Ice and rock collapse triggered deadly flood in Nepal

A large flash flood in Nepal’s Rasuwa district on Wednesday morning was likely triggered by a massive ice and rock collapse, according to mbl.is. Nearly 2,000 people are missing and at least 579 have died.

The Icelandic Meteorological Office said the latest analysis indicates that the collapse caused a catastrophic flood near the border between Nepal and Tibet. The flood carried huge amounts of water, mud and boulders, causing extensive damage and many deaths.

The event was detected by seismometers and was initially thought to be a tectonic earthquake. Further analysis by the United States Geological Survey found that the signal was caused by the collapse of glacial ice and rock, followed by a mudflow.

The Icelandic Meteorological Office said the exact sequence of events remains unclear. Satellite imagery indicates that the collapse began on the northern slopes of Langtang Lirung, where a large volume of glacial ice and rock fell more than 1,000 metres to the valley floor.

“Such an event generates enormous friction, which can melt large amounts of glacial ice and cause a catastrophic flood like the one seen in the news,” Daniel Ben-Yehoshua, a landslide expert at the Icelandic Meteorological Office, said. He added that the resulting flood wave can become particularly high and powerful when water has only a narrow valley channel through which to escape.

Seismic measurements and satellite images can help clarify the sequence of events, even when a collapse begins in a remote and difficult-to-reach location. Monitoring such areas is challenging, and it can be difficult to identify in advance the changes that may lead to a sudden collapse.

The Icelandic Meteorological Office said that although conditions in Iceland differ from those in the Himalayas, there have been cases in Iceland where landslides have fallen onto glaciers or into lakes and rivers, triggering flood waves, blocking river channels or causing floods.

In 1967, the Steinsholt landslide occurred when a large rockslide fell onto Steinsholtsjökull glacier, carrying glacial ice with it and partly entering a glacial lake. This caused a powerful flood down Steinsholtsdalur valley.

When a large rockslide fell into Lake Öskjuvatn in 2014, it created a wave that reached 20 to 30 metres above the lake’s surface, and even higher in some places.

“The Icelandic Meteorological Office is systematically mapping and monitoring slopes that show signs of instability,” Ben-Yehoshua said.

The office said that glacier retreat and thinning can change the support available to mountain slopes and expose steep, fractured terrain. Warming can also contribute to the melting of permafrost in high mountains and increase water flow through cracks. Under certain conditions, this may increase instability and the risk of ice and rock collapses.

However, it is not yet possible to conclude that climate change caused this particular event. Investigations are still under way.