Heavy monsoon rainfall and a glacial lake outburst flood (GLOF) that caused severe flooding downstream – that’s the cause of the human-aided natural disaster in Sikkim that washed away part of the Teesta 3 dam – damaged parts of Teesta 5, wrecked bridges and drowned roads.

It was not immediately clear as to what may have triggered the GLOF on Lhonak lake in northwest Sikkim, although this is a risk that has loomed large for at least a decade. “As the GLOF occurred at around 1am (on October 4) on Lhonak lake, a lot of boulders, debris and water came downstream, flooding Teesta. There is no rainfall data available for this remote region from the India Meteorological Department (IMD). Some Swiss scientists have also suggested that an earthquake yesterday may have triggered the GLOF. We are trying to assess possible triggers,” said a senior official of the Central Water Commission (CWC).

A GLOF is caused when the moraine (the debris accumulated over the years by a glacier) that usually functions as a dam, creating a lake, is breached.

“The moraine dam of the lake may have been breached. That’s the understanding,” the official added. Parts of northeastern India  were jolted by an earthquake on Monday, and parts of northern India, by another on Tuesday.

The IMD  said vigorous monsoon conditions are impacting eastern India. “There is exceptional rainfall over West Bengal and Sikkim mainly due to a low-pressure area, which is lying over the region. The winds move up to the top of the hills. This is called orographic uplifting of air, which can cause a sudden and very heavy downpour. Extremely heavy rainfall will continue for at least one more day over north Bengal and parts of Sikkim,” explained IMD director general M Mohapatra.

“The region where GLOF occurred is extremely remote and we do not have automatic weather stations there, so it’s very difficult to tell how much it rained there,” he added.

According to CWC, Singtam, Rangpo, Lachen and surrounding towns may have been inundated by the flood.

Miriam Jackson, glaciologist and programme coordinator, cryosphere at Kathmandu-based International Centre for Integrated Mountain Development, said Indian scientists have not yet confirmed that it is a GLOF event. All recent satellite images are cloud covered.

Due to a rise in average temperatures globally, the Himalayan region has become more vulnerable to GLOF events. “As the climate warms, glaciers shrink in size and a lake may be formed, often at the side of the glacier (glacier-dammed lake) or at the front of the glacier, between the glacier and a moraine ridge (moraine-dammed glacier) or another type. As the climate continues to warm, these lakes may grow in size. There is a lot of evidence that there are more lakes than previously. This implies strongly there will be more GLOFs and GLOFs will be bigger (and more damaging),” she added. “We haven’t yet verified this as a GLOF. It could be a flash flood triggered by intense rain, or may be intense rain has triggered a GLOF.”

Jakob Steiner, High Mountain Asia hazards and hydrology expert and fellow of ICIMOD’s Himalaya University Consortium, said: “This is, incredibly sadly, another classic case of a cascading hazard chain that amplifies as you go downstream. It is possible that strong permafrost degradation in the vicinity of the lake may have destabilised the dam, which may have then failed upon an exceptionally strong rainfall event. Downstream, a hydropower structure was ripped away, further contributing to the massive impact that has resulted in yet more loss of life and damage to homes and infrastructure in what has been just a devastating monsoon season in the mountains of the Hindu Kush Himalaya. Tragically, what we’ve seen this summer, in terms of flash floods, is likely to pale in comparison with what’s to come, unless we limit temperature rise.”

“The catastrophic Glacial Lake Outburst originating from Lhonal Lake in Sikkim is a stark reminder that glacier hazards are going to breach thresholds and all thanks to the warming climate. While the frequency and intensity of these events are ravaging lives and livelihoods, there is a dire need to revisit the current state of disaster preparedness and climate readiness. As per studies, there has been a 25% increase in GLOF events in the last two decades; it becomes imperative to conduct hyper-granular risk assessments that map the identified hot spot lakes and its probable extent of damage into current disaster management plans,” said Abinash Mohanty, Sector Head Climate Change and Sustainability, IPE-Global- an international development organization and an expert reviewer of the IPCC’s sixth assessment report.

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