Sand waves are rhythmic seabed patterns which develop as a result of the interaction between tidal flow and perturbations in the seabed. Due to their size and dynamic behaviour sand waves can pose a threat to offshore infrastructure such as cable trenching. To ensure the safety of these cables, it is crucial to understand the interactions between sand waves and trenches. Residual currents, currents that are not induced by the tide, can exert a significant influence on sand wave dynamics due to the non-linear relationship between flow velocity and sediment transport. Hence, this thesis aims to investigate the effects of residual currents on the morphological interactions between sand waves and trenches. A Delft3D FM model is used to simulate a case study of a trench within a group of sand waves in the Belgium Continental Shelf. The influence of residual currents, mostly induced by wind and atmospheric pressure, is studied during a period of a storm and a period with representative residual currents. It is found that during the tidal cycle at the peak of the storm, the residual currents cause an increase of 66% in sediment infilling into the trench compared to tide-only conditions. Since this is in the same order of magnitude as the background infilling due to tidal dynamics, this is less than expected. The residual currents are however strong enough to reverse the sediment transport patterns of the sand wave and trench system. During the representative residual currents, the contribution of residual currents is found to be insignificant, especially compared to the observed influence of the spring-neap tide. This study concludes that the overall effects of the residual currents, induced by wind and atmospheric pressure, can alter sediment transport patterns but the magnitudes are insignificant in the case study under investigation. It is recommended, for further research and for effective design and maintenance of offshore infrastructure procedures, to keep in mind that morphodynamics of sand wave and trench systems are highly subject to the variations in tidal dynamics induced by the spring-neap tide.

Wave runup observations are key data for coastal management, as they help validate predictive models for the inundation frequencies and erosion rates. Efforts to develop automated algorithms that effectively identify the instantaneous water line from video imagery have led to a plurality of methods. However, under dissipative conditions, the presence of a seepage face often hinders proper extraction and requires time-intensive data quality control or manual digitization. In this study, we explore traditional color contrast (CC) preprocessing techniques and develop a novel method that incorporates a measure for texture roughness — specifically, local entropy — alongside saturation. The CC-model showed good agreement with the manually digitized water line (0.12 m root-mean-square-
error (RMSE) and correlation coefficient (r) of 0.94), and with its runup statistics (0.08 m
RMSE, and r of 0.97 for the 2% runup exceedance, R_2%). The timing aspect of the method also
showed good agreement (3.88 s RMSE, and r of 0.70 for T_m−1,0). Concurrently, a convolutional
neural network (CNN) informed by CC-preprocessed images was cross-validated using nine manually labeled video time series, each lasting 1 hour and 30 minutes. The CNN model demonstrated good agreement during cross-validation with manually labeled time series (0.10 m RMSE and r of 0.96 for the full-time series, and 0.09 m RMSE and r of 0.97 for R_2%). The temporal dimension of the CNN estimate was also satisfactory (3.51 s RMSE, and r of 0.79 for Tm_−1,0). The observed R_2% values showed the best agreement with the formula for extremely dissipative conditions from Stockdon et al. (2006), with RMSE-values lower than 0.13 m and r-values that exceeded 0.70 for all three methods. When applied to other datasets, the CNN method occasionally failed to accurately capture the water line due to specific characteristics of the new timestack images. These results validate our ML method as a viable proof of concept and challenge us to enhance its adaptability and accuracy across varied environmental conditions. Despite these limitations, the CNN method can be effectively implemented for long-term runup analysis. Additionally, the CC method is anticipated
to be applicable across similar beaches along the northern Gulf of Mexico for long-term extreme value analysis and wave-by-wave analysis. Both methods demonstrate potential in reducing the time required to extract the instantaneous runup from video imagery under dissipative conditions and enhance real-time monitoring, enabling better predictive modeling of coastal processes.

This study focuses on the infragravity (IG) waves at 5 measurement sites (Brouwershavense Gat 2 (BG2), Oosterschelde 4 (OS4), Cadzand, Hansweert and Bath) in the Scheldt region, examining their characteristics in these waters as well as to what extent they can be modelled using SWAN (Simulating WAves Nearshore). To facilitate this, data collected during the occurrence of four storms (Corrie, Dudley, Eunice and Franklin) that traversed the North Sea in January and February 2022 has been used.

A spectral analysis has been used to provide insight into the IG and sea-swell (SS) wave field. To this extent, the significant wave height for the IG (0.005 – 0.04 Hz) and SS (0.04 – 0.33 Hz) frequency bands have been derived, in addition to the storm averaged wave period. Measurement device BG2 documented the largest storm averaged significant wave height for both the IG and SS frequency bands (0.256 m and 3.46 m during storm Corrie, respectively). The measurement device with the smallest storm averaged significant waveheight values was Hansweert, which measured 0.024 m and 0.217 m for the IG and SS wave frequencies, also during storm Corrie. Most of the wave energy for the measurement devices in the Western Scheldt, Hansweert and Bath, can be attributed to waves with frequency values larger than the upper boundary for SS waves (T < 3 s). The fraction of the total wave energy contained within the IG and SS frequency bands ranged from 0.1 to 0.56 for these two measurement devices. Conversely, the IG and SS frequency bands contain 0.81 to 0.96 of the total wave energy for measurement device Cadzand, BG2 and OS4. The correlation between the IG and SS significant wave heights is mostly strong for the BG2, OS4 and Cadzand measurement devices (0.657 to 0.956) and generally moderate for Hansweert and Bath (0.474 to 0.87), with one outlier equivalent to 0.128). A bispectral analysis was used to offer more insight regarding the components contributing to the total IG wave field. The results at Bath proved to be erroneous for all storms except Corrie, as the derived free IG (FIG) contribution was negative. The origin remains unclear, although it appears to be related to unexplained spurious bursts in the wave elevation signal. The bispectral analysis for the remainder of the locations lead to maximum TIG wave heights ranging from 0.05 m to 0.12 m for Hansweert, 0.15 m to 0.39 m for BG2, 0.11 m to 0.35 m for OS4 and 0.21 m to 0.34 m for Cadzand for the 4 storms. The contribution of the bound IG (BIG) and FIG energy at Bath and Hansweert appears to fluctuate heavily, undergoing rapid changes on an hourly basis. For BG2, OS4 and Cadzand, the FIG energy contribution is strongly dominant as it generally amounted to > 0.75 of the TIG energy.

