Deep in the rugged mountains of Arunachal Pradesh, where snow-fed rivers descend through steep valleys before entering the Brahmaputra plains, one of India’s most ambitious infrastructure projects is taking shape. The 2,880 MW Dibang Multipurpose Project (DMP), being developed by NHPC Limited on the Dibang River, is poised to become a landmark in India’s hydropower history.
When completed, the project will feature India’s tallest dam, at 278 metres, and the country’s largest hydropower-generating capacity. More importantly, it will serve multiple national objectives, including renewable energy generation, flood moderation, water security, regional development, and strategic strengthening of India’s position in the Brahmaputra basin.
The Dibang River: A powerful tributary of the Brahmaputra
The Dibang River originates near Keya Pass on the Indo-China border at an elevation of more than 5,000 metres in Arunachal Pradesh’s Dibang Valley district. From its Himalayan source, the river flows southward for about 195 kilometres through the mountainous districts of Dibang Valley and Lower Dibang Valley before joining the Lohit and Siang rivers near Sadiya in Assam. Thereafter, the combined flow is known as the Brahmaputra.
In its upper reaches, the river traverses deep gorges and difficult terrain in the Mishmi Hills before entering the plains at Nizamghat. The basin is fed by several major tributaries, including the Mathun, Tangon, Dri, Ithun, and Emra rivers, along with numerous smaller streams such as Ahi, Aripani, Ilupani, Ashupani, Iphipani, and Deopani.
The catchment area upstream of the proposed dam site covers approximately 11,276 square kilometres, including about 640 square kilometres of permanently snow-covered terrain. While a small part of the basin extends into Tibet, the overwhelming majority lies within Arunachal Pradesh. The river carries an average annual runoff of more than 28 billion cubic metres and contributes nearly 8.5 percent of the flow recorded at the Brahmaputra’s Pandu gauging station near Guwahati.
After descending from the mountains, the river spreads across a broad floodplain, often forming a braided channel between four and nine kilometres wide. Frequent changes in channel alignment cause annual flooding, erosion, and loss of valuable agricultural land.
The Dibang project: A multipurpose vision for energy and flood control
The Dibang Multipurpose Project is located near Munli village in Lower Dibang Valley district, approximately 1.5 kilometres upstream of the confluence of the Ashupani River and the Dibang.
Originally conceived by the Brahmaputra Board as part of a basin-wide flood moderation strategy, the project has evolved into one of India’s most significant multipurpose river valley developments. While clean electricity generation remains a key objective, flood moderation has been incorporated as a major design feature.
The project envisages the construction of a 278-metre-high concrete gravity dam, extending 798 metres across the river valley. The structure has been designed to withstand extreme hydrological and seismic conditions while creating a large storage reservoir capable of regulating floods and generating electricity throughout the year.
Upon completion, the project will install twelve Francis turbine generating units of 240 MW each, producing a total capacity of 2,880 MW. Annual energy generation is expected to exceed 11,223 million units in a 90-percent dependable year.
Engineering on a monumental scale
The engineering complexity of the Dibang project is comparable to that of some of the world’s major hydropower developments. The combination of a very high dam, extensive underground works, and challenging Himalayan terrain makes it one of the most technically demanding hydropower projects ever undertaken in India.
Located about 1.5 km upstream of the confluence of the Ashupani River and the Dibang, the project sits in steep, rugged terrain with a favourable combination of geology and topography. Constructing a dam of this magnitude requires the temporary diversion of the Dibang River away from the dam foundation. During construction, the river will be diverted through five 12-metre-diameter tunnels designed to handle a 1-in-25-year flood of 8,680 cubic metres per second (cumecs). These works will be supported by upstream and downstream cofferdams that isolate the construction zone from the river. The successful management of river diversion will be a critical milestone in the project’s execution.
Measured from the deepest foundation level, the 278-metre concrete gravity dam will be the highest in India. The dam’s crest elevation will be EL 540.00 m.
