Nepal’s deadly “tsunami of dirt” may have begun with a glacier collapse

A vast wall of debris swept through Nepal’s Bhote Koshi River valley, killing more than 150 people and leaving hundreds missing, according to reporting published by Phys.org. The flow traveled more than 170 kilometers from the mountains toward the China–Nepal border, with at least 34 Australians reported missing. The surge carried water, ice, mud, rock and trees. By the time it reached Gyirong Port, a China–Nepal border crossing, the debris reportedly resembled a tsunami made of earth. Early estimates put the wave at roughly 30 meters high. The event generated seismic energy equivalent to a magnitude 4.4 earthquake, initially making its scale appear similar to an earthquake on seismographs. The precise cause has not been established. Early analysis described in the Phys.org report suggests that an abrupt glacier collapse may have followed a huge ice-and-rock landslide. Satellite imagery indicates the landslide may have stripped away substantial portions of a glacier, setting off its collapse. Another possible mechanism, a glacial lake outburst flood, has been considered. Such floods occur when meltwater held behind a natural dam is suddenly released. However, the early analysis cited in the report suggests that this was not what happened in this case. The disaster occurred in a region where warming can affect both glaciers and permafrost—the rock, soil and ice that remain frozen for many years. As frozen ground thaws, it can weaken the material binding steep mountain slopes together. An earthquake, rockfall or other disturbance may then trigger a rapid landslide, although the report does not identify one confirmed trigger for Nepal’s collapse. The Bhote Koshi valley is a major route linking Kathmandu with Lhasa. A smaller flood struck the same Nepalese district the previous year and killed nine people, according to the report. The episode has renewed attention on early-warning systems that monitor river levels, heavy rainfall and seismic activity. Such systems helped authorities evacuate residents before a glacier-related debris flow destroyed the Swiss village of Blatten, the report says. Nepal also received funding announced by the Green Climate Fund for improved monitoring systems, though installing and maintaining this technology can be difficult for lower-income countries.