Sediment transport in low-energy rip current systems

Sediment transport in rip currents is described based on field observations made at Palm Beach, NSW, Australia in April and June, 1994. Direct measure ments of sediment transport using streamer traps mounted on portable racks were made in three low-energy rip currents. Time-averaged sediment flux was found to increase with increasing rip current velocity (u(r)) and decreasi ng depth suggesting that maximum transport is associated with the fastest f lowing rips at low tide. Sediment grain size exhibited a significant fining upwards trend in the rip channel flow with up to 50% of the sediments tran sported in the bottom 10% of flow. Gross sediment transport rates were foun d to be strongly related to u(3). Examination of the Shields parameter (the ta) indicated that waves are more important than currents in the entrainmen t of sediments, but that currents are responsible for subsequent transport of the sediments. Using a Bagnold-type approach as a conceptual basis, net transport in the feeder channel was found to be inhibited at all times duri ng a tidal cycle, whereas offshore transport in the rip-neck occurred at al l times. The relative roles of waves and currents in rip sediment transport therefore contributes to the infilling of the feeder channels and incision of the rip-neck channel observed during an almost complete cycle of low-en ergy intermediate beach state evolution as described by the model of Wright and Short (1984).