Glaciers are melting at an alarming rate due to climate change, leading to the formation and expansion of glacial lakes. While these lakes may appear serene, they pose a serious threat of sudden floods known as Glacial Lake Outburst Floods (GLOFs). In the Eastern Himalaya, particularly in Sikkim, the risk of GLOFs is increasing, endangering communities and infrastructure downstream.
Recent studies have examined the extent of this risk, identified the most dangerous glacial lakes, and proposed measures to mitigate the potential disasters. A comprehensive study by Kaushik et al., conducted in the Sikkim Himalaya employed satellite imagery and advanced modelling techniques to create an updated inventory of glacial lakes, assess their susceptibility to outburst floods, and model potential flood magnitudes under different scenarios.
Why is Sikkim at high risk?
Sikkim, a small Himalayan state in India, is home to more than 200 glacial lakes and 446 glaciers. The region is witnessing rapid glacier ice loss due to rising temperatures, leading to the formation of large, unstable glacial lakes. Additionally, the steep terrain, frequent seismic activity, and heavy monsoon rains increase the chances of lake outbursts. Many settlements, bridges, and hydropower plants lie in the potential flood paths, making the situation even more critical.
The study identified 232 glacial lakes in the region and classified them based on their potential risk. Among these, Khanchung and South Lhonak lakes were found to be the most hazardous, with possible peak flood discharges exceeding 9,500 cubic meters per second.
How do GLOFs occur?
A GLOF occurs when a glacial lake suddenly releases a massive volume of water due to the failure of its natural dam, which is often composed of loose moraine materials. The triggering factors for such an event are varied and complex. Intense monsoon rains can increase water levels beyond the lake’s holding capacity, leading to overtopping and subsequent dam failure.
Large-scale rockfalls or ice avalanches can create displacement waves that destabilise the dam, while glacier movements and seismic activity can weaken the moraine structure, making it susceptible to collapse. Once the natural dam breaches, a surge of water rushes down the valley, carrying debris, rocks, and ice, thereby amplifying the destruction.
Findings of the study
The study revealed that glacial lakes in the Sikkim Himalaya have been increasing in both number and size over the past few decades, a trend that is closely linked to rising temperatures and glacier retreat. The updated glacial lake inventory recorded a total lake area of 22.23 km², reflecting a significant expansion over previous estimates.
The study identified that while some lakes remain relatively stable, others exhibit rapid growth, increasing the likelihood of catastrophic outburst floods. The risk assessment classified one lake as very high risk, eight lakes as high risk, and twenty-two as medium risk, indicating the urgent need for monitoring and mitigation efforts.
The dam break flood simulations conducted for seven of the most dangerous lakes provided alarming insights into potential disaster scenarios. In extreme cases, the peak discharge from the Khanchung and South Lhonak lakes could reach up to 9,504 and 8,421 cubic meters per second, respectively, leading to widespread destruction in downstream areas. Even in less extreme scenarios, substantial flood volumes would reach major settlements like Chungthang town, placing over 10,000 people in immediate danger.
The modeling results also suggested that an outburst event from the lakes near Ponggu La, Za La, and Gurudongmar could trigger cascading failures, significantly amplifying flood magnitudes and extending the range of devastation.
The study also highlighted that the expansion rate of some lakes has been alarmingly fast, with South Lhonak Lake expanding by nearly 60% over the past three decades. This rapid growth suggests increasing instability, which, combined with the fragile nature of moraine dams, makes these lakes highly vulnerable to failure. The simulations further estimated that if a breach were to occur, the floodwaters would reach densely populated areas within hours, providing little time for evacuation unless early warning systems are in place.
Furthermore, the study found that increased frequency of extreme precipitation events and rising minimum temperatures have contributed to the destabilisation of lake dams. The region's seismic activity further exacerbates the risk, as even moderate earthquakes could trigger landslides or structural weaknesses in moraine dams, leading to sudden and uncontrolled water release.
The analysis underscored the interconnectedness of climatic, geological, and hydrological factors driving GLOF hazards in the region. The risk is further compounded by unplanned infrastructure development, such as roads and hydropower projects, which often increase vulnerability by altering natural drainage patterns and weakening slopes.
Impact of GLOFs in Sikkim
If a major GLOF occurs in Sikkim, the consequences would be catastrophic. The impact would not be limited to immediate flooding but would also include significant erosion, loss of fertile land, and damage to hydropower projects, transportation networks, and human settlements. The study estimates that over 10,000 people are directly at risk, while approximately 1,900 homes, five bridges, and two hydropower plants are vulnerable to severe damage.
Furthermore, the Teesta River basin, a critical hydrological system in Sikkim, would be particularly affected, disrupting water supply and energy production across the region. Historical data from past GLOFs in the Himalayas suggest that such events can wipe out entire villages, highlighting the urgent need for proactive risk management.
What can be done?
Given the increasing risk, urgent action is necessary to mitigate potential disasters. The study emphasises the importance of early warning systems equipped with real-time monitoring technologies that can detect shifts in lake stability and provide timely alerts to communities downstream. Controlled water drainage systems and engineered spillways can help reduce lake levels gradually, minimising the risk of sudden outbursts. Infrastructure planning must consider flood-prone areas, ensuring that new settlements and critical facilities are constructed outside of high-risk zones.
Additionally, raising awareness among local communities about evacuation plans and emergency responses can significantly enhance disaster preparedness. Long-term strategies should focus on addressing climate change, as reducing global carbon emissions and promoting sustainable development can slow glacier melt and decrease the frequency of GLOF events in the future.
Conclusion
GLOFs pose a severe and escalating threat to Sikkim and other high-altitude regions worldwide. The study conducted in the Sikkim Himalaya provides crucial insights into the increasing number and expansion of glacial lakes, highlighting the urgent need for mitigation measures. Every simulated scenario in the study demonstrated significant flood volumes reaching densely populated areas like Chungthang town.
The findings stress that immediate action is required to prevent large-scale destruction. The integration of advanced modelling techniques with up-to-date inventory data presents a strong foundation for mitigating GLOF hazards. By prioritising risk assessment, implementing early warning systems, and adopting sustainable environmental policies, Sikkim can better safeguard its communities and infrastructure against these devastating natural disasters.
The full paper can be accessed here