Depth-averaged idealised modelling of the hydro- and morphodynamics of tidal inlet systems and estuaries

Tidal inlet systems and estuaries are two very common features in coastal areas all over the world. Tidal inlet systems are typically observed at barrier coasts and they consist of a backbarrier basin and a strait, connecting the basin to the open sea. Estuaries on the other hand form a transition zone between riverine and maritime environments and they can be found at areas where rivers debouch into the coastal waters.

From a morphological point of view, tidal inlet systems and estuaries are highly dynamic. Large amounts of sediment are eroded when the ebb and flood currents are strong enough. Subsequently, the sediment is transported by means of diffusive processes (driven by the sediment concentration differences) and/or advective processes (driven by the currents). Due to gravitational forces, the sediment can settle again, resulting in an alteration of the bed, which in turn influences the water motion. This interaction between the bed topography and the
water motion often results in beautiful and complex bed patterns, forming in a range of different time and space scales. In many tidal inlet systems a branching, fractal-like structure of channels and shoals is observed, while estuaries are more often characterised by a braided pattern of meandering channels, separated by shoals and bars. 

This variety in deeper and shallower areas, together with the tidally varying water levels and the salt-fresh water interface are characteristics that define tidal inlet systems and estuaries as unique ecological environments, which are often of international importance, e.g. as breeding and feeding location of (migratory) birds. Local fishery, aquaculture and tourism are important sectors that profit from the ecological richness of these systems. Additionally, several ports are situated along estuaries, forming a crucial, logistical link between ocean transport and the
hinterland. In order to reconcile the various functionalities of tidal inlet systems and estuaries in a sustainable way, it is necessary to understand the processes that drive the development of these areas, such that the impact of human interference (e.g. land reclamation, dredging) and long-term changes (e.g. sea level rise) can be estimated, evaluated and possibly mitigated.

Therefore, the main objective of this thesis is to develop a depth-averaged, idealised, process-based model to gain a better understanding of the main processes, driving the hydro- and morphodynamic behaviour in tidal inlet systems and estuaries. A process-based model is based on first physical principles (e.g. conservation of mass and momentum) and describes the physical processes by mathematical equations. An idealised model only retains the physical processes believed to be essential to reproduce the phenomena of interest. Finally a depth-averaged model, describes these phenomena in a two-dimensional, horizontal way, by averaging the three-dimensional equations over the vertical dimension.

The water motion in our model is described by the depth-averaged shallow water equations and is being forced at the seaward side of the domain by a prescribed tidal motion, which consists of the semidiurnal lunar tidal constituent (M 2 ) and possibly its first overtide (i.e. a secondary tide of higher frequency than the principal tide): the quarter diurnal lunar constituent (M 4 ). These overtides are also generated
inside the basin, due to nonlinear interactions of the tidal constituents with the bathymetry or amongst themselves. Overtides may cause asymmetry in the water motion, affecting the net sediment transport. The sediment transport is governed by a depth-integrated advection-diffusion equation that describes the transport of suspended sediment. Finally, the bed evolves due to divergences and convergences of tidally averaged bedload and suspended load transports.