EPFL · Wetenschap

What triggered the devastating flash flood in Nepal?

A massive flash flood swept away hundreds of homes in a mountain region in Nepal, close to the border with Tibet, on August 26. Initial assessments point to the collapse of part of a nearby glacier, but the exact cause is still under investigation by geomorphologists and glaciologists.

Professor Christophe Ancey, an expert in natural disasters from EPFL, explained that while various theories exist, it's too early to determine the precise cause. Images show a wave over 20 meters high, characteristic of a natural dam bursting. Such dams can form from accumulated ice and sediment, or from sudden events like a mountainside collapse. A combination of rapid snowmelt, heavy rainfall, and the bursting of a subglacial water pocket can also trigger large flows of water and sediment.

The wave in Nepal was estimated to be 90% water and 10% debris, with a flow rate around 20,000 cubic meters per second, vastly exceeding the river's usual flow. This differs from a debris flow, which is at least 80% sediment and moves more slowly.

Ancey contrasted this with an event in Blatten, Switzerland, where a glacier collapse caused an ice and rock avalanche that dammed a river. However, the accumulated debris was not large enough relative to the river's flow to cause a catastrophic outburst.

Switzerland has long-standing monitoring and alert systems for natural disasters, including sensors on mountainsides and civil engineering structures to contain or divert debris flows. Rivers are equipped with debris barriers, and scientists monitor glaciers and lakes to prevent outburst floods.

While a direct link between past Swiss natural disasters and climate change is not clear, the current rapid global warming may lead to more intense and frequent events. Thawing mountain peaks, melting permafrost, and retreating glaciers release sediment, while warmer air causes more extreme precipitation, creating conditions ripe for debris flows.

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