Context: The Union Ministry of Environment, Forest and Climate Change (MoEF&CC) has approved the proposal for rebuilding the Teesta-III dam, despite concerns over its design and stability. The new structure will be a 118.64-metre-tall concrete gravity dam.
Relevance of the Topic: Prelims: Questions based on rivers, tributaries and dams associated.
About Teesta-III Chungthang hydroelectric dam
- In October 2023, the original 1,200 MW Teesta-III Chungthang hydroelectric dam was destroyed in a flash flood.
- The flood was triggered by a Glacial Lake Outburst Flood (GLOF) from the South Lhonak Lake, which washed away the 60-metre-tall concrete dam, claiming 40 lives.
- Reason for failure of dam: Moraine on the South Lohank lake’s flank suffered a slope failure, weakening the terminus. The failure sent rocks tumbling into the lake generating strong ripple, the event also set off multiple landslides about 30 to 40 kilometers downstream.
Links between dam failure and global warming:
- Reducing albedo: Particulate matter, especially black carbon, also known as soot, reduces the albedo of ice and leads to absorption of solar insolation.
- Accelerating melting of ice: Global warming and rise in the global average temperature accelerates the melting of ice and glaciers, this leads to the rise in the water levels. (E.g., South Lhonak lake itself was formed in the early 1960s and grew to 167 hectares by 2023)
- High rate of glacial retreat: Glacial retreat has also been known to destabilise extant geological formations and create new sources of risks.
- Rise in glacial lakes: As per the Central Water commission, the number of glacial lakes in Himalayan region grew by 10.8% from 2011 to 2024.
Concerns in the construction of Teesta-3 2.0 Dam:
- The dam’s design and structural aspects are still pending approval from the Central Water Commission (CWC), the Geological Survey of India, and the Central Soil and Materials Research Station.
- Reports on the risk assessment fail to account the erosion, sediment transport and riverbank collapse that have a significant impact on the flood behaviour.
- Local communities and environmental groups like affected citizens of Teesta have highlighted potential risks like Glacial lake outburst floods during project planning stages.
Measures Undertaken
- Enhanced spillway design:
- The dam design is altered from a concrete face rockfill dam (a commonly used structure worldwide) to a concrete gravity dam, a design that relies on its own weight to bear loads.
- The company claims that the complete concrete design will lead to a rise in the spillway capacity from 7000 cubic metres per second (cumecs) to 19946 cumecs. This enhancement will offer greater resilience to flash floods and GLOFs.
Suggested Measures:
- Development and implementation of a robust Early Warning System (EWS) for flood alerts in the river catchment.
- Conduct thorough environmental impact assessments (EIA) that incorporate climate change projections, glacial behaviour and potential GLOF risks to inform project design and location decisions.
About South Lhonka Lake
- South Lhonak lake is a glacial lake in North Sikkim.
- Formation: The proglacial lake formed due to the retreat of Lhonak Glacier.
- The National Remote Sensing Center showed a 40% increase in its size over the past three decades.
About Teesta River
- Origin: Teesta river originates from the Tso Lhamo (lake) in North Sikkim.
- Course: Flows through Sikkim and West Bengal in India and then enters Bangladesh. Here it merges with Brahmaputra river (known as Jamuna in Bangladesh). The total length is 414 km.
- Tributaries: Ranget, Lachung, Lachen and Dikchu rivers.
- Hydropower project: Teesta-III, Teesta-IV and Teesta-V dams in Sikkim.
- Water dispute: Teesta water sharing treaty with Bangladesh, a key bilateral agreement that has been pending between the two countries for over a decade.
