Scientists have said the flood water became a fast-moving mixture of material capable of destroying bridges, buildings, and radically reshaping the river channel, ITV News' Science Correspondent Martin Stew reports
The after-effects of a glacial collapse may have caused a flash flood that has left more than 1,300 people missing in Nepal and Tibet, including 33 British tourists.
Prime Minister Andy Burnham said the government will do “everything we can” to support British nationals in Nepal following the floods.
But how did such devastating floods start?
Although this occurred at the height of the monsoon season, significant rainfall does not appear to have been the trigger.
The US Geological Survey (USGS) initially reported a magnitude 4.4 earthquake early on Wednesday, about 66 kilometres (41 miles) north of Kathmandu, near the border.
But after further analysis, it was determined that the seismic event was caused by the impact of a glacial collapse, not an earthquake, which sent debris flowing downstream.
ITV Science Correspondent Martin Stew said the glacier collapsed five kilometres above sea level and may have caused it to crash into a lake below, which had formed following a previous landslip.
This created a huge wave of mud, ice, and meltwater, which may have caused the devastating floods.
Before the avalanche, river levels were about seven metres. But minutes after the avalanche, levels rose by another five metres - around 15 feet.
The flooding left towns and cities submerged under up to five feet of mud and sediment - a loose material such as sand, dirt, or small pieces of rock that settles at the bottom of a river.
River levels at Galchhi reportedly rose by as much as nine metres within thirty minutes.
Why was the flood so destructive?
Scientists have said that sediment would have made the cascades of water so dangerous.
Associate Professor in River Science at the University of Auckland, Jon Tunnicliffe, said: “Sediment is a crucial part of why these cascades are so destructive. Once a surge begins to entrain rock, gravel and boulders, it can grow enormously downstream.”
He said that the torrent was no longer simply a flood of water, but a fast-moving mixture of material capable of destroying bridges, buildings, and radically reshaping the river channel.
“Narrow mountain valleys concentrate that energy, while roads, settlements and hydropower infrastructure are often concentrated on those same valley floors,” Tunnicliffe added.
“The danger also does not necessarily end when the initial surge passes. Fresh deposition can block tributaries, raise riverbeds, divert channels and create new temporary dams."
He also warned that a major flood changes the landscape in ways that can “generate further hazards over the following days, weeks and sometimes years”.
"Climate change can amplify hazards that already exist by altering the thresholds at which one process triggers another," Tunnicliffe said.
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