An Unstructured Wave-current Coupled Model And Its Application In The Nearshore Mass Transport

Mass transport in the nearshore involves two important issues of the marine environment and sediment movement, and its range of up to tens or even hundreds of kilometers. The core dynamics of mass transport in nearshore is water flowing, which includes the contribution of tidal current, wave-induced current and wind-induced flow. To build a hydrodynamic platform which is suitable for calculating mass transport in large-scale region has become the focus of attention in mass transport research.In the previous study, wind-induced flow could be considered in tidal current model by adding boundary conditions, and there are two main methods to calculate wave-induced current field:one is wave-current unified model, such as the wave-induced model based on Boussinesq model; another is by adding the radiation stresses and other factors which is computed by the wave model into the current field model, also called coupling model. The unified model is only applicable to simulate wave-induced current in small-scale region; the coupled model can consider wave-current interaction, and also adapt to calculate the current field in large-scale area, which is the realistic way to study large-scale wave-current coupling.The basic equation of large-scale wave model is the wave action balance equation, and the wave diffraction effect can be simulated by combining with the eikonal equation. In addition, the wave and current parameters in numerical simulation could interact directly by using the same unstructured meshes, and the finite volume method could ensure the flux conservation of discrete in unstructured grid. There is a lack of relevant research of wave model, which is based on the wave action balance equation and eikonal equation by using finite volume method under unstructured meshes. And the related research of wave-induced current by coupling the above wave model and current model was also lacking.In this paper, a large-scale wave-current coupled module with wave-induced current was built. Some researches and application in actual areas were conducted combined with the mass transport equation and particle tracking methods.(1) Based on the modified wave action balance equation and the eikonal equation, a new large-scale spectral wave model was built. The model could account for diffraction in most situations, which would improve the accuracy of the wave propagation. The spectral action balance equation had been split into four equations for numerical simulation, and the numerical implementation in space used the finite-volume schemes on unstructured grids. The numerical model considered the effect of non-uniform current to wave propagation, and the discrete of wave parameters were based on the crossover control bodies, which provided great flexibility for coupling the wave with current field. Many studies were simulated to demonstrate the applicability and accuracy of present model.(2) Coupled the unstructured wave model with three-dimensional current model, the wave-current coupled model was constructed. The wave and current were implemented on the same unstructured meshes in the different control bodies, which could capture the coupling action accurately and improved the computational efficiency. In the coupling process, the hydrodynamic model was affected by the three-dimensional radiation stresses and the wave turbulence mixing effects due to the wave breaking and propagation, also the current fields and surface elevation computed by the current model were provided to the wave module. Additionally, the boundary conditions of the hydrodynamic module needed to be modified to account for the wave-current interaction. The coupled model was evaluated in studying the different wave-induced current fields, the vertical structure of the wave-induced current, the wave-induced current in actual region and the wave-current interaction. Many examples were calculated to verify the coupled model.(3) Combined the wave-curent coupled model with the mass transport equation, the mass transport model under the wave-current interaction was built. Based on the above model, the change of dispersion coefficient in wave propagation and the transport of pollutants by wave-induced current were simulated, and the numerical results were contrasted with analytical values and the experimental data. The research showed the effect of wave on pollutant diffusion was as the same order of magnitude as the current field, and the wave-induced current was one of the main factors of pollutant transport in coastal area.(4) Combined with Lagrangian particle tracking method and conservative pollutant transport model, the coupled model was used to study the mass transport in actual sea, including Dalian Bay and Hupo Bay. In Dalian Bay the simulated contents included the wave-current coupled hydrodynamic, the tidal-induced residual current, the conservative pollutant transport, the trajectories of characteristic particles, and the influence of different artificial islands to conservative pollutant transport. The research showed that the wave-induced current had little effect on the flow field in the gulf, and the tidal current was the main driving force of pollutant migration in the Bay. The direction of conservative pollutant migration was the same as the residual current direction; the self-purification capacity of conservative pollutants in estuary of gulf was better than that in bottom, and it in north was better than that in south; the pollutant in the sub bay was difficult to diffuse out. The influence of different artificial islands to the mass transport in Dalian Bay would vary in different regions.In Hupo Bay, the conservative pollutant transport and the trajectories of characteristic particles before and after the project of a new waterway were the focus of research. The results showed that a tidally-induced vortex was exist behind the bay mouth, the wave had little impact in the bay, the wave-induced current would make the flow field change outside the gulf in nearshore, but had little impact on mass transport; the self-purification capacity of conservative pollutants in the vortex was better than that in the middle and bottom of the bay, the new waterway would increase the tidal volume and strengthen the water self-purification capacity of the bay, especially in the bottom.