Landslide blocks diversion tunnel at NHPC's Subansiri hydro project

North Lakhimpur: A massive landslide at NHPC's much-delayed 2,000 MW Subansiri hydro project along the Assam-Arunachal Pradesh border on Friday completely blocked the only functional diversion tunnel, thereby significantly obstructing water flow through the river, officials said.

A NHPC Subansiri Lower Project spokesperson told PTI that the diversion tunnel (DT) number 1, which was the only one in use, was blocked due to a landslide at about 11.30 am.

There has been no report of any human casualty due to the incident.

"Out of the five DTs of 9.5 metres in diameter each, four were already closed. We are at our last stage of completion and would have closed this last tunnel also, but before that, this natural disaster struck," the spokesperson said.

He explained that as the main dam spillway was not ready and the river was flowing through the DT, the entire water flow has been impacted after the landslide.

"As a result, the river flow to the downstream is highly reduced. The dam spillway bay level is 145 metre mean sea level (msl)," he said.

The spokesperson said that the river is presently carrying around 1,000 cubic metre per second (cumec) water and it is being stored in the reservoir, where the water level is gradually increasing.

"At 1 pm, the water level reached 139 metres. It is expected the water level to reach 145 metres by the evening and the river will again flow through the spillway to the downstream normally," he added.

An official from the local administration said that at present only 5-10 cumec water is flowing downstream and the river bed has almost dried up.

"People are walking on the river and catching fish on the dry river bed. This has brought immense dangers to the aquatic life of the river," he said.

A villager living in the downstream area said that he has never seen the Subansiri river, a tributary of Brahmaputra, drying up in this way in his entire life.

"We heard from our parents that the Subansiri bed last dried up in 1950 when the great earthquake had struck. After that, the river was always full of water and its aquatic life. It also has about 30 river dolphins in this area," he said.

The Subansiri Lower Hydro Electric Project, which faced multiple bottlenecks since its inception, has been delayed and its deadline for commissioning has been revised multiple times.

In June, the commissioning of the project was postponed further till the end of 2023-24 fiscal. More than 90 per cent of the total work of the project has already been completed so far.

On completion, the Subansiri project, which is a run-of-river scheme, will generate around 7,500 million units of power annually in "90 per cent dependable year."

According to a company estimate in January 2020, the cost of the mega project, which was originally scheduled to be commissioned in December 2012, had escalated to around Rs 20,000 crore from the initial Rs 6,285 crore.

NHPC commenced the construction work of the Subansiri project in January 2005 after obtaining forest clearance in October 2004.