Rapid growth in water requirements makes it necessary to increase the amount of water drawn from rivers. The dams necessary for capturing river water have to be built to resist damage when large floods occur, and an idea of the possible destructive power of floods is given by the front photograph. The need for protection results in thick sill structures fitted with gates, and "upstream" and "downstream" cut-off walls. Sediment transported by rivers settles forming deposits behind dams, where flow velocities decrease. On the other hand, where flow velocities are high through hydraulic machinery (pums and turbines) fed from the dam, it can be necessary to remove even fine sand from the water, and also to remove floating debris. Various hydro-mechanical installations (including gates and screens) are introduced into the flow circuits to deal with sediment and debris problems.Many emperical solutions to definition of very important details complement standard design procedures. Understanding of their use is facilitated by numerous illustrations.
