On 7 February, an ice-capped piece of rock broke off in the Himalayas, triggering an avalanche and unleashing a devastating flood in the Chamoli district of Uttarakhand, India. As of late February, at least 70 people were killed, more than 100 remain missing, and two hydropower plants under construction were damaged.
Although the avalanche and the flood were not the result of a glacial lake outburst, their devastating effects illustrate the threats posed by climate change and increasing development in the region, experts said.
An Avalanche “Not of Snow but of Rock”
Observations indicated that a piece of a hanging, ice-capped rock broke off from nearby Nanda Devi glacier at about 5,600-meter elevation and fell to a spot 3,200 meters below, where glacial meltwater and debris had already formed a lake. The lake, situated on a steep slope, breached its shores in no time with the additional avalanche debris, said Manish Mehta, a scientist at the Wadia Institute of Himalayan Geology in Dehradun.
Satellite images dispelled initial impressions that Nanda Devi itself broke. David Petley, an Earth scientist, author of AGU’s Landside Blog, and vice president for innovation at the University of Sheffield, analyzed 3-meter-resolution satellite images of the site a day before the collapse and one image taken a few hours after. He said the images showed that “a large block of rock weakened on the mountainside over the weeks leading up to the disaster. On the day [of the disaster], it finally collapsed.” Petley explained that the large block “instantaneously fragmented and turned into an avalanche, not of snow but of rock, which headed down the valley.”
Chittenipattu Rajendran, a geologist at the National Institute of Advanced Studies in Bangalore, agreed with Petley and said the disaster began as a rock slope detachment, with a huge piece of icy rock rolling down the cliff at tremendous speed and landing on the top of a hanging glacier in the Raunthi glacier area. “It instantly transformed the glacier ice into water. The water mixed with rocks and other debris…[and] the pressure of debris further squeezed the water-sodden sediment, [which flooded] the river valley in cascades.”
Disasters are not new to the Himalayas, which were formed 40–50 million years ago when the northward moving Indian plate crashed into the Eurasian plate. Tectonics are still very active in the region—every year, the Indian plate migrates northward by about 5 millimeters, and the Himalayas are elevated by about 1 centimeter. The continuing movement has resulted in numerous faults, with sheets and slabs of deformed and sheared rocks that are weak and loosely bound.
The February 2021 flooding disaster is the second to strike Uttarakhand in less than a decade, and geologists fear greater fallout from the twin impacts of global warming and a spate of development projects such as roads, dams, and power plants.
A 2014 report from India’s National Institute of Disaster Management cautioned that climate change is likely to aggravate the frequency of flash floods due to high-intensity rainfall, glacial lake outburst floods, and landslides.
The disasters in Uttarakhand can be attributed fundamentally to “the warming-induced untimely melting of permafrost, the permanently frozen ice in the cracks and crevices between the rocks that holds them together and helps stabilize steep slopes,” Rajendran said. “If warmer temperatures cause destabilization of steep slopes in traditionally cold climate regions, there will be more high-risk areas for rockslides and rock avalanches in the future.”
Geologists have been warning against tampering with the slopes, especially digging for mining and roads. Such disturbances could further destabilize the earth and trigger rockfalls, landslides, and debris avalanches during heavy rains.
“Road building is endemic in the Himalayas and is causing huge damage,” said Petley. “Roads cut slopes without proper engineering in many cases. They disrupt drainage.” The waste rock from such construction projects is dumped on the slopes or in the river, which can further alter the landscape and disrupt drainage.
The Chamoli flood has damaged several hydrological projects by the sheer force of gushing water or by siltation, said Rajendran. “These projects have been opposed from the very beginning by the public, as they were perceived to endanger the sensitive ecosystems of the Himalaya,” he said. “The Chamoli event is yet another reminder of what to expect in the future.”
—T. V. Padma (@tvpadma), Science Writer