Syllabus: GS-III & V: Infrastructure

Why in the News?

After a 20-year wait, the Subansiri Lower Hydroelectric Project (SLHP) has started generating power, with one unit synchronised with the National Power Grid on December 1, 2025. This marks a crucial step in India’s push towards clean and stable hydropower.

A Defining Moment in India’s Hydropower Journey

The Subansiri Lower Hydroelectric Project, located at Gerukamukh on the Assam–Arunachal Pradesh border, is India’s largest run-of-the-river hydropower project under construction. Implemented by NHPC, it will generate 2000 MW through eight units of 250 MW each.

• Unit 1 and Unit 2 have started test generation.
• Three more units are expected by December 2025.
• The full commissioning is targeted for March 2027.

Once operational, the project will supply 7.421 billion units of renewable energy per year, supporting India’s carbon-neutrality ambitions.

Understanding the Project: Key Technical Features

• Type: Run-of-the-river scheme – uses natural river flow with minimal storage.
• Developer: NHPC Ltd. (National Hydroelectric Power Corporation)
• Dam Height: 116 metres, concrete gravity dam.
• Reservoir Length: 34.5 km (storage: 1365 MCM + 442 MCM flood cushion).
• Diversion System: 5 diversion tunnels.
• Spillways: 9 spillway bays for controlled release.
• Powerhouse: Underground structure with 8 × 250 MW turbines.

These design features help balance power generation, flood moderation, and ecological flow.

About the Subansiri River

• The Subansiri River, also known as Chayul Chu in Tibet, is the largest tributary of the Brahmaputra, contributing nearly 7.92% of its total flow. 
  • It is a right-bank tributary of the Brahmaputra.
• It is a trans-Himalayan river flowing through Tibet (China), Arunachal Pradesh, and Assam before joining the Brahmaputra at Jamurighat in Lakhimpur district.
• Known as Gold River for its historic gold dust (Xiphoid sand).

Origin and Course

• The river originates in the mountains of Tibet, where several “Chu” streams (notably Sikung Chu) form its upper course .
• Total length: ~442 km
  • ~192 km in Arunachal Pradesh
  • ~190 km in Assam
  • Rest in Tibet
• It enters India near Taksing at Upper Subansiri district of Arunachal Pradesh.
• It then flows along the Miri Hills, and descends into the Assam plains at Dulangmukh.

Geographical Features

• Drainage basin: ~32,640 sq km
• Known for its steep gradients, rapid flows, and braided channels in the plains.
• Important tributaries include Rangandi, Dikrong, and Kamala.

Project’s Long Journey: 20 Years of Challenges & Resilience

1. Halt in Construction (2011–2019)

Assam witnessed massive protests led by civil groups, environmentalists, and experts citing:

• Dam safety concerns,
• Seismic vulnerability,
• Downstream ecological impact,
• Risk of sudden floods.

All work stopped for eight years, leading to cost escalation from ₹6,285 crore (2002) to nearly ₹26,000 crore.

Safety Reforms and Expert Panels: How the Project Was Strengthened

To address public concerns, the Union Government constituted multiple high-level committees:

1. Technical Expert Committee (TEC), 2012

• Rejected fears of “dangerous hydrological design”.
• Recommended increased safety margins.

2. Dam Design Review Panel (DDRP), 2013

Suggested major upgrades:

• Widening dam base from 171m → 271m
• Additional cut-off walls
• Strengthened grouting and anchoring
• Enhanced seepage protection

3. National Committee for Seismic Design Parameters (NCSDP)

• Approved upgraded seismic safeguards.

4. Cumulative Impact and Carrying-Capacity Study (CWC, 2014)

Mandated:

• Minimum environmental flow of 240 cubic metres/second
• Long-term river health monitoring
• Downstream ecological protections

Ecological & Safety Measures for Downstream Assam

Key Protection Measures

• A 15-meter flood cushion to absorb sudden inflows.
• 442 MCM buffer capacity for flood moderation.
• 60 km of riverbank protection downstream.
• Raised carrying capacity (15–20%) of the Subansiri channel.
• ₹470 crore community welfare and infrastructure package for affected villages.

These steps aim to reduce flood anxiety among downstream communities.

Power Allocation: Who Gets What?

• Assam:
  • Standard allocation: 208 MW
  • Additional preferential: 300 MW
  • 25 MW free electricity
• Arunachal Pradesh:
  • 12% free power as the “host state”

Remaining power is allocated to other states, including those in the Northeast.

Importance for India’s Energy Transition

The project contributes directly to:

• India’s National Hydropower Policy goals
• Paris Agreement commitments
• 2030 renewable energy targets
• Stabilising the Northeast’s grid
• Reducing coal dependence

Exam Hook: Key Takeaways

• India’s largest run-of-river hydropower project under construction.
• 2000 MW capacity; 8×250 MW units.
• Crucial safety redesign after public protests.
• Subansiri: Trans-Himalayan river, meets Brahmaputra near Majuli.
• Reservoir: 34.5 km; dam: 116 m high.
• Environmental flow mandated: 240 cumecs.

Mains Question

“The Subansiri Lower Hydroelectric Project illustrates the complex balance between development, ecology, and public consent in the Northeast. Discuss.”

SOURCE