The capabilities of the SWAN model were evaluated by simulating FIG waves in the North Sea basin. The model relates incident SS wave energy to reflected FIG wave energy with FIG source lines based on the reflection parametrization of Ardhuin et al. (2014). The hourly SS wave inputs were provided by Copernicus Marine Service. The predictive skill was used to qualitatively assess to what extent the model is able to reproduce FIG waves in the Scheldt region. The model was unable to correctly predict the FIG waves Hansweert and Bath, where the predictive skill values for all storms ranged from 0.0003 to 0.0009 for Hansweert, and were 0 for Bath. Better predictive skill values were obtained for BG2 (0.5044 to 0.6565), OS4 (0.3327 to 0.4293) and Cadzand (0.6466 to 0.7510), but there is still room for improvement. The lacking skill values can be attributed to not enough FIG wave energy being able to penetrate the estuarine waters. Reflective lines were implemented along the coastline of the Western Scheldt in an attempt to force more FIG energy into the estuary. This only led to a minor increase for the predictive skill values (O(0.002) for Hansweert, 0 for Bath and O(0.0001) for BG2, OS4 and Cadzand) but from a spatial standpoint, it appears that there are locations in the estuary where the absolute increase was O(0.02), which is significant as there are areas in the estuary where the significant FIG wave height has doubled as a consequence of the implementation of the reflective lines.

Dunes often act as the primary line of defence for low-lying hinterland against storm surges with dune erosion in the swash-dune collision regime. In the swash-dune collision regime, an elevated total water level, consisting of tide, surge, and wave setup, temporarily submerges the beach. As a consequence, the incident swash may run up to the dunes and collide with the dune face. The damage to dunes following from these collisions can be severe. In the most extreme of cases, storm surges with dune erosion in the swash-dune collision regime can lead to dune breaching or failure with flooding of the hinterland as a consequence. These floods can have devastating economic effects and potentially lead to loss of life.

The risk assessment of areas protected by dunes is often performed using predictive dune erosion models. Such models first use possible storm conditions as input, then use a set of physics based and empirical equations to model the physical processes based on that input, and finally use a measure of impact to the dunes from the model results to estimate the amount of damage to the dunes. However, not all physical processes are currently fully understood, complicating accurate reproductions in models and the risk assessment of areas protected by dunes.

This dissertation aims to study two such physical processes that are relevant to dune erosion during storm surges in the swash-dune collision regime. The first process is the suspension of sediments in the inner surf zone. The total amount of sediment in the water column affects the sediment transport rates. As a consequence, sediment concentrations can have a substantial impact on the magnitude and speed with which sediments eroded from the dune face are transported offshore, and thus on the total amount of dune erosion.

The second process studied is sediment transport due to soil instabilities, i.e. the slumping (or avalanching) of sediments from the dune face. Sediment transport due to soil instabilities leads to dune scarping and a gradual retreat of the dune face. If this type of transport persists for a prolonged period of time, the retreat of the dune face can continue until there is no more dune to erode. As a consequence, the dune breaches and enters the overwash regime, and complete failure of the dune may follow.

To study both processes, a prototype scale field experiment was conducted in the winter of 2021-2022. Two artificial dunes were constructed in close proximity to the high water line on a sandy beach. This increased the probability that the total water level driven by a storm event would result in dune erosion within the swash-dune collision regime. The dunes were monitored for a period of three months and within this period three such events occurred.

The suspension of sediments in the inner surf zone was studied by comparing variability in measured, wave-averaged (i.e. 20 min mean) suspended sediment concentrations. These variations were compared to variability of hydrodynamic drivers that are known from literature to govern sediment suspension during storm conditions. Overall, sediment suspension due to bore turbulence appeared the dominant suspension driver during energetic events representative of storm conditions. During such events, wave energy was saturated in the inner surf zone, and almost all waves were breaking and contributed to the generation of bore turbulence at the free surface. The outcome of the first study suggests that, based on the events analysed, dune erosion models may achieve more accurate results if computations of the magnitude of suspended sediment concentrations were to include a bore-induced turbulence term. If such a term is already included, models should properly address the relative importance of bore-induced turbulence when compared to other drivers.

Sediment transport due to soil instabilities was studied by analysing profile crosssections of the dune face during two storm events. Overall, the morphodynamic behaviour of the upper dune face and dune crest was primarily steered by the morphodynamic behaviour at the dune base. The morphodynamic behaviour (i.e. erosion rate) of the dune base correlated well with the elevation difference between the dune base and the incident total water levels, specifically the square of the total water level that was exceeded for 2% of the time. The slumping events that occurred during both storms likely occurred when sediments from previous slumps at the dune base were nearly depleted by the persisting erosion rate. As a consequence, under similar erosion rates, a new slumping event occurred sooner when the volume of the preceding slump was smaller. A clear relationship could not be established between hydrodynamics seaward of the dune and the volume of individual slumps.

