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.

Since 1990, the Dutch coastline is maintained within the ‘Dynamic Preservation’ program, according to which the coastline is maintained seawards from a reference line, mainly by applying nourishments. Research into the maintenance of the Dutch coast is continuous and causes the content of the coastline preservation program to change constantly since the initiation in 1990. In recent years, the switch was made from yearly nourishment programs to the use of multiannual nourishment programs, in which an interim nourishment planning is included for 4 years. Next to the nourishments following the ‘Dynamic Preservation’ program, ten large reinforcements were applied along the Dutch coast in the past decade according to the ‘Zwakke Schakel’ project. After reinforcement, the coastline at the ‘Zwakke Schakel’ locations needs continuous maintenance to remain at the desired position.

The combination of the long term maintenance at the ‘Zwakke Schakel’ locations and the multiannual nourishment program, leads to a more or less fixed character of the nourishments program with recurring maintenance nourishments in each period. The question is to what extent also the adjacent coast is maintained by sediment transported from these recurring maintenance nourishments. A situation in which the adjacent coast can be sufficiently maintained by long term application of nourishments at the ‘Zwakke Schakel’ locations, would lead to an even more fixed character of the nourishment program. At this moment, knowledge on the contribution of sediment transported from beach nourishments to the maintenance of the adjacent coast is insufficient.

The research presented in this thesis focusses on one case study. Along the coastal stretch between Scheveningen and IJmuiden, three distinct ‘Zwakke Schakel’ reinforcement nourishments were applied at Scheveningen, Katwijk and Noordwijk. At all locations the coastline was migrated seawards, with varying distances of 60 to 100 meters. The coastal stretch between Scheveningen and IJmuiden is part of the Holland coast and bounded by the breakwaters of the Scheveningen and IJmuiden harbours. Along the Holland coast, sediment transport is dominated by wave related processes wherein longshore transport is the most important sediment transport process. Gradients in longshore sediment transport are therefore an important cause of erosion and accretion.
Results of the yearly measurements done along the entire Dutch coast already show a positive effect of the maintenance nourishments in the area. With a refined version of an existing Unibest-CL+ model the effect of the recurring maintenance is further assessed for the long term. In the model, the longshore sediment transport volumes and resulting coastline evolution are modelled for a timescale of 55 years, starting in 2006 before application of the ‘Zwakke Schakel’ reinforcement nourishments and including the effect of possible sea level rise of 0.2 to 1.5 cm per year. The model is validated by comparing transport quantities (volumes and gradients) and coastline development with real measurement results and results from earlier research.

In order to maintain a positive coastline position along the adjacent coast, the autonomous erosion needs to be sufficiently compensated by the accretion related to the long term maintenance. At several locations in the area of interest initial erosion is expected, after which the erosional trend switches into a seaward migrating trend on the long term, partly under influence of the maintenance nourishments. This process is expected to occur at both Wassenaar (between Scheveningen and Katwijk) and Noordwijkerhout (north of Noordwijk) in the upcoming decades, although the inclusion of some uncertainty in amongst others sea level rise shows that it is unsure whether a positive development at Noordwijkerhout will really occur. The erosional trend at Bloemendaal and Zandvoort, close to IJmuiden, cannot be compensated by the sediment transported from maintenance nourishments. On the time scale of 55 years, the region of influence of the maintenance nourishments does not reach Bloemendaal and Zandvoort. The regions of influence of all ‘Zwakke Schakel’ maintenance nourishments are expected to cover the area from Scheveningen up to around 10 kilometres northwards from Noordwijk in 2060. Individual regions of influences are expected to reach a size of 15 to 24 kilometres up to 2060.

Although in most cases the trends of coastline development within the regions of influence are expected to become positive on the long term, the coastline position itself may be located too much landwards due to the initial erosion. In order to solve this problem, additional (shoreface) nourishments need to be applied at Wassenaar and Noordwijkerhout. At both locations shoreface nourishments are already applied in the past decades, which supports the outcome of the model results. At Bloemendaal and Zandvoort, additional (shoreface) nourishments will surely be needed in order to maintain the coastline.