With large-scale human interventions and climate change unfolding as they are now, coastal changes at decadal timescales are not limited to incremental modifications of systems that are fixed in their general geometry, but often show significant changes in layout that may be catastrophic for populations living in previously safe areas. This poses severe challenges that are difficult to meet for existing models. A new free-form coastline model, ShorelineS, is presented that is able to describe large coastal transformations based on relatively simple principles of alongshore transport gradient driven changes as a result of coastline curvature, including under highly obliquely incident waves, and consideration of splitting and merging of coastlines, and longshore transport disturbance by hard structures. An arbitrary number of coast sections is supported, which can be open or closed and can interact with each other through relatively straightforward merging and splitting mechanisms. Rocky parts or structures may block wave energy and/or longshore sediment transport. These features allow for a rich behavior including shoreline undulations and formation of spits, migrating islands, merging of coastal shapes, salients and tombolos. The main formulations of the (open-source) model, which is freely available at www.shorelines.nl, are presented. Test cases show the capabilities of the flexible, vector-based model approach, while field validation cases for a large-scale sand nourishment (the Sand Engine; 21 million m3) and an accreting groin scheme at Al-Gamil (Egypt) show the model’s capability of computing realistic rates of coastline change as well as a good representation of the shoreline shape for real situations.

Prediction of the shoreline response behind offshore breakwaters is essential for coastal protection projects. Due to the complexity of the processes behind the breakwaters (e.g., wave diffraction, currents, longshore transport), detailed modelling needs high computational efforts. Therefore, simplifying the process effect in a simpler coastline model could be efficient. In this study, the coastline evolution model ShorelineS is used. A new routine was implemented in the model to adjust the wave heights and angles behind the offshore breakwaters. Two approaches from the literature and a newly introduced one were tested in this study. The model free grid system was used to simply track the breaker line; such an advantage also helped to form tombolo, which is not common for these types of models. The tests showed promising results for single and multi breakwaters systems; however, the newly introduced approach still needs further testing and refinement for better performance and less computational cost.

Namhangjin beach is protected by multiple submerged breakwaters (SBWs) which were built to protect the ~4 km long sandy beach. A coastline model (UNIBEST) was used to investigate the long term effect of the SBW structures on the beach. The model computes long-term shoreline changes due to coastal structures as a result of the strong longshore sediment transport gradients at the structures. Bathymetry data of the shoreface and nearshore profiles were obtained from a field survey, while wave conditions from offshore WAM hindcast (Wave Modeling Group) were transformed towards the nearshore with the Delft3D+SWAN modelling system. Local wave sheltering by the SBWs was included in the wave model. A situation with and without the SBWs was modelled. A rapid adjustment of the shoreline was observed in the model as a result of the wave conditions in the first two years. After that, the shoreline shape stabilized without significant changes both for the situation with and without SBWs. A smooth curved coastline shape was obtained in the model without SBWs, while the model with SBWs shows a similar overall shoreline shape with undulations of the shoreline shape behind the breakwaters. A similar undulating shoreline was observed in the Sept-2013 imagery at Namhangjin beach. The local accretion behind the SBWs may induce some erosion in the lee area of the SBWs, causing distortions of the shoreline shape. Most sediment accreted at the first SBW (i.e. the northern most SBW where alongshore transport from the North was trapped), while the coastline change rate gradually decreased towards the South. The effectiveness of the SBWs at the considered shoreline section for maintaining the shoreline is somewhat ambiguous as local areas with accretion or erosion are present with respect to the situation without the breakwaters. The results show that multiple SBWs need to be carefully designed to protect beaches as local distortions of the shoreline shape may be present directly downdrift from the structures.

Large perturbations in the coastline, such as the 'Sand Motor' nourishment (∼21 million m³) at the Holland coast, can initiate considerable spatial and temporal changes in the median grain size (D₅₀) of the sea bed on the lower shoreface. The relevance of hydrodynamic conditions for the development of the heterogeneity in D₅₀ at large-scale nourishments was assessed with a numerical model (Delft3D), which required a validation against 2.5 years of D₅₀ measurements. A good representation of the observed spatial pattern of D₅₀ was obtained independent of a 2DH or 3D approach and initial condition for the D₅₀ of the bed. Five sediment size fractions and a multi-layer administration of the bed composition were used. The extent and magnitude of the coarsening of the bed is related to the velocity of the horizontal tide, while a far less pronounced coarsening takes place during energetic conditions (i.e. H_m0≥ 3 m). Differential suspension behaviour between the size fractions, which are all mobilized at the bed, causes a preferential transport of fine sediment (in alongshore direction) away from the Sand Motor at the lower shoreface (i.e. seaward of MSL -6 m). Storm conditions may induce a partial removal of the coarse top-layer due to mobilization of all of the size fractions and mixing with the relatively fine substrate material. Simulations also show that transport of the fine sand fraction extents to much deeper water than for the medium and coarse sand fractions. Models with multiple sediment fractions are therefore required for the assessment of environmental impacts of large-scale coastal structures or land reclamation's and sediment transport on the lower shoreface.

