Daisuke Yoshikawa

Effects of Beach Topography Variation on the Generation of Nearshore and Offshore Currents near the Shoreline

Tokuzou Hosoyamada

Detached currents are strong offshore currents generated by waves returning from the shore, with velocities exceeding 1 m/s and sometimes reaching 2 m/s during large waves. These currents surpass human swimming ability and are difficult to detect due to their complexity and the rapid increase in wave intensity in localized areas, making them a major cause of water accidents. Despite the absence of periodic variation in the longshore direction, offshore-facing high-speed currents are observed alongside regular cusp topography, causing sand drift and significant changes in near-shore topography. However, the detailed dynamics of offshore currents are difficult to observe, and their generation and growth processes remain poorly understood. As a result, basic research on the mechanisms of rip current generation is limited.
This study aims to visualize the relationship between detached currents and regular cell structures, such as cusps, in the littoral direction through numerical calculations. Since controlling these phenomena experimentally is extremely difficult, a numerical method was used to sequentially calculate the time-averaged fields of offshore and beach currents while minimizing the influence of shoreward variations (topography, structures, and wave direction). The results confirmed that water level reduction near the mean water surface berm generates offshore-directed currents. Dye input calculations revealed that wave-like cell structures form in the longshore direction, and the parameter dependence of these structures was consistent with previous studies. Analysis of offshore velocity vectors clarified the relationship between self-organized cell structures and rip currents, demonstrating that rip currents can form even on planar topography with uniform gradients.
However, since this study did not incorporate a wave-breaking model and was conducted under idealized conditions, further research is needed to apply these findings to real coastal environments. Future work should compare the results with empirical equations and consider more realistic topography and wave conditions to enhance the understanding of rip current generation mechanisms.