The beach recovery process determines the resilience of a sandy coast and is an important aspect of the coastal safety. Sediment stored underwater due to storms is transported onshore by the migration of subtidal and intertidal bars under mild wave conditions. The intertidal zone is an im-portant interface, connecting the marine and aeolian zone and facilitating the transition of hydro-dynamic to aeolian sediment transport. The aim of this study is to investigate the cross-shore mor-phodynamics of intertidal sandbars. The dominant cross-shore sediment transport processes can mainly be divided into surf zone and swash zone processes. The surf zone processes are primarily determined by the balance of wave nonlinearities, undertow and infra-gravity waves. In the swash zone, the cross-shore sediment transport is determined by the balance between the turbulent uprush and the gravity induced backwash. A recent terrestrial laser scanning (TLS) measuring campaign conducted at Kijkduin, the Nether-lands, provided new insight in the intertidal bar behavior. The results of one cross section are ana-lyzed. In a period of 6 weeks, two distinctive intertidal bars formed, grew and migrated onshore during mild wave conditions and eventually eroded again during a storm. Within 5 days, the upper intertidal bar migrated onshore over a distance of 25m and grew with a height of 0.3m, attributed to swash zone processes. Onshore sediment transport fluxes reached values of nearly 2 m3 per meter width in one tidal cycle.
The findings are compared with two XBeach models (surf beat model and hydrostatic swash model) which are used to reproduce the observed morphological behavior of the upper intertidal bar. Both models partly reproduce the onshore migration but show deviating results regarding the final growth of the intertidal bar. In contrast to the surf beat model, the morphological changes in the hydrostatic swash model are primarily induced by swash zone processes, which is comparable to the processes in the TLS measurements. Finally, a conceptual model is developed in which four intertidal bar regimes are classified based on the tidal water level. The distinction determines the dominant cross-shore processes for the for-mation, migration, growth and destruction of intertidal bars. The model shows that the swash zone processes are dominant for the onshore migration and growth of intertidal bars in the overwash regime, while the surf zone processes are dominant in the submersion regime. The findings presented in this study provide a better understanding of the intertidal bar behavior. Although the XBeach models did not reproduce the observed behavior completely, there are some pronounced similarities. Further research is required to increase the knowledge of intertidal bar behavior at a variety of sandy coasts and to improve the performance of rocesses-based models like XBeach.

Information about the hydrodynamic, sediment transport and morphodynamics is limited in pocket beaches (Dehouck et al., 2009). However, a good understanding is necessary for effective management of coastal areas (Scholar et al., 1998). A small artificial pocket beach has relatively large shadow zones, therefore, diffraction might be an important process. Designs of pocket beaches are now based on equilibrium beaches and morphological changes and sediment losses during storm conditions. The numerical model XBeach is used to predict the beach development and losses. However, so far diffraction has not been included in these simulations. Therefore, the main objective of this research is to gain better insight on the effect of including short wave diffraction in the simulations on the morphological development in pocket beaches. Various modes of XBeach give the opportunity to variate including short wave diffraction or not. A simplified pocket beach and a case study beach has been used to research the influence of short wave diffraction on the morphological development within a storm and the underlying hydrodynamic and sediment transport processes. The simplified beach used, has a length of 500 metres and perpendicular groynes. The shadow zone covered about 30% of the embayment area. The case study beach is a stable designed, real, curved pocket beach in Constanta, Romania, with a similar surface area as the simplified beach but a total shadow area of 40%. In both cases the effect of short wave diffraction appeared to be a wider circulation cell than without diffraction and the outflow velocities along the groyne are lower with diffraction. Sediment transport patterns are similar to the circulation patterns, however, without diffraction an abrupt decrease of sediment transport can be found at the boundary of the shadow zone. Erosion and sedimentation patterns are dispersed gradually over the embayment in case diffraction is included in the simulations. In case diffraction is neglected the transitions of erosion and sedimentation are very abrupt. The exact results of the morphological development in the case study look more reliable than the simplified pocket beach results. Both cases, with and without diffraction, show erosion, however, with diffraction the total loss of sediment is 10% less relative to without diffraction. With diffraction, the exposed shoreline shows less erosion (10%), the shadow shoreline shows more erosion (90%), the shadow embayment shows less sedimentation (30%) and the exposed embayment shows more sedimentation (350%). Without diffraction abrupt vertical changes of about one metre exist at the boundary of shadow to exposed zones. The results with diffraction look promising however, a real validation of the results was not possible with the available data. In conclusion, diffraction appears to be an important phenomenon in relatively small pocket beaches. The sedimentation and erosion pattern is much more gradually dispersed over the area. The main alongshore velocities and sediment transport direction in the exposed zone and the gradual development of sediment transport into the shadow zones cause these gradual transitions. The abrupt transitions in the sediment transport patterns without diffraction result in very abrupt changes in the bed level alterations.

