Numerical model on the flow dynamics around the sediment-water interface in the tidal coastal area

•We develop a numerical model of the coupled flow around the sediment-water interface in a tidal coastal area.•A fully coupled simulation well represents the hydrodynamics around the sediment-water interface under reciprocating flow.•The velocity distributions obtained by the proposed model were more accurate than those obtained by a wall function model.•The numerical model shows a good performance in simulating the turbulent parameters around the sediment-water interface.

Investigating flow dynamics around the sediment-water interface is one of difficult issues in marine environment. It has important significance to study the physical, chemical and biological activities between the overlying water and sedimentary layer with tidal forcing. In this study, we established a numerical model to investigate the flow dynamics around the sediment-water interface in the tidal coastal area. The model reflected the hydrodynamics of periodic reciprocating unsteady flow in the overlying water layer and the fully coupled simulation of flow movement between the overlying water layer and sedimentary layer. A sedimentary layer, an overlying water layer, and an air layer were all treated as the fluid zone. The unsteady Reynolds-averaged Navier-Stokes equations, and the Reynolds stress model with porosity, were solved by the finite volume method. Moreover, the drag source term in the momentum equation was modified as the Darcy-Forchheimer extended form. The model showed a strong performance in simulating the hydrodynamics of coupled flow around the sediment-water interface with tidal forcing in a coastal area. With considering the sedimentary layer, the benthic boundary layer velocity distributions were more accurate than those obtained by a wall function model. Around the sediment-water interface, there was inertial loss in the flow. Furthermore, velocity increased with increasing porosity, and velocity gradient became larger. Compared with models with Darcy's Law, the numerical model in this study had better performance in the turbulent characteristics of sediment-water interface layer. The model can lead to better understanding of the exchange mechanisms of oxygen, nitrogen and nutrient between overlying water and sediment.