In times when the Dutch coastline faces pressure from coastal erosion due to the growing challenges of sea-level rise, sustainable coastal policies are essential. Increasingly large and more voluminous maintenance works are required. New sand nourishment strategies, such as mega-nourishments, are being tested along the Dutch coast. Due to the enormous quantities of sand, local safety standards increase. One example of such a project is the Hondsbossche Dunes (HBD). Although its primary function is the realization of local flood safety near the former sea dike, it also feeds nearby coast and dunes caused by natural forces like waves, currents, tides, and wind. Understanding rates and quantities of sediment dispersion to see how coastal state indicator evolve (e.g. coastal volume, beach width and depth contours) is crucial for policymaker. They must maintain the coastline as its main function of flood defense for the future, while also accommodating recreation and nature.

Literature demonstrates that one-line alongshore transport models can predict general coastline evolution trends at mega-nourishment sites well. However, their accuracy at other coastal state indicators can be limited due to, for example, the exclusion of cross-shore sediment redistribution. Recent efforts have focused on coupling such a one-line transport model (ShorelineS) with a new cross-shore redistribution model (Crocodile). Compared to other model types, the advantage of this coupling is its ability to calculate large spatial and temporal scales with minimal computational time. Additionally, the coupled model now accounts for a depth-dependent component by including this Crocodile model for the redistribution of sediment in cross-shore direction.

A data analysis of the coastal evolution at the HBD for the first 9 years was carried out. It shows that most volumetric changes occur in the initial 3 years. Furthermore, the data reveal that cross-shore activity varies over the depth. This becomes most evident from quantitative differences in volumetric changes for different vertical coastal cells (e.g. dune, beach, surfzone, shoreface). Also the differences in their correlation to the shoreline change suggest depth-dependent activity. For the models, this implies that the coupled model is likely to represent the magnitude and behavior of the volume (cells) and depth contours better.

The model comparison consist of the ShorelineS model, and ShorelineS+Crocodile (coupled model). An existing HBD ShorelineS model is extended with 4 years from 2020 to 2024, and includes a dune module, which enables the models to simulate the dunefoot and therefore provide insight in the beachwidth indicator. For the coupled model, Crocodile is used as a cross-shore module within ShorelineS that is configured with a dynamic equilibrium, and a alongshore varying and curved equilibrium grid, for its best performing run (SC5).

The results of the study suggest that the SC5 simulation improves ShorelineS on the volumetric evolution indicator; it halves the error to 250 m/m3 in 3 years time and it followings the temporal trend at the erosional zones very well. Additionally it adds new value and insight with the development of the cross-shore profile at every timestep. Although the magnitude is not yet always accurate, the SC5 simulation is able to represent independent evolution of depth contours. Despite its shown potential, the model results are constrained due to significant instabilities that appear in the model. These instabilities stem from a growing gradient-the difference in slope of the initial and equilibrium profile-which is posed by a bad translation of the equilibrium grid to a profile.

This research therefore recommends first and mostly to improve and solve the spatial and temporal stability as described above. This is necessary to make long-term predictions and contribute to future coastal maintenance strategies in the face of rising sea levels. Other relevant recommendations include creating a more realistic diffusion coefficient in Crocodile that balances subaerial and subaqueous changes better, integrating subaerial volume changes of Crocodile with ShorelineS' dunemodule in order to make a comprehensive comparison, and finally including location specific nourishments via Crocodile module and consequentially account for profile adjustment.