The humongous volume of water in the flood came from several sources. The first was water released from the melting of glacial ice that fell from the mountain at an elevation of 5,150 metres above mean sea level onto the valley floor at about 3,750 m. This water was then carried forward by the river.
When the rock and ice fell onto the valley floor, it generated enormous energy, producing seismic waves that were recorded as an earthquake-like signal by detectors. The mechanical energy of the moving debris melted glacial ice, which became the primary source of water in the flood.
A second source was water stored beneath the glacier. A third was water from thawing permafrost. A fourth came from the melting of buried ice on the valley floor, while a fifth came from the pressure wave generated by displaced river water ahead of the debris flood.
The combination of ice, rock, water and sediment travelled about 35 km downstream, causing widespread destruction. The resulting floodwater travelled much farther through the Bhote Koshi and Trishuli river systems, reaching areas close to India.
The WWA study analysed the disaster as a combination of factors, rather than as a single extreme weather event. The research team included experts in glaciology, mountain hydrology, climate science, humanitarian aid, seismology and social science.
The scientists examined why the initial rock and ice collapse happened, and what role warming and climate change may have played. They found that while the underlying geological structure controlled where and how the slope failed, longer-term warming and changing precipitation from snow to rain may have reduced slope stability.
This could have happened by weakening ice-filled fractures and rock-ice contacts, and by increasing water pressure. “Climate change is thus best understood as a destabilising factor acting on a pre-existing geological predisposition, rather than the fundamental cause of the failure,” the scientists said.