Backwash is the flow of water down a beach under the influence of gravity after the breaking of a wave and its associated swash. As this water returns to the breaker zone it carries beach material with it. Steep waves, which break almost vertically on to a beach, have an extremely powerful backwash and move much material out to sea. Backwash contributes to longshore drift. Research Backwash
Longshore drift is the gradual movement of beach materials such as sand and shingle along a shore. This process is the result of waves breaking at an angle on to a beach: driven by obliquely breaking waves, a thin layer of turbulent water known as the swash moves diagonally up the beach, carrying sand and shingle landward and depositing them when it runs out of energy. Some of this material is then carried back down into the breaker zone by the returning backwash. The net effect is that material is moved in small steps sideways along the beach, a process which may be aided by longshore currents.
Longshore drift may result in extensive beach erosion, the material removed being ultimately redeposited further down the coast. In some seaside areas attempts have been made to stop such erosion by erecting groynes, man-made barriers which extend down the beach and into the sea, and behind which sand and shingle moving along the shoreline are trapped. Research Longshore Drift
Swash is the movement of a thin layer of turbulent, foaming water up a beach following the breaking of a wave. As this water rushes along it carries sand and shell fragments with it, depositing them when it runs out of energy. Flat waves, which have a very strong swash, often move a lot of material on to a beach in this way. Some of the swash soaks into the beach, while the rest returns to the breaker zone as backwash; the edge of a zone of swash action is often marked by a line of seaweed and driftwood. Research Swash
Abbreviations
Research Backwash
