FRIDAY — NEPAL: WHAT REALLY HAPPENED

FRIDAY — NEPAL: WHAT REALLY HAPPENED?

🇳🇵 NEPAL: I'VE BEEN DIGGING INTO THIS STORY

I'm going to talk about Nepal again today because I've been following this story very intently and trying to figure out exactly what happened.

And now that we're starting to put the pieces together, it is actually fascinating from a scientific standpoint — despite the horrible tragedy and the enormous loss of life.

The death toll is already staggering, and it is likely that the final numbers will be much higher once search-and-rescue and recovery operations are complete.

But this is also a good example of why we need to be careful about what we believe on social media.

Everybody jumps to conclusions nowadays.

“Here's what happened!”

“No, THIS is what happened!”

And sometimes those explanations are being made before scientists even have a chance to examine the evidence.

Now, after studying satellite imagery, seismic data, temperature observations and the terrain itself, scientists are beginning to develop a much clearer picture of what actually happened.

And it is quite remarkable.

🛰️ SATELLITES SHOW US WHAT HAPPENED

This is where satellite imagery becomes incredibly important.

Scientists have been able to compare the mountain before and after the August 26 disaster.

The Copernicus Sentinel-2 satellite captured the region before the event, while Landsat-9 captured imagery approximately two hours after the collapse.

That gives scientists something incredibly valuable:

A picture of the landscape before the mountain came down — and a picture of it after.

And when you put those images side by side, you can actually see the enormous change in the source area and the debris that moved downstream.

Satellite imagery: Landsat-9 image from August 26, approximately two hours after the collapse, compared with Sentinel-2 imagery from August 24.

The European Space Agency's imagery is particularly useful because the shortwave-infrared imagery helps distinguish ice, water and debris in an area where clouds can make ordinary satellite imagery difficult to interpret. (European Space Agency)

🏔️ THIS WAS MORE THAN A GLACIER COLLAPSE

Initially, there were different ideas about what happened.

Some of the early reports suggested that perhaps a glacier simply collapsed, sending an enormous amount of water and ice downstream.

But the newer satellite analysis tells us the story is much more complicated.

Scientists examining higher-resolution Planet Labs imagery have found that a large portion of the failed area was actually bedrock, with glacier ice also involved.

In other words, we're talking about a massive rock-and-ice avalanche caused by rapid slope failure.

The approximate source area is around 28.2925°N, 85.5302°E, near the Langtang Lirung/Tsangbu Ri region.

And the satellite images show the missing section of the mountain.

That's the smoking gun.

🪨 A WHOLE SECTION OF THE MOUNTAIN CAME DOWN

The Himalayas are an extraordinary geological environment.

The mountains were built by the collision of enormous tectonic plates, which produced intense folding, fracturing and deformation of the rocks.

And in these high mountain environments, ice and water can become part of that geological system.

Water gets into cracks and fractures in rock.

It freezes.

It expands.

It freezes and thaws over and over again.

That process can weaken rock over time and contribute to what scientists call frost cracking.

And there is evidence that temperatures near the source area had been unusually warm in the period leading up to the disaster.

One nearby climate station recorded temperatures approaching 86°F (30°C) the day before the event, while unusually rapid temperature swings occurred during the three days before the collapse.

Those rapid temperature changes may have contributed to instability in the rock and ice.

But — and this is important — we should not say that warmth alone caused the disaster.

Scientists are still working out exactly how all of the pieces fit together.

The evidence points toward a combination of unstable high-elevation rock, ice, temperature changes and the unique geology of the Himalayas. (ArcGIS StoryMaps)

💥 THEN THE MOUNTAIN LET GO

At approximately 8:37 a.m. Nepal time on August 26, the slope failed.

And when it failed, an enormous volume of rock and ice came crashing down the mountain.

Scientists estimate that the avalanche may have contained somewhere between roughly 197 million and nearly 500 million cubic meters of material, although those estimates remain preliminary and will undoubtedly be refined as more research is completed.

To put that into perspective, even the lower estimates represent an absolutely enormous amount of material.

This wasn't a little landslide.

This was a mountain-sized event.

The collapse also generated seismic energy equivalent to approximately a magnitude 5.2 earthquake.

That's an important point because the seismic signal initially created some confusion about whether an earthquake itself might have triggered the disaster.

But scientists subsequently determined that the signal was generated by the enormous mass movement rather than a conventional tectonic earthquake. (USGS)

🌊 ROCK + ICE + WATER = A GIANT SLURRY

Now comes the really incredible part.

As that enormous mass of rock and ice descended from the mountains, the material was being broken apart and mixed with water.

Ice melted.

Frozen material was incorporated into the flow.

Rock and sediment were pulverized.

And the whole thing transformed into an extraordinarily dense, sediment-filled flow.

Basically, you had a gigantic slurry of rock, ice, dirt and water racing down the valley.

And because these mountains are so steep, gravity was working with an enormous amount of force.

The material entered the existing river channels and continued picking up additional water, sediment, boulders and debris as it traveled downstream.

This is how an event that started high in the mountains became a catastrophic flood and debris avalanche many miles away.

🚨 AND IT MOVED INCREDIBLY FAST

The resulting flood and debris flow traveled approximately 100 kilometers — about 62 miles — downstream.

And scientists have been able to reconstruct portions of its movement using satellite imagery, seismic data, river gauges, CCTV footage and eyewitness reports.

