Glacial Lake Outburst Floods
Glacial lakes are usually dammed by terminal moraines, which are mounds of rubble carried down the valley by glaciers and deposited in the form of ridges across the glacier valley as the glaciers melt. Since these moraines are not well consolidated and frequently contain an ice core that slowly melts, they are unstable. This means that glacial lakes are in danger of bursting through their moraine dams to cause a catastrophic flood in the valley below. A glacial lake outburst flood (GLOF) is thus a type of flood that occurs when water dammed by a glacier or a moraine is released abruptly. The dam can consist of a terminal moraine or glacier ice. Failure can occur due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, volcanic eruption under the ice, or when a large portion of a glacier breaks off and displaces massive quantities of water in the glacial lake at its base. A water body that is dammed by the front of a glacier is called a marginal lake, and a water body that is capped by the glacier is called a sub-glacial lake. When a marginal lake bursts, it may also be called a marginal lake drainage. When a sub-glacial lake bursts, it may be called a jökulhlaup – an Icelandic term that originally referred to a glacial lake outburst flood in Vatnajökull (southeast Iceland) triggered by volcanic eruptions, but was subsequently adopted into the English language to describe any abrupt and large release of sub-glacial water.
It is now generally accepted that climate warming is having a significant impact on the high altitude tropical glaciers. One of the effects of warming is that glaciers are thinning and retreating throughout much extent. This is accompanied by formation of melt-water lakes, both on the glacier surface and in front of them. Such high-altitude glacial lakes are hazardous to humanity and infrastructure as they can drain instantaneously and create devastating floods in the downstream. The formation of moraine-dammed glacial lakes and glacial lake outburst flood (GLOF) is major concern in countries like Bhutan, Tibet, China, India, Nepal and Pakistan. Glacial lake volumes vary, but may hold millions to hundreds of millions of cubic meters of water. Catastrophic failure of the containing ice or glacial sediment can release this water over periods of minutes to days. Peak flows as high as 15,000 cubic meters per second have been recorded in such events, suggesting that the v-shaped canyon of a normally small mountain stream could suddenly develop an extremely turbulent and fast-moving torrent some 50 meters (160 ft) deep. Effects of glacial lake outburst floods are often compounded by a massive river bed erosion in the steep moraine valleys. As a result, the flood peaks increase as huge quantities of sediment are transported downstream and deposited within the channel to obstruct the exit of floodwaters.
GLOF Threat in the Himalayan Region: Glacial lake outburst floods (GLOFs) are a serious and potentially increasing threat to livelihoods and infrastructure in most high-mountain regions of the world. In the Indian Himalayan region the first glacial lake outburst flood was reported in 1926, when flood caused by the Shyok glacier in Jammu and Kashmir destroyed Abudan village and surrounding areas as far as 400 km from the burst site. Another report by scientists from National Remote Sensing Centre in Hyderabad showed sudden emptying of some moraine-dammed lakes of Shaune Garang glacier in Himachal Pradesh in 1981 and 1988, based on high discharge measured downstream. The report stated that GLOF studies are limited and not well understood in the Indian Himalayan region.
The Indian Himalayan region is home to over 7,000 glaciers, covering an area of 8,500 km2. These glaciers play an important role in shaping and influencing the environmental conditions in India. Siachen, Gangotri, Zemu, Milam, Pindari, Bhagirath Kharak and Satopanth are some of the important glaciers located in the Indian Himalayan region. Approximately 968 glaciers drain into the Ganga basin in Uttarakhand and over 4,660 glaciers feed the Indus, Shyok, Jhelum and Chenab river systems. The Ravi, Beas, Chenab and Sutlej river systems are fed by 1,375 glaciers and 611 glaciers drain into the Tista and Brahmaputra basins and contribute between 50 to 70 per cent of the annual discharge.
The inventory of glacial lakes in Sikkim Himalaya, prepared using temporal satellite data, shows the presence of 320 glacial lakes. Analysis of satellite data has also revealed that a lake has formed at the snout of South Lhonak glacier, that is at a height of about 7,000 metres. The lake, bound only by loose soil and debris called moraine, could cause havoc downstream if it ruptures, according to scientists Babu Govindha Raj and his colleagues at the National Remote Sensing Centre (NRSC) in Hyderabad. A sudden outburst "can create devastating floods downstream," they warn, adding that the probability of this happening "is very high". The glacial lake hazard vulnerability studied in Shako Chhu lake in Sikkim Himalaya shows a high risk of GLOF.
In Himachal Pradesh, current glacial lakes and modelled future lakes are expected to form as glaciers retreat. Current hazard, vulnerability, and exposure indices are combined to reveal several risk ‘hotspots’, illustrating that significant GLOF risk may in some instances occur far downstream from the glaciated headwaters where the threats originate. In particular, trans-national GLOFs originating in the upper Satluj River Basin (China) are a threat to downstream areas of eastern HP. Various studies suggest that many of the Himalayan glaciers have receded in recent decades due to climate forcing. Temporal satellite data analysis shows that the Milam Glacier in Goriganga Basin, Kumaon Himalaya receded 1328 m laterally and 90 m vertically during 1954–2006.
Already several glacial lakes in the Himalaya have reached alarming proportions. These include the Imja Tsho Lake in eastern Nepal and Tsho Rolpa lake in Central Nepal, which have become more than two kilometres long and about 100 metres deep. The Himalayan states, Uttarakhand, Himachal Pradesh and Jammu and Kashmir, are surrounded by about 200 potentially dangerous glacial lakes formed by glacial melt. Till date no early warning system is in place to evacuate people in case these lakes breach their thin walls of debris and loose soil.
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Department of Geology
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