Flow Kinematics of Low-energy Rip Current Systems

Abstract

Measurements of hydrodynamics and topography were obtained during two separate field experiments at Palm Beach, NSW, Australia in order to assess flow characteristics within rip current systems during accretionary conditions. At an evolutionary time scale (days-weeks), feeder channel flow decreased and rip-neck flow velocity increased during a cycle of intermediate beach state evolution. At shorter time scales (hours), rip current velocity is inversely related to changes in water depth and is clearly modulated by the tide. Spatially, rip current velocity increases progressively both longshore from the feeders to the rip-neck and offshore within the rip-neck itself. Flow velocity is not constant across the rip channel and increases towards the middle and deeper section of the rip channel. Vertical velocity profiles within the rip-neck and feeder channels show an initial increase in flow magnitude above the bed, with maxima towards the middle of the flow and then a subsequent decrease towards the surface. Evidence of pulsatory rip flow behaviour was found at infragravity frequencies in both rip-feeder and rip-neck environments. Infragravity energy was dominant in the feeder channels during the observed beach state evolution and increased in the rip-neck as the system evolved towards transverse bar and low tide terrace and rip beach states. There is evidence to suggest that topography may exert direct controls on rip flow. The results of this study provide both new quantitative information on the flow kinematics of rip currents and confirm findings of previous studies.