Cofferdam

A cofferdam is a temporary, watertight enclosure, built from sheet piling, earthen embankments or other barrier material and then pumped dry, that allows construction work such as a bridge pier, dam foundation or ship repair to proceed in an area that would otherwise be underwater or waterlogged. Cofferdams have been used since antiquity, with early forms recorded in ancient river and harbor construction, and they remain a standard modern technique wherever foundation work must be carried out below a river, lake or sea level. Once the structure being built inside it is complete, a cofferdam is typically removed or flooded, its purpose being purely to hold water back during construction rather than to remain as a permanent structure itself.