Different model approaches are currently being used to implement sediment transport due to soil instabilities. These different approaches all use the persisting erosion rate of the submerged part of the dune base to steer the erosion of the upper dune face. Therefore, according to the field experiment results, these different approaches may all be able to achieve accurate dune erosion volume magnitudes. Still, depending on the application (e.g. one-dimensional versus two-dimensional modelling), one approach might be more suitable than others.
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Sand wave is one of the large-scale seabed morphological features with a typical wave height of metres and a wavelength of hundreds of metres. Its mobile nature causes a nuisance to offshore infrastructure and marine transport. Pre-sweeping is a construction procedure commonly involved in submarine utilities installation to dredge existing sand waves but recovery of sand waves is usually observed after dredging. The underlying processes of sand wave recovery remain relatively underexplored. Given the potential implications for improved dredging planning, this thesis aims to investigate the drivers of sand wave recovery and examine the modelling performance of Delft3D FM on the recovery process through a case study. The numerical model is the first 3D morphological model specifically for simulating sand wave recovery after pre-sweeping. With the incorporation of DelftBlue and MORFAC, computational efficiency has been significantly enhanced. The results show that the transverse slope effect and the oblique current flow supply sediment into the dredged area causing sand wave recovery. This study also elaborates on the two stages in the recovery: an adaption period referring to the morphological adaption of the dredged profile in response to the forcing and a subsequent regeneration period referring to the growth of a feature developed during the adaption period. Although the precise estimation of the recovery duration by Delft3D FM remains challenging, this pioneering research provides an explanation on how tidal flow drives the recovery and reveals the accomplishment of Delft3D FM in modelling the phenomenon. Further exploration of this subject, such as the impact due to storm and the influence of trench configuration, is suggested to further enrich our understanding of sand wave recovery.

To reduce computational efforts, surrogate models have been developed for dune erosion prediction. Current surrogate models can describe the relationship between the XBeach input and output (Athanasiou, 2022) and provides a prediction of a morphological indicator based on a parameterized input (profile shape parameters and hydrodynamics). In this research, first steps are taken to set-up a surrogate model that is able to deal with spatial input and the prediction of actual profile shapes along the Holland Coast. Using a convolutional neural network structure (U-Net), several model set-ups are explored and scaled-up to a realistic storm scenario along the Holland Coast.

Mangrove forests are important (sub)tropical intertidal ecosystems providing coastal protection and other ecosystem services like carbon storage and biodiversity. In view of the loss of these forests many rehabilitation projects have started, but unfortunately these efforts often fail. A main cause for coastal erosion is a disrupted (fine) sediment balance due to human interventions. This imbalance prevents establishment of propagules (floating mangrove seeds) at intertidal areas and the early growth of planted mangroves. Coastal solutions such as permeable dams have aimed to stop the erosion, but we need additional systematic methodologies to understand the coastal system.

The objective of this thesis is to extend the physical and ecological understanding of coastal mangrove systems, especially of currents, sediment pathways and propagule pathways, to improve rehabilitation strategies using Demak (Central Java, Indonesia) as study area. Demak's coastline suffers from erosion problems, originating from extensive land use by fish ponds and land subsidence. An idealized morphostatic 3D model was set up in Delft3D-4 to simulate the hydrodynamics of the area, influenced by the interaction between flows due to river discharge, tides and wind. The simulations are used as input for the SedTRAILS particle tracking model to compute sediment and propagule pathways.

The model results support that sediment supplied by alongshore currents and riverine sediment could serve as source for the coastline. The river plume is at the same time obstructing as well as trapping sediment and propagules, because stratification prevents mixing of the sea and river water. However, sediments suspended near the bed could be transported underneath the river plume towards the coast by estuarine circulation. Another lesson learnt is that sediment (suspended in the water column) and propagules (floating around the water surface) do not follow the same pathways, as for propagule dispersal the wind direction is of much larger influence than tidal currents. The simulations confirmed that seasonal variability in wind direction could significantly influence the establishment area and availability of propagules, and old fish pond bunds can increase the establishment area for propagules. Sensitivity analysis show that the timing of propagule release within one tidal cycle (flood-ebb) influences the probability to successfully establish on the nearby coast. A propagule survival module is developed (based on inundation-free period and bed shear stress) as stormy seasons may increase the sediment establishment, but the larger bed shear stresses may lower the survival of propagules.

Although this modelling study is focusing on Demak, there are many other similar mangrove coasts around the world where an idealized model would enable to understand where the sediment and propagules are going and the travel time from source to sink. Permeable bamboo dams would be recommended to add to the model in further research, because they could increase the survival of propagules by reducing bed shear stresses and increase the establishment of propagules by trapping them. In conclusion, this research has made a start in counteracting coastal erosion and improving the success of mangrove forest rehabilitation by studying sediment and propagule pathways, and developing a propagule survival module.

Mangrove forests are important wetland ecosystems that inhabit intertidal zones within tropical and sub-tropical latitudes. They offer a diverse array of ecosystem services, including coastal protection and carbon sequestration. Despite their significance, global mangrove cover has declined in recent decades, prompting various restoration initiatives, most of which often fail due to a lack of system understanding of the local mangrove habitat and associated physical and biological processes. A method gaining prominence involves the use of permeable dams to restore the sediment balance along eroding coastlines, thereby facilitating the restoration of mangroves in the area. These dams help dissipate the approaching waves and reduce current magnitudes, thus creating a low energy environment behind them, ideal for sediment deposition. However, a comprehensive understanding of the system-wide implications of utilisation of permeable dams remains largely unexplored.

The study is focused on examining the response of a coastal system to structural presence, based on currents, waves and sediment and propagule pathways, to optimise restoration strategies. The coastline of Demak in Indonesia is chosen as the system to be modelled due to the persistent erosion problem in the region and the ongoing use of permeable and impermeable structures to mitigate the problem. A nested model was set-up in Delft3D-4 to estimate the flow hydrodynamics resulting from river discharge, tide and wind forcing in the area. Wave propagation in the region was also modelled using Delft3D-4 with a standalone nested wave model. Output from the hydrodynamic models served as input for the Lagrangian particle tracking model, SedTRAILS, to compute the sediment and propagule pathways.

The hydrodynamic models demonstrate a decrease in both current magnitude and wave activity behind the structures, with a greater reduction observed in the case of impermeable structures. However, the radius of influence is constrained to less than a kilometre from the structures. The sediment pathways reveal reduced sediment movement behind the structures. However, the sediment imported due to the structures are sourced from within the intertidal basin which could trigger coastline retreat in the region. Additional tests uncover impermeable structures with larger length to opening size ratio, placed closer to the mangrove fringe to be able to retain sediment with comparatively higher efficiency. In case of propagules, the trapping behaviour of the structures is less apparent, with dependencies on the relative location of the structure to the sources and the direction of wind. In Demak, when structures are located relatively offshore to the propagule sources, they are able to trap the propagules, thereby creating a possibility for mangrove restoration. The study concludes that coastal structures enhance sediment and propagule retention, with varying efficacy depending on the type, location and length of the structure, suggesting their potential utility as a valuable tool for mangrove restoration.

