cofferdam

Etymology

coffer + dam

noun

1. (engineering) A temporary watertight enclosure used to create a dry foundation for building bridges and other structures over water.
   He stood full six feet in height, with noble shoulders, and a chest like a coffer-dam. 1851, Herman Melville, Moby-Dick
   Downstream there was a small wooden cofferdam and when he woke there were naked children splashing in the pool there and he rose and wrapped his jacket about his waist and walked out along the bank where he could sit and watch them. 1992, Cormac McCarthy, All the Pretty Horses
   Where the viaduct crosses the lake, the piles will be bored directly into the lakebed, using a cofferdam to hold back the water while the pier is built. January 12 2022, “Network News: First pier cast for HS2's Colne Valley Viaduct”, in RAIL, number 948, page 10
2. (nautical) An empty space that acts as a protective barrier between two floors or bulkheads on a ship.