6.2.3.2 Canals

From the intake the water is conveyed either directly to the turbine through a pressure pipe or by a canal. In a canal the flow is a function of its cross-sectional profile, its slope, and its roughness. The application of hydraulic theory yields reasonably accurate results to man-made canals where the cross-section is regular in shape and the surface roughness of the construction materials - earth, concrete, steel or wood - is well-documented.

The velocity of water in a canal should be kept above a minimum value to prevent sedimentation and aquatic plant growth, but below a maximum value to prevent erosion, especially in unlined canals. At the end of the canal, just before the entrance to the penstock, there is the forebay. Although it can be designed to provide water storage, a forebay normally provides only enough storage to provide extra volume needed during turbine start-up. It should include a spillway, a purging outlet, a trash-rack and an air vent.

6.2.3.3 Penstocks

From the forebay the water is conveyed to the turbine via a pressure pipe or penstock. Penstocks can be installed over or under the ground, depending on factors such as the nature of the ground itself, the penstock material, ambient temperatures and environmental requirements. A flexible and small diameter plastic penstock for instance, can be laid on the ground, following its outline with a minimum of grade preparation. Otherwise larger penstocks should be buried, provided there is not too much rock excavation.

The sand and gravel surrounding the pipe provide good insulation, and eliminate anchor blocks and expansion joints. Buried penstocks must be carefully painted and wrapped to protect the exterior from corrosion, but further maintenance is minimal. From the environmental point of view, this solution is optimal because the ground can be returned to its original condition and the penstock does not constitute a barrier to the movement of wildlife.

The materials, diameter, wall thickness and type of joint characterize a penstock: