Fully coupled morphological modelling under the combined action of waves and currents

The non-linear interactions of tides, waves and sediment transport in shallow coastal waters require coupled modelling of waves and currents, as well as the consideration of feedbacks between flow, sediment transport and morphology. However, most of the previous work about the wave-current morphological modelling lacks full consideration of the feedbacks of bed deformation on flow or/and assumes instant sediment adaption, which may be subjected to uncertainties. To bridge this gap, a two-dimensional depth-averaged (2DH) coastal model that couples an advanced fully coupled flow-morphological model and the phase-averaged SWAN model by a two-way coupling method has been developed. The feedback impacts of bed deformation on the flow, and the spatial and temporal scales required for sediment adaptation to the capacity regime are fully considered in the fully coupled flow-morphological model. For efficient and accurate modelling, a hybrid local time step/global maximum time step (LTS/GMaTS) method and the Open Multi-Processing (OpenMP) technique are used together with the shock-capturing finite volume method and the Harten-Latex-van Leer-Contact (HLLC) approximate Riemann solver. The applicability and accuracy of the present coastal model has been tested by four cases including the wave-induced currents at a tri-cuspate beach, longshore currents and sediment transport over transverse bars, channel infilling and migration under wave-current actions, and surge-wave driven morphology in a bay-inlet system. Reasonable agreement between the computation and measurement can be obtained if empirical and numerical parameters of the model are sensibly specified. The fully coupled morphological approach is suggested especially for extreme wave-current conditions due to its completeness in physics and negligible extra cost in computation. In addition, the hybrid LTS/GMaTS method can also effectively accelerate the computation in the wave-current environment.