At one location roughly 88 kilometers downstream, the Trishuli River reportedly rose approximately 9 meters — nearly 30 feet — in only 30 minutes.

Think about that.

A river suddenly rising nearly 30 feet in half an hour.

That's why people downstream had so little time to react.

The USGS estimates that the overall debris flow and flooding traveled nearly 100 kilometers through the Lende Khola and Trishuli River systems. (USGS)

🛰️ THE SATELLITE IMAGES ARE INCREDIBLE

And this is where I think the satellite imagery really brings the story home.

Researchers have obtained high-resolution imagery from Planet Labs showing the source area before and after the collapse.

The images can actually be annotated to distinguish the bedrock portion of the collapse from the glacier ice.

That is extremely important because it shows us that the event wasn't simply:

“The glacier melted and fell.”

It was much more complicated.

A massive section of the mountain failed.

Rock came down.

Ice came down.

The material transformed as it descended.

And then that enormous mass entered the river system.

Planet Labs imagery: approximately 3.7-meter resolution, showing the source region before and after the collapse.

The researchers at the Center for Land Surface Hazards specifically note that the satellite imagery shows a large portion of the collapsed region was bedrock, with some glacier/ice, which is an important distinction from the early descriptions of the event. (ArcGIS StoryMaps)

🌊 WHAT ABOUT THE RIVER BEING BLOCKED?

There was also an early theory that the initial collapse created a temporary dam in the river, allowing water and debris to build up before suddenly releasing.

But the newer analysis suggests that this probably wasn't the main mechanism responsible for the initial catastrophic flood wave.

The timing is one of the reasons.

The seismic evidence indicates that the initial rock-and-ice avalanche occurred only about seven minutes before floodwaters reached a nearby community.

That's simply not enough time for a massive lake to form, fill and then catastrophically drain in the way originally proposed.

Instead, researchers now believe that melting of ice and frozen material within the initial avalanche, combined with the enormous amount of rock and sediment being incorporated into the flow, played a major role in creating the downstream disaster.

Interestingly, new lakes did form later because debris blocked portions of the river system.

Satellite imagery showed two such lakes forming after the initial event.

So there really were debris dams and lakes — but they appear to have developed after the main avalanche and flood had already begun. (ArcGIS StoryMaps)

🛰️ YOU CAN ACTUALLY SEE THE FLOOD FROM SPACE

The satellite imagery isn't just showing us the mountain.

It also shows what happened downstream.

Sentinel-2 imagery from August 12, before the event, can be compared with imagery from August 27, the day after the disaster.

The difference is remarkable.

The river system is dramatically different, and the sediment and floodwater can be traced downstream.

This is why satellite technology has become such an important part of disaster science.

When you can't get people safely into a remote mountain valley, space can give you the view you need.

The ESA says satellite imagery is being used to map the inundation, assess damage and understand how the landscape changed before and after the event. (European Space Agency)

❓ CAN WE PREDICT SOMETHING LIKE THIS?

That's the million-dollar question.

Can we predict when another mountain slope is going to fail?

I'm not sure we're there yet.

We can identify unstable slopes.

We can monitor glaciers.

We can watch temperatures.

We can use satellites to look for changes in the landscape.

We can monitor seismic activity.

But predicting the exact moment when hundreds of millions of cubic meters of rock and ice will suddenly let go?

That's a whole different ballgame.

And that's one of the lessons from Nepal.

There are warning signs that scientists can study, but turning those warning signs into an exact prediction of when a mountain will collapse remains extraordinarily difficult.

🔎 SO WHAT REALLY HAPPENED?

When you strip away all the social-media speculation, the emerging scientific picture looks something like this:

A massive section of unstable Himalayan mountainside suddenly failed.

⬇️

Rock and glacier ice came crashing down the mountain.

⬇️

The enormous avalanche generated seismic energy equivalent to a magnitude 5.2 earthquake.

⬇️

Rock, ice, frozen material, water and sediment transformed into an enormous debris flow.

⬇️

That material entered the river system and picked up even more water and debris.

⬇️

The resulting flood/debris flow raced downstream for roughly 62 miles.

⬇️

Communities, infrastructure and lives were devastated along the river valleys.

That's the picture we're getting now.

And the satellite imagery is helping prove it.

🧠 THE BIG LESSON

For me, this is another reminder that nature is much more complicated than the first explanation we see on social media.

The first explanation isn't always the correct explanation.

The first headline isn't always the final scientific answer.

Sometimes you have to wait.

You have to look at the satellite pictures.

You have to look at the seismic data.

You have to look at the temperatures.

You have to look at the geology.

And then you start putting the puzzle together.

That's what scientists are doing in Nepal right now.

And honestly, as terrible as this disaster has been, what they're learning from it could eventually help us understand — and perhaps better prepare for — similar disasters in the future.

🇳🇵 REMEMBERING THE PEOPLE OF NEPAL

Ultimately, though, this isn't just a scientific story.

It is a human tragedy.

Thousands of people have been killed, injured, displaced or remain missing, and entire communities have been devastated.

So while I'm fascinated by the science of what happened, we also have to remember the people who were caught in this disaster.

My thoughts are with everyone affected by this terrible event.

And that's it for my blog this Friday.

🌎 WEATHERMADNESS BRIEF™

Making Weather Make Sense.

I'm Meteorologist Henry Margusity with your WeatherMadness Brief™.

Stay curious, keep asking questions — and don't believe everything you see on social media. Sometimes Mother Nature requires a little more digging. 🌎🛰️