Sea walls, and sloping walls called revetments, stop erosion of coastal lands but they refocus the erosion onto the beach. This causes beach erosion and, eventually, beach loss. Beaches need sand Healthy Beaches have abundant sand. When sand supply is restricted beaches will erode. Beaches get sand from both the ocean and the land.
Ocean currents can move sand along the coast to build beaches.
Dunes and other land-ward sand deposits give sand to a beach in response to the forces of wind and waves. To absorb the additional wave energy, beaches and dunes give up sand to the waves which carry it seaward and drop it on the bottom. This raises the seafloor and flattens the overall profile of the beach. Waves then shoal and break further offshore, minimizing their erosive effects. This typically happens in response to seasonal shifts in wave energy.