3D modelling of hydrodynamics and mixing in a vegetation field under waves

Vegetation contributes to the sustainable development of aquatic environments. It provides food and shelter to many organisms and controls the ecological system in rivers, estuaries and coastal areas. In estuarine and coastal areas, wind waves can affect significantly the hydrodynamics and mixing processes there. In this work the wave-induced mixing process in a vegetation field is investigated by using a σ-coordinate 3D model. In the governing equations the vegetation field is represented by momentum sink terms. A random walk model is used to derive an expression for the mechanical dispersion coefficient used in the mass conservation equation. The numerical model is first validated through the simulation of the propagation of random waves, the attenuation of random waves over vegetation, as well as the flow and mixing in a unidirectional flow through vegetation. The numerical model is then used to simulate the mixing of a tracer in a vegetation field under regular and random waves. The results show that the mechanical dispersion and the reduced advection generated by the wave-vegetation interaction lead to a broader lateral spread and a narrower longitudinal spread of tracer plumes. The degree of randomness of waves will not affect the mixing significantly, as long as the peak period and the total energy of the waves remain unchanged.