Bar formation due to wave groups and associated long waves

Field observations reveal that waves propagating onto a beach from deeper water have a grouped structure. Associated with this pattern of alternating high and low waves is a long wave forced by radiation stress variations due to the changing short wave height. As the wave groups propagate onshore, the relationship between the variation in the short wave height and the long wave motion is complicated by breaking of the short waves, reflection of the long wave and the generation of a free long wave by time variation of the breakpoint position. Nevertheless, there may still be a fixed phase relation between the short-wave envelope and the long wave motion at any particular location, resulting in a net transport of sediment over the timescale of the short wave groups. This transport arises from sediment grains mobilised by short waves and moved by the long wave motion. Model predictions of wave group propagation onto a beach and the associated long wave motions are presented which suggest that sediment transport due to short-wave/long-wave coupling may result in the formation of a bar within the inner surf-zone and the possibility of additional bars further offshore, outside the surf-zone. This mechanism for bar formation may provide a new framework for understanding bar morphodynamics.