How Dams Work
A typical dam is a wall of solid material built across a river to block the flow of the river thus storing water in the lake that will form upstream of the dam as water continues to flow from the river upstream of the dam.
The main purpose of most dams is to create a permanent reservoir of water for use at a later time. The dam must be watertight (ie impermeable or impervious to water) so that water does not leak out of the dam and escape downstream. An essential part of a dam is therefore the "impermeable membrane", ie the watertight part of the dam that prevents water leaking out. As we shall see later, it is not necessary that the entire dam wall be watertight. The natural earth or rock on which the dam is built (ie the dam foundation) must also be watertight as must the river valley in which the storage reservoir forms. If these natural areas (dam foundation and storage area) are not watertight then water could leak out of the reservoir even if the dam itself is watertight.
As well as being watertight a dam must also be stable ie the dam wall must have sufficient strength to firstly, stand permanently under its own weight especially when at least part of the dam wall is saturated with water and secondly, resist the water pressure in the lake upstream of the dam. This water pressure exerts a force on the dam wall tending to push it downstream. The higher the dam, the greater the depth of water stored behind the dam and the greater the water pressure on the dam wall. The dam must also have sufficient strength to resist other forces to which it may be subjected from time to time eg shaking from earthquakes. The threat that earthquakes pose to dams varies widely depending on the region of the world in which the dam is located.
A dam must have some way of releasing water in controlled amounts as it is needed ie an outlet valve of some type. Depending on the purpose of the dam the water may be released into a pipeline to supply a city with water, or into a hydro-electric power station to generate electricity or the water may simply be released into the river bed downstream of the dam and allowed to flow naturally down the river, eventually to be pumped out and used for irrigation of crops further downstream. The outlet valve must be connected via a pipe or tunnel to some type of intake structure where the water is actually drawn from the storage reservoir.
When the river on which the dam has been built floods a very large volume of flood water will flow into the storage reservoir. Usually this is very, very much more water than can be released through the outlet valve. A dam must have some means whereby these large volumes of flood water can flow around the dam without causing damage to the dam itself; ie a spillway which, in most cases, is an open cut channel large enough to carry the flood water around the dam. If the dam is built of concrete the spillway may form part of the dam wall itself. However, if the dam is built of earth and/or rock fill (ie soil and broken rock) the spillway must be a separate structure because flood waters cannot be allowed to flow over the top of a fill (or embankment) dam which would be quickly washed away by the flood water if this was to happen.
A large dam project may involve many types of construction apart from building the dam wall itself eg tunnelling for diversion or outlet works; road building to replace roads flooded by the reservoir; quarrying to obtain rock fill and other construction materials; excavation of open cuts for the spillway, access roads and road deviations.
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