The coastal safety of the dunes are of high importance for low laying countries (Chen et al., 2017). This is especially true for the Netherlands as it is used officially as a safety measure against flooding from extreme weather since 1984 (Rijkswaterstaat, 1989). As of current 1D modelling of the waves is an acceptable method for the Dutch coast to assess the safety of the dunes (Den Heijer, 2013). For assessing the dune coastal safety the 98t h percentile of the runup R2% (Stockdon et al., 2006) is used for defining the wave response during a storm on the dunes. The current dune coastal safety studies consider bound waves (infragravity and sea- swell waves) propagating from offshore to nearshore. However, due to the Dutch coast being located at a very large semi-enclosed basin additional free infragravity waves can be present during a storm next to the bound waves that are reflected from other coasts (Daouk, 2020) as leaky waves (Herbers et al., 1995), and potentially increasing the R2% magnitude. Here we show that the R2% with the additional free infragravity waves in the wave domain can have a significant response on the dune coastal safety. The additional free infragravity waves can have tens of centimetres of R2% magnitude increase that is dependent on the slope of the domain. For the additional free infragravity waves a higher R2% is observed for steeper slopes, while a larger R2% growth is observed at the milder slopes when the free infragravity waves are initially introduced to the slope. The dune coastal safety assessments conducted thus far do not consider the effects from leaky waves from other coasts, therefore underestimating the safety provided from the dunes against flooding. With the research conducted by considering additional free infragravity waves with the traditionally bound waves originating in a storm, a better insight can be provided for dune coastal safety. As a result, companies and governmental bodies such as Rijkswaterstaat in the Netherlands that assess the coastal safety of the dunes can benefit by having a more complete representation of the coastal hydrodynamics that can as a result prevent flooding or provide dune nourishment after the episodic event (Sallenger, 2000).

The Building with Nature approach has been gaining ground in hydraulic engineering, increasing the importance of understanding the cross-shore morphodynamic processes. The intertidal zone, where marine and aeolian processes come together, is an important link in the transport of sediment from the sea towards the dunes. The grain size distribution affects the sediment supply in the intertidal zone. This research investigates the effect of marine processes on the cross-shore variations of the grain size distribution in the intertidal zone by using a one-dimensional non linear shallow water XBeach model.
The intertidal zone is subject to shoaling, surf and swash zone processes. The grain size influences the beach slope, the initiation of motion and settling to the bed. The cross-shore sediment transport is the combination of sediment that is stirred up from the bed and subsequently transported. Breaking induced turbulence enhances stirring of sediment from the bed and keeps sediment in suspension. The amount of stirring and the transport direction depends on the wave conditions.
Input and control data for the model study was provided by the Scanex 2020 fieldwork campaign at Noordwijk, the Netherlands. The ADV velocity data combined with a pressure signal has been used for the tidal and incoming wave signal. Cross-shore profiles have been determined in Matlab based on terrestrial laser scans. Soil samples of the intertidal zone were taken with a sand scraper and analyzed with a sieve tower. For the initial grain size distribution is the average distribution of 14 samples on a transect was used. Based on wave, wind and soil sampling data a model period from 29-2-2020 02:00 to 10-3-2020 13:00 was selected.
The XBeach model used is as described by Reniers et al. (2013), but with a time-averaged turbulent kinetic energy and a different implementation of the Riemann boundary. The model consisted of a 176 x 3 grid with a grid size of dx=1 m and dy=5 m. For the initial bathymetry the laser scan of 29-2-2020 02:00 was used. The initial grain size was imposed on all the model grid cells. Additional to the standard run, runs have been performed to research the effect of a storm, the model sensitivity and the effect of aeolian transport.
The model shows a pattern of cross-shore grain size variations with coarser sediment from x=20 to x=56 m, finer sediment from x=57 to x=105 m and fluctuating grain size from x=106 to x=136 m compared to the initial grainsize. After 24 h a grain size pattern establishes with a clear deposition of fine sediment on the upper beach. The pattern remained stable for nearly the full model period. After 200 hours the fines become less prominent and move onshore. On the intratidal scale sediment becomes coarser when submerged and finer when emerged, except near the high water line where fine sediment is deposited.
The model reproduced the same pattern of grain size variations over the cross-shore as was found in the soil samples of 10-3-2020. As the cross-shore grain size pattern remained stable during the model period, processes on the spring-neap time scale or storm time scale seem to govern the cross-shore variations of the grain size. For the aeolian transport this would imply that for this model period the fine sediment supply is controlled on the same time scales. Nevertheless, considering that aeolian transport could have resulted in coarsening of the fines in the upper intertidal zone, processes over a single tide, could be more important than was visible in the model result.

