Return the used water to the stream through a tailrace that carries the full flow without backing up, and never divert the whole stream. The carrier limits diversion to 50 to 70% of the dry-season flow while the installation page sizes the plant at 70 to 80% of minimum flow; at 5 L/s of dry flow and 10 meters of head that is 175 to 245 watts against 245 to 280 watts.
Part of Micro-Hydro Turbine
The tailrace is the channel that carries water from the turbine back to the stream; environmental flow is the share of the stream left in its own bed between the intake and the outfall.
Two Limits On What You May Take
The carrier says to return water to the stream below the turbine through a tailrace channel, to leave environmental flow always, and never to divert 100% of the water: fish, wildlife and downstream users need it. Its rule is to divert no more than 50 to 70% of dry-season flow. The Installation page says to size the system for 70 to 80% of minimum flow, again for both ecological and legal reasons.
The two ranges overlap only at 70%. They are quoted here as two sources, not averaged, and the corpus does not say which is right. A careful default is the smaller share until you can see what the stream below the intake looks like in a dry spell, and what downstream users say. Between the intake and the outfall the stream carries only what you leave it, and that reach can be hundreds of meters long.
Worked Example: What The Choice Costs In Watts
Dry-season flow 5 L/s, head 10 m, power = Q × H × 7.
| Share diverted | Flow to turbine | Power |
|---|---|---|
| 50% | 2.5 L/s | 175 W |
| 70% | 3.5 L/s | 245 W |
| 80% | 4.0 L/s | 280 W |
The span from 50% to 80% is 105 W. For comparison, 5 × 10 × 7 = 350 W is the whole stream, which the carrier forbids. Hydro power is linear in flow, so every 10% of the dry flow you leave in the stream costs 35 W here (Power Calculation). Agree the share in advance with anyone who uses the water below: see Water Rights.
Tailrace: Let The Water Out Faster Than It Arrives
The tailrace must drain freely. Two things go wrong when it does not:
- Lost head. The installation page says that back-pressure in the tailrace reduces effective head. A tailwater 0.5 m too high on a 10 m site costs 5% of the power.
- A drowned runner. The Crossflow Turbine page requires a tailrace that carries the full flow without backing up, because a crossflow runner runs in air and loses power when submerged.
Method: Size And Place The Channel
Step 1: Fix the flow. Use the design flow, not the dry flow, since the tailrace must pass the most the turbine can take.
Step 2: Choose the cross-section. Flow = area × speed. For 20 L/s through a channel 0.30 m wide and 0.10 m deep, area = 0.03 m² and speed = 0.020 ÷ 0.03 = 0.67 m/s. A deeper, narrower channel is faster at the same flow; a wider one is slower.
Step 3: Set the outlet height. Set the channel floor at the turbine outlet above the highest water level you expect in the stream at the outfall. If the stream floods higher than that, the tailrace backs up and the turbine is submerged.
Step 4: Protect the outfall. Line the outfall with stones so the returning water does not scour the bank or undermine the powerhouse foundation.
Step 5: Return the water to the stream, not to a field. Downstream users and the stream’s life depend on it (Race Construction has lining methods for a longer channel).
Common Mistakes
- Diverting the whole dry flow — the bypassed reach goes dry.
- Fixing the share without telling downstream users.
- A tailrace sized for the dry flow — a flood or full-flow day backs it up.
Safety
Outfall currents
Water leaving a tailrace can be fast and deep against a steep stream bank. Keep the outfall bank edge fenced or marked, and keep children away from the channel while the plant runs.
What’s Next
- Learn next: Generator & Electrical for the machine the water has just driven
- Combine with: Seasonal Variation to see how the dry flow changes the share