Beach erosion is a reduction in the amount of sand a particular beach has. On a global level, sea level rise causes beach erosion. Beaches get sand from both the ocean and the land.
Larger waves move sand from the coastal sand dunes off into the ocean.
This raises the seafloor, flattens the overall profile of the beach, and, therefore, causes waves to break further offshore. This, in turn, minimizes the waves' impact on coastal lands. Beaches recover from these seasonal shifts when the waves move the sand back onto the beach and the winds blow the deposited sand into dunes. These dunes will store the land-based sand until the next large wave event. Rather than pulling sand from a landward supply in order to promote waves breaking further off-shore during the seasonal high wave period, the seawall or revetment prevents this natural phenomena from occurring.