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Cofferdams are temporary enclosures built within or across a body of water, allowing the impounded area to be pumped dry. This creates a dry working environment for construction or repair work below the normal water level. They are commonly used in the construction of bridges, dams, piers, and other offshore structures, as well as for repairing existing structures in water.

The design of a cofferdam depends heavily on factors such as water depth, flow rate, soil conditions, and the type of construction to be undertaken. Common types include earth-fill cofferdams, which are essentially temporary dikes made of soil, and sheet pile cofferdams, which use interlocking steel sheets driven into the ground to form a watertight barrier. Other types include rock-fill and cellular cofferdams.

Construction typically involves driving piles or sheet piles into the riverbed or seabed to create the enclosure. Once the cofferdam is in place, pumps are used to remove the water. The area is then dewatered, allowing workers access to the dry space for construction or repair. After the work is completed, the cofferdam is removed, and the water is allowed to refill the area.

Cofferdams must be designed to withstand the hydrostatic pressure of the surrounding water and any potential environmental forces like currents or waves. Careful consideration of structural integrity and dewatering capacity is crucial for the success and safety of the project.