[Anchor]

The massive flood, powerful enough to swallow entire buildings in an instant, was fundamentally different from a flood caused simply by heavy rain.

A debris flow, mixed with soil and rocks as well as water, 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 escalated is because a debris flow, rather than plain water, swept through.

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

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

Looking at footage filmed in Ilgraben, Switzerland, in the Alps, the energy of a debris flow is so immense that even giant boulders tumble down effortlessly.

A debris flow has a density 1.5 to over 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 doubled, meaning it possesses twice the destructive power.

This is why entire buildings vanished, bridges were torn apart in a single blow, and power plants were completely destroyed.

Particularly, much like a rolling snowball gathering mass, a debris flow also increases in volume the farther it flows.

[Kim Jae-jung, Head of the Geotechnical Disaster Experiment Team, National Disaster Management Research Institute: (Debris flows) entrain and drag along the remaining ground materials around the valley as they move. Unlike typical landslides, the very scope of the damage is 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 U.S. Geological Survey classifies a debris flow as very fast if it exceeds 10 meters per second, or 36 kilometers per hour.

However, analysis suggests that the speed of this massive flood in Nepal reached 70 kilometers per hour, and over 100 kilometers per hour in some fast sections.

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

By the time one realizes a debris flow is cascading down, it is already too late, requiring early detection of precursor signs for prior evacuation; however, this was also far from easy since the glacier collapse occurred across the border in China.

[Video Editing: Lee Hong-myung | PD: Kim Do-gyun, Han Seung-ho | XR: Park Chun-woong, Kim Min-young, Choi Jae-young]