Earth scientists study Sikkim flood in India to help others prepare for similar disasters

On Oct. 3, 2023, a multi-hazard cascade in the Sikkim Himalaya, India, was triggered by a permanently frozen (permafrost) lateral moraine -- debris from erosion along a glacier -- collapsing into South Lhonak Lake.

"A landslide went into a lake and that triggered a wave that eroded a dam at the end of the lake, which resulted in a slurry-like flood for hundreds of kilometres," explains Dr. Dan Shugar, PhD, an associate professor with the Department of Earth, Energy and Environment in the Faculty of Science.

Known as a glacial lake outburst flood, it killed at least 55 people, left dozens more missing, damaged agricultural land and destroyed a hydropower dam.

The Sikkim flood was declared one of the worst climate-related disasters to have occurred on the continent that year by the World Meteorological Organization's State of the Climate in Asia 2023 report.

A new paper in the journal, Science, presents a collaborative effort by scientists, academics, government departments, non-governmental organizations and others to investigate the event.

Shugar, a geomorphologist who's a co-author on the paper, says it's important to forensically analyse what happened and what can be learned from it as rapid climate warning affects mountain regions around the world.

The paper looks at the drivers and causes and assesses the downstream impacts of the hazard cascade using high-resolution satellite imagery, seismic data, meteorological data and field observations. It also explores the triggers of the flood and reconstructs its hydraulic dynamics, evaluates downstream implications and considers the long-term impacts of the event.

"The assessment indicates that the high hazard level arises not only from the flood itself but also from the subsequent processes it triggers," says the paper.

Dr. Ashim Sattar, PhD, the study's lead author who's an assistant professor in the School of Earth, Ocean and Climate sciences at the Indian Institute of Technology, adds that the Sikkim flood had devastating effects for downstream regions.

"This is high time to be building resilience in downstream regions that are exposed to such potentially catastrophic events in the Himalaya," he says in a statement.

"The threat of these catastrophic events is growing, urging us to act with urgency in protecting both our environment and communities in the Himalaya and similar regions around the world."

Sattar says glacial lakes are growing in number and size, so they need to be critically and urgently evaluated for downstream hazards and damage potential.

"Climate change is leading to changes in permafrost temperatures, increasing the risk of slope failures that can lead to avalanches or trigger glacial lake outburst floods in the high mountains."

In Canada, for example, a glacial lake outburst flood in British Columbia's southern Coast Mountains destroyed forest and salmon spawning habitat in November 2020.

Shugar, who has studied both events, says they can have serious implications for people and infrastructure.

"This study (on the Sikkim flood) is a good example of a mountain disaster galvanizing the global Earth science research community to work together on a common goal," he says.

"Advances in Earth observation technologies over the last decade have dramatically improved our ability to understand these sorts of events, and ultimately, leading to disaster risk reduction."