How a flood in the Himalayas destroyed everything in its path

The landslide happened on Feb. 7 some 20 kilometres west of Nanda Devi, India’s second-highest peak, and resulted in a flash flood that authorities say killed more than 200 people, destroyed villages, and swept away two hydro-electric projects.

Avalanches and flash flooding in the Himalayas are common during summer and monsoon months, as melting snow and heavy rains combine. But incidents like this are rare so early in the year, alarming scientists studying climate change that is rapidly heating the world’s highest mountains.

Experts are still studying the details of exactly what happened, but detailed satellite imagery helps to build a comprehensive picture of the events that unfolded.Scientists said heavy snowfall followed by bright sunshine led to snow-melt in the area. That could have triggered a chain reaction that led to the avalanche and heavy flow of ice, water, rocks and debris surging down the Dhauliganga river valley. While at least 70 bodies were recovered, the state government has formally declared that another 136 people missing are presumed dead.

“The area witnessed a heavy snowfall and then solar rays [sunshine] resulted in the melting of ice,” said Ravi Chopra, director of the non-profit People’s Science Institute in Uttarakhand state, where the incident took place.

Dave Petley, Professor of Geography at The University of Sheffield, told Reuters there was a reduction in the amount of snow, which could have helped trigger the landslide, but not enough to cause downstream flooding.

“There was clearly a thaw event happening at the time of the landslide. But this looks to be superficial snow only, not large volumes.”, said Petley, who studies landslide events.

A deluge of debris and water

Satellites captured images of the torrent of water, rocks and debris barrelling down the valley. The images below from Planet Labs show the entire valley on the day of the disaster and provide more evidence of what unfolded.The dust plumes show how extensive the landslide was.

Much of the damage downstream was caused by a torrent of water making its way through the valley.Initial news reports suggested this was due to a Glacial Lake Outburst Flood (GLOF) - a sudden release of water from a glacier resulting in a rapid downstream flow. But Prof. Petley disagreed. “This event was definitely not a GLOF - there is no evidence of a breached glacial lake,” he said. “We know for certain that a large block, consisting mostly of rock but with some ice, detached from the mountainside.”

“It remains possible that there was considerable buried ice in the valley floor that was picked up and melted, but only fieldwork would tell us the answer,” he said.It remains unclear where the large volume of water which caused the flooding originated. Fieldwork by the Indian government is ongoing.

Destruction

The Dhauliganga River valley bore the brunt of the flooding. As the debris raced through the valley it reached as far as Joshimath, an important military outpost for the Indian army and a religious site for Hindus.

The flood caused major damage to two hydro power projects on the Rishiganga and Dhauliganga River valleys. The Rishiganga Hydroelectric Project, owned by Indian conglomerate Kundan Group, and the state-owned National Thermal Power Corporation’s (NTPC) Tapovan Vishnugad project were constructed within 5 kilometres of each other.

The village

The torrent skirted the village of Raini and washed away a bridge and the 13.2 megawatt Rishiganga project on the banks of its namesake river, a tributary of the Dhauliganga.

The bridge was a supply lifeline for residents of the neighbouring village of Raini Chak Lata on the other side of the river. Perched high on a hill, the majority of the villagers survived, but the flooding washed away worker accomodation for the hydro project on the valley floor and left the surrounding area almost barren.

The dam

About 5 kilometres downstream, the 520 megawatt Tapovan dam - still under construction as part of the larger Tapovan Vishnugad hydro power project - bore the brunt of the flood. The water descended from Raini to the dam, ramming through the 22 metre-high barrage and overcoming its 12 metre-high water gates. The barrages survived and remained standing amidst a vast expanse of slush.

Trapped in the tunnel

As the area flooded, water trapped multiple construction workers in a tunnel which was designed to divert the flow of the Dhauliganga. As it surged down past the dam, the flood resulted in a major blockage of the tunnel entrance, located 234 metres upstream from the Tapovan barrage. Dozens were inside working at the time.

It is unclear how deep the workers were in the tunnel. Rescuers were using a combination of bulldozers and excavators to unblock the tunnel, but with limited success.

Since then, rescue efforts have waned with those missing in the floods now presumed dead.

The tunnel is a 11.77 kilometre-long channel on the barrage’s bank that diverts water towards the underground power generation house downstream merging with the Alaknanda River, according to an Environmental Impact Assessment (EIA) prepared by the NTPC for the Asian Development Bank in 2007.But as the project was still under construction, it remains unclear how deep the tunnel was, and why workers were unable to get to the other end to escape.

Incidents like the one on the Dhauliganga are rare in February, the month with the lowest amount of water flowing through the Dhauliganga, according to a 2007 environmental assessment carried out by the company constructing the dam. As such, the disaster has prompted calls by environmental groups for a review of power projects in the ecologically sensitive mountain area.Though the precise cause of the flooding remains unknown, some point towards climate change, rapidly heating the world’s highest mountains. Experts say rampant construction is adding to the burden weighing on rural communities across the Himalayas, especially in areas close to glaciers.

A spokeswoman for India’s power ministry said the country has strict measures in place regarding the planning of hydropower projects and the rights of local people are always considered. The use of explosives in construction in the region was criticised after devastating floods in the state in June 2013, dubbed a “Himalayan tsunami” that claimed more than 5,000 lives.