Earthquke Preparedness

2-8 May 2014 #705

Anatomy of a Himalayan tsunami

Kunda Dixit

Two years after the deadly flashflood on the Seti River, scientists say it was a warning of cataclysmic events to come

Text by: KUNDA DIXIT

Multimedia by: Ayesha Shakya

It was a bright and sunny Saturday morning on 5 May, 2012 and many people from Pokhara had come to cool off in the Seti River or to take a dip in the scalding hot springs on its banks.

High above them, Capt Alexander Maximov was piloting a two-seater plane on his second tourist sight-seeing flight of the morning. Suddenly, Maximov noticed a billowing brown cloud below Annapurna 4. He immediately understood that this was something totally different from a snow avalanche and turned his plane around. Minutes later, he saw that the normally white Seti River below him had turned turbid brown. A wall of water carrying mud, boulders and tree trunks was surging down the Seti gorge. Maximov, turned his plane around and raced the flood, radioing ahead a warning to Pokhara airport.

Point where a huge wedge of rock broke off the south ridge of Annapurna 4 and fell nearly 3,000m vertically on the ice shelf below. Photo: Jeffrey Kargel/ University of Arizona

At Sardi Khola on the banks of the Seti, 13-year-old Chahana Pun and her family were among hundreds bathing in the river when they heard what sounded like thunder. There were screams as people climbed to higher ground. Chahana and a sibling were lucky, but the surging river washed away their parents and a baby brother. Altogether 72 people were killed that morning, including three Ukrainian tourists. Many of the bodies were never found.

In Pokhara, Maximov downloaded the video from his wingtip camera and was astounded to see the brown cloud, providing first indications to Prof Dave Petley of Durham University that the flood was not a result of a glacial lake outburst as originally thought, but caused by huge rockfall on Annapurna 4.

Halfway around the world at Columbia University’s Lamont Doherty Earth Observatory in New York state, scientists had detected a 4.3 magnitude earthquake triggered by the rockfall at 03:24:56 GMT (09:09:56 Nepal time) on 5 May and precisely calculated the volume and direction of the slope failure on Annapurna.

In the two years since, the Seti flood has been closely analysed by Nepali and international experts. Two studies published recently on NASA’s Earth Observatory website by University of Arizona hydrologist Jeffrey Kargel and in the Journal of the Nepal Geological Society by a team led by German geologist Jörg Hanisch conclude that a chunk of the south ridge of Annapurna 4 broke off and tumbled down to the Seti gorge unleashing the flood either by causing an impounded lake to burst, or from the melting of falling ice.

Kargel teamed up with ICIMOD scientist Sharad Joshi, helicoptered up the Seti gorge after the event, interviewed locals, pored over satellite imagery, analysed silt and rocks downstream. Hanisch worked with Nepali geologist Achyuata Koirala and Netra P Bhandary of Ehime University in Japan and used satellite imagery to conclude that the disaster was caused by a ‘sturzstrom’, a rock-ice avalanche that fell into the Seti.

Forests in the slope above the Seti gorge were flattened by the shockwave from the sturzstrom. A waterfall plunges into the Seti gorge which is still covered in ash-coloured dust and debris six months after the rock and ice avalanche. Photo: Jeffrey Kargel/University of Arizona

Farmers told Kargel’s team that two weeks before the flood, the Seti had suddenly stopped flowing. They concluded that the river must have been dammed by a small landslide that can be seen in aerial photographs.

Both groups of scientists have pieced together the probable sequence of events. A gigantic wedge of the south ridge of Annapurna 4 estimated at 15 million cubic metres fell nearly 3,000m vertically on an ice shelf pulverising itself and releasing the cloud of dust that Maximov saw. Within two minutes, the mixture of rock, ice, debris and fell another 2,000m along a 2km swathe to the Seti gorge below, creating shockwaves that flattened forests.

Landslide expert Dave Petley of Durham University thinks the rock and ice went straight across the plateau falling into the Seti’s gulley system.

The Seti gulley system is so narrow, even the bottom is not visible. Avalanches and rockfalls from the steep slopes of Machapuchhre and Annapurna on either side frequently block the river. The ice and rock from the event two years ago sent debris plunging into the river at very high speed. [The photos were taken seven months later.] THIS PHOTO WAS TAKEN TWO WEEKS AGO Photo: Olav Myrholt

“The debris flow consisted of ice, water and sediment from the main collapse event, so there was no need for another landslide to create the flood,” Petley explained.

Whatever the origin of the water, the flood roared downstream at 50km/h and within 30 minutes had arrived at Kharapani where villagers didn’t stand a chance.

The Seti has seen catastrophic floods before. One was 12,000 years ago, and another probably in 1255 AD when an earthquake could have blocked the river with a landslide, and the lake located in the Sabche Cirque subsequently burst. Both caused cataclysmic floods downstream, depositing debris 60m thick where Pokhara city is located today.

In his paper titled The Pokhara May 5th Flood Disaster: A Last Warning Sign Sent By Nature, Hanisch warns that a flood like the one 750 years ago would have an apocalyptic impact: about half a million people live in Pokhara Valley today. Experts want a detailed investigation of the reasons for the 5 May flood and the possibility that global warming could have triggered the rockfall. Both previous Seti floods happened as the earth warmed naturally at the end of ice ages.

Capt Maximov possibly saved hundreds of lives by radioing the warning, and the lesson for the future is to have permanent early warning systems on Himalayan rivers vulnerable to flash floods especially after earthquakes. It is also a warning not to have large settlements and not locate many big projects along the same river.

In his report, Kargel warns: ‘Our findings do not bode well for the future of the small settlements scattered along the riverside, and there would seem to be a strong case for resettlement.'

Read also:

Lessons of Seti, KUNDA DIXIT

The Seti settles CINDREY LIU

Seti's orphans CANDICE NEO

After the flood

Eye in the sky

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