New insights into combined surfzone and estuarine bathing hazards

Abstract. Rip currents are the single largest cause of beach safety incidents globally, but where an estuary mouth intersects a beach, additional flows are created that can exceed the speed of a typical rip current, significantly increasing the hazard level for bathers. However, there is a paucity of observations of surfzone currents at estuary mouth beaches, and our understanding and ability to predict how the bathing hazard varies under different wave and tide conditions is therefore limited. Using field observations and process-based XBeach modelling, we demonstrate how surfzone currents can be driven by combinations of estuary discharge and wave-driven rip currents under various combinations of wave and tide forcing. While previous studies have demonstrated the high hazard that rip currents pose, typically during lower stages of the tide, here we demonstrate that an estuary mouth beach can exhibit flows reaching 1.5 m/s – up to 50 % stronger than typical rip current flows – with a high proportion (>60 %) of simulated bathers exiting the surfzone during the upper half of the tidal cycle. The three dimensional ebb shoal delta was found to strongly control surfzone currents by providing a conduit for estuary flows and acting as a nearshore bar system to generate wave-driven ‘river channel rips’. Despite significant spatiotemporal variability in the position of the river channels on the beach face, it was found to be possible to hindcast the timing and severity of past bathing incidents from model simulations, providing a means to forewarn bathers of hazardous flows.