The coastline of Demak, Indonesia has been eroded during the last 15 years. To restore the natural coastal protection which existed out of mangroves forest, permeable dams, consisting of bamboo poles with a brushwood filling, have been built to attenuate the waves and facilitate sedimentation behind the dams and thus creating a habitat for mangroves. However these designs required a lot of maintenance, so a new type of design is proposed without a filling of brushwood, containing only vertical bamboo poles. Next to this the possibility to include aquaculture in the design is proposed. Therefore this study assesses the wave transformation by the new designs in Demak, Indonesia with the numerical wave model SWASH. In order to do that first the hydrodynamic conditions are analyzed to obtain the design conditions for the structures, and then SWASH is validated against laboratory experiments to find the right drag coefficient values. The design wave conditions are based on the local water depth, offshore wave heights and periods and local bathymetry. The offshore waves are based on a dataset from WaveWatch III, which compares well with measurements of a local storm event. The dataset of WaveWatch III is filtered and extrapolated to determine the wave conditions for return periods(R) of 1 and 5 years. This resulted in offshore waves with Hm0 = 2.08 and 2.39m, and Tp = 6.9 and 7.5 s respectively. The water level is mainly influenced by the tidal elevation and the surge levels. The surge levels are obtained from a risk assessment of the coast of North Java (Willemsen et al., 2019), which vary between 0.63 to 0.68mfor R = 1 and 5 years respectively. The tidal levels are determined using the water depth measurements of two transects from Van Domburg et al. (2018) and analysed with Matlab tool Utide by Codiga (2011), resulting in a spring tidal range of 85 cm and a neap tidal range of 50 cm.
Four different configurations from the experiments of Jansen (2019) are used to validate SWASH: single row, longitudinal(the spacing in flow direction is longer than the lateral spacing), open uniform and dense uniform configuration. These configurations are modelled in the numerical wave model SWASH by use of the vegetation module. The most influential factors are: the drag coefficient, the way to describe mass conservation and the number of stems(cylinders) per m2. For the drag coefficient the bulk drag coefficient of Gijon Mancheno et al. (2021) is used, which contains factors for sheltering, blockage and the KC state of the flow. For the densely packed configurations the bulk drag coefficient proved to have a better agreement than the drag coefficient of a single cylinder. The sensitivity to the number of stems per m2 is small when implementing the longitudinal configuration as an average amount of stems perm2 or by specifying the individual rows of the configuration and so locally increasing the number of stems. Three methods of describing the mass conservation in SWASH are evaluated: by means of a cross sectional approach that is expressed by the blockage factor in the bulk drag coefficient Gijon Mancheno et al. (2021), by a volumetric approach due to activating the porosity in SWASH which means that the blockage factor cannot be included in the drag coefficient and by a combination of these two. For the longitudinal configuration the best agreement is found the cross sectional approach and for the single row configuration the best agreement is found by the volumetric approach.
Once SWASH is validated, the designs of several structures are investigated. Firstly, a design consisting out of two rows of bamboo poles is considered where the spacing between the rows is varied to find an optimum distance. The transmission rate Et /Ei decreased from 75% to 55% with a spacing sx = 0.42 m to 5.8 m, larger spacings did not result in less transmission. If one wants to be conservative at least three rows are needed to have a lower transmission rate of 50 %. When mussels are considered, the structures have to be placed in deeper water as mussels can only grow between MLWS and 40 cm above the bed. To provide enough space for mussel growth, the poles have to be placed more sparsely. The effect of a larger water depth in combination with a limited pole length and sparse structures is larger than the extra drag and frontal area provided by mussels, especially since they did not cover the whole pole length, and resulted in high transmission rates. It is thus recommended to, or place a high number of rows of mussel poles or place a few rows without mussels followed by poles for mussels. This decision however also depends on benefits that mussel poles may bring and the cost of the materials, therefore a cost/benefit analysis is required.
This thesis found an efficient design that can be used in reducing wave attenuation along muddy coasts without the need of a brushwood filling. Hereby it provides an economically and user friendly alternative with respect to the current design, as it requires less material and maintenance.

Recent studies have shown that flood risk contributes to a major part of the total risk caused by natural hazards in Western Europe. Especially in the United Kingdom, flood risk has been identified as a substantial threat, which is likely to continue to grow due to (coastal) urbanisation, sea level rise and climate change. A separate study has shown that common analysis practice may underestimate flood risk due to the exclusion of compound flood events. These are flood events with multiple flood causes, such as a combined coastal and pluvial flood event. Floods are often caused by a complex system of multiple dependent flood driving mechanisms, which vary and interact as time progresses. Various fundamentally different methodologies have been developed to estimate the susceptibility of an area to floods. There is no singular best methodology which can be applied to every problem. Due to the complex nature of flood events, it is vital to understand these methodologies and their characteristics to represent the susceptibility to flooding correctly. Misrepresentation of flood susceptibility can lead to inefficient decision making, or worse, insufficient flood safety. The goal of this research is to compare several flood susceptibility estimation methodologies (including coastal, pluvial, and compound flooding), to observe the results and clarify notable differences in outcomes. A case study is conducted on the coastal town of Eastbourne, which has various common, but non-trivial characteristics. The flood susceptibility of Eastbourne is estimated using various estimation techniques, such as statistical analysis and numerical modelling. Not only did this give insight into the flood susceptibility of Eastbourne, but it also illustrated the capabilities of the selected methodologies. Eastbourne has a history of coastal and pluvial/fluvial flooding. In order to include the physical mechanisms of these flood types, it was decided to use relatively computationally expensive flood susceptibility estimation methodologies. Hence, use was made of sensibly chosen storm scenarios, based on extreme value analysis. The statistical analysis started with a general exploratory data analysis in order to find notable dependence structures between variables. Afterwards, two variants of extreme value analysis on reanalysed historical data were used to create "smart" scenarios. First of all, the peak over threshold approach (POT) was used to create scenarios in which a singular flood type was dominating. Secondly, the conditional approach was applied to create compound storm scenarios. In total, 8 different scenarios were created. These scenarios gave the input conditions for the models. At the coastal boundary, a spectral action balance resolving wave model SWAN was used to estimate close shore conditions based on offshore scenario input. These nearshore conditions where then used to estimate overtopping rates into the inundation domain via three different overtopping methodologies. First of all, a empirical equation (the "new" overtopping formula of the EuroTop manual) has been applied. Secondly, a Gaussian process emulator (Bayonet GPE), an approach similar to a neural network, has been applied. And lastly, use was made of a numerical hydrodynamic model SWASH to simulate wave transformation and overtopping. This last approach is especially interesting since numerical models have the ability to improve upon spectral action balance resolving wave models. Numerical models can implicitly account for infra gravity waves, a physical mechanism that is known to impact overtopping rates, something that spectral action balance resolving models struggle with. The numerical hydrodynamic model HEC-RAS, is used for the simulation of inundation caused by the storm scenarios. All 8 scenarios are calculated twice, in order to compare simulations made with two sets of governing equations: The diffusive wave approximation of the shallow water equations (DSW) and the full shallow water equations (SWE), also known as the Saint Venant equations. The DSW approach has the advantage of being relatively computationally efficient, whilst SWE approach has the benefit of including a more accurate representation of physics. Since the DSW approach is more computationally efficient, it allows for solving on a higher spatial resolution. The model was applied to the part of Eastbourne, which is deemed to have the highest flood risk. It was then fed with overtopping discharges at the coastal flood defences and used precipitation events as defined by the respective scenarios. After the simulation, flood maps have been generated to compare the inundation patterns. Inundation curves have been made as well, for a more objective and apprehensible comparison of the severity of the flood events. The overtopping results showed that all three approaches agreed reasonably well, and were able to estimate plausible overtopping rates. There were no overtopping measurements for validation purposes, but the overtopping magnitude does agree with the coastal flood history of Eastbourne. The numerical method is more advanced than the empirical formula and the GPE. However, the results also showed a large sensitivity with regards to the (vertical) discretisation. Furthermore, the numerical approach, as applied in this research, did require explicit inclusion of refraction and fitting of breaking behaviour based on computationally expensive simulations, in order to give reasonable estimations. The numerical approach is promising and has the potential to become more accurate than the other methods considered. However, in its current state the method does seem as uncertain as the other considered approaches, whilst being more complex to implement. Lastly, regarding the comparison of governing equations for the estimation of inundation, it was found that the low spatial resolution SWE approach systematically underestimated inundation severity with regards to the high spatial resolution DSW approach. In addition to this, it was found that the times between the storm peaks and the inundation peaks was systematically lower for the SWE simulations than for the DSW simulations. Both these phenomena can be explained as a consequence of using a different spatial discretisation. When using the same spatial discretisation these effects disappear and the inundation curves agree well. There are, however, still some smaller local differences between simulations visible on the inundation maps. In the case of Eastbourne, modelled as described in this report, it is thus more important to accurately include topological features than to accurately represent the flow physics. This could, however, be different when considering higher spatial resolutions.

