On the intervening night of October 3-4, 2023, South Lhonak, a glacial lake in the Sikkim Himalaya burst, causing flood and huge destruction downstream. More than 150 people reportedly died, and critical infrastructure, including the Teesta-III dam and agricultural land, were damaged in the devastating event. More recently, on August 16, 2024, a glacial lake in the Nepal Himalayas flooded Thame village in the Khumbu region of Nepal Himalayas, destroying properties, including 20 houses, an elementary school, and a clinic and displacing 135 people. Rapid deglaciation due to climate change in the Himalayan region has given rise to an increased number (and area) of glacial lakes. In addition, high seismic activity, and steep topography in the Himalayan region, have rendered these lakes highly prone to catastrophic flood events.
Glaciers are known to be highly sensitive to climate change and are visible indicators of rising global temperatures. A lake is formed in the foreground of some of the retreating glaciers. These lakes are formed and typically dammed by unstable and unconsolidated materials (rocks, boulders, pebbles, cobble, soil, etc.) from the valley floor and glacier deposits. Glaciated mountainous regions, especially the Hindu Kush Himalayas (HKH), are witnessing a rise in catastrophic flood events called the Glacial Lake Outburst Floods (GLOFs). A GLOF involves the abrupt release and downstream movement of water and sediments from the glacial lakes. GLOFs can cause massive damage to both lives and infrastructure.
Recent global estimates suggest the presence of more than 1,10,000 glacial lakes with an area of ~15,000 sq km. The area of the glacial lakes has increased at a rate of ~22% per decade between 1990 and 2020. Between 850 and 2022, more than 3,000 GLOFs were recorded worldwide. More than 10 million people are globally exposed to the impacts of GLOFs and it has been estimated that more than half of the globally exposed population to GLOFs is found in just four countries: India, Pakistan, Peru, and China. Among these HKH has the highest potential for GLOF impacts. An estimated 18% of these GLOFs have been recorded in this region.
The International Centre for Integrated Mountain Development (ICIMOD), Kathmandu has documented about 700 GLOFs in the HKH since the 1833 killing more than 7,000 people. The number of GLOFs recorded globally has increased in most glaciated mountain regions. In the Himalayan region, the number of GLOFs is projected to triple by the end of this century due to the changing climate.
A March 2023 report by the National Remote Sensing Centre (NRSC), Hyderabad, mapped a total of 28,043 glacial lakes (>0.25 ha) in the Indus, Ganga, and Brahmaputra basins covering parts of India and the transboundary region. These lakes are distributed in the Brahmaputra basin (64.19%) followed by the Indus basin (19.02%) and Ganga basin (16.78%), with a total extent of 929.91 sq km, 173.95 sq km, and 206.85 sq km, respectively. Almost 7,500 of these lakes are located within the Indian Himalayan region.
Many of these lakes are susceptible to GLOFs. According to the latest research, more than 500 glacial lakes spread across the Himalayas were identified as very high hazard-prone. NDMA has identified 189 glacial lakes in the Indian Himalayas as high risk. Most of these lakes are concentrated in the Central and Eastern Himalayas. Given the high potential to cause damage to life, property, and infrastructure, mitigation and resilience becomes imperative. Regular monitoring of the size and water level of such lakes, and real-time monitoring of the meteorological conditions are necessary for any early warning system.
However, glacial lakes are generally situated at very high altitudes in remote locations, and setting up monitoring equipment is often a monumental challenge. Controlled discharge to lower the water level is another method to mitigate outburst risks. This method too encompasses some practical and engineering challenges.
Most of the people living in the proximity of these dangerous lakes are among the most marginalised and vulnerable populations across the Himalayas. Mitigating the risks and building resilience requires a fair amount of resources. Raising community awareness, identifying the potential influence zones of GLOF, and developing emergency response and evacuation plans are crucial.
All high-risk lakes may need to be monitored in real-time/near real-time by installing and maintaining advanced equipment such as tripwire sensors, lake water-level monitoring cameras, and automatic weather stations. Early warning systems downstream of these lakes should be installed and maintained efficiently throughout the year. While several initiatives taken by the government, including the recently announced National Glacial Lake Outburst Floods Risk Mitigation Programme (NGRMP) with a budget of `150 crore are commendable, proactive steps by the implementing agencies, in association with local administration and communities, will help mitigate and build robust resilience against GLOFs.
Dr Sandip Tanu Mandal, Research Associate, Mobius Foundation, New Delhi
Dr Aditya Joshi, Advisor, Mobius Foundation, New Delhi, IFS (Retd), Ex-PCCF/HoFF & CWLW, Manipur