Climate change has led to increased temperatures, sea level rise, increased natural hazards, etc. Glaciers are the very sensitive to climate change and its affects can be seen in glaciated zones around the globe.
Himalaya has huge repositories of glaciers that are reportedly retreating leading to glacier thinning. This glacier thinning due to melting has resulted in the development of new glacial lakes and the magnification of existing ones due to the accumulation of meltwater behind loosely consolidated end moraine dams that had formed from debris left out by glaciers while retreating. These moraine dammed glacial lakes are potential source of catastrophic disaster as they are inherently unstable (ICMOD, 2011; UNDP, 2013).
Fast melting of glacier or an extreme weather event such as heavy rainfall can easily trigger these glacial lakes to burst and can cause damage to vulnerable people and property living downstream in the valleys.
The flash flood due to sudden burst of a glacial lake produce the violent flow of water and associated debris and is known as a Glacial Lake Outburst Flood (GLOF). Lake Outburst and debris flow disaster in Kedarnath, Uttarakhand in June 2013 was one the destructive disaster occurred in Himalaya and still various potential disastrous lakes exists and are developing in this region (Allen et al, 2015).
Glaciers work as a water tower, sustaining the lives of millions downstream. The volumes of these glaciers vary – remaining sensitive to global temperature conditions. The glaciers have embedded within it many lakes which follow a seasonal pattern of freeze and thaw. With continuing warming trends, many glaciers are melting rapidly, giving birth to a large number of glacial lakes. These ‘moraine dammed’ lakes are comparatively feeble and its unexpected outburst is a threat to life, asset and infrastructure, downstream.
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 and the threat is real.
Glacial lake monitoring and preparedness for disaster risk reduction are the prime most need of these fragile regions now. Glacial lake are needed to be mapped and monitored to recognize the GLOF risks associated with them. Indian Space Research Organisation (ISRO) among many other organisations working in the area, has also taken the responsibility and has been engaged in glacial lake monitoring and water bodies in the Himalayan region of Indian River Basins.
ISRO has increased its capabilities to show the potential of satellite remote sensing to monitor the glacial lake and water bodies. High resolution data such as Cartosat-2 Panchromatic, Resourcesat – 2 LISS VI multispectral and RISAT-1 SAR Radar images was used to monitor regularly two lakes – Lhonak lake and Pareechu lake during 2013 to 2015.
Also, glacial lake monitoring and water bodies with water spread area more than 50 hectare for on monthly basis for June to October for 5 years (2011-2015) has been performed.
Inventory of glacial lake and water bodies with water spread area more than 10 hectare has been prepared. According to inventory there are total of 2026 glacial lakes and water bodies in Indian River basin of Himalaya out of which 503 are glacial lakes. It has been found that about 1600 glacial lakes and water bodies have water spread area between 10 and 50 hectare and about 200 water bodies have between 50 and 100 hectare.
Relation between elevation and water spread area
A comparison with elevation has revealed that more than 50 per cent i.e. about 1167 glacial lakes and water bodies are located within the elevation range of 4000 to 5000m. A basin wise inventory showed that Brahmaputra basin has most number of glacial lakes and water bodies i.e. 1391 followed by Indus (351) and Ganga basin (284). Brahmaputra Basin has 295 glacial lakes and 1096 water bodies whereas Ganga basin has 179 Glacial lakes and 105 water bodies. Least number of glacial lakes lies in the Indus basin.
ISRO has done remarkable work by inventorying the glacial lake monitoring and water bodies. They are also regularly monitoring and providing information on inventory and monthly changes through their Bhuvan and India-WRIS portal. All this together is very useful for identification of potentially dangerous lakes prone to GLOF and giving early warning to mitigate disasters. This monitoring is also helping in prioritizing of glacial lake monitoring for GLOF studies and climate change studies.