Some of the sediment which is eroded from the Dutch coast ends up in the Wadden Sea. Because of this the morphological development of the Wadden Sea is important, not only for the Wadden Sea itself but also for the maintenance programs of the adjacent coastlines. This is one of the reasons that the morphological development of the Wadden Sea is extensively studied. Some of these studies use the ASMITA model to make predictions of the morphological development with accelerating sea level rise. The Groningerwad is a part of the Wadden Sea consisting of a number of smaller tidal basins which has not been modelled with ASMITA. It has not yet been necessary to structurally nourish the coastlines surrounding the Groningerwad. However, as sea level rise increases it might well be possible that the coastal profiles surrounding the Groningerwad require nourishment. Therefore this thesis aims to study the Groningerwad with ASMITA to make a prediction of how the area will develop with accelerating sea level rise. To do this a morphological study is performed to determine the current morphological developments. This morphological study, based on available literature and bathymetry measurements of the area, finds that the Groningerwad is a highly dynamic area. It also determined the area and characterizing volume of each of the tidal basins, which have been used to set up the ASMITA model. For each basin in the Groningerwad an ASMITA model is set up using the information from the morphological study. The ASMITA model is used to make predictions for the development of the intertidal, channel and delta volumes of each of the Groningerwad its basins. The required parameters for the model have been derived from relevant formulas and the assumption that the Groningerwad is currently in a morphodynamic equilibrium. This was done because the time period for which bathymetrical measurements are available are to short to allow for a proper calibration procedure for these parameters. With this setup the ASMITA models show that all basins will lose intertidal sediment volume with rising sea levels. The larger basins of the Groningerwad also will not reach a new dynamic equilibrium state with large levels of sea level rise rate increase. When comparing these results to other basins in the Wadden Sea, it appears that the basins in the Groningerwad respond a lot slower than other Wadden Sea basins. Given the difference between the Groningerwad and the Wadden Sea and the fact that the time period over which bathymetrical data is available was to short to fully calibrate the model the recommendation is made to revisit this study when more data is available and it is possible to calibrate the relevant parameters.

The behaviour of tidal basins has been predicted using a modelling approach in which a morphological equilibrium for an entire basin was used frequently in the past. Because tidal basins have, to an certain extend, a fractal nature, it is expected that this approach could be used with a morphological equilibrium for subbasins as well.

In the first part of this thesis the morphological equilibrium for subbasins is examined. A numerical simulation using tracers is conducted in order to find the watersheds of the Ameland Inlet. The watersheds are used to divide the basin into subbasins of different scales. These subbasins are used to find a morphological equilibrium between the channel volume and tidal prism. The applicability limitations of the morphological equilibrium relation are also investigated in this part of the research.

In the second part of the research the newfound equilibrium is applied in an Asmita model. The modelling exercise is done in order to showcase a proof of concept of the multi element modelling approach. In this modelling approach both a 6 and 10-element model are used to look for the spatial differences in morphological behaviour within a basin.

In the first part of this thesis an equilibrium for the channel volume with respect to the tidal prism of subbasins in the Ameland inlet is found. This equation can be applied to basins (Channel and flat combined) with a minimum area of 40 km². This limitation is for both the equilibrium study as the Asmita modelling. The modelling exercise proofs that the multi element Asmita modelling is possible and gives a good insight into the spatial differences in morphological behaviour within the Ameland Inlet.

