Wave dissipation patterns as an indicator of rip current hazard

Abstract

Rip currents (rips) are hazardous offshore-directed flows in the surfzone of beaches worldwide. Rips are a major hazard for recreational beach use and are the dominant cause of beach rescues and drownings. It is therefore important to understand what conditions make a rip most hazardous, in order for beach safety practitioners to mitigate the risk. The aim of this thesis was to determine how patterns of wave breaking influence rip channel hazard on beaches. In order to quantify wave breaking, video imagery from three hydrodynamically diverse case studies was used, and validated at two sites against Lagrangian GPS drifter data. This thesis firrst developed pre-processing techniques for video imagery which then subsequently improved the reliability of rip channel detection. Hitherto, the noise inherent in such signals has made automated detection of rip currents problematic. Wave breaking patterns could then be identified with the novel application of synoptic typing methods to the imagery, resulting in a classification scheme for rip currents based on wave breaking. Two dominant types were identified: (1) open channels, whereby the rip channel has free connectivity to the region beyond the surfzone; and (2) closed channels, where wave breaking across the seaward extent of the channel effectively closes this connectivity to the offshore region. Investigation of Lagrangian data for each of the prevailing states shows that under open conditions, drifters were highly likely to be transported beyond the edge of the surfzone by the rip current, with exit rates reaching 100 % at times. Under closed conditions, drifters were more likely to be retained in the surfzone, with typical exit rates between 0 and 35 %. A rip current that exits the surfzone is more hazardous to bathers, and therefore, this thesis subsequently investigated the prevalence of open and closed rip channels in records of rip rescue events. Over two sites for which data were available, upwards of two thirds of major rip rescues occurred when the channel could be classified by this new method as open. Furthermore, the majority of surfer and bodyboarder rescues occurred under open conditions. Despite their over-representation in the rip rescue record, the overall prevalence of open channels over a year is only around 40 %. Normalising the number of rescues in open rips by their occurrence shows open rips to be twice as hazardous as closed channels. This new approach provides a quick and inexpensive means to assess high risk surf conditions at rip beaches worldwide, with the deployment of only a small (often mobile) imaging system.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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