Answer is: repetitive action of swash and backwash transports sediment sideways along the coast.
Longshore drift is a geological process of the transportation of sediments.
Wind squeezes water along the coast and generates a water current which moves parallel to the coast.
Longshore drift plays a large role in the evolution of a shoreline.
There are formulations (Bijker formula, The Van Rijn formula ...) that explain intensity of longshore drift.