Field observations on the morphodynamic evolution of a low-energy rip current system

Abstract

A field experiment was conducted at Palm Beach, NSW, Australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions. Observations and measurements were obtained from nearshore surveys, multiple dye releases, oblique photographs, and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven-day period in rip and feeder channels. The dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of Wright and Short (Mar. Geol., 56, 1984, 93–118) for intermediate beaches. The morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width, an increase in channel depth, a reduction in the cross-sectional area available for rip flow, and an increase in morphological relief. Rip flow velocity increased during the observed evolution and was also modulated by the tide, experiencing maximums at low tide and minimums at high tide. Morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state. The existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip-head over the seaward slope of the opposite longshore bar. Net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip-head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system. A direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross-sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of Wright and Short. The results of this study are restricted to a subset of commonly occurring beaches, but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity, the nature of which can be linked to morphologic control. In addition, the rip-head bar is identified as a potentially key component of low-energy rip system evolution.