backwash

The return flow of water down a beach after a wave has broken and run up the shore — the seaward pull that follows each swash. Backwash carries sand, shells, and sediment back toward the sea, sorting material by size and weight. The balance between swash and backwash determines whether a beach is building or eroding. The word also describes the turbulent water behind a moving boat, and the secondary consequences of any disruptive event.