Tragedy in Nepal: How do flash floods happen?

Sudden and unexpected, the disaster in the China-Nepal border region unfolded within 10 minutes on Wednesday, with massive volumes of water and debris racing 20 kilometers downstream, causing devastating loss of life. Flash floods are a common occurrence in the Himalayas, as Niels Hovius, head of the Geomorphology Section at the GFZ Helmholtz Centre for Geosciences in Potsdam, explains. Hovius, who has extensively worked in the region, is an expert on early warning systems.

The disaster began with a glacier collapse in the northern Langtang massif, near the Nepal-China border. As the ice broke off, it formed a massive ice avalanche that rapidly descended the slope. The friction between the ice and the ground melted the ice, turning it into water that carried debris and mud downhill at speeds comparable to passenger cars on German highways.

Seismological instruments recorded a 4.4 magnitude earthquake at the time of the collapse, but Hovius clarified that it was not a typical tectonic earthquake, but rather the force of the falling glacier.

Several factors contribute to the devastating nature of flash floods in the Himalayas. The topography around glacial lakes can change suddenly, causing a sudden release of water that turns into torrents rushing downstream. Landslides, too, can trigger deadly mudflows even without glaciers or glacial lakes. All three types of events—glacier collapses, glacial lake outburst floods (GLOFs), and landslides—are regular occurrences in the Himalayas, as per Hovius.

The narrow valleys of the Himalayas further exacerbate the situation, as the water and debris have limited space to spread out, increasing their depth and speed. Additionally, the closer local populations are to the source of the flash flood, the more widespread the devastation. Nepal, one of the poorest countries in Asia, faces significant challenges in protecting its people from flash floods. Many people live along vulnerable river valleys, and there are limited early warning systems in place.

Hovius points to a flash flood in the same region last year, which, while smaller, was still severe enough to give people just an hour to evacuate if they had received timely warnings. Efforts are underway to establish local acoustic early warning systems, but their coverage is limited to only one percent of at-risk areas. Hovius suggests using mobile phones to send push notifications as a potential solution.

He also emphasizes the importance of reassessing settlement planning, as many communities have expanded downriver over the decades, increasing their vulnerability.

Regarding the impact of climate change, Hovius notes that rising temperatures make glaciers less stable, leading to larger volumes of water accumulating in glacial lakes and increasing the likelihood of GLOFs. While it is difficult to attribute individual events to climate change, Hovius believes that flash floods in mountainous regions will become more frequent as global temperatures continue to rise. Despite the Himalayas being accustomed to flash floods, Hovius warns that future disasters could be even more severe.