The Cross‐Shore Component in the Vertical Structure of Wave‐Induced Currents and Resulting Offshore Transport. Issue 10 (18th October 2021) - Lu, Jing
Abstract: 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. Plain Language Summary: Rather than examining longshore flow, which has been extensively investigated, this study focused on cross‐shore flow due to waves. It was found that in addition to the longshore flow (vertically mean), a wave‐induced offshore flow component extended a long‐distance offshore. Moreover, the vertical and horizontal gyres (eddies) generated by waves greatly contributed to sand loss from a beach. Specifically, a vertical gyre eroded sediment and drove its offshore transport in the lower layer of water. Furthermore, it was determined that the vertical and horizontal gyres were both affected by highly variable topographic slopes, and that the fast‐rotating rim of a horizontal gyre ensured that suspended sediment was retained in the center of the gyre. The results suggest that a breakwater extending from the eastern headland could prevent the erosion of sediment in the bay, as simulations showed that the pattern of wave‐induced currents was mainly influenced by the length of barriers (headlands) and the bottom slope. Key Points: In addition to vertically mean longshore flow, there was a prevalent wave‐induced offshore component of flow Wave‐induced vertical gyres were found to erode sediment and drive its offshore transport Numerical experiments showed that a breakwater at the eastern headland of the bay could prevent the erosion of sediment