RAMESH PRASAD BHUSHAL
KATHMANDU: US scientists have said that the massive flash flood in the Seti River at the beginning of the month was due to a huge rock and debris avalance on the flank of the Annapurna IV Mountain.
According to two US professors, who have been studying the cause of the such floods, a global seismological network in the area had recorded a small tremor due to the avalance prior to the flood. It is clear that the landslide is a huge rock slope failure – note that this cliff is about 2000 m high, on the flanks of Annapurna IV,” said Dave Petley, Professor of Geography at the University of Durham. “The mass appears to have fragmented when it reached the lower angled slopes at the foot of the steep section, and to have run out as a rock and debris avalanche.”
Even after three weeks of the flood that killed 72 people, experts in the country have not been able to establish the cause of the flood before a month of annual monsoon rainfall. US scientists claimed that the rock slope failure had a volume of about 22 million cubic metres. “At the toe of the steep slope, the mass fragmented and transitioned into a rock avalanche that flowed down the very steep slope. As the gradient started to decline, deposition initiated,” scientists said. In the beginning, Nepali Army said that the landslide blocked the river causing the flood, after which no agencies have expressed opinion differing to the Army till date.
Scientists Dave Petley and Colin Stark said they have been working in close coordination with National Aeronautics Space Agency (NASA) to further reveal the facts about the rock failure. “The landslide consisted of a main detachment event from the steep 2000 metre high rock slope flanking Annapurna IV,” they have written on the blogosphere of American Geophysical Union.
However, the scientists said there is one remaining oddity they are yet to resolve fully. They said the image (satellite and real) shows that the landslide did not reach the main channel – indeed even the most distal lobe is some considerable distance from the main channel. So, how did the landslide generate the huge debris flow that travelled down the main channel is the question.
“At this moment, we can only speculate, but the most likely explanation is that a small proportion of the debris entered and then flowed down one or more of the steep gullies that descend about 2000 metre to the main channel,” Petley further said.