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.

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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.

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Shoreface nourishments are commonly applied for coastal maintenance, but their behaviour is not well understood. Bathymetric data of 19 shoreface nourishments located at alongshore uniform sections of the Dutch coast were therefore analyzed and used to validate an efficient method for predicting the erosion of shoreface nourishments. Data shows that considerable cross-shore profile change takes place at a shoreface nourishment, while an impact at the adjacent coast is hard to distinguish. The considered shoreface nourishments provide a long-term (3 to ~30 years) cross-shore supply of sediment to the beach, but with small impact on the local shoreline shape. An efficient modelling approach is presented using a lookup table filled with computed initial erosion-sedimentation rates for a range of potential environmental conditions at a single post-construction bathymetry. Cross-shore transport contributed the majority of the losses from the initial nourishment region. This transport was driven partly by water-level setup driven currents (e.g., rip currents) and increased velocity asymmetry of the waves due to the geometrical change at the shoreface nourishment. Most erosion of the nourishment takes place during energetic wave conditions (Hm0 ≥ 3 m) as milder waves are propagated over the nourishment without breaking. A data-model comparison shows that this approach can be used to accurately assess the erosion rates of shoreface nourishments in the first years after construction. @en

Tide and waves redistribute sediment that is placed at the coast. This holds especially for a large-scale nourishment like the Sand Motor. However, not all of these sand grains are transported at the same pace. Fine grains typically move much faster than the coarse grains, but the dynamics of sand particles along exposed beaches are not well understood. Some studies have even argued that the coarser grains are more exposed to the flow and therefore more easily picked up than the fine grains. As most of these studies took place in a laboratory, it would be useful to see evidence of this behavior in the field. If the fine and coarse sand grains move differently, then areas with a coarser or finer bed composition will develop, as sand is removed from some regions and accumulates in others. In this sense, the Sand Motor can be seen as a big sandsorting machine, which can provide us with fundamental understanding of the dynamics of sand grains. Such knowledge is essential if we are to protect and manage the coast in the future. This is particularly important to manage and protect fish populations, since changes in the composition of the sandy bed affect their habitat. For example, juvenile flatfish are known to favor the fine sand over coarser sand, since they can more easily hide from predators (Page 119).@en

Increasingly large sand nourishments are used for the maintenance of sandy coasts around the world. There is, however, still very limited understanding of their behaviour (i.e. redistribution) and effects on the marine environment. This PhD thesis explores the morphological reshaping of shoreface nourishments (i.e. long bunds of sand placed at the sub-tidal bar with volumes of 1 to 5 million m3) based on data at 19 field sites as well as the reshaping of mega nourishments (i.e. temporary land reclamations) using data of the 'Sand Motor' at the Dutch coast (with a volume of 21 million m3). Considerable cross-shore profile change takes place at shoreface nourishments, consisting of a landward movement of the nourishment crest and erosion of the seaward edge of the nourishment, erosion directly landward of the shoreface nourishment (in the first 100 to 150 m) and some accretion in the inner surfzone (at MSL -2m). Especially the water-level gradient driven currents and onshore transport due to wave skewness are responsible for the morphological change, which could be modelled with Xbeach using a lookup table with initial sedimentation-erosion rates for possible climate conditions. Mega nourishments, on the other hand, reshape predominantly in alongshore direction as a result of the alongshore wave-driven current. Design graphs showing the erosion rates, life span and maintenance volumes were made for the mega nourishments, which can be used for the planning phase of projects. Making a differentiation between the non-rotating foreshore and active surfzone proved to be essential for an accurate representation of the wave-driven alongshore transport in 1D coastline models. Furthermore, the lifetime of the nourishment is related to the sensitivity of the alongshore wave-driven transport to a shoreline rotation, which is affected especially by the wave energy. The recent upscaling of the sand nourishment volume in the last decades (i.e. to Sand Motor scale) and increasing anthropogenic pressure also comes with questions regarding the impact that is made on the natural environment. This thesis investigated the development of the bed sediment composition at the 'Sand Motor' using field measurements and numerical modelling, since bed composition is relevant for marine ecology. Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) a coarsening of the lower shoreface of the exposed part of the Sand Motor (+90 to +150 µm) and (2) a deposition area with relatively fine material (50 µm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the D50 is most evident outside the surfzone (i.e. seaward of MSL -4 m), while alongshore variation in D50 was relatively small in the surfzone itself (i.e. landward of MSL -4 m). Preferential erosion of the finer sand fractions takes place during mild to moderate wave conditions, while a reduction of the local armouring of the bed takes place during storms which mobilize all sediment fractions and mix the the top-layer of the bed with the relatively finer substrate. A 3D multi-fraction morphological model gave a good hindcast of 2.5 year of observed spatial and temporal changes in D50 at the Sand Motor, which showed that the coarsening of the bed after construction of the Sand Motor is mainly due to the tidal contraction at the Sand Motor. The currents transport especially the fine grains of the sand mixture, which are more easily suspended than the coarse grains. This difference in suspension behaviour is the main cause of the observed bed composition changes at the lower shoreface. Within the surfzone the difference in suspension behaviour of the size fractions will be smaller, as the energetic conditions can suspend all size fractions. The current findings imply that large-scale bed composition changes can take place at any coastal structure which has a considerable impact on the tidal currents. @en

