How a Collapsing Glacier Triggered a Biblical Flash Flood in Tibet and Nepal

Scientists have identified the likely cause of yesterday’s horrifying flash flood in Tibet and Nepal. After surveying seismic records and satellite images, the U.S. Geological Survey (USGS) has pinpointed an ice avalanche as the likely cause of the destruction.



Initially, it was unclear what had happened. Officials registered seismic activity of 5.2 on the Richter scale near the site. At first, it seemed probable that an earthquake had caused a landslide, which then triggered the flood.

But subsequent analysis by the USGS found that a massive glacial collapse and debris flow generated the seismic signal, rather than an earthquake. An extraordinary chain reaction of intensifying ice, rock, water, and sediment followed as the mass moved downstream.



Related: The Glaciers Aren’t Melting—They’re Collapsing





The Collapsing Glacier 

Preliminary data suggests the avalanche began at roughly 5,000 meters (16,404 feet), below the towering summits of Tsangbu Ri (6,781 m, or 22,247 feet) and Langtang Lirung (7,234 m, or 23,734 feet), the latter of which is also the world’s 99th highest summit. 

Satellite imagery indicates that a substantial section of the glacier’s tongue broke away and began hurtling down the mountainside, picking up rock, ice and debris along the way. “It looks like a glacier in the area basically snapped off in a clean break, allowing it to fall just over a vertical kilometer to the valley bottom,” explains Daniel Shugar, a geologist with the University of Calgary.



Researchers estimate that the collapsed area of ice covered roughly 0.2 square kilometers. After the initial flow of ice and rock, reporting diverges on what happened next. Some officials described the event as potentially involving an avalanche-induced outburst flood. As a result, some news agencies reported that water built up behind the debris, like an artificial dam, and then burst. However, recent reporting suggests that the initial landslide simply had so much force that it continued down valley, gathering water and mud.

The slide continued for over 100 km (62 miles) down the Trishuli River Valley. Reported damage continued up to 130 km (118 miles) downstream. The steep, narrow river valley contributed to the flood’s severity, as floodwaters were tightly constricted as they moved downstream. Moreover, in the first 65 km (40 miles) of its descent, the flood descended an astonishing 2500 meters (7,874 feet) of elevation. 



Related: NASA: Glacier Collapse Buries Swiss Village



The Aftermath

The aftermath of the landslide is, in some places upstream, complete and utter annihilation. Little remains of the Gyirong Port, the border crossing between Tibet and Nepal, which was the first significant settlement in the flood’s path. CCTV footage from the site is apocalyptic. Areas downstream fared slightly better but still suffered large-scale destruction. 

Over 1,300 people are reported missing, with over 160 declared dead. Authorities are still in the early stages of the rescue effort. However, the death toll is likely to surpass 1,000 in the coming days. Rebuilding the hundreds of destroyed buildings and houses will take many years. And questions remain as to whether they should be rebuilt, given the current situation.  



Was Climate Change to Blame?

It’s too early to blame climate change for this glacial collapse directly. However, it’s nearly certain that climate change was at least partly responsible. 

No matter what, the collapse wasn’t entirely the fault of climate change. Glaciers collapse for many reasons, and collapses happen whenever and wherever glaciers exist. Even if the world were cooling rather than warming, glaciers would occasionally collapse because of steep terrain, fractures within the ice, changes in meltwater, earthquakes, or many other factors.  

Unfortunately, climate change is shifting conditions to an extent that these disasters are occurring where they have never occurred before, at least not on a human timescale. The Himalayas are warming rapidly (the pace of glacial melt has at least doubled since 2000). Often, meltwater can build up around or underneath a glacier. Occasionally, the result is a sudden lapse in the friction holding the mass to the mountainside, with no warning. 

In areas where glaciers have recently melted, slopes can often be unstable and filled with moraine and other loose debris. Additionally, large glacial lakes can build up in upper valleys, posing a similar threat to the recent flood. All it takes is an avalanche or landslide to quickly overflow the banks. Seconds later, hundreds of millions of tons of water are barreling downstream.



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