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.