The spillway section consists of eleven blocks, each 14.0 m wide and containing one spillway opening. Two levels of orifice-type spillways are proposed: six openings measuring 6.0 m × 8.0 m, with their crests at El. 454.2 m, and five openings measuring 9.0 m × 12.0 m, with their crests at El. 485.3 m. Together, they are designed to pass a flood of 19,000 cumecs. Each spillway bay will have a dedicated hydraulically operated radial gate housed in the dam body. A bulkhead gate will be provided to allow maintenance of the radial gates. The peak discharge of the probable maximum flood (PMF) hydrograph is 26,230 cumecs. At the full reservoir level (FRL), the spillway’s design capacity of 19,000 cumecs is considered sufficient to route the PMF through the reservoir while keeping the reservoir level within the maximum water level (MWL).
The water-conductor system comprises six headrace tunnels (9.0 m in diameter and 300–600 m long), six pressure shafts (7.5 m in diameter and 231 m long), and twelve penstocks (4–5.2 m in diameter) to carry water to the turbines. After power generation, the water will be returned to the river through six tailrace tunnels (9 m in diameter and 320–470 m long).
Large underground caverns will house the powerhouse, transformer galleries, and associated equipment. The underground powerhouse complex has three main caverns: the MIV cavern, measuring 17 m (W) × 29.1 m (H) × 250 m (L); the powerhouse cavern, measuring 24.5 m (W) × 56.3 m (H) × 419.0 m (L); and the transformer, GIS, and draft-tube cavern, measuring 19 m (W) × 31.5 m (H) × 407.8 m (L). The installed capacity will be 2,880 MW, generated by twelve 240 MW Francis units at a net rated head of 222.5 m.
A reservoir with multiple benefits
The concrete gravity dam across the Dibang River will create a reservoir covering approximately 35.64 square kilometres. The maximum water level (MWL), full reservoir level (FRL), and minimum drawdown level (MDDL) of the proposed project are EL 538 m, EL 530.3 m, and EL 489.2 m, respectively. The corresponding storage capacities are 3,510.0 million, 3,247.9 million, and 1,965.3 million cubic metres, respectively. The reservoir’s backwater will extend up to 41 km along the Dibang River and its tributaries, including Airi Pani, Ilu Pani, Imu Pani, Ahi, Ithun, and Emra.
Apart from generating electricity, the vast reservoir will help regulate seasonal river flows and improve connectivity to remote upstream settlements that currently remain difficult to access. Water transport opportunities may increase significantly, creating new socio-economic possibilities for local communities.
More importantly, the reservoir will provide substantial flood moderation benefits for downstream areas that have historically experienced severe monsoon flooding.
Environmental safeguards and ecological flows
Recognising the ecological significance of the Dibang River system, the Ministry of Environment, Forest and Climate Change imposed several safeguards when granting environmental clearance to the project.
One important condition requires the continuous release of a minimum environmental flow of 20 cubic metres per second downstream of the dam. Additionally, at least one generating unit must operate continuously throughout the year to help maintain aquatic habitats and riverine ecology.
Extensive studies were undertaken on biodiversity, wildlife, fisheries, forest ecology, and downstream impacts before environmental approval was granted.
Construction materials
Approximately 216.0 lakh cubic metres of coarse aggregate, 108.0 lakh cubic metres of fine aggregate, 2.95 lakh tonnes of reinforcing steel, and 65.0 lakh tonnes of cement are required for construction of the Dibang Multipurpose Project. Against the stated total requirement of 447.0 lakh cubic metres of coarse and fine aggregate, 582.0 lakh cubic metres of suitable aggregate material is available from nearby river shoal deposits. The project proposes to use materials from these riverbeds, as this is expected to have a minimal environmental impact.
Addressing sedimentation and geological challenges
Like all Himalayan rivers, the Dibang carries significant quantities of sediment. Detailed sedimentation studies indicate that approximately 11.3 million cubic metres of sediment may enter the reservoir each year.
