The tidal flow in an estuary, and consequently the morphodynamics and ecology, is highly dependent on the basin geometry (Ridderinkhof et al. 2014).The characteristics of tidal wave propagation in prismatic basins are well known, and also the effect of converging basins has been studied to some depth. The influence of tidal flats on tidal wave propagation has been studied using 1D models, while in this paper, both a 1D and a 2D model will be applied. The latter approach allows to account for the water motion above the tidal flats, in contrast to a 1D model where the tidal flats are modelled as storage areas.
To gain some fundamental insight, an idealized model (which is fast, compared to complex procesbased models and easy to analyse) has been used, that describes the water motion in a semi-enclosed (converging) basin by means of the shallow water equations, forced by prescribed free surface elevations at the entrance (x=0). The main focus of this paper is to study the influence of tidal flat geometry on the spatial structure of different tidal harmonics and of tidal asymmetry between ebb and flood periods. This work represents a first, validation step in the development of a 2D idealized model for the identification of morphodynamic equilibria. The Finite Element Method (FEM), was used to spatially discretize the governing equations, in which the physical variables are expanded in their tidal constituents. The rectangular and converging tidal basins, that are considered here, can thus be easily extended to more general geometries.
After a favorable comparison with other models in literature, the tidal hydrodynamics was studied for different values of bottom roughness and for different widths of the tidal flats. As an example, the free surface elevation amplitude of the internally generated overtideM4 (quarter-diurnal), relative to that of theM2 (semi-diurnal) component, prescribed at the entrance, is given in Fig. 1, for a converging channel. Though discrepancies exist between the 1D and 2D results, both models show that the amplitude ratio increases towards the closed end of the basin (x=L).