Large morphological features, so-called sand ridges, with an average height and length in the order of 10 meters and 3 kilometers respectively are located at future offshore wind farm locations in the Dutch North Sea. Recent literature, considering relatively deep water conditions (kh 1, with k the wave number and h the water depth), has shown that a wave field propagating over a shallowing depth transition shows an increase in kurtosis. Kurtosis is a measure for the vertical deviation from the commonly applied linear ocean wave model, and therefore describes a higher probability of occurrence of "rogue" waves. These "rogue" waves are known to the general public from sailors' tales and thrilling videos. Scientifically, rogue waves are described as statistically abnormal waves and are generally classified as waves that exceed twice the significant wave height. Because these waves have an abnormal height, they are associated with extreme forces. Considering the sand ridges at future offshore wind farm locations, and possible increased damages to wind turbines due to rogue waves, this thesis researches if the increased probability of occurrence of rogue waves due to the local bathymetry as observed in the relatively deep water literature also applies to relatively shallow water conditions and the Dutch North Sea. To achieve this objective, physical experiments have been performed in the Hydraulic Engineering laboratory of the Delft University of Technology. Also, an existing statistical model that describes a rogue wave mechanism due to the presence of depth transitions is evaluated. The model describes the rogue wave mechanism as the interaction between the transmitted wave field and second-order components: the free sub- and super-harmonic components generated by the depth transition. As the model assigns a prominent role to these sub- and super-harmonics, it was ensured that these components are generated correctly during the physical experiments. Following, this thesis also assesses and contributes to the second-order wave generation at the TU Delft laboratory facility.

For both developed and developing nations, coastal zones form an attractive location for urban settlements. With the expected increase in the earth’s population, coastal areas will experience a further increase of inhabitants. Floating city development could therefore be an interesting alternative for land-based urban expansion on land [41]. Expanding an urban settlement towards the ocean however, will make it more susceptible to extreme forces such as a tsunami waves. By generating more knowledge on how a floating structure interacts with a tsunami wave, it can show the potential value that a floating city can bring. If a floating city limits the effect of (extreme) events such as tsunamis, it can protect the coastal zone which is located near a floating city. To provide an answer to this question, this research will focus on how a floating structure can reduce the transmission of a tsunami. With the construction of an analytical model, representing the floating structure and the tsunami wave as simplistic as possible, the system can be understood more quickly. If the problem is solvable by generic programming language, this would mean that it can solve a larger range in the spectrum of the problem. The conceptual model features two options: one where the platform has no freedom of movement, the other where the platformcan move vertically. They both assume that hydrostatic pressure holds during wave propagation and a linearization of the momentum equation describing the water particle interaction. For each option, the transmitted wave height is determined based on varying the floating structure dimensions. This gives an indication on which parameters are of influence in the transmission of the wave. First, the conceptualmodel is analysed by changing the platformdraft and the length for both the motionless and the vertically moving platform. Both options are influenced most by the length of the structure. The situation with the motionless platformshows this effect earlier, by a higher wave attenuation percentage for the same platform length. Whether the draft also has an influence is strongly dependent on the value representing the length of the platform. The difference between the two platform movement options shows that the effect of changing the allowed movement of the platformis significant. Next to the reduction in transmission, the conceptual model shows signs of resonance. The moving platform option in the model is formed by a second order differential equation. Fitting this equation, resonance is evident and therefore visible for certain combinations of the platformdimensions. In addition damping is present, ensuring that there are some parts where, despite the natural frequency pointing there, no resonance occurs. The amount of damping is strongly linked to the platformlength, with a higher level of damping for a longer platformlength. Finally, the results from the conceptual model are compared to the outcome in SWASH. This numerically based model has the possibility to simulate a tsunami wave in its development towards the coast and also features a buoyancy function for structures. This comparison serves as a validation of the conceptual model. In general, the conceptual model always results in a less reduced wave attenuation percentage and can be said to be more conservative. Due to the assumptions of leaving out the non-hydrostatic pressure and a lower level of detail, a maximum of 5% deviation in both model results occured. This however, matches with the fact that it is a less detailed model and adds to the reasoning that the conceptual model provides what it is meant for. Next to the effect of the structure itself, the positioning of the structure is also of large importance. Wave height and intensity of the wave will vary due to the surrounding local coastal features. Next to that, the local water depth is determinant in compressing the wave, therefore decreasing the wave length when the water depth decreases. The maximum wave attenuation that can be achieved according to both models is 10%, considering platform dimensions and location variations. The conceptual model appears to work for which it is intended: modelling the resulting transmission between a floating structure and a linear tsunami. It is expected that modelling programs can be expanded and/or improved, so that more realistic floating structures can be modelled. However, it will remain difficult to accurately model a tsunami as it is complex in behaviour. Yet, this research brought the field one step closer to evaluating the transmission of tsunami waves when interacting with a floating structure of certain dimensions.

In future decades, coasts will be exposed to increasing risks because of climate change and sea level rise, which poses an increased threat of coastal inundation, erosion and ecosystem loss. The natural variability of coasts can make it difficult to identify these impacts. Most beaches worldwide show evidence of recent erosion. Sea level rise is currently not necessarily the primary driver, but will become very relevant in the future. Understanding both the short-term and long-term development of a coast is considered essential for coastal managers, in order to maintain safety in the future. For a generic evaluation of the state of the coastal zone, it is also important that not only the coastline changes are examined, but also the behavior of the dunes. The interactions between transport mechanisms should be investigated in order to make a good prediction of the large-scale and long-term development of the coastal zone. Therefore, the adaptation of the dunes and the coastline to interacting cross-shore and longshore sediment transport processes must be studied. To get a comprehensive view on those interactions, a new coupled surfzone-dune model is developed and validated.
This coupled model consists of a combination of coastline model ''Unibest-CL+'' and coastal profile model ''the CS-model''. The coupled model has the capability to simulate beach and dune evolution at decadal to centennial timescales including the relevant sediment transport processes that impact the beach and dune evolution: dune erosion and overwash, aeolian dune build-up, beach erosion and accretion due to gradients in longshore sediment transport, adaptation to sea level rise and response to nourishments. The model is tested quantitatively based on the sediment balance, in a very simple and a more complicated academic case focused on the individual effects of all transport mechanisms included in the coupled model. All test cases in the simple case -- which consist of the smallest scale situation the coupled model is able to simulate -- demonstrate sediment conservation and therewith confirm that the implementation of the different transport equations and their interaction in the coupled model is correct. In the complicated test cases – covering a varying number of cross-shore profiles along a larger stretch of coastline – the accuracy of the predicted sediment fluxes and the subsequent coastline and dune volume changes have been determined. The accuracy of every individual transport mechanism included in the coupled model is found to be sufficient and the different sediment fluxes within the coupled model are balanced.
After model testing, the model is used to hindcast the 22-year long coastal evolution near IJmuiden. The dune evolution in this highly dynamic case is simulated reasonably. Furthermore, the coastline evolution is predicted in accordance with the data. Overall, the coupled model is found to quantitatively and qualitatively perform satisfactory. The model is capable of quantifying the long-term beach and dune evolution with interacting longshore and cross-shore processes. Though coupling the models and therewith simulating the longshore sediment transport gradients instead of deriving them from data induces model uncertainties, the application of the model on the IJmuiden case shows that it is possible to simulate the longshore sediment transport gradients using a schematized wave climate to predict the dune volume evolution sufficiently. Furthermore, the interaction between the cross-shore and longshore sediment transport processes enables the simulation of the redistribution of nourishments, improving the predictive ability of both model components with regard to the coastline evolution as well as the evolution of the dune volume. For future research it is recommended to use the coupled model to look into nourishment strategies and their effectiveness on the long-term, climate change scenarios and their effect on decadal to centennial scale dune and beach evolution and the relative importance of different transport mechanisms on the long-term beach and dune evolution.

Recent analysis on measurements in the North Sea has shown a large amount of wave energy ranging in infragravity bands in the North Sea during storm events. To better understand the generation and propagation of free infragravity waves, the SWAN model is used to inspect the infragravity wave pattern, the infragravity wave origins, and the corresponding contribution from different coastlines in the North Sea. The local equilibrium between sea swell wave and infragravity wavebands is applied to parameterize the source infragravity wave along the coastlines bordering the North Sea following the approach of Arduin et. al. (2014). The empirical parameter α1 representing the radiating energy level from each coastline and bottom friction coefficient χ representing the dissipation strength are combined to calibrate the model. 10 storm events with an observed infragravity wave height over 10cm are chosen to gain detailed insight into the evolution of free infragravity waves under extreme conditions in SWAN. A correlation between α1 and χ is found. A relatively good explanation between measured and predicted infragravity wave height can be obtained as long as a large α1 and a large χ are implemented in the model or vice versa, showing an accuracy of up to 75% in the open sea. Based on a proper combination of two variables, the contribution of infragravity energy from each coastline varies spatially in predicted results. The Danish coastline is the dominant region radiating infragravity waves to the open sea and the reflected free infragravity waves could reach the adjacent coastlines such as Netherlands and UK. Behind the peak of an extreme storm event, the excitation of free infragravity energy is observed along the south of the UK coast originating from Denmark. The north of Holland coasts also radiate large amounts of free IG energy but are unable to have a prominent impact in the center of the North Sea due to energy dissipation. These results imply that the radiated infragravity waves can be influential in distant coastlines in the North Sea during storm events and the changes of local coastal dynamics can be attributed to a different generation region due to the storm characteristics.

Climate change puts under pressure existing and future coastal interventions. Growing threats like sea-level rise and intensity of storms require solutions to be adaptable and resilient. Nature-based solutions have shown to tackle these challenges while providing social, environmental, and economic benefits. The role of vegetation in coastal protection is increasingly recognized. Aquatic vegetation reduces erosion, storm surge, and incoming wave height.

Large-scale modeling of waves with spectral wave models such as SWAN is indispensable for the design of coastal structures and the assessment of flood risk. Wave dissipation due to vegetation can be modeled in SWAN as increased bottom friction (implicit modeling) or as an additional dissipation function (explicit modeling). The second assumes that vegetation can be represented as rigid cylinders or plates (canopies) with different properties. While some studies concluded that implicit modeling reproduces the spectral evolution of field measurements more closely, others concluded the opposite.

