180 km/h Concrete Slurry Buries Nepal

What caused the Nepal disaster was not ordinary water, but debris flow with properties similar to concrete slurry. Its density and viscosity are much greater than water, making it impossible for people to avoid or swim in. Professor Kim Ki-bum explained, "What state is this in? It's like concrete slurry. Yes. You can think of it as having almost that consistency," revealing that the debris flow was powerful enough to push house-sized rocks.

This debris flow moved at a record-breaking average speed of 180 km/h. Professor Kim Ki-bum emphasized, "The flow speed calculated here is an average of 180 km/h," stating that this is incomparable to typical floods. Such a speed is considered an extremely rare case in recorded history.

Landslide is a broad term encompassing various geological phenomena. Professor Kim Ki-bum explained, "We call it all a landslide, but geological terms express it in more detail," adding that creep refers to very slow surface movement, landslide means an entire mass sliding down, lahar refers to a mixture of volcanic ash and water, and debris flow signifies a mixture of mud, rocks, and water.

The Nepalese government initially announced that a magnitude 5.2 earthquake triggered the landslide, but the U.S. Geological Survey (USGS) analysis differed. The USGS revised its conclusion, stating that the impact of the enormous landslide caused the magnitude 5.2 earthquake. Professor Kim Ki-bum explained, "It's not that the earthquake caused the landslide, but rather, the energy from the mountain collapsing due to the landslide, hitting the ground, generated a 5.2 magnitude earthquake. They revised it this way."

Because it is difficult to accurately determine the depth of an earthquake solely from seismic records, it is essential to analyze differences in energy waveforms, such as fault movements and explosions. Through such analysis, the USGS ultimately concluded that this magnitude 5.2 earthquake was caused by the landslide. Professor Kim Ki-bum said, "The conclusion was an earthquake caused by a landslide."

A magnitude 5.2 earthquake signifies the amount of energy; the greater the amount of collapsed material and the longer the fall distance, the larger the magnitude. Professor Kim Ki-bum explained, "Magnitude 5.2 is the amount of energy. The more material that collapses and the greater the fall distance, the higher the height. In this case, it is said to have fallen about 3 km." It is estimated that in this disaster, a mixture of glaciers and rocks fell approximately 3 km, causing an earthquake of this magnitude.

Nepalese flood forecasting authorities became aware of the flood at 9 a.m., but this was already after major damage had occurred. Authorities relied on CCTV monitoring sections and downstream water level observation stations, but these proved insufficient due to the sudden scale and speed of this disaster. Professor Kim Ki-bum said, "The flood forecasting authorities became aware that a flood had occurred at 9 o'clock. It's hard to call it a man-made disaster." He emphasized the unpredictability, stating that this incident "can be considered an event that exceeded the scale of natural disasters we could imagine." This is regarded as a case where the disaster transcended the typical scale, making it difficult for existing warning systems to respond.

This debris flow traveled a total distance of 19.6 km from its origin to the CCTV observation point in just about 7 minutes. This calculates to an average speed of 170-180 km/h. Material composed not just of water, but a combination of rocks and glaciers, moving at such a speed, carried immense destructive power.

The debris flow reached the downstream hydropower plant section from the accident site in about 6 minutes. Although CCTV confirmed the debris flow passing through the plant, no adequate warning could be issued in that area. Professor Kim Ki-bum stated, "It can be considered practically impossible for people to actually evacuate this area in 6 minutes," pointing out that evacuation in such a short time was virtually impossible.

Typical flash floods occur with rainfall, and water levels gradually rise, allowing for observation. Professor Kim Ki-bum explained, "Still, it has to rain. So, even if rainwater increases rapidly, something gradual is observed." However, this Nepal disaster occurred on a clear day without rain, showing a distinct difference from typical floods.

This landslide was enormous, with the collapsed section spanning 1.5 km in width and the sunken area extending about 3 km. Professor Kim Ki-bum explained, "It was completely covered by rock debris, and this area is almost 1.5 km wide. Yes. The width alone is 1.5 km, and the sunken area itself is about 3 km." The entire surrounding area was covered in muddy water, demonstrating the severity of the damage.

Initially, international experts pointed to a Glacier Lake Outburst Flood (GLOF) as the cause of this disaster. However, Professor Kim Ki-bum stated, "It was not confirmed that a glacier lake existed at the foot of Mount Langtang Lirung, where the incident occurred," indicating that there was no glacier lake near Mount Langtang Lirung, the site of the accident. Additionally, the physically insufficient time for a GLOF to occur simultaneously with a debris flow also undermined the credibility of this hypothesis.

Since the debris flow reached the affected area just 7 minutes after the earthquake, there was physically too little time for the collapsed ice chunks to melt into water, for this water to penetrate the voids between the soil heaps, and for it to become viscous like cement slurry. Professor Kim Ki-bum explained, "In just 7 minutes, the collapsed ice chunks melted into water, and that water penetrated the pores between the collapsed soil heaps to make it completely viscous like cement slurry and flow down. The time was too short for that."

Professor Kim Ki-bum stated that if the dangerous lake upstream had been identified, early warnings or water drainage measures could have reduced the damage. He added that deploying heavy equipment with military helicopters or using small explosives to secure water channels would be easy from an engineering perspective. However, it is analyzed that such technical responses were realistically difficult due to Nepal's national capacity and geographical accessibility limitations.

There are significant differences in the topographical environment between a similar disaster in Switzerland and the Nepal accident. In Switzerland, the debris flow covered a flat village, which reduced its speed, but in Nepal, the path was a steep slope from beginning to end, so the speed did not decrease. Professor Kim Ki-bum explained, "In Switzerland, the speed decreased as it covered a flat village, but in Nepal, it traveled at full speed because it was a steep slope from beginning to end," adding that Switzerland successfully issued evacuation orders through prior observation.

A small earthquake of magnitude 2.6 occurred at 8:20 a.m., 17 minutes before this large-scale landslide. Professor Kim Ki-bum analyzed, "A magnitude 2.6 earthquake occurred at 8:20 a.m., 17 minutes before the main landslide. This earthquake likely triggered the landslide or created cracks earlier, leading to the main collapse." Given that the Himalayan region is seismically active with frequent earthquakes, it is possible that this small earthquake played a role in triggering or weakening the landslide.