In late April and early May 2012, what was usually a roaring Seti river in Northwestern Nepal had slowed to a trickle. The milky-white turbid water had turned blue and clear. And then suddenly on May 5, 2012, the flooded river laden with slurry of sediment, rock, and water surged through the Seti valley in the Kaski district, obliterating dozens of homes and sweeping 72 people to their deaths. The floods waters were upto 30 m high at places. It reminded of the sequence of events leading to Tsunami. Questions swirled about where the water had come from and how it arrived with so little warning and that too in a non-monsoon season.
NASA Earth Observatory (acquired October 7, 2013)
Now, twenty months after the disaster, experts like Dr Jeffrey Kargel, hydrologist at University of Arizona, are in a position to say what happened. Dr. Kargel has concluded that it was not just one event but a series of event that combined to produce the devastation.
NASA Earth Observatory site describes the affected landscape as: “The landscape in this part of Nepal is shaped by a cycle of landslides and subsequent erosion. As the tectonic collision of India with Asia pushes the Himalaya upward, ice, water, and gravity, assisted by sporadic earthquakes, combine to grind the mountains down. The channel of the SetiRiver itself is cut into the remnants of a much larger debris flow, perhaps 1,000 times as big. 60 to 100 meters (200–330 feet) thick, the landslide deposits are composed of the same limestone as the peaks to the north. Likewise, the May 2012 flow left behind jagged fragments of limestone, carried from the crest of the Himalaya to the foothills in a single event.”
‘Seti’ in local languages means white, and Seti is the ‘White River’, its water is glacial white, turbid and laden with sediment.
The high intensity floods in May 2012 came in waves, and the first wave alone had around a quarter of a million cubic meters of water in just a few minutes. There were about 27 waves in all over the next hours, according to eyewitnesses, so several million cubic meters of water flowed overall.
NASA Earth Observatory image (acquired Dec 22 2013)
However, the series of event started weeks before the flood with a series of rockfalls that sent debris tumbling into the SetiRiver, backing water up in the extremely deep and narrow gorge. The last of these landslides occurred just a week or so before the flood. The situation grew dire on May 5, 2012, when an unusually powerful ice avalanche and rockfall tumbled down a vertical cliff on a ridge just south of Annapurna IV Mountain peak. The total drop from the Annapurna IV ridgeline to the bed of the Seti below Pokhara is about 6,100 meters (20,000 feet) spread over a distance of only 40 kilometers. The distance between the landslide dam and sight of worst floods was about 29 km.
Prof Jeffrey Kargel, writes (http://earthobservatory.nasa.gov/blogs/fromthefield/2014/01/24/setiriverclues/), “A flash flood—what geologists call a hyper concentrated slurry because it was thick with suspended silt—had torn through some villages along the Seti River, in north-central Nepal, just north of the country’s second largest city, Pokhara. It was immediately recognized as a very deadly event, but the death toll—and a tally of those who remain missing but were clearly also killed—was not known exactly for several months. 72 souls lost. Though not large on the scale of global disasters, this event was terrifying for the fact that it seemed to come from nowhere—literally from beneath a blue sky.”
AFTER IMAGE: shows the area on May 6, 2012, roughly 25 hours after the landslide
BEFORE IMAGE: the same area on April 20. The diagonal lines are gaps in the data, due to a partial failure of the satellite.
One scenario (see: http://earthobservatory.nasa.gov/IOTD/view.php?id=78070&eocn=image&eoci=related_image) estimates that roughly 22 million cubic meters of rock broke off the slope of Annapurna IV. Colin Stark at Lamont-Doherty Earth Observatory at ColumbiaUniversity said: “There’s a drop of about 2,000 meters into the canyon, so we’re talking an enormous gain in momentum. Then I think the debris ran down the canyon at speeds upwards of 30 meters per second—a guess but what we see for the landslide itself.” Stark estimated that events unfolded in a matter of minutes with no time for a temporary dam to form.
