How ignored landslide warnings led to Subansiri running dry

Subject: Geography

Section: Places in news

Context:

A large part of the hill on the left side of the Subansiri Lower Hydroelectric Project collapsed into its reservoir.

Details:

The deposits blocked the only functional diversion tunnel and stopped the flow of water downstream of the dam into the Subansiri River, a major tributary of the Brahmaputra.
It will be India’s largest HPP when completed.

How a dammed river flows:

Typically, once a location is selected for a dam, a temporary earthen barrier (coffer dam) and a few diversion tunnels are built immediately upstream to bypass the dam construction site.
Once the dam is ready, the diversion tunnels (DTs) are closed, and water starts to flow through the multiple spillways.
In a hydel project, a set of tunnels are also built to carry water from the reservoir to the powerhouse. The water turns the turbines and then goes back into the river. Once the powerhouse is operational, this becomes the main path of the water, and the spillways are used occasionally.
What happens to Subhansiri?
- The flow downstream was highly reduced after a landslide blocked the only DT in use.

The ‘mistake’ NHPC committed:

The Central Electricity Authority (CEA), the statutory body that advises the government on policy relating to electricity systems, recommended in April 2022 that the impact of the DTs on the slope stability of the project site should be examined.
NHPC Ltd, which is implementing the Subansiri Lower Project, ignored the recommendation.
The site’s history of landslides:
- This is the sixth landslide event on the lower Subhansiri project site.

What will NHPC do now?

The project will now focus on slope stabilisation.
The likely option would be to build a temporary sheet pile dyke — a metallic barrier with steel bracing — inside the reservoir to isolate the under-construction spillway gates, and to finish the work after the slope stabilisation process is over.

About Subhansiri river:

It is a trans-Himalayan river and a tributary of the Brahmaputra River that flows through Tibet’s Lhuntse County in the Shannan Prefecture, and the Indian states of Arunachal Pradesh and Assam.
It is approximately 518 kilometres (322 mi) long, with a drainage basin 32,640 square kilometres (12,600 sq mi).
It is the largest tributary of the Brahmaputra contributing 7.92% of the Brahmaputra’s total flow.
It enters India near the town of Taksing and flows east and southeast through Miri Hills, then south to the Assam Valley at Dulangmukh in Dhemaji district, where it joins the Brahmaputra River at Jamurighatin Lakhimpur district. Small tributaries of the Subansiri include Rangandi, Dikrong and Kamle.

Source of this article: Indian Express