Risk assessments of rip current hazards on typical rhythmic beaches

Rip currents (offshore flows) are common hazards on the open coast with topography anomalies. The present research investigates the rip current risks on the typical rhythmic beaches, such as transverse-bar beaches, rip-channel beaches, and crescent-bar beaches. A linear beach-instability analysis model is derived to generate these rhythmic beaches from the situ-based initial bathymetry, and the simplified and GPU-accelerated (graphic processing unit) version of the fully nonlinear mild-slope equation model (NMSE-GPU model) is adopted to simulate the Lagrangian rip current field and display the surface flow behavior features. Considering the effect factors of rip current intensity, range, and circulation regime, comprehensively, a new method of assessing quantitatively the rip hazard index RHI (i.e., risk index of the rip current hazard) is proposed. The “recirculation flow” pattern is found dominant for these rhythmic beaches by introducing the hourly drifter retention rate to present the circulation regime. The rip hazard index RHI is proven to increase with increasing topography perturbation amplitudes, but decrease with increasing wave incident angles and perturbation wavelengths. The sensibility of RHI to the individual tests was examined to validate the repeatability and applicability of the present assessment method. Furthermore, considering different body states of drowning swimmers, the effects of applying Eulerian velocities (underlying flows) on the rip hazard index assessment were also investigated.