What Triggered Devastating Flash Flood In Nepal?

Was it a mudslide, debris flow or a glacial lake outburst flood? On Wednesday, 26 August, a massive flash flood swept away hundreds of homes in a mountain region in Nepal, close to the border with Tibet. Initial assessments point to the collapse of part of a nearby glacier. But what exactly occurred from a geophysical standpoint, and could we see a similar natural disaster here in Switzerland? We spoke with Prof. Christophe Ancey, head of EPFL's Environmental Hydraulics Laboratory and a specialist in fluid mechanics.

A number of theories are currently making the rounds. At this point, what do we know for sure about the cause of the flood?

First of all, it's important to keep in mind that media outlets are throwing out a number of terms and theories - some of which are a little outlandish. So take those statements with a grain of salt. It's still too early to know the exact cause of this catastrophe. Subglacial water pockets, debris flow and mudslides do all occur, but it could take geomorphologists and glaciologists several months to figure out the precise chain of events leading up to the disaster.

Do we already have an idea based on the first images?

Yes. The images and videos we currently have show a wave over 20 meters high moving very rapidly. That's typical of waves created when a natural dam bursts. Such dams are formed by the build-up of ice and rocky sediment in a river, which can accumulate gradually over time or pile up suddenly, such as when a mountainside collapses. We could call this a debris dam.

Sometimes, several processes occur simultaneously, such as the rapid melting of the snow cover, heavy rainfall and the bursting of a subglacial water pocket, which is a pool of water trapped underneath a glacier. Such a confluence of processes can cause large masses of water and sediment to start flowing. It's similar to what happens when a man-made dam bursts - the huge amount of water that's released carries along a lot of sediment and debris. In natural dams, this can be called a dam burst.

What was the wave made of, and why did it destroy everything in its path?

Waves formed by a debris blockage and subsequent breach contain at least 90% water. The remaining 10% is made up of sediment, plant debris and other objects carried in the water. The images from Nepal show that the flowing mass was very liquid and moved very quickly. Assuming that the wave was 20 meters high, then the flow rate was probably around 20,000 cubic meters per second - which is roughly the same as the Amazon River. By comparison, that river in Nepal usually flows at a rate of a few cubic meters per second.

Is this what is referred to as debris flow?

No, debris flow is different - it consists of a large flowing mass of at least 80% sediment and 20% water. It's akin to a huge stream of liquid concrete. It moves much more slowly than water but can destroy everything in its path. Debris flows occurred in Chamoson, in Valais Canton, in 2018 and 2019, and more recently in Val de Bagnes.

Is there a link between the natural disaster in Blatten and the one in Nepal?

What happened in Blatten was that part of the Birch Glacier collapsed, triggering an avalanche of ice and rock. The collapse was caused by an accumulation of rock on the glacier. This rock tumbled down into the valley, burying part of the village and depositing a huge mass - some 2 kilometers long and 200 meters wide - into the Lonza river bed. The mass dammed up the river and a small lake formed upstream of the deposit. But after a few days, the water carved a path through the deposit. The volume of accumulated debris is so big relative to the size of the lake and the Lonza river's flow rate that there was little chance that the mass would burst and break up.

Does Switzerland have a monitoring and alert system for these kinds of events?

Yes, of course, and it's been in place for a very long time. The technology has changed over the years, but many of the mountainsides and areas considered to be at risk in our country are equipped with sensors. If you've driven along mountain roads, you've probably seen the traffic lights connected to sensors that can detect an avalanche or debris flow. And to protect communities, civil engineering structures have been built to contain or divert ice or debris flows. In addition, many rivers in Switzerland are equipped with debris barriers, which are dams containing slits that filter out debris and let the water pass through. Scientists monitor glaciers and natural lakes - especially lakes formed by retreating glaciers - to help prevent outburst floods and the bursting of subglacial water pockets.

Does climate change mean we'll see more of these events in Switzerland?

Historian Christian Pfister made a list of the natural disasters that have occurred in Switzerland over the past thousand years. There's no clear link between these events and climate change. For instance, numerous natural disasters occurred in the 19th century, but the average global temperature was 3°C lower than it is today. That said, the swift pace of global warming means that such events will probably be more intense and more frequent. Large amounts of rocky sediment are being released as mountain peaks thaw, the permafrost melts and glaciers retreat. At the same time, the warmer air is leading to more intense extreme precipitation. This is creating a perfect storm of exposed debris and more frequent weather events that can put all that debris into motion.