6.2.3 Civil engineering works

Once the site has been selected and the basic layout decided, it is necessary to develop the scheme in detail. The following section describes the function of all the possible basic components and shows how they may be designed. Figure 6.2 shows the esential features of the civil works.

6.2.3.1 Weirs and intakes

Due to the high costs involved in their construction, dams are rarely used in small?scale systems. However, provided that topographical conditions are favourable, the construction of a small gravity dam to store water during periods of low demand can be justified. In diversion schemes, a weir made of mass concrete or stone masonry, with a crest one meter or more above the river bed, will be enough to create an adequate depth of water at the intake of the canal or pipeline.

The function of the intake is to conduct water into the penstock or power canal under controlled conditions. The intake serves as a transition between a stream, which can vary from a trickle to a raging torrent, and a flow of water controlled both in quality and quantity. Its design, based on geological, hydraulic, structural and economic considerations, requires special care to avoid unnecessary maintenance and operational problems that cannot be easily remedied and would have to be tolerated for the life of the project.

The major criterion in the design of an intake is its orientation with respect to the stream, as a means of controlling the quantity and quality of water entering. Rivers tend to deposit sediments in the inner sides of bends. Thus, the intake should not be located on the inside of the bend, to avoid blockage of the entrance by sediment, nor to the outside, because water borne debris can impair its functioning. The ideal diversion site is a relatively straight section, stabilized by rock outcropping, in which the weir could be embedded. Trash-racks should also be placed at the intake entrance to prevent the ingress of floating debris and large stones.