Changes

'''Dams and intake'''<br>

A dam is usually constructed of concrete, blockwork or masonry, preferably founded on rock. Rock, or some other impermeable material, should also form the basin of the impoundment. Twin intake pipes (one in use, one in reserve) are built into the wall of the dam; on the upstream side of the dam, they have strainers or screens; on the downstream side they are fitted with control valves. A scour pipe is also built into the dam, at low level, with a stop valve on the downstream side, and is used periodically to drain the pond and to clear accumulated silt, etc.