The
first sign of trouble for residents along the upper Seti River in Nepal was the
water. In late April and early May 2012, what was usually a roaring river had
slowed to a trickle. And the milky-white water—colored by rock flour from
glaciers upstream in the Sabche Cirque—had turned blue and clear.
Residents
regarded the river changes as bizarre, but nobody connected the slowdown to
anything dangerous upstream. So it came as a surprise when a slurry of
sediment, rock, and water suddenly surged through the valley on May 5, 2012,
obliterating dozens of homes and sweeping 72 people to their deaths. In the
chaotic aftermath of the flood, questions swirled about where the water had
come from and how it had arrived with so little warning.
Had
a natural dam formed and then abruptly burst? Had debris from a rockslide
dammed the Seti into a glacial lake and then failed? Had water long trapped in
subterranean caves found its way out? Did an avalanche high on Annapurna
triggered the deluge? “There were all sorts of theories in the beginning, but
they were mostly speculation,” said Jeffrey Kargel, a University of Arizona
hydrologist who has been studying the disaster. “We didn’t have hard data from
the field that could prove or disprove most of them.”
Now
they have that data. Twenty months after the disaster, experts like Kargel have
made enough observations in the field, conducted enough tests in the lab, and
analyzed enough satellite data to say quite definitely what happened. He has
concluded that it was not just one event but a series of them that combined to produce
the devastation.
It
began weeks before the flood with a series of rockfalls that sent debris
tumbling into the Seti River, 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. The total drop from the Annapurna IV ridgeline to the bed of the
Seti is about 6,100 meters (20,000 feet) spread over a distance of only 40
kilometers (25 miles)—more relief than anywhere in the continental United
States.
“You can imagine what an enormous amount of force accompanied the
avalanche,” said Kargel. “It was so powerful that huge amounts of snow melted
due to the friction, and it produced a blast of hurricane-force winds that
flattened old-growth forests near the Seti River Gorge.” As the force of the
avalanche and winds poured into the gorge, it overwhelmed the natural dam
created by the earlier rockslides. The dam burst and sent a surge of pent up
water and avalanche debris rushing downstream.
Evidence
of the massive rockfall and avalanche was still visible on December 22, 2013,
when an astronaut on the International Space Station snapped a photograph (top)
showing debris still coating the slopes below Annapurna IV. Next to the
snow-covered slopes of the cirque, the debris-covered surfaces appear tan. The
second image, a photograph by Kargel, shows the vertical cliff where the avalanche
began.
Kargel’s research group has taken four separate
trips to the area since 2012. In the process of piecing together how the
disaster occurred, the team also analyzed whether a similar event could
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