A wave-resolving modeling study of rip current variability, rip hazard, and swimmer escape strategies on an embayed beach with irregular rip channels

Abstract. Drownings due to rip currents are a major threat to beach safety. In this study a high-resolution Boussinesq model with a modified wave-resolving Lagrangian tracking module has been applied to a 2-km-long embayed beach, Dadonghai of Sanya, Hainan Island, with the purpose to study rip current variability, real-time rip hazard identification, and the optimal swimmer escape strategies. Beach stage plays an important role in the occurrence and strength of rip currents. Satellite images of the Dadonghai Beach shows that crescentic bars and shore-connected transverse bars emerge alternatively in the embayed beach, which results in long-term modulation of rip strength periodically according to the modeling. A series of tests are designed and confirms that rip current strength is closely related to wave properties and tidal levels. Spectral analysis of output time series at specific points shows that rip currents fluctuate on the orders of 1 min and 10 min, which reflects the effects of wave-group and VLF (Very Low Frequency) motions. Real-time evaluation of rip hazard is crucial for providing the lifeguards and general public with appropriate information on the occurrence and location of the rip currents, and how to set patrolled area. However, fine-scale numerical modeling using the existing hydrodynamic models are computing-demanding due to a cascade of basin-scale meteorological and wave climate modeling down to the wave-resolving scale. In this study, an attempt of using GPU-accelerated Boussinesq model embedded with the spectral wave model (WAM6) has been made, which enables a faster and more complete description of rip hazard. Lagrangian tracking of virtual swimmers demonstrates that multiple factors contributing to the survival of swimmers caught in the rip currents, include surf-zone bathymetry, rip strength, flow patterns, and swimmer’s position. For weak-to-moderate rip currents and longshore currents, swim onshore consistently seems the most successful strategy across all the scenarios in this study. For swimmers within the inner surf zone, the successful rate is satisfactory by taking strategies of either swim onshore or swim parallel to the beach. Higher surf-zone exit rate along the Dadonghai Beach are not favorable to stay afloat action, which put swimmers at a higher risk of being expelled to deeper water. Pulsation of rip currents in wide rip channels can form swirls or eddies which can also be hazardous to swimmers with weak swimming ability. One of the differences of the present study from the previous works is that the random, wave-resolving modeling was adopted for Lagrangian tracking of virtual swimmers with 1-m resolution. Virtual trajectories yielded by the wave-resolving and wave-averaged velocities are generally consistent with each other. However, using Boussinesq model shows its superiority in studying fine-scale nearshore circulation and its variability, as well as understanding the effects of wave randomness and directional spreading on surf-zone flows.