PROR environmental impact and schematic

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

 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.