The Hollandsche IJssel plays an important role in the freshwater provision of the province Zuid-Holland. Consequently, for Rijkswaterstaat it is key that salt intrusion is minimal in the Hollandsche IJssel. Recent studies noted that salt intrusion in the Hollandsche IJssel is limited due to a phase difference between tidal velocities in the main channel, the Nieuwe Maas, and the side channel, the Hollandsche IJssel. Earlier research investigated the impact of phase differences between branches and found it can lead to increased dispersion in the main channel, through a process known as tidal trapping. At the same time, this phase difference can prevent the saltiest water from entering the side channel, as was found at the Hollandsche IJssel. Because of this role, it is relevant to find out how this phase difference may be influenced by sea level rise, more extreme river discharges and particularly how it depends on the geometry of the main and the side channels. Especially the latter could help Rijkswaterstaat to minimize salt intrusion at locations relevant to freshwater intake, such as the Hollandsche IJssel.

The main objective of this thesis is to investigate how the geometry of the side and main channel influences the tidal phase difference between these two channels, and how this may impact the salt dispersion in the side channel. For this, an analytical model is developed describing harmonic wave propagation in multi-branch systems and this is used next to results from a 3D numerical model for the Rhine Meuse Delta (RMM3D). First, the influence of changes in geometry and forcing is systematically investigated for a network containing a single junction. This shows that the length and depth of the side channel are the most significant variables. The depth is one of the main variables impacting friction, which governs the type of wave which can form in the system. A decrease in friction allows a wave to transform into a standing wave pattern as the return wave becomes more important, while increased friction transforms it into a propagating wave. The length also controls the type of wave which can form as it determines the distance along which the friction can work. Additionally, the length also governs potential resonance in the side channel.

Next, the phase differences of the M2, M4 and M6 tide are determined for the junction with the Hollandsche IJssel in the Rhine Meuse Delta (RMD) based on the RMM3D model. The main tidal constituent regarding tidal trapping was found to be M2. However, this does not fully represent the time difference between flow reversal at the Hollandsche IJssel and the Nieuwe Maas, which was found to be around 75 minutes. Additionally, the phase difference at the Lek was investigated. For the M2 tide at the Hollandsche IJssel and Lek, a phase difference of 55⁰ and 31⁰ was found, respectively. These phase differences prevent salt intrusion in the respective side channels. The inflow of the side channels starts while the main channel still flows to the sea during the ebb. At this moment, the salt concentrations in the main channel have already returned to background levels...

A rise in the global mean temperature induced by climate change is expected to have a large impact on ecosystems in all regions of the world. One of the threats is accelerated sea level rise (SLR). This may induce the loss of intertidal areas in tidal inlet systems. The long-term morphological response of tidal inlet systems can be modelled using reduced complexity model ASMITA (Aggregated Scale Morphological Interaction between Tidal inlets and the Adjacent coast). The ASMITA model simulates morphological development on an aggregated spatial and temporal scale by imposing a morphological equilibrium condition. As such the model is fast, allowing for multiple long-term simulations. The model is physics-based and the parameters can be related to field values.

Currently, salt marshes are not implemented in ASMITA. However, salt marshes could be of importance to the morphological development in tidal inlet systems. Moreover, it is relevant to assess the resilience of the ecologically important salt marshes by themselves. The aim of this research is to implement salt marshes in ASMITA to assess their influence on the rest of the tidal inlet system and to gain insight into the long-term morphological response of salt marshes to accelerated SLR.

Salt marsh development is governed by horizontal and vertical processes. The marsh height increases by capturing mineral sediment and by the accumulation of plant biomass. Autocompaction and deep subsidence lead to a decrease in marsh height. The implementation of salt marshes in ASMITA relies solely on the input of mineral sediment. At the marsh edge, generally, a cyclic behaviour of sedimentation and cliff erosion occurs. Due to the high degree of spatial aggregation, cliff erosion is excluded from the model extension. The governing processes for salt marsh development in the ASMITA model extension are mineral sedimentation, sediment availability and relative SLR.

The spatial and temporal aggregation of governing processes for salt marsh development are included in the aggregated advection-diffusion equation and model parameters for the horizontal & vertical exchange of sediment, and sediment availability. Data analysis on hydrodynamic conditions and salt marsh development was conducted for the derivation and calibration of these model parameters. To verify the salt marsh implementation, three ASMITA models were created. A one-element salt marsh model consisting of only a salt marsh element, and two different multiple elements models, which contain the ebb-tidal delta, channels, tidal flats and salt marshes.

It can be concluded that ASMITA can model the mineral sedimentation on a salt marsh but depicts a large sensitivity to the parameter setting, particularly for the sediment concentration. Based on the chosen parameter configuration, the Oosterkwelder salt marsh is preserved when subjected to SLR rates below 16 mm/year.

The ASMITA salt marsh extension can be employed to obtain an expeditious first impression of long-term morphological salt marsh development. However, due to the lack of incorporation of detailed processes, the model should not be employed for in-depth analyses of salt marsh development. The interaction between the salt marsh element and the remaining tidal inlet system components requires further model improvements.

Estuarine intertidal flats comprise valuable ecosystems and act as an important sediment source for the adjacent salt marsh systems. However, accelerating sea-level rise threatens the mudflats and associated ecosystems, where the mudflat accretion lag behind sea level rise. A reliable forecast on the morphological developments of the mudflat under sea-level rise scenarios is of vital importance to assess sea level rise impact on the estuarine system.

Different tools exist that can predict the long-term evolution of the mudflats, viz. Delft3D, ASMITA and the hybrid model (Delft3D-ASMITA). Since the hybrid model is newly developed, the comparison among the various approaches has not yet been available. However, it is significant to know if they can produce the same results.

The research aims to compare the three modeling approaches (Delft3D, ASMITA, and the hybrid model) based on a case study in South San Francisco Bay. This comparison will reveal the strengths and weaknesses of the three approaches as well as indications where the approaches may strengthen each other.

The research is conducted in three main phases. Phase 1 consists of the sensitivity analyses in Delft3D for the case of South San Francisco Bay; Phase 2 contains the calibration of one-element and multi-element ASMITA models to reproduce the Delft3D model; Phase 3 focuses on the potential to improve the simulation efficiency of Delft3D in the hybrid model.

Model result comparison shows that, after the calibration, the ASMITA and hybrid model can efficiently simulate the same cases as in Delft3D. However, the upper part (landwards end) of the mudflat is more sensitive to the water level changes in the hybrid model due to the different sediment transport computation modules. The power indicating the relation between the equilibrium and actual morphology as well as the reference level is the important calibration coefficient to adjust the steepness of a mudflat. It can be concluded that the Delft3D model is used as the foundation to calibrate ASMITA and the hybrid model, both of which can improve the simulation efficiency with simplifications, especially in the long-term morphological development.

The study provides clear insights into the comparisons among different modeling approaches in the case of the long-term morphological developments of mudflats by the impacts of sea-level rise. It is recommended to do further research on the configuration of a 2D model and the conduction of the combination of different modeling approaches in other similar cases to confirm the validation.