Mega-nourishments, aiming at providing long-term coastal safety, nature qualities and recreational space, have been applied recently at the Holland coast and are considered at various other places in the world. Methods to quickly evaluate the potential and lifetime of these coastal mega nourishments are therefore very much desired, which is the main objective of this research. Two types of mega nourishments can be distinguished: feeder-type mega nourishments may erode freely to feed adjacent coasts for a more natural, dynamic dune growth while permanent mega-nourishments are designed to preserve safety levels and need to maintain their size and shape and thus needs to be nourished themselves. The design and impact assessment studies for both types of mega nourishments require detailed morphological studies to determine the morphological evolution. In this paper 2DH (Delft3D) and 1D (UNIBEST-CL+ and LONGMOR) numerical models were calibrated using data of the Sand Motor mega-nourishment and were then applied to model a series of mega-nourishments with various width over length ratios and volumes in order to derive relations and design graphs for erosion rates, life span and maintenance volumes. These relations and design graphs can be used in project initiation phases and feasibility studies. The magnitude of the modelled wave-driven longshore sediment transport rates in 1D coastline models depend on the representation of wave refraction on the lower shoreface, since a distinction should be made between the non-rotating lower shoreface and active surfzone. It was shown that the life time of nourishments is mainly determined by the dimensions of the nourishment and incoming wave energy.

Sand nourishments are presently widely applied to maintain or enhance coastal safety and beach width. Over the last decades, global sand nourishment volumes have increased greatly, and the demand for nourishments is anticipated to increase further in coming decades due to sea level rise. With the increase in nourishment size and the request for more complex nourishment shapes, an adequate prediction of the morphodynamic evolution is of major importance. Yet, neither the skill of current state-of-the-art models for such predictions nor the primary drivers that control the evolution are known. This article presents the results of a detailed numerical modelling study undertaken to examine the model skill and the processes governing the initial morphological response of the Sand Engine and the adjacent coastline. The process-based model Delft3D is used to hindcast the first year after completion of the mega-nourishment. The model reproduces measured water levels, velocities and nearshore waves well. The prediction of the morphological evolution is consistent with the measured evolution during the study period, with Brier Skill Scores in the ‘Excellent’ range. The model results clearly indicate that the sand eroded from the main peninsular section of the Sand Engine is deposited along adjacent north and south coastlines, accreting up to 6 km of coastline within just one year. Analysis of model results further show that the erosional behaviour of the Sand Engine is linearly dependent on the cumulative wave energy of individual high energy wave events, with the duration of a storm event being more dominant than the maximum wave height occurring during the storm. The integrated erosion volume due to the 12 events with the highest cumulative wave energy density accounts for about 60% of the total eroded volume of the peninsula, indicating that the less energetic wave events, with a higher probability of occurrence, are also important for the initial response of the Sand Engine. A structured model experiment using the verified Delft3D model indicates that wave forcing dominates the initial morphological response of the Sand Engine, accounting for approximately 75% of the total erosion volume in the first year. The vertical tide is the second most important factor accounting for nearly 17% of the total erosion volume, with surge, wind and horizontal tide playing only a minor role.

Bed sediment composition, with a focus on the median grain size D₅₀, was investigated at a large-scale nourishment (The ‘Sand Motor’) at the Dutch coast (∼21.5 million m³ sand). Considerable alongshore heterogeneity of the bed composition (D₅₀) was observed as the Sand Motor evolved over time with (1) coarsening of the exposed part of the Sand Motor (+90 to +150 μm) and (2) a depositional area with relatively fine material (50 μm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the measured D₅₀ values was most evident outside the surfzone (i.e. seaward of MSL −4 m). Coarsening of the bed after construction of the Sand Motor was attributed to hydrodynamic sorting processes, because the alongshore heterogeneity of the D₅₀ showed a similar spatial pattern as the mean bed shear stresses. The observed alongshore heterogeneity of the D₅₀ and correlation of D₅₀ with modelled mean bed shear stresses suggest that preferential erosion of the finer sand fractions has taken place. The selective transport of finer sand fractions results in a coarser top layer of the bed at the Sand Motor. The preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport. The measurements also show the impact of a storm, which consists of a ∼40 μm finer D₅₀ of the offshore bed composition in front of the Sand Motor (i.e. where a considerably coarser bed was in place). Additionally, storms may generate a (temporary) zone with fine bed material at the toe of the deposition profile. This means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate.