Tailrace: The Channel That Carries Water Away

Tailrace — the channel, flume, or conduit that conveys water **away from** a hydraulic prime mover (waterwheel, turbine, hydroelectric plant) after it has passed through the runner/blades.

Counterpart: Headrace

• Headrace: Channel bringing water **to** the turbine (forebay → penstock → scroll case).
• Tailrace: Channel taking water **from** the turbine (draft tube → tailrace → receiving water body).

Design Considerations

• Hydraulic efficiency: Minimize exit velocity (recover kinetic energy via draft tube); avoid submergence fluctuations.
• Cavitation prevention: Maintain pressure > vapor pressure; draft tube design critical.
• Sediment management: Prevent deposition (reduces capacity) or scour (undermines structure).
• Environmental: Fish passage (downstream migration), thermal plume, dissolved oxygen, ramping rates.
• Ice: Cold climates — frazil ice blockage; aeration, heating, design velocity.

Types

• Open channel: Trapezoidal, concrete/riprap; most common.
• Closed conduit: Pipe/culvert — where space constrained.
• Natural stream: Direct discharge — requires habitat assessment.
• Surge tank / tailrace tunnel: High-head plants; pressure transients.

Scale Examples

• Micro-hydro (10 kW): 6" PVC pipe → creek.
• Grand Coulee (6,800 MW): 40-ft diameter tunnels × 4 → Columbia River.
• Three Gorges (22,500 MW): Massive surface tailrace channel → Yangtze.

Terminology Note

"Tailrace" applies to **any** water-powered machinery — historic gristmill, modern pumped storage, tidal turbine. The principle is universal: **energy extracted → water returned**.