The return flow of water downslope to the sea after the swash has moved upshore. Steep waves and long waves are linked with stronger backwash, and flatter beach gradients; see the great C. King (1959), and Hardisty et al. (1984) Marine Geol. 61, 1. When the path taken by the backwash differs from the path of the swash, longshore drift occurs. See Bird et al. (2007) Geogr. J. 173, 2, on backwash erosion.