Preprints
https://doi.org/10.5194/egusphere-2024-482
https://doi.org/10.5194/egusphere-2024-482
15 Mar 2024
 | 15 Mar 2024
Status: this preprint is open for discussion.

New insights into combined surfzone and estuarine bathing hazards

Christopher Stokes, Timothy Poate, Gerd Masselink, Tim Scott, and Steve Instance

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.

Christopher Stokes, Timothy Poate, Gerd Masselink, Tim Scott, and Steve Instance

Status: open (until 09 May 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
Christopher Stokes, Timothy Poate, Gerd Masselink, Tim Scott, and Steve Instance
Christopher Stokes, Timothy Poate, Gerd Masselink, Tim Scott, and Steve Instance

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Short summary
Currents at beaches with an estuary mouth have rarely been studied before. Using field measurements and computer modelling, we show that surfzone currents can be driven by both estuary flow and rip currents. We show that an estuary mouth beach can have flows reaching 1.5 m/s and have a high likelihood of taking bathers out of the surfzone. The river channels on the beach direct the flows and even though they change position over time, it was possible to predict when peak hazards would occur.