One sixth of the world's coastline consist of coral reefs and provide natural flood defence for the people who live in the coastal region behind the reef. However, a rising sea level, changing wave conditions and degradation of corals threaten the coastal safety of these reefs.Numerical models can be applied to study the reef-hydrodynamics and the effects of coral degradation on the reef-hydrodynamics. When non-linear processes are important or the individual waves need to be determined, a phase resolving model is preferred. Within this thesis two issues regarding the application of non-hydrostatic models to coral reefs were studied.

Due to the large bottom gradient in front of a reef, the offshore boundary has to be located in deep water, which means that frequency dispersion becomes important. The accuracy of frequency dispersion within non-hydrostatic models depends on the number of vertical layers. However, the addition of a vertical layer increases the computational time extremely. Therefore, a reduced two layer non-hydrostatic model (XBeach-nh+) was developed with the assumption of a constant non-hydrostatic pressure in the lower layer. In theory, XBeach-nh+ is capable of modelling the wave transformation from deep to shallow water, but the applied boundary conditions cannot force deep water waves. On top of that XBeach-nh+ has never been properly validated for reef environments.

Furthermore, the corals (growing on the reef flat) have a large effect on the reef-hydrodynamics by dissipating a large part of the wave energy. There exist different formulations to include vegetation into a non-hydrostatic wave model, but these formulations are mainly applicable for cylinder shaped geometries, whereas corals are more complex in shape. Apart from the shape, the in-canopy velocity can be significantly different from the free stream velocity. Therefore, a porous in-canopy model was implemented to model the in-canopy velocity, which was used to determine the canopy-induced force on the depth-averaged flow computation.

Firstly, the inclusion of the second reduced layer improves the dispersion relation up to a relative depth ($kh$) of 5 for linear waves. A simulation of biochromatic waves over a plane beach showed that XBeach-nh+ is capable of modelling the energy transfer between the major wave components. Both steeping and reflection of the sub-harmonic were modelled according to the measurements. Furthermore, the validation of random waves over a fringing reef showed the capability of XBeach-nh+ to model the reef-hydrodynamics for different wave conditions (rel. bias of -0.003 for total wave height, -0.081 for LF-waves and -0.103 for the setup). Moreover, the addition of the second reduced layer gives a more robust prediction for all modelled wave conditions, whereas the one-layer model contains more scatter.

Secondly, the in-canopy model captures the canopy-induced force when the canopy parameters were known. Both the in-canopy flow of unidirectional and oscillating flow fields was accurately modelled when the results were compared to the measured velocity though cylinders and corals. Although, the canopy parameters were not always known, it was shown that an un-calibrated in-canopy model, based on porosity and canopy height, gives a competitive result compared to a fully calibrated shear stress formulation. The applicability of XBeach-nh+ in 2-dimensional domain with a coral covered reef flat was shown by modelling a 5 day Swell event at Ningaloo Reef. Reasonably accurate results were achieved when using the in-canopy model, based on the canopy properties.

This thesis discusses the morphological modelling of the gravel revetment on artificial composite beaches. A composite beach is a combination of a sandy lower beach and a gravel upper beach. These beaches are found in nature, but can also be created artificially by placement of a gravel structure on a sandy beach. Artificial composite beaches are widely acknowledged as an effective type of shore protection against runup and overtopping (Blenkinsopp, 2016), but an assessment of such a beach with a numerical model has not been attempted yet.

Therefore this work explores to what extent the morphological developments of these artificial beaches can be modelled with a numerical model. The numerical models that were used in this work are XBeach and XBeach-G. The aim is to reproduce the short term (3 to 6 hours) cross-shore morphological developments of the DynaRev experiments (a series of physical model experiments to investigate the stability of an artificial composite beach under sea-level rise), with a focus on the beach section on which the gravel revetment is placed. The first step was to find the relevant processes that are missing in the currently available numerical models. The missing processes were identified by combining knowledge acquired through the modelling of composite beaches with the currently available models, substantiated by analysis of the DynaRev experiments and literature research.

It was concluded that the numerical models were lacking two important processes: namely the absence of a gravel transport formulation in XBeach and transport of sand in the gravel revetment. The missing processes were implemented into XBeach and the updatedmodel’s performances were verified with the DynaRev experiments as benchmark. The implementation of the XBeach-G gravel transport formulation in coherence with the existing XBeach code for sand transport required under-the-hood adaptations to improve the switches that are already in place for multiple sediment fractions. It was found that a combination of sand transport and gravel transport is possible, but this combination presents difficulties regarding suspended sediment transport in combination with the hydrodynamics and the groundwater dynamics.

