Two Hours on the Teesta: The South Lhonak Outburst, the Loss of Teesta-III and the Case for Coupling Early Warning to Dam Safety

B.A.P-I Retrospective, 2020 to 2024. This paper looks back at 2023 and the lesson it holds for the protection of India's critical infrastructure.

1. Introduction

On the night of 3 to 4 October 2023 a glacial lake high in North Sikkim emptied into the Teesta valley, and within about two hours the dam of the 1,200 MW Teesta-III project at Chungthang had been washed away. The South Lhonak glacial lake outburst flood (GLOF) has since been reconstructed in close scientific detail, with Indian institutions at the centre of that work, and the parliamentary record now shows how the national response has developed. The event can therefore be revisited with a clarity that was impossible in the first weeks.

This paper asks what official and scientific sources have established, and what the event teaches about two instruments on which India relies: early warning for Himalayan hazards, and the Dam Safety Act, 2021. The argument is that the disaster was a cascade across systems governed separately, and that India's reform agenda, already under way, should be completed by coupling the monitoring of upstream lakes to the operation of downstream dams.

2. What Happened: The Scientific Reconstruction

The most complete account is the study led by Ashim Sattar of IIT Bhubaneswar, published in Science in early 2025. It found that a landslide carrying 14.7 million cubic metres of frozen sediment fell into South Lhonak Lake, raising a wave about 20 metres high that breached and eroded the frontal moraine and released around 50 million cubic metres of water; 55 people died and 74 were reported missing, and damage reached as far as 385 km downstream across Sikkim, West Bengal and Bangladesh (Newcastle University, 2025, Jan. 31). The International Centre for Integrated Mountain Development (ICIMOD), a partner in the work, added that the unstable lateral moraine had been moving more than 15 metres a year between 2016 and 2023, and that permafrost thaw may have contributed (ICIMOD, 2025, Feb. 4).

The timing is equally instructive. The triggering landslide occurred at about 22:13 local time on 3 October, and the flood reached Chungthang, site of the 60 metre high Teesta-III dam, at about 00:30 on 4 October (Petley, 2025, Jan. 30). Roughly two hours separated cause from impact at the dam. That interval is the most important number in the whole event, because it defines what an early warning system can achieve. ICIMOD also observed that current models "often underestimate risks by failing to account for sediment entrainment and riverbank collapses" (ICIMOD, 2025, Feb. 4): a flood gathering material as it travels grows beyond what a lake-volume calculation would predict.

3. A Hazard India Had Already Identified

South Lhonak was not an unknown hazard, and Indian agencies had been working on it for years. In September 2016 the Sikkim Department of Science and Technology and Climate Change, with the state disaster management department and C-DAC, led an expedition that installed three siphon pipelines of 140 HDPE pipes to draw the lake down, recorded a depth of about 130 metres at its centre, and installed a water-level sensor developed by C-DAC as an experimental warning system (Government of Sikkim, 2016, Sep.). In February 2024 the Ministry of Earth Sciences told the Lok Sabha that NDMA had issued GLOF guidelines in 2020 with Swiss partners, and that on 16 September 2023 NDMA-led expeditions had installed two solar-powered, twin-camera automated weather stations at two high-risk lakes in Sikkim, South Lhonak and Shako Cho, and identified sites for a sensor array for an end-to-end early warning system (Lok Sabha, 2024, Feb. 7).

The full system was still being built when the lake failed. An NDMA member explained that the lake sits near the India-China border, is accessible only from July to September, and required heavy equipment, border clearances and agreement among several agencies on a single date (Reuters, 2023, Oct. 11). These are genuine constraints of high-altitude work. The reform lesson is that a hazard with a known address and a three-month access window is best handled as a protection task with a fixed deadline, with clearances and logistics settled well before the season opens.

4. Response, Rescue and the Dam

The national response on the ground was immediate. The Army, NDRF, Border Roads Organisation, state disaster response teams, paramilitary forces and police mounted a large search and rescue operation, while Army and Indian Air Force helicopters flew supply and evacuation sorties in poor weather; the Army also lost soldiers and vehicles at Burdang (All India Radio, 2023, Oct. 7). By 9 October nearly 400 tourists had been brought out of North Sikkim, 354 of them by the IAF in 10 sorties of Mi-17 and Chinook helicopters (PTI, 2023, Oct. 9).

For the dam itself, the operator's reply to the National Green Tribunal recorded that the dam had been "washed away upto river bed level", with damage to intake, spillway tunnel and chute spillway gates and debris entering the underground powerhouse (Sikkim Urja Limited, 2024, May 19). The project, whose generating station had entered commercial operation in February 2017 (CERC, 2020, Jan. 9), had served the state for more than six years. Its restoration is a test of how far design standards for Himalayan hydropower will now account for upstream glacial hazards.

The Dam Safety Act, 2021, which came into force on 30 December 2021, created a National Committee on Dam Safety, a National Dam Safety Authority, State Committees and State Dam Safety Organisations, with dams classified by vulnerability and hazard and offences punishable by imprisonment of up to one year, or two years where loss of life results (CWC, 2022, Jun. 17). The Authority directs the intervals at which emergency action plans are updated and risk assessment studies undertaken (SCC Online, 2021, Dec. 16). The Act is a landmark that gave India, for the first time, a statutory dam safety institution. Its framework is built around the dam as the source of hazard to those downstream, and South Lhonak showed the value of extending it to hazards that arrive from upstream.

5. What Changed Afterwards

The Government of India has acted on these lessons. A High Level Committee of the Ministry of Home Affairs approved Rs 150 crore under NDMA's National GLOF Risk Mitigation Programme for four states, including Rs 40 crore for Sikkim, and the Department of Water Resources set up a committee to study the morphological changes in the Teesta after the floods (Lok Sabha, 2025, Feb. 13). In August 2024 it was reported that the government would reassess existing and under-construction dams vulnerable to GLOFs, as well as new dams in catchments with glacial lakes (Swarajya, 2024, Aug. 9). These are the right priorities. The next step is to make the outcome of that reassessment binding through standards issued under the Dam Safety Act.

6. Recommendation: Bind the Lake to the Dam

What comes through from all of this is that South Lhonak was a problem of coupling. The science of the lake, the instruments on its shore, the dam's design basis and the gate operators at Chungthang sat with different institutions, and the two hours that mattered fell between them. The lesson for India's critical infrastructure is that a hydropower station in a glaciated catchment should be treated as part of a single hazard system that begins at the highest lake above it.

Three measures follow. Every specified dam with a glacial lake in its catchment should hold, through the National Dam Safety Authority, a GLOF-specific emergency action plan with pre-authorised gate operations triggered by upstream sensors. Monitoring of NDMA's high-risk lakes should be completed against fixed dates, with security clearances settled in advance. And performance on both should be measured and published, which is what a composite instrument such as the Bharat National Resilience Index is designed to do (Dash, 2025, Oct. 28). Two hours is not much. With the system India is now building, it is enough.