Tidal Inlet Systems: bottom pattern formation and outer delta development

Tidal inlet systems are found around the world’s coastline. They consist of a backbarrier lagoon or basin which is connected to the adjacent sea by means of a narrow strait. The basin is bounded by land and/or tidal watersheds. The motion in these systems is predominantly driven by tides (compared to other forcing agencies such as waves), examples are the inlets in the Dutch Wadden Sea. The focus of this thesis is on two aspects of tidal inlet systems: 1. the physics of morphological pattern formation in the inner basin and 2. the hydro- and morphodynamic interaction between the inner basin and outer sea, including the dynamics of the outer delta (or ebb-tidal delta). Values used in the thesis are representative for the Frisian Inlet system, located in the Dutch Wadden Sea. The results show that bottom patterns in the inner basin can form as free instabilities of the system. Both global patterns (channel/shoal patterns covering the whole basin) and local patterns (tidal sand bars near the entrance of the basin) were found. The latter are characterised by dominant advective sediment fluxes, whereas global patterns are found when advective and diffusive fluxes are of the same order of magnitude or when diffusive fluxes are dominant. Different physical mechanisms of pattern growth are described for each case. The advective transport of sediment is mainly due to transport of the tidally averaged concentration by the residual current. Analysis of the interaction between the basin and the outer sea shows that the residual water motion inside the basin is strongly influenced by a shore-parallel tidal wave in the outer sea. The effect of earth rotation is much less in this respect. The interaction of the current through the strait and the along-shore current causes asymmetries in the water motion but not in the erosion and deposition pattern. Therefore, long-term simulations were performed to study the morphological development of a recently formed inlet. The actual geometry of the Frisian Inlet (a double inlet system) is used here with a flat bottom topography. The Engelsmanplaat, situated between the two subsystems, is not included in the geometry. The results show the development of two distinct outer deltas and channels and shoals in the system. Apparently the width of the inlet allows for a double inlet systems, so that the Engelsmanplaat is not a necessary condition for the stability of the Frisian Inlet. Initially the two systems evolves symmetrically, but after some 200 years asymmetries can be observed both on the outer delta (orientation of the main channel and different sizes of the two outer deltas) and in the basin (faster development of the eastern system than the western one). As waves are not included in the forcings, this means that tides alone can cause a preferred orientation of the main channel on the outer delta.