The behaviour of subtidal sandbars can be strongly influenced by the introduction of sand nourishments in the coastal system. This study focuses on the impact of nourishments on subtidal bar behaviour at spatio-temporal scales beyond a single nourishment project. It aims to determine the long-term behaviour of subtidal sandbars along an entire coastal cell, taking into account both the unnourished and nourished regime, and covering various types of nourishments. The analysis is based on over 50 years of sandbar evolution along the Delfland coast, a 17-km long coastal cell at the Dutch North Sea coastline protected by groynes and maintained with frequent sand nourishments. Observations reveal clearly different sandbar behaviour during the unnourished (first 20 years) and nourished periods of the dataset. Introduction of the first beach nourishments (nourished sand primarily placed at the subaerial beach) was found to stimulate sandbar development along previously unbarred sections of the coast. Shoreface nourishments (nourished sand placed at the seaward face of the pre-existing subtidal sandbar) tended to migrate shoreward rapidly at a rate of 20 to 60 m/year at this coast, thereby forcing the pre-existing sandbar to weld to the dry beach. An abrupt transition of sandbar dynamics was observed following a major nourishment operation (∼ 37.5 Mm3 of nourished sand) that covered the entire coastal cell. A new, shallow sandbar formed with a degree of alongshore variability that was unprecedented at the Delfland coast over the full study period. These results imply that individual nourishments can influence the formation and migration of individual sandbars, while continued nourishments can fundamentally change long-term sandbar dynamics along an entire coastal cell.@en

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.@en

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.

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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.

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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.

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The presence of complex nearshore sand bar patterns (i.e. alongshore bathymetric variability) has an impact on local currents, affecting recreational safety and nearshore mixing processes. This study assesses the evolution of alongshore bathymetric variability along the Delfland coast in The Netherlands, over the first 5 years after construction of a mega-scale beach nourishment (the Sand Motor) in the central part of the coastal cell. A total of 38 bathymetric surveys was conducted over this period. Alongshore variability was quantified by subtracting an alongshore averaged bathymetry from the actual surveyed bed levels for both the intertidal and subtidal zone. From 2 years after construction onwards, the subtidal nearshore bathymetry at the Sand Motor is considerably more alongshore variable than the adjacent parts of the Delfland coast. Intertidal variability tends to be high in areas where beach groynes are present.@en

Impact of spatial variability in the nearshore bathymetry on net sediment transport rates has been investigated for a selection of observed beach states at the Dutch coast. The beach states comprise a longshore bar trough and two transverse bar rip situations, which were present at the large scale Sand Motor nourishment at the Holland coast. These observed bathymetric features were then applied multiple times next to each other along a longer stretch of coast to obtain a repeating pattern of the considered beach state. The wave transformation towards the shore, alongshore wave-driven and water-level setup driven currents and sediment transport were computed with the Delft3D model, which has been applied successfully for many other studies at the Sand Motor. It was found that net sediment transport is considerably influenced for the most pronounced transverse bar rip configuration, which was most prominent for conditions with small wave angles (i.e. less than 10° from shore-normal). Furthermore, a decrease in transport rates is found for conditions from larger angles of wave incidence (i.e. 30 to 45° from shore-normal). Impacts of the bathymetries of longshore bar trough and the less pronounced transverse bar rip system on net sediment transport rates were much smaller. The actual cause for the enhancement (or decrease) of the net transport for the transverse bar rip configuration is expected to be related to 1) the oblique orientation of the rip-channel for the considered configuration as well as to 2) a more diffusive pattern of the wave breaking as a result of the refraction on the spatially variable bathymetry.@en

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.@en