Uttarakhand: Breach, avalanche? Cause of flood divides scientists
There was no consensus till Monday evening on what exactly may have triggered the disastrous flash flood in the Rishiganga valley on Sunday. At least 26 people have died and 171 were still missing till the time of going to print.US-based scientists who looked at satellite images suggested it was caused by a landslide onto a glacier which led to debris flooding the river, while Dan Shugar of the University of Calgary suggested a landslide triggered an ice avalanche.
Not everyone agrees. Jimmy Kansal, deputy director, The Snow and Avalanche Study Establishment of the Defence Research and Development Organisation, said: “There was no landslide or avalanche. There could be a glacial lake which led to the breach, but we are still investigating that.”
The glacial lake theory (which means the flooding was what is called a glacial lake outburst flood, or Glof) is supported by scientists at the Divecha Centre for Climate Change at Indian Institute of Science, Bengaluru, who say they have found clues to the origin of the flash floods. The team has concluded that release of water from an underground glacial lake led to flash floods and inundation in the valley.
Glacial lakes are typically formed when a glacier retreats, with the water collecting in the area where the glacier used to be, and dammed by the moraine (or material such as rocks left behind by the glacier). There are also instances of glacial lakes being damned by ice. When the dam bursts, it causes a Glof, resulting in flooding downstream, which is exactly what happened in this case.
The team at Divecha Centre used a tool to map the depression in the bedrock below the north Nanda Devi glacier and said the data suggests a depression of 25 ha upstream of the glacier terminus.
This underground lake has a capacity to store 4.5 million cubic metres of water. The lower part of the ablation zone (zone of the glacier which has melted or calved and formed a lake), the scientists added, receives a significant amount of water from a tributary glacier located at the northern side of the Nanda Devi glacier. “If this depression filled with water develops appropriate hydrostatic pressure, it can accelerate the lower part of the ablation zone, possibly releasing water from the underground lake. It could be the potential reason for the flash flood,” a note prepared by the IISc team said.
“This is a new tool developed in IISc by our team which can be very useful to study such disasters. The tool is based on Laminar flow equation and surface slope; known as Himalayan Glacier Thickness Mapper (Highthim). The tool was used successfully to map depressions below South Lhonak lake in Sikkim and further estimate the future expansion of the lake,” said Professor Anil Kulkarni, distinguished scientist, Divecha Center for Climate Change.
Responding to the US-based scientists who said the floods may have been triggered by a landslide, Kulkarni said “It appears to me that they looked at the adjacent valley. Initial reports suggest that flash flood was caused due to breaking of Nanda Devi glacier. The observation was widely published by numerous news media and also supported by reconnaissance survey carried out by the Indian Air Force. We went deeper to see what happened under the surface.”
But the landslide theory has other subscribers.
Uttarakhand chief minister Trivendra Singh Rawat said that scientists of Indian Space Research Organisation (ISRO) have revealed to him that the Chamoli tragedy was caused by a mix of snow and rock avalanche and that satellite images have shown no glacier breakage.
A glacial break usually means a piece of the glacier breaking off as a large piece of ice (quite simply, an iceberg). That does not seem to have happened in this case.
Rawat spoke to reporters on Monday after chairing a meeting of senior government functionaries, senior officers of Army, ITBP and ISRO scientists.
Rawat said ISRO scientists told him that a few days back there was snowfall in the affected area. “And from one trigger point due to rockfall at the top, it all slid downwards. Due to all this, lakhs of tonnes of snow came gushing down which led to the tragedy. The ISRO scientists said the satellite imagery doesn’t show any glacier breakage and that this area is not avalanche-prone.”
Pradeep Srivastava, scientist at Wadia Institute of Himalayan Geology Dehradun said that preliminary data suggests the Chamoli tragedy was not caused by any glacial outburst, but due to sliding of the rock mass exposed on the top of the glacier and an avalanche.
“Due to freezing and thawing, a rock mass broke away and slid down. Data suggests a 0.2km-long rock mass broke away and slid down, adding to the debris accumulated on the glacier. This created a melting layer near debris. An avalanche was evident from the huge clouds of dust that came down with the flowing muck and debris,” he said.
Srivastava added that on February 6 there was snow in the area and on February 7, the average temperature was 5 to 6 degrees Celsius, enough to melt the snow.
“Satellite imagery is showing snow on February 6 and no snow on the slopes on February 7. So it seems that it was mix of snow avalanche and rock avalanche which led to the disaster,” he concluded
As for the water, it can “can come from surface snow, from big cavities in glaciers or that trapped in cervices. Besides water also melts due to pressure from the base of the glacier”.
Srivastava said a team from Wadia Institute is inspecting the area to find out “what exactly happened.”
Interestingly, the breach of dams, especially moraine ones, has been known to be caused by rockslides, avalanches, rain, or a glacier break that causes a large piece of ice to fall into the glacial lake -- so it is possible that both theories hold in this case.