South Lhonak Lake remains highly vulnerable to future GLOF events, says scientists
GANGTOK,: A newly published scientific study has confirmed that a massive collapse of 14.7 million cubic meters of lateral moraine into South Lhonak Lake (SLL), on the night of October 3-4, 2023, sparked a 20-metre-high ‘tsunami-like wave’, breaching the lake’s moraine dam and releasing approximately 50 million cubic meters of water downstream, equivalent to 20,000 Olympic sized swimming pools.
The disaster led to widespread destruction in the Teesta Valley, affecting Sikkim, West Bengal, and Bangladesh.
The research, published in science.org is a collaborative study by involving scientists, non-governmental organizations, and diverse stakeholders, on the SLL GLOF and its cascading hazards highlighting how the flood and its triggered processes increase Teesta Valley’s vulnerability to future events.
Drivers and causes of the 3 October 2023 GLOF from South Lhonak Lake:
On 3 October 2023, a massive hazard cascade in the Sikkim Himalaya began with the collapse of 14.7 million m³ of lateral moraine into South Lhonak Lake, triggering a 20 m tsunami-like wave. This wave breached the frontal moraine, creating a 165 m-wide, 55 m-deep opening that drained 50 million m³ of water. The Glacial Lake Outburst Flood (GLOF) eroded vast amounts of sediment, peaking at a discharge of 48,500 m³/s, and travelled downstream, reaching Chungthang within two hours.
Seismic and satellite data confirmed the event timeline, with numerical models reconstructing the process. The disaster, a rare event with a return period exceeding 200 years, underscores the need for improved hazard assessment and risk management.
Moraine failure conditioning factors: The moraine failure at South Lhonak Lake was driven by long-term glacier mass loss due to climate warming, glacier-lake interactions, and permafrost degradation. Since 1950, annual temperatures have risen by 0.08°C per decade, with glacier mass loss accelerating in recent years. Permafrost warming weakened the moraine, making it susceptible to failure. Pre-GLOF mapping showed progressive slope instability, with increasing deformation over the years. While heavy rainfall from a cyclonic system coincided with the collapse, no extreme cloudburst was detected, suggesting that long-term landscape changes had primed the moraine for failure.
GLOF-induced erosion, channel aggradation, and landslides: The GLOF triggered 45 secondary landslides along the Teesta Valley, mapped using high-resolution satellite imagery and field observations. Most landslides resulted from the flood's lateral erosion destabilizing valley slopes, with the most intense erosion occurring 40-45 km downstream, where flow velocities were highest.
A major landslide dammed the Teesta River at 35 km downstream, creating a lake that persisted as of May 2024. The total estimated eroded volume was ~270 × 10? m³, with the majority occurring upstream of Chungthang. Severe impacts were observed in Rangpo, Geli Khola, Teesta Bazaar, and Bardang, where debris buried infrastructure.
Impacts on population, infrastructure, agricultural land, and transboundary implications: The GLOF cascade severely impacted populations, infrastructure, and agriculture in India and Bangladesh. In India, 25,900 buildings were damaged, mostly downstream of Chungthang, with 59% built in the last decade. Around 276 km² of agricultural land was flooded. Infrastructure damage included 31 major bridges, 20 small pedestrian bridges, and 18.5 km of roads. The disaster destroyed the 1200 MW Teesta-III hydropower dam and affected four other dams downstream.
In Sikkim, 100 villages across Mangan, Pakyong, Gangtok, and Namchi were affected, leading to 55 deaths, 74 missing persons, 7025 displaced individuals, and significant livestock losses. Landslides worsened the damage, affecting roads and buildings.
In Bangladesh, transboundary flood waters impacted 17,000 buildings and 168 km² of farmland, with severe damage in Rangpur, Lalmonirhat, Kurigram, Gaibandha, and Nilphamari districts. The Teesta River’s water level rose dangerously close to the flood threshold, mainly due to the GLOF, compounded by heavy rainfall (300 mm/day) from 5-7 October. Sediment discharge at Dalia station surged 17 times higher than pre-flood levels, increasing river turbidity and worsening downstream impacts.
