The Cross-Shore Component in the Vertical Structure of Wave-Induced Currents and Resulting Offshore Transport

A comprehensive field survey was conducted, and a numerical model based on the Princeton Ocean Model was established to investigate the vertical structure of current induced by surface waves and identify its effect on sand loss from a beach. Previous studies have examined wave-induced longshore transport, whereas this study focused on cross-shore transport caused by waves. The introduction of vertically dependent radiation stress revealed that wave-induced vertical gyres existed with onshore and offshore flows in the upper and lower layers, respectively, besides the well-known wave-induced longshore flow. It was found that the vertical gyres were caused by the onshore wave force (a gradient of radiation stress) in the upper layer and the offshore wave force usually existed in the lower layer, as well as the horizontal imbalance of local wave force. The locations of wave-induced vertical gyres were found to correspond to the location of sediment erosion, and thus the lower part of the vertical gyres drove the offshore transport of eroded sediment. Moreover, the vertical gyres were located in areas with a highly variable topographic gradient (e.g., at a bar), and their driven offshore currents (undertow/return flow) extended far beyond the surf zone. The topographic slope had a profound effect on the vertical structure. Numerical experiments showed that an extended breakwater from the eastern headland of the bay could prevent sediment from being eroded. Furthermore, the shear front at the rim of a horizontal gyre could retain suspended sediment in the center of the gyre.

Automated summary

This study investigated the vertical structure of current induced by surface waves using a numerical model and found wave-induced vertical gyres in the upper and lower layers which drive offshore sediment transport. The study highlights the importance of understanding wave-induced cross-shore transport for beach erosion prevention and sediment management.