South Lhonak GLOF (2023)
Also called: Teesta flood 2023 · Topic: Natural Hazards and Disaster Management in India · NCERT: Beyond NCERT
Meaning
The South Lhonak GLOF (2023) was a sudden burst of water from South Lhonak lake in north Sikkim on 3-4 October 2023. South Lhonak is a lake held back by a wall of loose glacier debris. The flood rushed down the Teesta river, destroyed the Teesta III dam at Chungthang and killed many people, including soldiers. It is India's most recent major example of a cascade hazard, where one hazard sets off the next. It also showed that a dam built below a known high-risk lake can become part of the disaster.
Explanation
What a GLOF is
- GLOF (glacial lake outburst flood): a sudden release of water from a lake held back by a glacier or by glacier debris. The Class 7 NCERT calls it a "glacial burst".
- Moraine: the heap of rock, gravel and soil that a glacier carries and then drops. A dam made of moraine is loose and weak.
- South Lhonak is a moraine-dammed lake. Only a weak natural wall of debris held its water back.
How the 2023 flood happened (cause → process → result)
- Lake grows
- Warming melts the glacier.
- Meltwater collects behind the moraine dam.
-
The lake gets bigger every year.
-
Dam breaks
- A moraine dam can break for several reasons: the lake overfills, heavy rain or snow falls, an avalanche or landslide drops into the lake and pushes up a big wave, or an earthquake shakes it.
-
Once the wall gives way, the stored water escapes all at once.
-
Flood rushes down
- A huge mass of water and debris surged down the Teesta valley within hours.
- Debris turned the water into a debris flow (a fast, thick mix of water, mud, rock and boulders).
- The surge hit the Teesta III dam at Chungthang and destroyed it.
Why it is called a cascade
- Cascade hazard: one hazard sets off the next, like a row of falling dominoes.
- Here, the chain was: a growing glacial lake, then a dam breach, then a flash flood (a flood that rises within minutes or hours with very little warning), then a dam failure downstream.
- One trigger high in the mountains caused deaths and damage far down the valley.
Conditions that make such floods likely
- Warming: glaciers melt, lakes grow, and glacier ice and permafrost (ground that stays frozen all year) get weaker.
- Fragile slopes: the Himalayas are young, steep and prone to earthquakes, and they are covered in loose debris.
- Exposure: hydropower dams, roads and settlements sit inside the flood path.
- Season: GLOFs are most common in summer and the monsoon, when there is the most meltwater.
Where it occurs
- Source: South Lhonak lake, a moraine-dammed lake in north Sikkim.
- Flood path, the Teesta river:
- The Teesta rises in the glaciers of north Sikkim.
- It flows south through Sikkim and West Bengal.
-
It then enters Bangladesh and joins the Brahmaputra (Jamuna).
-
Main damage site: the Teesta III dam at Chungthang, in Sikkim.
- Another Indian GLOF example: Chorabari lake, above Kedarnath in Uttarakhand. It breached in June 2013 and sent floods down the Mandakini.
- Policy framework for GLOFs in India:
- NDMA Guidelines on Management of GLOFs (2020).
- A national GLOF risk-mitigation programme (2024) for high-risk lakes (verify current). It aims to assess lakes, set up automatic monitoring and early warning, and lower lake levels.
- NRSC (National Remote Sensing Centre, under ISRO) keeps an inventory of glacial lakes mapped from satellite images.
- CWC (Central Water Commission) monitors glacial lakes and water bodies.
Don't confuse with
- Chamoli flood (7 February 2021, Uttarakhand): this was a rock-and-ice avalanche from Ronti peak, not a lake outburst. It came in winter, and its path was Ronti Gad → Rishiganga → Dhauliganga [1]. South Lhonak was a true lake outburst in Sikkim.
- Kedarnath disaster (June 2013, Uttarakhand): here, extreme monsoon rain, made worse by a western disturbance (a low-pressure system that travels east from the Mediterranean region), caused Chorabari lake to breach. The flood went down the Mandakini, not the Teesta.
- Cloudburst: here the water comes from rain, which the IMD defines as 100 mm or more in one hour over about 20-30 sq km. In a GLOF, the water comes from a glacial lake that was already stored.
- Avalanche: in an avalanche, snow (sometimes with rock and ice) slides down a slope, mostly in winter and early spring. An avalanche can trigger a GLOF by falling into a lake, but it is not a GLOF itself.
Prelims Hooks
- South Lhonak lake (north Sikkim) → Teesta → Teesta III dam at Chungthang destroyed (October 2023).
- South Lhonak is a moraine-dammed lake, held back by loose glacier debris and not by solid rock.
- The Teesta's course: north Sikkim glaciers → Sikkim → West Bengal → Bangladesh, where it joins the Brahmaputra (Jamuna). Trap: it does not join the Ganga.
- "Glacial burst" (Class 7) means the same thing as a GLOF.
- NDMA GLOF Guidelines came out in 2020. NRSC (ISRO) keeps the satellite inventory of glacial lakes.
- Which of these was not a lake outburst? Chamoli 2021, which was a rock-and-ice avalanche. South Lhonak 2023 and Chorabari 2013 were both lake breaches.
Mains Points
- Cascading risk (GS-III):
- In South Lhonak 2023, Kedarnath 2013 and Chamoli 2021, a trigger high in the mountains turned into a debris flood far downstream.
- Planning must cover the whole valley, not just the hazard site.
-
Shared river basins call for cooperation with Nepal, China and Bhutan.
-
Development in hazard zones:
- The Teesta III dam was built downstream of a known high-risk lake, and it became part of the disaster.
-
Possible responses: cumulative impact assessment for each river basin (studying the combined effect of all projects in one basin), keeping dams out of the paths of high-risk lakes, and building climate-resilient infrastructure (designed to withstand floods and extreme weather).
-
Early warning gap:
- What exists: the NRSC lake inventory, CWC monitoring, the NDMA 2020 guidelines, and the national programme for high-risk lakes.
- What is still needed: automatic sensors at the lakes, lowering of lake levels, and sirens and evacuation drills in villages.
- This matches the Sendai Framework's push for "early warning for all".