n.
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. See also caisson .