The Wadden Sea is the largest system of tidal flats and barrier islands in the world extending from the northern Dutch coast to the coast of Denmark. Such a large natural area is ofgreat importance to bothhuman beings and ecosystems. Over 10,000 species of flora and fauna are found in theWadden Sea being a perfect habitat due to the relatively calm environment and high food availability. However, accelerating sea-level rise and human interventions, such as gas extraction, may induce an unwanted morphological change in theWadden Sea posing a threat to the natural habitats. This study investigates the morphological response of the Wadden Sea to SLR and subsidence induced by gas mining. A new hybridmodel whose aggregation level is between a process-based model
(Delft3D) and a aggregatedmodel (ASMITA) is applied in this thesis. The two-dimensional hybrid model applies depth integrated shallow water formulations and the advection diffusion equation for the transport ofsediments like Delft3D, but calculates the exchange ofsediment betweenbed andwater columnbymeans ofan equilibriumbathymetry concept under scenarios disturbing these equilibrium conditions. The Ameland inlet is chosen as the research area because it is a relatively autonomous
and undisturbed basin. The high robustness of the hybrid model makes it possible to apply a high morphological scale factor and a coarser grid compared to the processbased model, which shortens the computation time by orders of magnitude. However, the equilibrium concept also fixes the shoal-channel structure and suppresses channel migrations. Three different scenarios of sea-level rise rate (4, 6, 8 mm/year) are applied over a simulation period of 100 years. The general morphological response simulated by the hybrid model shows that the channels and the ebb-tidal delta erode acting as the main source ofsediment for accretion of the intertidal flats. The erosion/sedimentation is more pronounced with a higher sea-level rise rate. Sensitivity analysis shows a significant influence of the sediment diameter on the
channel erosion and sediment supply to the intertidal flats. Diffusivity plays an important role in the horizontal sediment exchange between the channel and the flat but only slightly influences the sediment import. Global equilibrium concentration and power n are similar to diffusivity in affecting morphological activity. The adaptation time scale is inversely proportional to both of these two parameters. The possibility of using more than one sediment fraction is proved and it can reproduce a more realistic sediment distribution as the observation. The hybrid model is also applied to simulate the morphological response to local
subsidence and the restoration after subsidence stops. The center of the subsidence circle dosen’t lower as much as the subsidence rate, which indicates that sediment is transported to the center. Sediment is supplied to the area ofsubsidence by the adjacent main channel. The result proves the sediment transport principle underlying the hybrid model that sediment is always transported along the gradient of the sediment demand.

The coastline near Domburg, in the southwest delta of the Netherlands, has been preserved with sand for three decades. Maintenance was conducted on the beach between the low water line and the dune foot every 4 years. To extent this interval, a shoreface nourishment was implemented in 2017 and was supplemented with a beach nourishment in 2019. Such a parallel shoreface bar is not a naturally occurring morphological feature, as the Domburg coast only features transverse bars.
This research investigates the hydrodynamic and morphological effects of different nourishment strategies at the Domburg coast. The study focusses on the application of the shoreface nourishment, as this strategy has not been previously applied at a coast without shore-parallel bars. The study aims at gaining more insight and knowledge on important mechanisms responsible for the spreading of nourished sediment, because this is not fully understood. Additionally, the performance of a numerical model to hindcast erosion and sedimentation in coastal zones is evaluated.
To this end, a morphological analysis based on measurements and a model analysis are conducted for three nourishment scenarios. The morphological analysis uses an extensive dataset of bathymetric surveys to compute volume changes, sediment fluxes, bar migration rates, momentary coastline (MKL) positions and nourishment longevities (defined as the period that the volume in between the dune foot and mean low water level is greater than before nourishment). The numerical model analysis is based on the output of a morphostatic XBeach model with simplified boundary conditions which computes the hydrodynamics and sediment transports.
The longevity of beach nourishments was found to be on average 3.3 years and the MKL regressed 3.9m/yr on average two years after construction. The eroded sediment did not accrete on the shoreface but was likely transported in the direction of the net sediment transport. The model output indicates that a beach nourishment only has a significant local effect on the hydrodynamics and transport rates.
The shoreface nourishment transforms from a landward skewed triangular shape into a more rounded body without the formation of a trough. The bar migrates onshore but not alongshore and the bar volume remains constant. The model shows a contraction of the tidal flow due to the shoreface bar, increasing the seaward velocity and causing a sheltered zone at the leeside and downdrift of the bar. Consequently, the alongshore transport gradients are increased. This causes extra erosion on the shoreface seaward of the bar while accretion is seen at the downdrift shoreface. The shoreface bar contributes little to the offshore dissipation of wave energy. No evidence for a wave-driven salient effect was found at Domburg from the model output.
The longevity of the 2019 beach nourishment is not prolonged by the presence of a shoreface bar, as this was found to be 3.1 years. Likewise, the MKL measured a regression of 4.3m/yr, similar to previous beach nourishments. Positively, the shoreface bar captures eroding beach sediment because accretion was found in the surf zone, as opposed to the 2014 beach nourishment. Therefore, a shoreface nourishment is moderately beneficial on maintaining the MKL but contributes to the sediment balance of the coastal cell.
Additionally, an alternative nourishment strategy was evaluated through the numerical model. A mega nourishment to the west of Domburg is a viable option as it is likely to feed the updrift and downdrift coastlines which have a sediment demand. It is recommended to further evaluate this nourishment strategy with different numerical models.