Future GLOF hazard in the Teesta Valley: The South Lhonak Lake (SLL) remains highly vulnerable to future GLOF events, particularly due to instability in the northern lateral moraine. Despite the October 3, 2023 failure, the northern moraine continues to deform rapidly, with surface velocities reaching up to 15 m/year. Glacier retreat and debuttressing are key factors in slope destabilization, increasing the risk of further failures, especially in Zone 1 on the northern lateral moraine. The southern moraine appears stable but could become unstable with continued warming and permafrost decay.
Post-GLOF, the Teesta Riverbanks were severely weakened, leading to ongoing collapses, particularly near roads and settlements. Landslides like L17 and L43 continued to slump, with significant damage to NH-10, blocking trade routes and isolating mountain communities. Subsequent monsoon floods have worsened erosion, posing long-term hazards.
Flood deposits in the Teesta Valley remain at risk of further erosion and debris flow, raising the riverbed and increasing flood risks in adjacent areas. The landslide-dammed lake (L6) has partially drained, but remaining deposits continue to be a hazard, potentially amplifying future GLOFs. These sediment risks are often overlooked in GLOF assessments, underscoring the need for continuous monitoring and mitigation efforts.
Summary and perspectives: The 3 October 2023 GLOF disaster from South Lhonak Lake (SLL) highlights the complexities of multi-hazard cascades in mountainous regions. The flood was not solely triggered by rainfall but was exacerbated by climate-induced glacier retreat and sediment transport, leading to widespread destruction in Sikkim, West Bengal, and Bangladesh. The Teesta Valley’s geomorphic conditions intensified the disaster, with heavy rainfall preconditioning slopes for failure, triggering numerous landslides and increasing flood severity.
The GLOF released 50 million m³ of water and entrained 270 million m³ of sediment, overwhelming infrastructure, hydropower projects, and communities. Current GLOF models often underestimate sediment transport and downstream impacts, making flood predictions inaccurate. The erosion rate along the 70 km flow path was extraordinarily high (3850 m³/m), further emphasizing the need for better hazard assessment tools.
The disaster underscores the urgent need for comprehensive Early Warning Systems (EWS) across the Himalaya, improved hazard modelling, and regional cooperation for transboundary risk mitigation. Despite previous mitigation efforts like siphons and monitoring, additional structural defences such as check dams and deflection walls are needed. Strengthening regulatory frameworks for hydropower projects is also critical, given the Teesta Basin’s high project density (47 hydropower plants, >5300 MW capacity), which increases exposure to GLOF risks.
Ultimately, this event highlights the limits of adaptation in the Himalaya, necessitating stronger risk assessments, infrastructure resilience, and community preparedness. A paradigm shift in GLOF risk management, integrating advanced technology, robust land-use planning, and proactive disaster mitigation, is essential for safeguarding mountain communities and ensuring sustainable development in high-risk environments.
BOX
The study highlights multiple drivers of the disaster, including:
Glacier Retreat & Climate Change: Rising temperatures (0.08°C per decade since 1950) accelerated glacier mass loss, increasing moraine instability.
Permafrost Thaw: Warming permafrost weakened the moraine, making it susceptible to collapse.
Heavy Rainfall: A low-pressure system from the Bay of Bengal intensified the flood’s impact, though no extreme cloudburst was detected.
Sediment Erosion & Landslides: The GLOF eroded approximately 270 million cubic meters of sediment, triggering 45 landslides along the Teesta Valley.
Future GLOF Risks & Recommendations
Despite the October 3 disaster, South Lhonak Lake remains vulnerable to future GLOFs. Scientists warn that the northern lateral moraine continues to deform at a rate of 15 meters per year, posing a high risk of further collapses.
Key recommendations from the study include:
Enhanced Early Warning Systems (EWS): Immediate implementation of advanced detection and response mechanisms across the Himalaya.
Stronger Hydropower Regulations: Improved safety assessments for infrastructure near glacial lakes.
Adaptive Risk Management: Basin-scale hazard mapping and community education programs.
Infrastructure Resilience: Strengthening roads, bridges, and flood defences to withstand GLOF impacts.