cofferdam, temporary barrier for excluding water from an area that is normally submerged. Made commonly of wood, steel, or concrete sheet piling (see pile), cofferdams are used in constructing the foundations of dams, bridges, and similar subaqueous structures and for temporary drydocks. If double sheeting is utilized, the space between the sheets is usually filled with clay and gravel. When great strain or pressure is likely to be encountered, as in deep water, the pneumatic caisson is preferred to the cofferdam.

Watertight enclosure from which water is pumped to expose the bed of a body of water in order to permit the construction of a pier or other hydraulic work. Cofferdams are made by driving metal sheetpiling (a series of thin, interlocking panels) into the bed to form a watertight fence. Roman engineers used cofferdams to found the piers of their arch bridges and aqueducts. Seealso caisson.

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