An effort was made to couple FVCOM (a three-dimensional (3D), unstructured grid, Finite Volume Coastal Ocean Model) and FVCOM-SWAVE (an unstructured grid, finite-volume surface wave model) for the study of nearshore ocean processes such as tides, circulation, storm surge, waves, sediment transport, and morphological evolution. The coupling between FVCOM and FVCOM-SWAVE was achieved through incorporating 3D radiation stress, wave-current-sediment-related bottom boundary layer, sea surface stress parameterizations, and morphology process. FVCOM also includes a 3D sediment transport module. With accurate fitting of irregular coastlines, the model provides a unique tool to study sediment dynamics in coastal ocean, estuaries, and wetlands where local geometries are characterized by inlets, islands, and intertidal marsh zones. The model was validated by two standard benchmark tests: 1) spectral waves approaching a mild sloping beach and 2) morphological changes of seabed in an idealized tidal inlet. In Test 1, model results were compared with both analytical solutions and laboratory experiments. A further comparison was also made with the structured grid Regional Ocean Model System (ROMS), which provides an insight into the performance of the two models with the same open boundary forcing.
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 waveinduced 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.LU ET AL.
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