Key Terms & Concepts — UPSC Mains
Run-of-River Hydropower
"A type of hydroelectric project that generates power using the natural flow of a river with minimal or no water storage, contrasted with storage-based dams that impound large reservoirs."
A run-of-river (RoR) hydroelectric project diverts a portion of a river's natural flow through a turbine to generate electricity, using only the flow that is present at any given time rather than storing water in a large reservoir. Key structural features of RoR projects: - Diversion weir or barrage: A low structure across the river diverts a portion of flow into a headrace channel or tunnel - Forebay: A small settling basin before the penstock - Penstock: A pipe or tunnel carrying water under pressure to the turbines - Powerhouse: Contains turbines and generators (often underground in Himalayan RoR projects) - Tailrace: Returns water to the river after power generation RoR vs. Storage Dam comparison: - Storage: RoR has minimal reservoir (small forebay only); storage dams impound millions of cubic metres - Environmental impact: RoR causes lower submergence, less displacement, lower ecological disruption; storage dams cause large-scale displacement and river ecology changes - Power output: RoR output varies with river flow (lower in dry season); storage dams can regulate output year-round - Cost: RoR projects generally cost less and build faster - Examples: Kalai-II (1,200 MW, Lohit River, RoR); Subansiri Lower (2,000 MW, RoR); Bhakra-Nangal (storage); Tehri (storage) Himalayan RoR projects: Most new Himalayan hydropower projects (in Arunachal Pradesh, Uttarakhand, HP) are RoR because: 1. Himalayan rivers have steep gradients, high head for power generation without needing large storage 2. Environmental clearance is easier for RoR (less displacement) 3. Geological risks of large reservoirs in active seismic zones Policy significance: After 2019, large hydro (>25 MW) was reclassified as renewable energy, making RoR projects eligible for Renewable Purchase Obligation (RPO) targets. This significantly boosted the economic case for new RoR projects.
GS3 Environment + Economy topic covering hydropower technology, environmental trade-offs, and India's renewable energy policy. GS1 Geography: Indian river systems, Himalayan hydrology. Important for Arunachal Pradesh hydro potential questions (50,000 MW theoretical potential) and CCEA approvals.
- 1 Definition: Uses natural river flow without large reservoir storage
- 2 Components: diversion weir → headrace → forebay → penstock → powerhouse → tailrace
- 3 Advantage over storage dam: lower submergence, less displacement, lower ecological impact
- 4 Disadvantage: output varies seasonally with river flow (not dispatchable year-round)
- 5 Himalayan RoR: steep gradient compensates for lack of storage; most new HP projects in AP are RoR
- 6 Classification change: large hydro (>25 MW) reclassified as Renewable Energy in 2019
- 7 Kalai-II (1,200 MW, Lohit River, Arunachal Pradesh), RoR, approved CCEA April 2026
- 8 Subansiri Lower (2,000 MW, Arunachal Pradesh), India's largest planned RoR project
The Kalai-II Hydroelectric Project on the Lohit River in Anjaw District, Arunachal Pradesh (approved by CCEA, April 2026, cost ₹14,105.83 crore, capacity 1,200 MW) is a run-of-river project, it will divert a portion of the Lohit River's flow through 7 turbines (6×190 MW + 1×60 MW) without creating a large storage reservoir, minimising submergence and displacement in India's border-sensitive easternmost district.
