Coastal defenses are objects and engineering techniques used to defend coasts against erosion and flooding. These may be hard engineering or soft engineering.
Hard engineering techniques involve building rock or concrete structures, most commonly sea walls, rock islands and rock aprons. Piles of large rocks are especially efficient at dissipating wave energy as they are able to move, and water can percolate through them.
Soft engineering most commonly involves encouraging the build up of land between the sea and the object being protected, most commonly a beach, but sometimes a salt marsh or dune system. Beach nourishment takes place at many coastal towns in western countries. Many places encourage sediment accumilation by constructing groynes, out of rock or concrete. These cause sand carried by longshore drift and sea currentss to be deposited.
The alternative to coastal defenses is managed retreat.