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

A field experiment was conducted at Palm Beach, NSW, Australia to investiga te 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 surv eys, multiple dye releases, oblique photographs, and the deployment of vari ous combinations of nine ducted impeller flow meters and five pressure sens ors over a seven-day period in rip and feeder channels. The dataset provide s a unique opportunity to assess and quantify aspects of the accretionary m odel of Wright and Short (Mar. Geol., 56, 1984, 93-118) for intermediate be aches. The morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width, an increase in channel dept h, a reduction in the cross-sectional area available for rip flow, and an i ncrease in morphological relief. Rip flow velocity increased during the obs erved evolution and was also modulated by the tide, experiencing maximums a t low tide and minimums at high tide. Morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transver se bar and rip state. The existence of a dominant feeder in a twin feeder a nd 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 ri p channel was balanced by net deposition in the feeder channels and bar cre sts and qualitative observations suggest that this rip-head bar both receiv es 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 lip velocity increasing as cr oss-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, bu t show that accretionary beach state evolution on intermediate beaches is i ndeed characterised by an increase in rip current velocity, the nature of w hich can be linked to morphologic control. In addition, the rip-head bar is identified as a potentially key component of low-energy rip system evoluti on. (C) 1999 Elsevier Science B.V. Al rights reserved.