Coastal defense strategies increasingly include hybrid dunes, which combine sandy dunes with hard structures. In the case of hybrid dunes, the sandy system and hard elements interact. Wave impact, reflection runup as well as dune erosion and scour are all being affected by both the hard elements and sandy parts. To better understand these interactions, we conducted a field experiment, with a hybrid dune setup to measure hydrodynamics and dune erosion during storms. The experiment setup contains four cross sections to compare the morphological development at a sandy dune (1), dike-in-dune (2), vertical wall-in-dune (3) and dike (4). The works took place in the winter of 2024/2025. The various storms that occurred caused strong erosion to the test setup. Preliminary results show that while the hard elements slowed down dune retreat behind them, the beach morphology in front remained remarkably similar. Erosion may have been governed by large-scale sediment availability in the system, beach elevation and hydrodynamics rather than by hard-soft interactions at the hybrid dune.

For reasons of computational efficiency, 1D XBeach cross-shore transect models are often applied on quasi alongshore-uniform coasts instead of a more computationally expensive 2DH modelling approach. In this paper we describe a practical methodology to allow 1D models to mimic infragravity wave growth observed in field conditions and represented in 2DH models and thereby significantly reduce the overestimation of infragravity wave heights near to the shore. We show that even if the coastal setting may support the 1D model assumption of alongshore uniformity in forcing and bathymetry, 1D XBeach models will overestimate infragravity wave growth relative to field conditions due to the underlying assumption of alongshore-uniform intra-wave (group) hydrodynamics common to all wave (group)-resolving models. Model analysis showed little variation in the optimum calibration value of this approach for a large range of wave forcing conditions (wave height, period and directional spread) and consistency with 2DH model results for a large range of Dutch coastal profiles. Comparison to field observations showed that the methodology significantly increased the skill of the 1D model to simulate nearshore IG wave heights. These results have implications on the ability to accurately predict the response of these coasts to energetic events, which is crucial to assess current and future flood risk and to design effective flood protection and mitigation measures.

For successful and sustainable management of barrier islands, a thorough understanding of the ebb-tidal delta dynamics and interactions with the adjacent shorelines are of the utmost importance. Such understanding requires detailed observations and interpretations of the morphodynamics of smaller-scale features such as the individual channels and shoals (referred to as intra-delta dynamics). The intra-delta dynamics of Ameland Inlet (the Netherlands) are studied through analysis of sixteen high-resolution bathymetric surveys, supplemented with an extensive dataset of hydrodynamic observations collected in 2017. The observations are compiled into a synthesis of the morphodynamics of the ebb-tidal delta and its neighboring shorelines, to provide a basis for present day and future coastal management.

Our observations show that Ameland Inlet as a whole can be classified as a typical mixed-energy, wave-dominated system. However, the ebb-tidal delta contains distinct areas that are wave or tide dominated, and these areas evolve with the changing morphodynamics of the ebb delta. Between 2005 and 2021, large morphodynamic changes have occurred on the ebb-tidal delta and continuous erosion of the island tips occurred. Limited wave-sheltering by the ebb-tidal delta exposes the shorelines of the adjacent barrier islands to significant wave-driven sand transports and sand losses. Sediment supply from longshore transport and the erosion of the updrift island Terschelling contributed to the formation, growth and migration of a series of ebb-chutes and lobes, which eventually led to complete relocation of the main channel on the ebb-tidal delta. This main channel relocation took 15 years to complete and is an example of the ebb-delta breaching model of sand bypassing. Changes in the sediment bypassing patterns result in a sediment starved western island tip of Ameland, necessitating repeated sand nourishments under the Dutch coastal maintenance policy. Our observations also confirm the role the ebb-tidal delta as a sand reservoir for the downdrift barrier island. The delta sand body is not a reservoir for the back-barrier basin, since the basin is predominantly supplied with sand eroded from the updrift island of Terschelling.

As demonstrated in this study, the intra-delta dynamics of an ebb-tidal delta are complex and can change drastically through time. Only through detailed measurements and observations can all the intricate interactions that take place be unravelled.