Nepal's Dangerous Himalayan Valleys

Trishuli Disaster

On 26 August this year, a wall of mud, water, and debris roared down the Trishuli River valley in Nepal, sweeping away whole villages in its path. Thousands are dead or missing. A video of the mud exploding around a building at the Gyirong border crossing between Tibet (China) and Nepal looks like an AI fake but apparently is real.

The Center for Land Surface Hazards created a storymap to summarize the geography of the flood. Here I display the topography of the valley to gain insight into the region affected. The valley itself is high in the mountains. In the topographic map (below left), the boundary between the orange and green contours is at an elevation of 3000 m. The enormous fall of rock and ice that initiated the disaster occurred on the north face of the mountain Langtang Lirung. The location is marked "Avalanche" on the map, at about 4000 m. The first two towns hit, at the Nepal-China border (white curve in map), were about 10 km around the curving valley. The debris had already descended more than a kilometer before it hit them. It continued another 50 km southward and down about another kilometer in elevation. Even Trishuli Bazaar, far down the valley, lost 60 houses and two concrete bridges. In the same town, an entire school was wiped away by the flood, just moments after a fast-acting principal evacuated all the students and staff.

The flood wiped out the main roads in the area, which tend to run along the banks of the river. Thus the harm from the disaster affected many thousands of people who were not themselves in the path of the flood.

The figure at right gives a perspective view of the northern part of the valley. Note that the one arcminute resolution of the topographic data - about 1.85 km - smooths out the peaks and valleys to some extent. All topographic data on this page is from the ETOPO 2022 global elevation data set.

Many Valleys

The valley in which the disaster struck is just one of many in Nepal. Nepal straddles the edge of the enormous Tibetan Plateau, most of which is over 4 km above sea level. As the map below shows, the southern edge of the Nepal is below 500 m and the northern edge follows the Himalayan Mountains, in which Everest and dozens of other peaks tower above 6 km. Much of the country is in the transition region sloping between lowlands and the Himalayas.

Just looking at the 3000 m contour in the map, at least 6 other major valleys are plainly visible. Each is vulnerable to a similar avalanche/debris/flood event like the one that occurred this year. While Kathmandu (the capital) and other major cities lie below 1500 m elevation, there are many villages, tourist destinations, and hydropower installations in the exposed highland valleys.

Another Climate Change Threat?

All around the world, glaciers are melting due to global heating. This raises the possibility that the flood was due to glacial melt. Some of the most-discussed mechanisms do not seem to be responsible however. One concern about melting glaciers is that meltwater lakes form in them; when the outer edge of the glacier melts enough, there is an explosive flood similar to a dam breaking. Something like this happened at the end of the last Ice Age, when the Scablands of Washington State were formed. Another flood mechanism is when a glacier on the side of the mountain is destabilized by melting ice, comes loose, and crashes down into the river below.

Some scientists now think that the Nepal flood was caused by a rock slide, in which part of the mountain sheared off, dropping a huge mass of both rock and ice a kilometer into the valley below. The energy of the long fall may have been enough to melt some of the ice and start the flood down the valley. There is also evidence that the landslide blocked an upper branch of the Trishuli River. This could have caused a temporary lake that then burst.

While landslides occur in an unchanging climate, climate change could have played a role in this disaster. Water from melting glaciers may have seeped into the rock and destabilized it. There is not enough information about this remote site to know if that was the cause in this case, but glacial floods have caused disasters from the Swiss Alps to India in recent years. Similar mechanisms are likely to precipitate future floods.