When there is a large distance between the water intake and the powerhouse, a high-level conveyance is usually used to bring the water closer to the powerhouse. This takes the form of a power canal or low-pressure headrace tunnel.

If the topography is fairly flat and at a suitable elevation, a power canal is usually more economical to construct. The intake to the canal normally comprises a trash rack and head gate, and does not need to be submerged, because any entrained air will escape at the free water surface of the canal. The downstream end of the power canal normally terminates in a head pond. This is equipped with an intake to the penstocks (the high-pressure water conveyances) and an emergency overflow weir, which discharges excess water when the head pond level rises too high due to the transient surges that occur in the canal each time the flow rate changes.

If there is high topography between the intake and the powerhouse, a headrace tunnel is used. The tunnel is normally paved with concrete so that maintenance and inspection vehicles can drive through when the tunnel is dewatered. The tunnel may be concrete-lined if the rock is poor, and steel lined in any places where the internal water pressure could hydro-fracture (burst) the surrounding rock. Otherwise, headrace tunnels are normally unlined, and the rock is stabilized with rock bolts and drains, and shotcrete reinforced with fibres or welded wire mesh. Most headrace tunnels flow full (i.e. there is no free water surface in the tunnel).

Transient surge pressures also occur in a headrace tunnel when the flow rate changes, so a surge shaft or surge tank is provided at the downstream end of the tunnel.

Published by hydrogray

Hydropower specialist

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