Long-term reservoir simulations indicate that substantial storage capacity will remain available even after decades of operation, supporting the continued functioning of both the flood-moderation and power-generation components.
The project site is underlain by complex geological formations consisting of gneisses, amphibolites, pegmatites, and metamorphic rocks. Extensive geological investigations have confirmed the suitability of these formations for construction of a major concrete gravity dam.
Building safely in India’s highest seismic zone
Because the project lies within Seismic Zone V, the highest earthquake-risk category in India, exceptional attention has been paid to seismic safety.
Advanced investigations, including local earthquake tomography and magnetotelluric studies, were carried out for the first time in India specifically for a major dam project. Site-specific seismic design studies were undertaken by IIT Roorkee, the Geological Survey of India, the Indian Institute of Geomagnetism, and the Central Water and Power Research Station.
The project’s seismic design was also reviewed by internationally renowned experts, including Dr. Martin Wieland, former Chairman of the Seismic Committee of the International Commission on Large Dams (ICOLD). After extensive scrutiny, the National Committee on Seismic Design Parameters approved the project’s seismic design criteria.
Studies indicate that reservoir-induced seismicity is unlikely given the prevailing tectonic environment in the Himalayas.
GLOF risk
Many glacial lakes in the Siang, Subansiri, Dibang, and Lohit river systems lie far upstream, often on the Tibetan Plateau. Longer travel distances, gentler gradients in some areas, and large reservoirs can help attenuate flood peaks before they reach major infrastructure.
Managing extreme floods
The Dibang River carries enormous flood discharges during the monsoon, making flood management a critical component of the project.
How a storage reservoir moderates a flood
The Dibang project is conceived as a storage-based scheme in which flood moderation is a key objective. It forms part of the Brahmaputra Board’s master plan to control flooding across the Brahmaputra’s tributaries. By operating the reservoir according to an approved rule curve, a flood cushion of 40.1 metres will absorb about 1,259 million cubic metres—roughly half the volume of three successive major floods. In the design scenario, the peak discharge of a 1-in-100-year flood, estimated at 12,757 cubic metres per second, is reduced to 3,000 cubic metres per second, the safe carrying capacity of the river below the dam.
For a 1-in-100-year flood, the reduction in water level is expected to be around 1–2 metres at locations more than 20 km downstream of the dam. This is significant, given the Dibang’s considerable width in these reaches. The project’s flood-moderation benefits are expected to be substantial downstream. Just upstream of the confluence with the Lohit, a 100-year return-period flood is projected to produce a 0.76-metre reduction in water level, a 0.77-metre-per-second reduction in velocity, and a 2,060-metre reduction in top width.
At the confluence, the flood-prone width is projected to shrink by about 1,608 metres (45 per cent), potentially bringing an estimated 8,040 hectares back into agricultural use. Paddy cultivation on this land is valued at about Rs 134 crore annually. Assam’s flood losses averaged Rs 3,534.70 crore annually between 2014 and 2021. Applying the Dibang’s 8.5 per cent share, the damage avoided is estimated at about Rs 300 crore a year.
Given the enormous economic losses caused by floods in Assam each year, the project’s flood-moderation benefits alone are presented as justification for substantial public investment.
Minimal impact of peaking operations
Hydraulic model studies indicate that peaking operations associated with electricity generation will have only minor effects on downstream river behaviour.
At the Dibang–Lohit confluence, water-level fluctuations are expected to be limited to about 17 centimetres and to occur gradually over approximately six hours. Such changes are considered insignificant in a river system of this scale.
The effects diminish further downstream and are not expected to adversely affect the ecologically sensitive Dibru-Saikhowa National Park.
From concept to reality: A two-decade journey
The development of the Dibang Multipurpose Project has involved more than two decades of investigations, public consultations, engineering studies, and regulatory approvals.
The Brahmaputra Board initiated project investigations in 2001 and prepared the first pre-feasibility report in 2002. Later that year, project implementation responsibility was transferred to NHPC Limited.
