Nepal floods: Scientists explain why mudslides are so deadly

Lethal surges of mud and water, described by experts as moving "like liquid concrete," are nearly impossible to predict and can’t be outrun, according to scientists studying recent deadly floods along the Nepal-Tibet border. The disaster, which occurred on August 26, killed at least 160 people and left hundreds missing.

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Lethal surges of mud and water that hit “like liquid concrete” are nearly impossible to predict and can’t be outrun.

While the exact cause of the landslide and subsequent flood is still under investigation, experts point to the extreme topography of the Himalayas as a compounding factor. Nepal's Foreign Minister Shishir Khanal initially suggested an earthquake triggered a landslide that blocked a river, leading to the flood when the obstruction eventually breached. However, the U.S. Geological Survey later indicated that seismic waves were generated by the landslide itself, not an earthquake.

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That defeats essentially every operational flood warning system in the world.

A preliminary assessment by the International Centre for Integrated Mountain Development (ICIMOD) found no anomalies on the Chinese side of the border. Sarthak Shrestha, an analyst with ICIMOD, believes a rock-ice avalanche in Nepal blocked the Lhende river, causing the cascading flood. Hydrologist Hatim Sharif noted the lack of rainfall beforehand, which defeats typical flood warning systems. He emphasized that by the time these mudflows are visible, escape is impossible, and climbing to higher ground is the only viable option.

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Running will not help.

Researchers like Dorothy Heinrich from the University of Reading explain that narrow river valleys in mountainous regions can rapidly accumulate water when blocked by avalanches or landslides, creating incredibly fast floods. Qianggong Zhang of ICIMOD warned of the potential for a second flood due to an ongoing blockage upstream. Nepal is also prone to glacial lake outburst floods (GLOFs), which are sudden, catastrophic releases of water from melting glacier-fed lakes, further increasing the risk of such hazards.

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In these mountain environments, the rivers can be quite narrow in areas. Anything that blocks them, be it an avalanche or landslide, will lead to a “big accumulation of water, then it creates these incredibly fast floods.”