Storm-driven changes in rip channel patterns on an embayed beach

Abstract

This paper introduces a semi-automatic computer algorithm designed to detect rip current channels in video imagery. As a case study, this method is applied to 3.3 years of video data from an embayed beach to demonstrate the link between antecedent surf zone morphology, wave energy and up/down transitions in beach state. An objective measure of rip channel change was developed to define six significant rip reconfiguration events and relate these events to wave energy. Over the period of study no complete resets of the nearshore morphology occurred. The analysis indicates that direct correlation of rip patterns with the instantaneous wave conditions is not a useful way to demonstrate how rips and waves interact. The average wave energy over a period of ten days, combined with storm duration were good indicators of rip channel change, demonstrating that in general, beach morphology responds with a time lag to changes in forcing. Rip channels with a short cross-shore length and narrow alongshore spacing responded faster to changes in wave conditions than rips with a long cross-shore length and wider alongshore spacing. To force changes in the rip morphology, longer rip channels required wave events of higher energy and/or a longer duration. Offshore islands protect the beach under certain wave approach angles, sometimes resulting in a dual-width surf zone, which was narrow at the sheltered end and wide at the exposed end of the beach. The wider surf zone end was characterised by three dominant and persistent rip channels, whereas the narrow surf zone section contained a number of smaller rips which evolved rapidly under wave forcing. Our observations demonstrate the importance of rip channel size in controlling the response time of nearshore morphology.