Environment ministry panel expresses concern over plan to redesign Sikkim dam washed away in 2023, defers nod

Government enterprise Sikkim Urja Limited has sought an amendment to the project’s old environmental clearance as it has proposed to redesign the project dam. It wants to replace the washed-away, concrete-faced rockfill dam with a concrete gravity dam. It has also proposed to enhance the spillway capacity from 7,000 cumecs to 19,946 cumecs, which it claimed can “accommodate both glacial lake outburst floods and probable maximum flood.”

However, the EAC was not convinced and during the deliberations held on November 30, it decided to seek the views of the Central Electricity Authority (CEA) on the proposed modifications. The committee also thought it prudent to carry out a thorough review of the proposed modifications to ensure the structural resilience and safety of the dam, particularly given the region’s susceptibility to extreme hydrological events, as per minutes of the EAC meeting.

Sub-committee formed

While deferring the proposal, it formed a sub-committee of two EAC members, and representatives of the CEA, the Central Water Commission and the Union environment ministry who will visit the project site.

On the intervening night of October 3 and October 4, an ice avalanche hit the South Lhonak Lake, located at an altitude of 5,200 metres, breaching the glacial lake. The avalanche created giant waves, leading to a glacial lake outburst flood (GLOF), and triggered a flash flood downstream in Teesta.

The force of the floodwaters breached the Chungthang dam, exacerbating the floods, killing 40 people across four districts of Sikkim. Across Mangan, Gangtok, Pakyong, and Namchi districts, about 100 villages were affected and 76 people were reported missing.

The Teesta-III project is in Mangan district and utilises a drop of about 800m in the Teesta River between Chungthang and Sankalang villages. The flash floods triggered by the GLOF event had overtopped the project dam and flooded the underground powerhouse, bringing power generation to a halt. Sikkim Urja and their environmental consultants R S Envirolinks have argued that the underground powerhouse and electro-mechanical equipment can be restored to their original condition in a year.

Company proposes starting partial generation

In fact, the company has proposed that partial generation can be started by constructing a coffer dam upstream. It has been proposed that the main dam can be restored in three years after partial generation begins, and the estimated cost of restoration is Rs 4,189.51 crore.

Following the GLOF event, the Centre and Sikkim government are collaborating on reducing levels of potentially risky glacial lakes in the upper reaches of Mangan district. The Centre has approved a Rs 150 crore National GLOF Risk Mitigation Programme and 189 lakes have been identified as high-risk glacial lakes, to mitigate the risks they pose.

The Sikkim government has already launched high-altitude expeditions this year to assess the susceptibility of Khangchung Chho, Gurudongmar Lakes A, B, and C, and Shako lakes. The Khangchung Chho Lake is located at 5,320 metres with a surface area of 183 hectares and an estimated volume of 106 million cubic metres.