating a high-speed debris flow.

"Water flows and ice-collapse debris flows differ in density, speed and the way they exert impact," Guo said. "High-speed debris flows contain large amounts of ice, rocks and sediment. In addition to the powerful impact generated by their high speed, they continuously scour and entrain materials along their path. Under the terrain and material conditions of this disaster, their capacity to strike, bury and block river channels was particularly strong."

This means that the occurrence of a disaster in a glacier-covered area alone is not enough to conclude that it was caused by a "glacial lake outburst." Determining whether a glacial lake has changed, where an ice mass originated, how materials moved through the channel, and what route the disaster body took requires a comprehensive assessment based on pre- and post-disaster remote sensing imagery, topographic data and field investigations.

Guo noted that there is a large elevation difference in the upper reaches of the Cuojian River, with the highest surrounding peaks rising above 7,000 meters. The glacier area where the ice collapse occurred may have been at an elevation of around 5,200 meters.

Glaciers are constantly moving. Slow and relatively stable movement normally does not directly cause disasters. But once part of an ice mass becomes unstable and suddenly collapses from a high elevation, the combination of a huge elevation difference and steep terrain can rapidly turn the event into a disaster.

Based on current assessments, only about seven minutes may have elapsed between the formation of the ice-collapse debris flow and its arrival at the port. Such a short time window highlights the difficulty of identifying and providing early warnings for high-altitude, long-distance chain disasters.

Why did the disaster occur here?

"The entire Himalayan region has complex geological structures and steep terrain. In addition, Gyirong Valley is affected by cryospheric processes such as glaciers and glacial lakes, as well as climate change," Guo said. "This disaster resulted from a combination of multiple complex factors."

The first factor was the terrain. The upstream area has high elevations, deeply incised valleys and steep slopes, creating favorable conditions for the rapid movement of the collapsing ice mass and the conversion of gravitational potential energy into kinetic energy.

The second factor was the availability of loose materials. Large amounts of glacial debris, rock fragments and soil are distributed along the channel. As the ice collapse moved rapidly through the channel, it strongly scoured the riverbed, rapidly increasing the scale of the disaster as it traveled downstream.

The third factor was activity within the cryosphere. High-altitude glaciers are not static. Ice movement, melting, the development of crevasses, and localized instability can all affect glacier stability.

In addition, rainfall was continuing in the area. Rainfall not only increases the amount of water flowing through channels but may also reduce the stability of loose deposits, increasing the risks of debris flows, landslides, and the destabilization of water bodies temporarily dammed upstream.

Guo stressed, however, that while these complex background factors can help explain regional risks, determining the specific trigger of an individual ice collapse requires further research based on more detailed remote-sensing analysis, meteorological data and field investigations. It would be inappropriate to simply attribute the event to any single factor based on the information currently available.