In the latest tragedy to beset the vulnerable Himalayan mountain region, which is acutely impacted by the ongoing climate crisis and suffers from a spate of excessive and untimely rainfall, floods, and landslides, a flood unleashed its devastating force upon North Sikkim on 4 October. According to the National Disaster Management Authority, while scientists are still investigating to pinpoint the precise cause of the flood, the primary factor contributing to the sudden surge appears to be a combination of unusually heavy rainfall event and the emergence of a glacial lake outburst flood (GLOF) originating from the South Lhonak glacial lake. Experts have also pointed to the 6.2 magnitude earthquake that struck nearby Nepal as a potential contributing factor.

The disaster has left an indelible mark of destruction in its wake. The death toll in the Indian state of Sikkim has risen to 55 as of 8 October 2023, and nearly 100 individuals continue to remain missing. It has been estimated that over 25,000 people have been directly affected by the disaster.

In its trail of destruction, numerous bridges and segments of National Highway-10, along with the Chungthang dam, were swept away. Houses suffered significant damage, and NHPC power stations had to be temporarily shut down. Although the precise financial toll is still under assessment, Sikkim Chief Minister Prem Singh Tamang declared that the Himalayan state had suffered losses of several thousand crore rupees.

In a tragic parallel, the Himalayan region experienced a similar catastrophe in 2013 when flash floods, accompanied by a GLOF triggered by the Chorabari Tal glacial lake, struck Uttarakhand. The widespread flooding severely impacted Uttarakhand, with the Kedarnath valley suffering the most, witnessing a devastating death toll exceeding 5,000.

ISRO carried out a satellite-driven assessment of the outburst of South Lhonak Lake in Sikkim through satellite images captured before and after the event.  According to ISRO, "The analysis reveals that the lake has experienced a rupture, resulting in the drainage of approximately 105 hectares of its area when comparing images from September 28, 2023, to those from October 4, 2023. This sudden release of water likely contributed to a flash flood downstream.”   Source: ISRO.

ISRO carried out a satellite-driven assessment of the outburst of South Lhonak Lake in Sikkim through satellite images captured before and after the event. According to ISRO, "The analysis reveals that the lake has experienced a rupture, resulting in the drainage of approximately 105 hectares of its area when comparing images from September 28, 2023, to those from October 4, 2023. This sudden release of water likely contributed to a flash flood downstream.” Source: ISRO.

Over the years, multiple studies have pointed out the rapid expansion of South Lhonak Lake due to glacier melting, making it susceptible to a GLOF. Located at an altitude of 5,200 meters, South Lhonak is one of 21 lakes identified in a 2021 scientific study as "potentially dangerous with a high outburst probability.” Due to glacier melt, the lake's size expanded nearly sixfold, growing from 17 hectares in 1977 to 99 hectares in 2008.

What is GLOF and why does it happen?

GLOF stands for Glacial Lake Outburst Flood. According to ICIMOD, it is a “sudden release of water from a lake fed by glacier melt that has formed at the side, in front, within, beneath, or on the surface of a glacier.” GLOFs typically result from melting glaciers, which can lead to water accumulation in these high-altitude lakes.

They are characterised by their sudden onset, which gives little warning to downstream communities. The floodwaters can carry large amounts of debris, including rocks, sediment, and ice, intensifying the destructive force.

They can be triggered by various events, including avalanches, earthquakes and volcanic eruptions. Heavy rainfall can exacerbate the situation and increase the risk of a GLOF event.

Another critical factor at play is the escalating climate change crisis. One of the primary drivers of GLOFs is the accelerated melting of glaciers, which is a direct consequence of rising global temperatures driven by climate change. As temperatures increase, glaciers worldwide are receding at an alarming rate. As glaciers melt, they contribute vast amounts of water to glacial lakes, making them larger and more prone to GLOFs. In some regions, climate change can also lead to changes in precipitation patterns, including more frequent and intense rainfall events. Heavy rainfall can contribute to the rapid filling of glacial lakes and increase the pressure on natural dams, increasing the risk of dam failure and GLOFs.In the case of the Sikkim disaster, the weather department reported that Sikkim received 101mm (four inches) of rain in the first five days of October, which is more than double the normal levels.