Over subsequent years, extensive surveys, environmental assessments, geological investigations, and design studies were completed. Public hearings experienced delays because of local concerns regarding displacement and environmental impacts. However, sustained engagement with stakeholders helped build confidence in the project’s benefits and mitigation measures.
The project also underwent judicial scrutiny when environmental clearance was challenged before the National Green Tribunal. In 2018, the Tribunal upheld the clearance, observing that adequate efforts had been made to balance development with environmental protection.
After all major statutory clearances had been obtained, the Government of India approved investment for the project in February 2023. Prime Minister Narendra Modi laid the foundation stone on 9 March 2024.
Project cost and completion schedule
The Government of India granted investment approval in February 2023 for NHPC Ltd. to implement the 2,880 MW Dibang Multipurpose Project in Arunachal Pradesh. The estimated cost was Rs 31,876.39 crore at May 2021 price levels, including budgetary support of Rs 6,159.40 crore for the flood-moderation component and Rs 556.15 crore for enabling infrastructure, such as roads and bridges. The estimated cost of electricity production is Rs 4.51 per unit in the first year, while the levelised tariff is Rs 4.46 per unit.
The project’s completion schedule is based on approval granted by the Cabinet Committee on Economic Affairs (CCEA). According to the approval, the project commencement date is 27 February 2023. The approved construction period is 108 months, with completion scheduled for 26 February 2032. Accordingly, the project is expected to be completed in February 2032.
Benefits for Arunachal Pradesh and Assam
The Dibang Multipurpose Project is expected to bring substantial, long-term benefits to both Arunachal Pradesh and Assam. For Arunachal Pradesh, the project represents a potential catalyst for socioeconomic development. The state will receive 12 per cent of the power generated free of cost throughout the project’s operational life, along with an additional 1 per cent allocated to the Local Area Development Fund. Significant provisions have also been made for rehabilitation and resettlement, community development initiatives, educational and healthcare facilities, road and bridge construction, and the preservation of the region’s cultural heritage. The project is expected to create employment opportunities, support local livelihoods, and improve connectivity and infrastructure in one of the country’s most remote and strategically important regions.
For Assam, the most significant and far-reaching benefit of the project lies in its flood moderation capability. Every year, floods and riverbank erosion cause widespread damage to lives, livelihoods, infrastructure, and agricultural land across the Brahmaputra valley. The storage capacity of the Dibang reservoir is expected to help regulate peak flood flows and reduce downstream flooding during extreme monsoon events. Detailed hydraulic modelling studies indicate measurable reductions in flood levels, flow velocities, and inundation extents in downstream reaches. Areas around the confluence of the Dibang and Lohit rivers, which contribute substantially to flooding in Upper Assam, are expected to experience notable reductions in flood impacts and erosion risks. By helping to moderate flood peaks, protect vulnerable communities, and reduce damage to farmland and infrastructure, the project is expected to strengthen the long-term resilience and economic security of millions of people in Assam’s flood-prone districts.
Conclusion
The Brahmaputra Valley has reached a point where the old arithmetic no longer holds. The floods are growing more severe, the climate is changing, and the country’s need for clean and flexible power is rising at the same time. The Dibang Multipurpose Project answers all three pressures with a single piece of infrastructure. It holds back water at the head of the valley, replaces coal-fired electricity with renewable power, and provides a means of regulating a river whose behaviour is changing with the climate. The project carries real costs, which must be addressed with honesty, scientific rigour, and fairness to the people most directly affected. But the cost of inaction is already being paid every monsoon in Assam. For the valley, the climate, and future generations, the Dibang Multipurpose Project is of utmost importance and must be completed with the greatest possible care and without avoidable delay.
A. N. Mohammed is a consultant in hydropower project development in Northeast India.
The views and opinions expressed in this article are solely those of the author and do not necessarily reflect the views or positions of Northeast Now.