LONGSHORE AND CROSS-SHORE SUSPENDED SEDIMENT TRANSPORT AT FAR INFRAGRAVITY FREQUENCIES IN A BARRED ENVIRONMENT

Field measurements of near-bed current velocities and sediment concent rations within a barred nearshore environment revealed a large flux co upling at far infragravity frequencies (<0.005 Hz). In the presence of strong longshore currents (time-averaged maximum = 0.92 m s(-1)), up to 30% of the longshore and 65% of the cross-shore suspended sediment transport can be attributed to far infragravity oscillations. While th e former was always directed with the longshore current, the latter wa s variable in direction both spatially and temporally. A number of fea tures of the measured far infragravity energy fit characteristics of a shear wave: (a) a large positive correlation between the long wave en ergy magnitude and the longshore current velocity; (b) the presence of a preferred frequency for the measured long wave; (c) a spatial varia bility in the ratio of cross-shore to longshore velocity at the prefer red frequency; (d) the relatively small surface expression of the long wave compared to the large magnitude velocities recorded at the prefe rred frequency.