How serious are the risks posed by GLOF?

According to a study, approximately 15 million people worldwide are at risk of being affected by the potential impacts of GLOFs. Those living in the High Mountains Asia (HMA) region are the most vulnerable among these populations. On average, they reside in the closest proximity to glacial lakes, with approximately one million people living within a distance of 10 kilometres from a glacial lake. Moreover, a significant majority of the people exposed to the risk of GLOFs, more than half of the total, are concentrated in just four countries: India, Pakistan, Peru, and China.

In the Himalayas especially, the rise in global temperatures has caused a significant increase in glacial melting. At present, there are 2,000 glacial lakes vulnerable to outbursts.

“Glaciers in the Himalayas are shrinking very fast at the rate of 20 metre per year due to global warming. This increases the threat of a GLOF.  Some glaciers may vanish in a few decades. In Jammu and Kashmir and Ladakh, most water requirements are met by glaciers. We need a proper strategy to preserve glaciers, so we don’t face water crises in the future,” stated Shakil Ahmad Romshoo, Vice-Chancellor of Islamic University of Science and Technology (IUST), Kashmir.

Likewise, according to the Sikkim State Disaster Management Authority, there are over 300 glacial lakes in the Sikkim Himalayan region, out of which 10 have been identified as particularly vulnerable to outburst floods.

How Satellites can Change the Flood Response Game

Research shows that despite numerous reports underscoring the risks associated with GLOFs in Sikkim, there have been insufficient efforts to monitor the situation actively. Scientists have also previously urged the need to assess the potential risks associated with glaciers and to bolster monitoring and early warning systems in high mountain regions.

There are numerous uncertainties in predicting and quantifying potential future catastrophic events, which could become increasingly prevalent and complex due to climate change. Gathering fundamental data is an immediate requirement for comprehending the hydrology, geology, and how Himalayan glaciers respond to climate change. Real-time monitoring is vital to provide policymakers with essential data that can be highly valuable. Satellite monitoring, in this regard, could play a crucial role in detecting, assessing, and monitoring glacial lakes, providing valuable data for understanding their size, status, and potential hazards.

Satellites equipped with various sensors are used to capture images from space, allowing for the identification of water bodies in glacial regions. This helps locate and catalogue glacial lakes, even in remote and inaccessible areas. They also provide precise measurements of the surface area and dimensions of glacial lakes. By comparing satellite images taken at different times, researchers can track changes in lake size over time. This is particularly important for identifying lakes that are rapidly expanding due to glacier melt as such, lakes are at a higher risk of triggering GLOFs.Satellite imagery can also capture the dynamic behaviour of glacial lakes, including water level fluctuations and changes in lake morphology. This information is crucial for assessing the stability of natural dams or barriers that contain the lake. Any sudden changes in lake size or shape can signal potential instability.

🛰️ According to a recent study conducted by scientists from IIT Kanpur in collaboration with the Department of Science & Technology, Government of India, satellite-based real-time monitoring of glacial catchments in the Himalayas can enhance our comprehension of flood risks in the region. This technology could be pivotal in establishing an early flood warning system, ultimately contributing to disaster mitigation and preserving human lives.

GLOFs remain a persistent threat to communities downstream and the local ecosystems. Scientists anticipate a rise in GLOF occurrences from the upcoming decades through to the early 22nd century. In response, countries vulnerable to these floods, with both population and infrastructure at risk, are urgently working to enhance disaster preparedness to prevent catastrophic consequences in the event of a GLOF. Satellite data will play a pivotal role in this endeavour, enabling us to effectively track alterations, create predictive models, anticipate disasters, and enhance our ability to respond swiftly and efficiently to these crises.