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 sheetpiling, usually steel in modern works, into the bed to form a watertight fence. The vertical piles are held in place by horizontal framing members that are constructed of heavy timber, steel, or a combination of the two.

The sides of a cofferdam must be sturdy and well-braced to withstand the shock of being hit by a heavy dredging bucket; they must also be able to withstand the horizontal forces from the surrounding body of water when the water inside is pumped from the cofferdam. The cofferdam goes back at least to Roman engineers, who built a number of these enclosures to found the piers of their stone-arch bridges and aqueducts. Compare caisson.