It Wasn't Just Water... Why the Devastating Flood Caused So Much Damage

[Anchor]

The recent massive flood, strong enough to swallow entire buildings in an instant, was fundamentally different from floods caused by heavy rain alone.

It was a debris flow, with a mixture of mud and rock sweeping down from the upper reaches of the river alongside water. Reporter Seo Donggyun explains just how destructive it was.

[Reporter]

The reason the damage from the massive flood in Nepal became so severe is that it was not just water, but a debris flow that swept through.

It is called a debris flow because soil, gravel, and chunks of rock are mixed in with water.

In high-altitude mountainous regions like Nepal, chunks of ice breaking off from glaciers can also get mixed in.

Looking at footage filmed at Illgraben in Switzerland, located in the Alps, the energy of a debris flow is immense, easily rolling down massive boulders as if they weighed nothing.

A debris flow has a density 1.5 to 2 times greater than ordinary water.

Because it is that much heavier for the same volume, even when colliding at the same speed, its momentum is twice as great—meaning it packs twice the destructive power.

This is why buildings vanished entirely, bridges were ripped apart in an instant, and power plants were completely destroyed.

In particular, much like a rolling snowball gathering mass, a debris flow grows in volume the further it travels.

[Kim Jae-jung / Head of the Geotechnical Disaster Experiment Team, National Disaster Management Research Institute: (Debris flows) sweep along and entrain remaining ground materials around the valley as they move. Unlike typical landslides, the scope of the damage itself is much wider.]

In a soil experiment conducted by the National Disaster Management Research Institute, a brick building was completely collapsed by a debris flow that traveled only a short distance.

The United States Geological Survey classifies a debris flow as very fast if it exceeds 10 meters per second, or over 36 kilometers per hour.

However, analyses indicate that the speed of the debris flow in this Nepal flood reached 70 kilometers per hour, and exceeded 100 kilometers per hour in some fast sections.

With an estimated 20 billion liters of massive debris flow striking at such high speeds, devastating damage was inevitable.

By the time one realizes a debris flow is gushing down, it is already too late, making it crucial to detect pre-signs and evacuate in advance. However, because the glacial collapse occurred across Nepal's border on the Chinese side, even this was difficult to manage.

(Video Editing: Lee Hong-myung, PD: Kim Do-gyun, Han Seung-ho, XR: Park Chun-woong, Kim Min-young, Choi Jae-young)
※ Please note: This article was translated by AI and may contain errors.