This flood appeared to behave like a glacier lake outburst flood, and the news media can be pardoned for having assumed that it was. Prof Kargel adds: “Seeing that the disaster occurred at the foot of the AnnapurnaRange, within the Greater Himalaya, probably every expert’s first thought was “glacial lake outburst flood” (GLOF), because these were common in Nepal’s Himalaya, and the news accounts of the disaster event resembled accounts of GLOFs”. However, available satellite images showed there were no such lakes. “It was, however, clearly a disaster that had its source in a high Himalayan amphitheatre-like bowl, a glacially-carved structure called the Sabche Cirque. This structure was rimmed by some of Nepal’s most famous, picturesque mountain peaks, including the storied, holy Machapuchare ( “fishtail” peak) & Annapurna IV, a 24,688 ft” mountain.
Annotated photo from Dr. Kargel
Thanks to video from winged camera of a 2 seater plane incidentally flying over the area at the time of the event, the disaster’s trigger was sourced on a ridgeline near Annapurna IV. Apparently part of this ridge—probably initially the glacier ice— collapsed, dropping ice and rock over 3000 m almost vertically onto unconsolidated rock debris (glacial moraines and ancient glacial lake silts and gravels) resting unstably in the deep bowl of the Sabche Cirque. Some of that loose debris was also swept up by the avalanche, and the mass flowed and dropped through an additional 1,500 m into the SetiRiver gorge. Indeed, the conversion of gravitational potential energy to heat could have melted roughly a tenth of the falling snow and ice by the time it reached the SetiRiver.
A view of the gorges and also the distant peaks of the Sabche Cirque and the ancient glacial deposits in between. The avalanche entered the gorge from the upper right corner of the scene. (Photo: Dr. Kargel)
One source seemed to be definitely involved, and that was a rockslide-dammed reservoir in the gorge. This was definitely not a GLOF, but was caused by a rockslide into the SetiRiver gorge, formation of an impoundment reservoir over a several week period due to damming of spring snow and ice melt, and then the final triggering event of the mighty rock and ice avalanche off Annapurna IV.
The source area for the avalanche of May 5, 2012. Annapurna IV is just off image to the upper left. (Photo: Dr. Kargel)
Warning: More such events could occur “There are good reasons to be concerned,” Jeffrey says, “Something like this will happen again. It’s inevitable.” The Seti River Gorge is unusually prone to dangerous blockages because of how narrow and deep it is. And the same processes that triggered the spring 2012 rockfalls and avalanche are still at work. “The only question is whether future events will be as destructive or whether people in the SetiRiverValley will have absorbed the lessons of 2012 and found ways to move their homes out of the flood plain.” India should not forget that we are downstream country in Seti basin and what happens there will flow down here too.
More significantly, this whole episode raises the question as to how much do we really know about the Himalayas. Uttarakhand disaster of June 2013 was a wake up call to understand the Himalayan ecosystem and its implications for future well being of the people and environment of India. However, in stead of learning any lessons, our governments in Uttarakhand (as also in Himachal Pradesh, Kashmir and North East) and at the Centre, with environmentally-challenged Mr Veerappa Moily as the environment minister, are playing into the hands of short sighted vested interests. This is also apparent in the recent decision to sanction the massive Lakhwar dam on YamunaRiver in Uttarakhand without so much as an Environmental Impact Assessment!
We still do not have full explanation for the Uttarakhand disaster of June 2013. This explanation of the Seti River Disaster in the same Himalayas, which happened just a year before the Uttarakhand disaster highlights the urgent need for more serious studies and applying precautionary principle while dealing with the Himalayas, its rivers and flood plains.
Compiled by SANDRP
Sources:
1. http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=82937&src=nha dated January 25, 2014
2. http://earthobservatory.nasa.gov/blogs/fromthefield/2014/01/24/setiriverclues/ This entry was posted on Friday, January 24th, 2014 at 10:34 am. See the absolutely amazing photos on the site that gives an insight into the situation in the head reaches of Seti River… possibly indicates how little we know about the catchments of our Himalayan Rivers.
3. http://earthobservatory.nasa.gov/IOTD/view.php?id=78070&eocn=image&eoci=related_image dated May 6, 2012
4. http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=78117&eocn=image&eoci=morenh dated May 23, 2012
5. http://blogs.agu.org/landslideblog/2012/05/23/understanding-the-seti-river-landslide-in-nepal/, dated May 23, 2012
6. http://www.icimod.org/?q=7377, dated June 25, 2012
