sea wall 
noun
A wall or embankment erected to prevent the sea encroaching on or eroding an area of land.A spokesman for the agency said that the sea wall protects a large area of low-lying land against the risk of flooding....
- To prevent future floods a sea wall was constructed and the level of the city elevated.
- Nowadays, though, the sea wall that was erected to keep the waves at bay is itself falling into the water.