Sediment flux in a rip channel on a barred intermediate beach under low wave energy

摘要

The classic model of water and sediment flux in barred surf zones is a net flux landward across a nearshore bar, alongshore in a feeder channel, and offshore in narrow jet-like flows in a rip neck cut through the bar; this circulation is frequently modulated by the tide, even under micro-tidal conditions. Water levels, waves, currents and suspended sediment transport (SST) were recorded at elevations of z = 0.13, 0.26 and 0.39 m in a rip neck on an intermediate bar-rip beach at a micro-tidal site, Bennett's Beach, NSW, Australia. Measurements revealed SST was driven by quasi-steady rip currents and by gravity and infragravity waves. The balance between these components determined the magnitude and direction of the overall net SST. Tentative conclusions are that: (i) the direction of the overall net SST rate in the rip neck was dominated, as expected, by offshore-directed mean cross-shore currents, especially around high tide; at this time the SST rates due to gravity and infragravity waves were relatively small and somewhat variable in direction. (ii) as the tide fell, relatively large SST rates were directed onshore by shoaling gravity waves propagating through the neck, which opposed and even exceeded the rip current transport. At mid-tide, the transport by infragravity waves complemented the gravity waves such that the overall net flux of suspended sediment was directed onshore into the rip cell at the two lowest elevations. (iii) whenever the SST by the rip current and by shoaling waves was close to a balance, it was the infragravity waves that controlled the rate and direction of the overall net suspended sediment flux.