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.