What 'dam' is the term used for a barrier built to keep away water from an area that is normally submerged?

Answer 1

you are probably referring to a cofferdam

Cofferdams

A cofferdam during the construction of locks at the Montgomery Point Lock and Dam.A cofferdam is a (usually temporary) barrier constructed to exclude water from an area that is normally submerged. Made commonly of wood, concrete or steel sheet piling, cofferdams are used to allow construction on the foundation of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam may be demolished or removed. See also causeway and retaining wall.

Answer 2

Dam, levy, like in N. O.