The second process was transport of sand in the gravel revetment. This was first analysed by looking at the initial transport rates of sand inside the revetment with a newly introduced equation for transport of sandinside a filter layer (Jacobsen et al., 2017). It was shown that transport fluxes of sand inside the revetment are likely to occur as results of the groundwater dynamics. To see bed level changes due to these transport fluxes this transport equation needs to be implemented into the XBeach code. This was not possible with the current architecture of XBeach and the way it accounts for multiple fractions. Therefore a new accounting system for multiple fractions was introduced in this work and implemented in XBeach, named the two-line model. In this experimental model sand transport gradients in the revetment are translated into visible sinking of the revetment (erosion) and settling of sand within the gravel revetment (deposition). In the model’s current state the erosive locations appear to match with the DynaRev observations, whereas locations where deposition occurs are no match. The two-line model was sensitive and showed instabilities, mainly due to high groundwater velocities that were caused by wave breaking in themodel. The results of the experimental model can possibly be improved by including vertical groundwater velocities to model sand transports, possibly also in combination with the addition of suspended sediment transport of sand inside the revetment.

Numerical process-based morphodynamic models are widespread in coastal engineering practice and have become the new standard when it comes to assessing the impact of natural or man-made structures on coastal environments. The most common practice among engineers is to focus on a single spatial and time scale, which means either neglecting certain processes under the assumption that they will average out, or performing detailed simulations for short time-spans in order to optimize the normally limited computational resources. Despite the efforts from several authors, at the moment there is a lack of a clear methodology which would allow incorporating the relevant physical phenomena only when required, hence optimizing the computational effort.

The above leads to the main research objective of this thesis: to gain insight in what is the added value of coupling process based morphodynamic models, regarding the morphological impacts near the beach. For this purpose two models that were originally conceived to resolve different timescales are selected; XBeach as a storm model, and the new suite from Deltares, Delft3D-Flexible Mesh (D3D-FM) as a long-term morphodynamic model. The area selected as study site is Anmok beach, located at the east coast of South Korea. The coastal erosion at this location is not yet well understood (mainly due to human interventions and storms) plus the micro-tidal wave-dominated environment makes this location ideal for this study. Recent researches on this site have found that there is a delicate balance between the stormy and calm periods, where the high energy wave events are the main drivers of local morphology.

One of the main findings in this thesis is that the coupling of independently calibrated models does not necessarily provide better morphodynamic results than the results obtained by running each model separately. Including different processes such as infragravity waves or Eulerian mass transport (which enhances the offshore sediment transport in the surf zone) during highly energetic events tend to generate large supratidal beach erosion. However, the post-storm recovery mechanisms present in long-term morphodynamic models are not sufficient to bring the sediment back to the beach. Therefore, it is recommended to include all the relevant physical processes (storm erosion and post storm recovery mechanisms) when following a coupling approach in order to have a coherent morphodynamic balance. Furthermore, the coupling of models can play an important role in identifying which processes are missing or are not fully represented by the different modelling packages.

The erosive effect of cumulative storms was shown to be relevant in the short to medium term and might become a key parameter when defining, for instance, the worst case scenario regarding shoreline retreat. Despite the fact that uncoupled long-term morphodynamic models produce better average results in the case of Anmok beach, the implementation of a coupled scheme was proven to be important when the erosion due to cumulative storm effects cannot be neglected.

Among the advantages of using D3D-FM for this work is the implementation of the Basic Model Interface, which meant an important reduction in bookkeeping efforts and the possibility to seamlessly couple D3D-FM with XBeach. This procedure allowed for the incorporation of more complex phenomena (such as infragravity waves) with an acceptable increase in computational time.

A research version of Delft3D (D3D) with specialized sediment transport equations in the swash zone was tested in an attempt to enhance the post-storm recovery mechanisms. The results obtained are promising in the sense that the accretion of the shoreline and lower dry beach was reasonably enhanced, especially when considering all the limitations involved in modelling the morphodynamics of the swash zone with a stationary wave model. Another important conclusion is that these models were capable of depositing the sediments at the lower backshore at best. Hence, there is still the need of a mechanism/process capable of transporting the sediments farther upslope into the dry beach or dunes, such as aeolian transport.

The decision to undergo with a coupled or uncoupled approach depends on case-by-case basis. For current practice, it is recommended to develop a coupled simulation in the medium-term, where both storms and calm periods have significant effects and where the intra seasonal variation could be a parameter of interest. For short-term simulations, an uncoupled storm model (e.g. XBeach) is recommended as is the most accurate for such a time span. For long-term simulations, the general recommendation is to run an uncoupled long-term model such as D3D. In this case the storm erosion and post-storm recovery processes are expected to average out, being the long-term model the most suitable package to obtain average morphological results.

For future work it is recommended to add an aeolian process-based model and incorporate the swash zone sediment transport module into D3D-FM as this would move us one step closer towards the development of a fully coupled model where all the relevant processes (storm erosion, post storm recovery and aeolian transport) are included.