Glacial lake outburst floods (GLOFs) are related to global warming. As the temperatures in the Himalaya soar, the glaciers retreat during the summer, leaving behind water filled, moraine dammed, precarious lakes holding huge amounts of water in a very unstable geomorphology. GLOFs have been known to occur in different parts of the world. In 1941, an outburst flood destroyed the city of Huaraz in Peru killing 4,500 people. Outbursts from a glacier-dammed lake in the Swiss Alps in 1968 and 1970 triggered debris flow and caused heavy damage to the village of Saas Balen. In the Himalayan realms, with its greatest concentration of glaciers outside of the poles, such an event would assume catastrophic proportions with urban inroads in higher altitudes, ever-expanding infrastructure and poor to non-existent integrated water management systems as opined in a paper by P Mukhopadhyay, 2011, titled ‘GLOF- A Threat Present and Real: Indian Summary’. According to the IPCC, 2001 assessments, the rising global mean temperatures by 2100 from 1.4° to 5.8°C, depending on the climate model and greenhouse gases emission scenario, would mean that up to a quarter of the global mountain glacier mass can disappear by 2050 and up to half could be lost by 2100 (IPCC, 1996).
GLOF threats to the Indian Subcontinent: The Himalaya, ‘third pole’ of the earth, comprises one of the largest collections of glaciers – around 9600, with a total glacier cover of 33000 km which holds the largest reserves of water in the form of ice and snow outside the Polar Regions. The climatic change/variability in recent decades has made considerable impacts on the lifecycle of the glacier with at least 20 significant GLOF events recorded in the last seven decades. The GLOF event in Nepal during 1981 damaged the Friendship Bridge of the China–Nepal Highway, destroyed the Kosi power station in Nepal causing significant economic losses. In fact the GLOFs at Dig Tsho in 1985 (Nepal) and Luggye Tso in 1994 (Bhutan) are considered ‘textbook’ case studies of such events globally. During August 2000, in the Tibetan Plateau, the GLOF occurred and destroyed more than 10,000 houses and 98 bridges – with financial losses of around Rs. 75 crore. In 2008, GLOF from Gulkin glacier, Karakoram Himalayas damaged a large number of properties.
In the Indian Himalayan region the first GLOF event was reported in 1926 – when a flood occurred as dammed waters were released by the Shyok glacier, Jammu and Kashmir, destroying the Abudan village and the surrounding land which were at a distance of 400 km from the source of the outburst. Another report by C V Sangewar et. al., 1999 titled ‘Reservoir within the Shaune Garang glacier, Kinnaur, Himachal Pradesh,’ Geological Survey of India; depicted sudden emptying of some of the moraine dammed lakes of Shaune Garang glacier, Himachal Pradesh in 1981 and 1988 based on high discharge measured downstream. As per the available records, the most significant GLOF event had occurred in Dig Tsho lake at Bhote Kosi in Nepal in 1985. It destroyed a nearly completed small hydropower project (estimated cost of Rs. 1.5 crore), numerous foot-bridges and trails – resulting in several fatalities (P K Mool 1993, ‘Glacier Lake Outburst Floods in Nepal’, WECS Bulletin). A huge landslide dammed lake in the Yigong river in China on 9 April 2000 caused a outburst flood (after about two months), which extended 500 km downstream into Arunachal Pradesh resulting in heavy damages (P Zhu, T Li 2001, ‘Flash Flooding Caused by Landslide Dam Failure,’ ICIMOD Newsletter). In a somewhat same manner, the June 2005 outburst of landslide dammed lake on Parechu river (China) caused a transboundary flood in Himachal Pradesh.
A study by Bajracharya et. al. 2009, titled ‘Land Cover Mapping in the HKKH Region: Cases from Three Mountain Protected Areas’, HimalDoc, refers to past records, which reflects that at least one catastrophic GLOF event has occurred at an interval of three to 10 years in the Himalayan region. Sikkim with its many glaciers – Zemu, Rathong and Lhonak has become the measuring stick of change in glacier patterns. The recent 2013 satellite-based study by scientists at the National Remote Sensing Centre (NRSC) in Hyderabad revealed that a huge glacial lake has formed at the snout of South Lhonak glacier, Sikkim which has a ‘very high’ potential to burst and cause devastation downstream. The glacial lake is about 630 m wide and 20 m deep covering an area of 98.7 hectares and contains 19.7 billion lt of water. Based on this study the scientists estimated that the Lhonak glacier had receded 1.9 km between 1962 and 2008. In another study by ‘Space Applications Centre (ISRO) (2010): National Wetland Atlas – Sikkim, the East Rathong Glacier has retreated significantly since 1965, marking a shift in its snout position and leading to the formation of a glacial lake behind its terminal moraines.
A long-term study ‘The Chinese Glacier Inventory’, by the Chinese Academy of Sciences reported that during the last 24 years there has been a 5.5 per cent shrinkage in volume of China’s 46,928 glaciers, equivalent to the loss of more than 3000 sq km of ice. The study predicts that if the climate continues to change at the present rate, two-thirds of China’s glaciers would disappear by the year 2050, and almost all would be gone by 2100 (China Daily, 23 September 2004). Within the last 25 years, some of the glacial lakes in China in Poiqu basin, which is a transboundary basin that joins with SunKoshi-BhoteKoshi in Nepal, lakes such as LumuChimi and GangxiCo have grown almost double in size. These lakes pose a potential threat of GLOFs.
An interesting work, the ‘Asia-Pacific Network for Global Change Research Inventory of Glaciers, Glacial Lakes and the Identification of Potential GLOFs Affected by Global Warming in the Mountains of India, Pakistan and China/Tibet Autonomous Region’, 2004-03-CMY-Campbell; records changes in the Tista River basin, Sikkim, Himachal Pradesh and Uttarakhand Himalayas. The Tista River basin contains 285 glaciers occupying about 576 sq km but has as many as 266 lakes and 14 glacial lakes that are potentially dangerous. The study states that the number of lakes and potential GLOF is much higher here as compared to other study regions. Himachal Pradesh in fact holds 2554 glaciers with the glacier area of about 4160 sq km but has only 229 lakes including 22 glacial lakes that may cause potential GLOFs.
Prediction and measures: In several high mountain ranges around the world, a grave uncertainty about the hazard potential of glacial lakes still exists, especially with respect to the effects of accelerating rates of glacier retreat as a consequence of atmospheric warming. Area-wise detection and modelling hazard potential is, therefore, a major challenge. There are several possible methods for mitigating and monitoring the impact of GLOF surges and for putting in place early warning systems. The most important mitigation measure for reducing GLOF risk is to reduce the volume of water in the lake in order to reduce the peak surge discharge. The GLOF prone area downstream need measures to protect infrastructure against the GLOF surge. Monitoring systems prior to, during, and after construction of infrastructure and settlements in such areas should be in place, which includes the signal transmission system, the glacial lake outburst flood sensing system, and the glacial lake outburst flood warning system.