BAR FORMATION DUE TO WAVE GROUPS AND ASSOCIATED LONG WAVES

Field observations reveal that waves propagating onto a beach from dee per water have a grouped structure. Associated with this pattern of al ternating 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 th e short waves, reflection of the long wave and the generation of a fre e long wave by time variation of the breakpoint position. Nevertheless , there may still be a fixed phase relation between the short-wave env elope and the long wave motion at any particular location, resulting i n a net transport of sediment over the timescale of the short wave gro ups. This transport arises from sediment grains mobilised by short wav es and moved by the long wave motion. Model predictions of wave group propagation onto a beach and the associated long wave motions are pres ented which suggest that sediment transport due to short-wave/long-wav e 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 fram ework for understanding bar morphodynamics.