Cofferdam

A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained.

Cofferdams are commonly used for construction or repair of permanent dams, oil platforms, bridge piers, etc., built within water.

These cofferdams are usually welded steel structures, with components consisting of sheet piles, wales, and cross braces.

These cofferdams are typically a conventional embankment dam of both earth- and rock-fill, but concrete or some sheet piling also may be used.

When complete, the cofferdam is removed and a similar one is created on the opposite side of the river for the construction of the dam's other half.

[citation needed] A cofferdam over 1 mile long was built to permit the construction of the Livingstone Channel in the Detroit River.

[citation needed] When all or part of the main deck of a sunken ship is submerged, flooded spaces cannot be dewatered until all openings are sealed or the effective freeboard is extended above the high water level.

One method of doing this is to build a temporary watertight extension of the entire hull of the ship, or the space to be dewatered, to the surface.

[6]: Ch.10 Complete cofferdams cover most or all of the sunken vessel and are equivalent to extensions of the ship's sides to above the water surface.

[6]: Ch.10 Small cofferdams are used for pumping or to allow salvors access to spaces that are covered by water at some stage of the tide.

[6]: Ch.10 Diving work on cofferdams often involves clearing obstructions, fitting, and fastening, including underwater welding, and where necessary, caulking, bracing and shoring the adjacent structure.

Open top cofferdams allow surface access to the work area below the waterline, and are at atmospheric pressure.