It Wasn't Just Water... Why the Devastation from the Massive Flood Was So Severe

[Anchor]

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

A debris flow, mixing not just water with soil and rocks, swept down from the upper reaches of the river. Reporter Seo Donggyun explains just how destructive it was.

[Reporter]

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

It is called a "debris flow" because water is mixed with soil, gravel, and massive rock fragments.

In high-altitude regions like Nepal, it can even include chunks of ice breaking off from glaciers.

Looking at footage filmed at Illgraben in Switzerland, located in the Alps, the energy of the debris flow is so immense that even gigantic boulders tumble down effortlessly.

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 torn apart in an instant, and power plants were completely destroyed.

In particular, much like a rolling snowball, 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) entrain and carry along additional ground materials located around the valley. Unlike general landslides, the scope of the damage itself is far more extensive.]

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 suggest that the speed of the debris flow in this massive flood in Nepal reached 70 kilometers per hour, and exceeded 100 kilometers per hour in some fast-moving sections.

With an estimated 20 billion liters of massive debris flow striking at such speeds, the devastation was bound to be catastrophic.

By the time a debris flow is noticed rushing down, it is already too late, meaning residents must detect pre-signs and evacuate in advance. However, because the glacial collapse occurred across the border in China, even this was far from easy.

(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.