--- title: "PROR environmental impact and schematic" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:pror-environmental-impact" source: https://www.bctengineer.com/q/semester_8/power_plant_design/pror-environmental-impact site: BCT Engineering --- # PROR environmental impact and schematic Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2079 Shrawan, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/pror-environmental-impact - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question Stating environmental impact of P-ROR type hydro-power plant, draw a schematic diagram showing all the necessary components and describe each of them. ## Short answer A peaking run-of-river plant causes daily flow fluctuations harming aquatic life, dewatered stretches, pond submergence, and sediment flush releases. The plant includes a diversion dam, intake, gravel trap, desander, daily pondage, headrace tunnel, surge tank, penstock, Francis turbine powerhouse, tailrace, and switchyard. ## Answer A **peaking run-of-river (P-ROR)** plant is an ROR plant with a small **daily pondage**, so that water collected during off-peak hours is used to run the plant at full capacity during the evening peak (typically 4–6 hours). #### Environmental impacts of P-ROR - **Hydro-peaking downstream:** flow below the tailrace rises and falls sharply every day; this disturbs fish, aquatic insects and people using the riverbank. - **Dewatered stretch:** between the dam and tailrace only the riparian release flows, reducing habitat, fish migration and water for irrigation/water mills. - **Pondage submergence:** a small area of land and forest is flooded; some houses or farmland may be affected. - **Sediment trapping and flushing:** the pond traps sediment; periodic flushing releases very turbid water downstream. - **Construction impacts:** spoil from tunnels, access roads, landslides, dust, noise and inflow of workers. - **Positive impacts:** clean peaking energy that replaces diesel/thermal imports, local roads, jobs and electrification. Mitigation: adequate riparian release (at least 10% of minimum monthly flow in Nepal), fish ladders, controlled flushing, re-afforestation and spoil management. #### Schematic of a P-ROR plant ```text River ==|== Dam/weir with spillway + undersluice | Intake (trash rack, gate) | Gravel trap -> Desander | [ DAILY PONDAGE ] (stores off-peak flow) | Headrace tunnel | Surge tank | Penstock (+ valve house) | Powerhouse: Turbine-Generator-Transformer | | Tailrace -> River Switchyard -> Grid ``` #### Components 1. **Diversion dam/weir:** raises water level; gated spillway passes floods; undersluice flushes sediment at the intake. 2. **Intake:** trash rack and gates admit the design discharge. 3. **Gravel trap and desander:** remove sediment (usually >0.2 mm) before water enters the pondage and tunnel. 4. **Daily pondage:** a reservoir sized to store the inflow of off-peak hours so the plant can run at full output during peak hours. Its volume is about $(Q_d - Q_{dry})\times t_{peak}$. 5. **Headrace tunnel:** pressure tunnel carrying water to the surge tank. 6. **Surge tank:** absorbs water hammer and meets sudden load rise. 7. **Penstock and valve house:** steel pipe to the powerhouse with an emergency butterfly valve at the top. 8. **Powerhouse:** turbines (Francis or Pelton depending on head), generators, governors, excitation, control and auxiliaries. 9. **Tailrace:** returns water to the river; may include a re-regulating pond to reduce hydro-peaking. 10. **Switchyard and transmission line:** step up and evacuate power to the grid. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which downstream impact is characteristic of a P-ROR plant? A. Flow below the tailrace stays perfectly constant B. Flow below the tailrace rises and falls sharply every day C. The river dries up completely below the tailrace D. Flow changes only once a year with the monsoon Answer: B. Storing water off-peak and releasing it in peak hours causes daily flow fluctuation (hydro-peaking), disturbing fish and riverbank users. (Key point: Hydro-peaking) 2. Which reach of the river is dewatered in a P-ROR scheme? A. Upstream of the dam B. Downstream of the tailrace C. Between the dam and the tailrace, where only the riparian release flows D. Only inside the pondage Answer: C. Water is diverted at the dam and returned at the tailrace, leaving the stretch between with only the riparian release. (Key point: Dewatered stretch) 3. Why does periodic flushing of a P-ROR pond harm the downstream river? A. It cuts off all flow downstream B. It floods the powerhouse C. It removes the riparian release requirement D. It releases very turbid water downstream Answer: D. The pond traps sediment, and flushing it releases a heavy sediment load that makes the river very turbid. (Key point: Sediment trapping) 4. Which is a mitigation measure listed for P-ROR impacts? A. Riparian release of at least 10% of minimum monthly flow in Nepal B. Releasing no water between dam and tailrace C. Storing water for several months D. Removing the desander Answer: A. An adequate riparian release, with fish ladders, controlled flushing and re-afforestation, reduces the environmental impact. 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