Channel–shoal patterns are often observed in the back–barrier basins of inlet systems and are important from both an economical and ecological point of view. Focussing on double–inlet systems, the initial formation of these patterns is investigated using an idealized model. The model is governed by the depth–averaged shallow water equations, a depth–integrated concentration equation and a tidally–averaged bottom evolution equation. Focussing on rectangular basins and neglecting the effects of earth rotation, it is found that laterally uniform morphodynamic equilibria can become linearly unstable, resulting in initial patterns that resemble channels and shoals. When the water motion is only forced by an M₂ tidal constituent, the existence of (laterally uniform) morphodynamic equilibria for which both inlets are connected strongly depends on the relative phase and amplitudes of the tidal forcing. If such equilibria exist, they can be either stable against small perturbations or linearly unstable. If these equilibria are linearly unstable, two instability mechanisms can be identified, the first related to the convergences and divergences of diffusive transports, the second mechanism related to a combination of advective and diffusive transports. In the former case, all eigenvalues are real and the bedforms grow exponentially in time. In the latter case, the eigenvalues are complex, resulting in bedforms that both migrate and grow in time. In case external overtides and a time–independent discharge are included, no diffusive instabilities are found anymore for the parameters considered in this paper. This implies that all instabilities are migrating in time. In all cases considered, the bed perturbations have only an appreciable amplitude at locations where the underlying laterally uniform equilibrium has a local minimum in water depth. This is consistent with observations from numerical models and laboratory experiments.

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A depth-averaged (2DH) exploratory model is developed to identify morphodynamic equilibria in short mesotidal inlet systems with arbitrary planform geometries. The water motion is forced by an M ₂ tidal constituent at the seaward entrance and is described by the depth-averaged shallow water equations, whereas the depth-integrated suspended sediment concentration follows from a diffusion equation, taking into account local inertia, horizontal eddy diffusion and topographically induced diffusive effects, erosion, and deposition. Based on a scaling analysis, it follows that the fine sandy bed evolution is dominated by the depth-integrated diffusive sediment transport. The depth-integrated advective contributions are one order smaller and therefore neglected. This observation also allows for the neglect of the advective terms in the governing equations. The associated morphodynamic equilibria are directly identified based on a continuation approach. By means of the exploratory model, the morphodynamic equilibria are studied in basins with a planform geometry characterized by width variations as a function of the distance to the seaward boundary. The model results show that in the case of a sufficient degree of widening in the landward direction, the equilibrium bed level exhibits significant lateral structures, characterized by shallow zones and deeper channels. The first channel bifurcation, as observed in many short tidal inlet systems, is forced by the planform geometry of the basin, and the associated physical mechanisms are explained. Furthermore, two mechanisms inducing asymmetric morphodynamic equilibria are investigated, of which the effect of an asymmetric basin planform seems to be dominant over that of the Coriolis force.

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To investigate the historical development of the tidally averaged transport of sandy sediments in the main branch of the Scheldt estuary over the last decades (1950–2013), a 2D exploratory model has been developed. This model comprises the depth-averaged (2DH) shallow water equations, driven by an asymmetric tidal forcing at the seaward side, as well as an advection-diffusion equation to describe the depth-integrated dynamics of the suspended sediment concentration. The tidally averaged sand transport results from a subtle balance between the various contributions; advective contributions due to internally generated and externally prescribed overtides and the diffusive and topographically induced contributions. A seaward tidally averaged sand transport is found near the open boundary, whose magnitude has increased since ca. 1950. Moving upstream, the magnitude of the seaward transport decreases and changes into a smaller landward transport with a local maximum near the landward boundary. This maximum has increased over the years. Varying parameters that capture changes in the environment, e.g. historical changes in the bathymetry, future mean sea-level rise or changes in tidal forcing, results in changes in the tidally averaged sand transport that are systematically analysed and related to changes in the various contributions. Our model shows that there is a competition, in terms of determining the magnitude and the direction of the tidally averaged sand transport, between the effects of historical bathymetric changes, changes in tidal forcing and (projected) SLR. Even small changes in the tidal forcing at the seaward boundary can have a large impact on the magnitude and the direction of the tidally averaged sand transport. This hampers accurate predictions of sediment transport and morphodynamic changes in tidal systems, due to the uncertainty in the response of the tidal dynamics to the projected sea-level rise.@en

A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics.

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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.@en