Numerical modelling of longitudinal dispersion in tidal flows with submerged vegetation

Three-dimensional simulations are performed to investigate flow and transport mechanisms through submerged vegetation under the influence of tidal oscillations. A global minimum least squares method is proposed to compute the spatially-averaged lateral and vertical diffusivities. Our results indicate that the longitudinal dispersion coefficients have sinusoidal behaviour and oscillate with a similar frequency as the tidal forcing. The dispersion coefficients are reduced at high water and enhanced at low water, which may be attributed to increased vertical shear and turbulence due to stronger bottom drag in the presence of vegetation. The effects of varying tidal amplitude and frequency are investigated. The revealed regularity of variation in the dispersion process suggests that an empirical formulation describing the longitudinal dispersion coefficient may be found. Our findings are applicable for specific types of vegetation with intermediate submergence, such as mangrove pneumatophores planted in estuaries.