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1.4.3 Cofferdams

A cofferdam is a temporary barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, steel, or concrete sheet piling, cofferdams are used to allow construction on the foundations of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam is demolished.

2 Spillways

A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have gates designed to control the flow through the spillway.

A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse-plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood.

It was the inadequate design of the spillway that caused the overtopping of a dam that caused the infamous Johnstown Flood.

3 Other considerations

The best place for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The most desirable sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

Significant other engineering considerations when building a dam include


Kenneth E. Boulding's poem The Ballad of Ecological Awareness (see External links) discusses the social and ecological impact of dam-building, beginning: "The cost of building dams is always underestimated" and concluding: "...cost-benefit analysis is nearly always sure/To justify the building of a solid concrete fact/While the Ecologic Truth is left behind in the Abstract."

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine monitoring of seepage from drains in, and around, larger dams is necessary to anticipate any problems and permit remedial action to be taken before structural failure occurs. Most dams incorporate mechanisms to permit the resevoir to be lowered or even drained in the event of such problems. Another solution can be rock grouting - pumping cement slurry into weak fractured rock under pressure.

4 Examples of dams

4.1 Failed dams



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