doi:10.1038/nindia.2013.20 Published online 8 February 2013
Satellite data has led space scientists to warn India of a time bomb in the form of a huge glacial lake ticking atop the Himalaya in the north-eastern state of Sikkim. If the lake bursts, it could cause havoc downstream, they say.
Satellite data between 1962 and 2008 has revealed that melt water from the receding South Lhonak glacier at a height of about 7000 meters in Sikkim Himalaya has formed a huge lake at its snout. According to Babu Govindha Raj and co-workers at the National Remote Sensing Centre (NRSC) in Hyderabad, this high altitude lake is dammed by 'moraine' – material such as rocks, boulders and loose soil left behind by the retreating glacier.
The scientists report that the glacial lake holds about 19.7 million cubic metres (or 19.7 billion litres) of water. It is about 630 meters wide, 20 meters deep and is spread over 98.7 hectares. A sudden outburst "can create devastating floods downstream," they warn in their report. The probability of this happening is 42 per cent, which according to them "is very high".
A 2003 report by the International Centre for Integrated Mountain Development (ICIMOD) shows that the glacial lake already had an outburst event evidenced from breach of the moraine dam. "This clearly indicates the hazardous nature of the glacial lake", the scientists say.
In the event of an outburst, the discharge of water from the lake could be as high as 586 cubic meters per second, which is more than the flow of water on the American side of Niagara Falls. However, the scientists note that more field-based investigations are needed to prove the hazardous potential of the lake.
Govindha Raj and colleagues used U.S. Landsat satellite data, declassified imageries of the American CORONA reconnaissance satellites (1960-1972) and India's Resourcesat-1 satellite. Data from NASA's Terra satellite launched in 1999 were also used for depth measurements.
Using the November 1962 CORONA imageries as baseline data, they estimate that the Lhonak glacier receded 1.9 km in 46 years (1962-2008). Also, that the lake started off as a small water body on top of the glacier, got separated and became larger with accumulation of melt water as the glacier receded. 2008 satellite imagery shows that the lake is still attached to the snout of the glacier but is expanding laterally and increasing in area due to the glacier retreat, the report says. "The rate of growth of the lake indicates possible developments of the hazard situation."
The scientists also point out that flash floods triggered by monsoon cloudbursts are common in the Himalayan region. Such cloudbursts over the moraine-dammed glacial lake can cause dangerous floods downstream.
With the Himalayan glaciers retreating fast, it is necessary to create an inventory of glacial lakes, establish their hazard potential using satellite data and in situ field surveys, and put an early warning system in place, they suggest.