Sediment Transport in Low-Energy Rip Current Systems

Abstract

Sediment transport in rip currents is described based on field observations made at Palm Beach NSW Australia in April and June, 1994. Direct measurements 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 decreasing depth suggesting that maximum transport is associated with the fastest flowing 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 transported in the bottom 10% of flow. Gross sediment transport rates were found to be strongly related to u³. Examination of the Shields parameter (θ) indicated that waves are more important than currents in the entrainment 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 during a tidal cycle, whereas offshore transport in the rip-neck occurred at all 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-energy intermediate beach state evolution as described by the model of Wright and Short (1984).