Collapsed glacier likely caused devastating Nepal-Tibet floods, scientists say

Preliminary investigations have pointed to a collapsed glacier as the cause of the flash flood on the Nepal-Tibet border, highlighting the issue of rapidly melting Himalayan ice, scientists have told the BBC.

Initial reports of an earthquake led many to believe that a tremor had caused a landslide, which in turn caused the flood that has killed at least 177 people.

However, the US Geological Survey then clarified that the disaster was caused by a "glacial collapse", which is when a glacier or part of a glacier disintegrates.

The impact of the collapsed glacier hitting the bottom of a valley was so powerful that it registered a magnitude of 5.2, according to USGS data and scientists who have analysed the event.

Dr Simon Cook, a glacier expert with the University of Dundee, told the BBC his team had detected that the lower part of a glacier in Nepal near the Langtang Lirung peak - which is approximately 15km (9 miles) west of the site of the floods - had collapsed.

He estimated that the glacier collapsed in a northwesterly direction and then travelled westwards downstream.

More than 800 missing in Nepal as authorities say glacial collapse triggered flash flood

An analysis , external by the International Centre for Integrated Mountain Development (ICIMOD), an intergovernmental scientific institution, found there could have been "an ice-rock avalanche originating from a high-altitude area".

This caused a "large volume of ice and rock debris" to enter the Lende Khola, a tributary of the Bhote Koshi river, which in turn generated a sudden surge in the river carrying water, sediment and large boulders downstream.

The centre found that water levels in rivers downstream rose between seven to nine metres within a span of 30 minutes, and several water monitoring stations were washed away or damaged.

Mike Searle, a professor of earth sciences at Oxford University, said that this whole process - from landslide to flood - could have taken hours or even days.

"The sheer wave of debris suggests there could have been a landslide first; the water built up, dammed the river, and then it burst in one huge great flood," he told the BBC.

The topography of the area could have also exacerbated the flood. Searle pointed out that Langsang Lirung sits on the crest of the high Himalayas and there are "incredibly steep valleys" on both sides of the mountain, which could have funnelled the water at higher speeds.

Searle said that Langsang Lirung, like the rest of the Himalayas, has been slowly rising due to long-term tectonic movement, where one plate has been pushing another plate up. "It's like putting a jack under the car, and then jacking it up."