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.
The original 1,200 MW Teesta-III Chungthang hydroelectric dam was destroyed in a flash flood on October 3-4, 2023. The flood, triggered by a Glacial Lake Outburst Flood (GLOF) from the South Lhonak Lake, washed away the 60-metre-tall concrete dam, claiming 40 lives.
The project is located on the main Teesta River in Mangan district, utilising a drop of approximately 800 metres between Chungthang and Sankalang villages.
In a meeting held on January 10, 2025 the environment ministry’s Expert Appraisal Committee (EAC) raised concerns about the extensive damage caused by the flood, including the washing away of the dam structure and the severe flooding of the underground powerhouse, which brought operations to a halt.
However, the project proponent, Sikkim Urja Ltd (SUL), assured the EAC that, aside from the dam, most other components of the project could be restored to their original condition within a year, according to the meeting minutes.
“The water conductor system remains largely unaffected by the flash flood. Since most components could be restored within a year, there is a strong case for rebuilding the dam and resuming operations at the earliest. We request an amendment to the Environmental Clearance (EC),” SUL stated.
The company proposed altering the dam design 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 claimed that this new design would offer greater resilience to flash floods and GLOFs.
The Bhakra Dam, India’s tallest gravity dam, is an example of the structural resilience of concrete face rockfill dam designs.
SUL argued that a concrete gravity dam would reduce the likelihood of failure due to overtopping.
“Spillway capacity has been increased from 7,000 cubic metres per second (cumecs) to 19,946 cumecs, considering both GLOF and probable maximum flood (PMF) scenarios. The spillway crest has also been lowered to enhance safety,” SUL stated during the meeting.
To improve worker safety during project operations, SUL has proposed relocating the dam control room to a higher elevation. Additionally, the operation of spillway gates will be synchronised with an early warning system to ensure they open in time to manage floodwaters.
SUL reported conducting a comprehensive study to assess future GLOF threats. The company identified 119 glacial lakes in the dam’s catchment area, of which 50, each covering at least 10 hectares, were shortlisted for further monitoring.
Among these, 13 glacial lakes with a water spread of 40 hectares or more were deemed potentially hazardous.
According to SUL, a worst-case scenario was assessed by studying different combinations of potential breaches. The highest volume outflow from a simultaneous breach of two lakes — 12,946 cumecs — was considered. “This capacity has been added to the PMF calculations for determining the spillway capacity,” SUL explained.
The EAC recommended an immediate study to map landslides within a 5-kilometre radius of the Lachen and Lachung catchment areas.
“Based on this study, a further action plan for monitoring potential landslides and implementing mitigation measures should be developed,” the committee advised.
The minutes of the meeting also noted that the Central Water Commission (CWC) had approved the GLOF value of 12,946 cumecs. However, the CWC’s design directorate has recommended a revised study of the PMF and diversion flood, which must be approved by the relevant hydrology directorate.
The dam’s design and structural aspects are still pending approval from the CWC, the Geological Survey of India, and the Central Soil and Materials Research Station.
The EAC also recommended the development and implementation of a robust Early Warning System (EWS) for flood alerts in the river catchment.
“The EWS should be integrated with dam gate operations to ensure public safety, protect human lives, and enhance safe dam operations. It should also be capable of transmitting warning signals to the State Disaster Management Authority, the National Disaster Management Authority, the Sikkim state administration, and downstream projects to alert communities and safeguard vulnerable areas. The EWS should be linked with the NDMA, the Sikkim State Disaster Management Authority, and the state government’s comprehensive warning system,” the minutes of the meeting stated.