Physics-based forecast modelling of rip-current and shore-break wave hazards

Abstract. Sandy beaches are highly attractive but also potentially dangerous environments for those entering the water as they can expose to physical hazards in the surf zone. The most severe and widespread natural hazards on beaches are rip currents and shore-break waves, which form under different wave, tide and morphological conditions. This paper introduces two new, simple, physics-based rip-current and shore-break wave hazard forecast models. These models, which depend on a limited number of free parameters, allow to compute the time evolution of the rip current flow speed V and shore-break wave energy E_sb. These models are applied to a high-energy meso- macro-tidal beach, La Lette Blanche, in southwest France where intense rip currents and shore-break wave hazards co-exist. Hourly lifeguard-perceived hazards collected during the patrolling hours (from 11AM to 7PM) from July 1 to August, 2022 are used to calibrate the two models. This data is also used to transform V and E_sb into 5-level scale from 0 (no hazard) to 4 (hazard maximized). The model accurately predicts rip-current and shore-break wave hazard levels, including their modulation by tide elevation and incident wave conditions, opening new perspectives to forecast multiple surf-zone hazards on sandy beaches. The approach presented here only requires a limited number of basic beach morphology metrics, and allows the prediction of surf-zone hazards on beaches where wave forecast is available.