Determining How Offshore Shoals Control Rip Current Generation in an Embayed Beach

Wu, G.; Liang, B.; Li H.; Lee, D.-Y., and Guo, W., 2017. Determining how offshore shoals control rip current generation in an embayed beach. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 224–228. Coconut Creek (Florida), ISSN 0749-0208.In this study, the generation of rip currents at Haeundae Beach, Korea, is investigated using an advanced, non-hydrostatic wave-resolving model SWASH. It is found that the presence of offshore shoals leads to strong wave refraction, creating significant alongshore wave height variations and mega-scale rip current systems. The modeled rip currents occurring locations correspond well with the low wave height areas. The rip currents persist even if the surf zone rip-channel topography is smoothed, with very similar spatial patterns and occurring locations compared with those generated under the original bathymetry. We argue that the mega-scale rip currents generated at Haeundae Beach are controlled by the offshore shoals, while the effect of the rip-channel morphology is minor. An idealized embayed beach with an offshore Gaussian-shape shoal is further designed to study the shoal characteristics on the resulting rip current patterns. Primary numerical results show that wave focusing and spreading happens in the presence of the shoal, leading to the formation of rip currents in the wave shadow area behind the shoal. However, a shallow shoal closer to the shoreline favors wave breaking on top of it and strong onshore-directed wave-induced currents, instead of offshore-directed rip currents.