Nepal’s deadly debris flow points to risks in other high mountain regions

When an immense and inexorable flow of rock, mud and water barreled through northern Nepal on August 26, it killed more than 1,200 people and left thousands still missing. The magnitude of the event caught even experts on the region’s geology off guard. That dense torrent reached speeds over 160 kilometers (100 miles) per hour and destroyed villages along the way.

What caused the bedrock to break in the first place is not entirely clear. But research and insights from scientists suggest that certain geologic, glacial and climate factors probably played a role in the disaster. Those insights underscore the potential threat to other high mountain regions, and how the risk of catastrophe may increase over time.

Human-caused climate warming is melting the ice that holds high mountains together, says glaciologist Ulyana Peña, also of the University of Colorado Boulder. But images taken by satellite soon after the event showed a freshly exposed, uniform slope had appeared on Langtang Lirung’s north face, at an elevation of around 5,150 meters. In reality, a slab of bedrock roughly 2.2 square kilometers wide had detached from the mountain, carrying the overlying ice down into the valley below.

At this point, scientists estimate that the detached chunk of glacier and bedrock amounted to a volume somewhere around 110 million cubic meters. That’s roughly 40 times the volume of the Great Pyramid of Giza. A swath of factors probably contributed to the breaking of bedrock on August 26, scientists say.

First, it’s possible that the magnitude 7.8 Gorkha earthquake that hit central Nepal in 2015 may have damaged the bedrock, mountain hydrologist Walter Immerzeel of Utrecht University in the Netherlands said at a Sept. 16 briefing about the factors behind the event. The temblor’s shaking triggered a landslide on Langtang Lirung that killed around 200 people in Langtang Valley. There’s also the matter of climate warming.

The ancient tectonic uplift of the Himalaya is thought to have left fractures now sealed with ice known as permafrost in rocks at the altitude where the bedrock failure occurred. Human-caused climate warming has softened much of the permafrost in these regions, he said. Climate change has increased the annual average temperature on the slope and in the region more generally by about 2 degrees Celsius since preindustrial times, according to a report released by World Weather Attribution on September 17.