Within the BE-SAFE project, field campaigns measured the spectral energy distribution over salt marshes in the Dutch Wadden Sea during several winter storms. The vegetated foreshore in front of the coastal dike got submerged over 2 m of water during high tide and storm surge. The measurements deployed wave gauges over the study transect, which was defined between the pioneer zone marsh edge and the near-dike location (300 m behind the salt marsh). Calibrating the implicit and explicit models in SWAN brought the modeled total wave energy decay closer to the measurement. Nevertheless, the spectral shape, which describes the energy distribution over frequencies, still showed significant and not yet understood differences near the dike.

A methodology was executed to investigate the mechanisms that could reduce the spectral mismatch between the SWAN wave model and measurements over vegetation. First, the literature highlighted possible mechanisms that could be incorporated for this purpose. Next, a new frequency-distributed explicit dissipation model of Jacobsen et al. (2019) was implemented in SWAN and compared to implicit and explicit models using lab and field measurements.

The results showed that the newly implemented model accurately captures the physics and the change of spectral shapes for all experimentally tested wave conditions and submergences. In contrast, the existing implicit and explicit dissipation models in SWAN reproduce the spectral evolution only under certain circumstances. In the validation and comparison to the field measurements with a much larger water depth than the vegetation height, the model of Jacobsen et al. (2019) correctly captured the vegetation's physical representation and the dissipation on the wind-sea frequencies. Nevertheless, the amount of energy on low frequencies was largely underpredicted by all frequency-distributed models. Therefore, the model of Jacobsen et al. (2019) was modified to include flexibility in a frequency-dependent reduction factor that reproduced the energy decay of the measurements in all frequency regions. Other mechanisms that could be responsible for the mismatch before and over the marsh are the redistribution of energy by non-linear triad interactions, generation of infra-gravity waves, and near-shore currents caused by horizontal variations on the vegetation properties.

The present research provides the range of conditions in which the tested explicit and implicit energy dissipation functions in SWAN are able to simulate the spectral evolution over rigid canopies and flexible salt-marsh vegetation. A new version of SWAN includes a new frequency-distributed explicit model that performed more accurately than existing models for rigid canopies. The physical insights from the research contributed to developing additional versions of SWAN, which performed closely to the energy distribution of the measurements over deeply submerged and flexible salt marsh vegetation species.

References:
Jacobsen, McFall, Van der A (2019). A frequency distributed dissipation model for canopies. Coastal Engineering, 150, 135-146.

This report presents the results of a research concerning long waves in the North Sea. ‘Long waves’ in this research is a collective name for various types of waves that are longer than the well-known sea-swell waves, here referred to as ‘short waves’. Wave types that are referred to as long waves are infragravity waves, meteo-tsunami’s, seiches and tides. Besides their extensive length, they distinguish themselves from regular sea-swell waves by the mechanisms responsible for their generation and the scale of their impact. Whereas short waves are generated by wind, long waves are generated by short wave-group forcing, the breaking of short waves nearshore and large atmospheric pressure variations. Long waves play a crucial role in the processes of coastline erosion and the breaching of flood defenses, as well as in the formation of rip currents and seiches. These events can lead to severe damage to hydraulic structures and possibly even casualties. A better understanding of them is thus vital for coastal safety.
Rijkswaterstaat has been collecting data regarding the occurrence of long waves on the North Sea for over a decade. Their measurement campaign started in a time when digital broadcasting signals and digital storage space were not well developed yet, making sending and storing the water surface elevation time-series not feasible. The data is therefore filtered by a FIR filter, reducing the size of the data, but also reducing the information that is contained in the data. All frequencies of approximately 0,0125 퐻푧 and higher are removed by the FIR filter. As a consequence a significant part of the infragravity signal is missing and it is not possible to perform a bispectral analysis (used to determine the bound long wave contribution). Nowadays, digital broadcasting signals and digital storage space have plenty of capacity to send and store the data, making the filter unnecessary.
Analysis of the spatial- and temporal distribution of long waves in the North Sea, shows that the majority of long waves have a yearly averaged significant wave height of 2,4 - 3,3 cm and a yearly averaged mean wave period of 125 to 140 cm. Based on data from the full decade, a clear trend in the spatial distribution of long waves cannot be distinguished, offshore and nearshore locations show approximately the same long wave properties. The seasonal analysis shows that the mean significant long wave height averaged out over all locations per season varies from 2,8 cm during summer, 2,9 cm during spring, 3,3 cm during winter to 3,5 cm during fall. For the mean wave period, the mean value averaged out over all locations per season varies from 127,5 s during summer, 128,1 s during spring, 135,6 s during winter to 136,3 s during fall. Predictions of the extreme wave conditions show that the significant wave height with a return period of 10.000 years (Dutch design condition for coastal flood defenses) are around 40-60 cm, although there is quite some uncertainty in these predictions. This uncertainty is due to a relatively short measurement period, usually 30 years of data is used for extreme wave predictions, and remarkable observations in the data.
Time-series analysis of the hourly significant wave height of long waves and predicted bound long waves (Hasselmann, 1962) (Hasselmann, 1963) shows that during mild weather conditions significant wave heights of long waves are low, with minimal contributions by the bound long waves. At the onset of a storm, the significant wave height of both long waves and bound long waves rapidly increase. At the end of the storm, they decrease to the pre-storm values. This shows that the generation of long waves occurs mainly during heavy weather conditions. Hourly significant wave heights of long waves, and especially bound long waves, show a strong correlation with significant short wave heights. The relative contribution of bound long waves (퐸푏푙푤/퐸푙푤) during a storm increases up to values of 60%, which is considerably higher than contributions of bound long waves found in other studies (Herbers et al., 1994). The high ratio of the relative bound long wave contribution, in combination with very low bound long wave activity during mild conditions, shows that the generation mechanism involving wave-group forcing is a major generation mechanism of long waves in the North Sea.

This report presents the methodology and findings of a study on the impacts of free long waves on dune erosion predictions in XBeach. In this paper a distinction is made between two types of infragravity waves: bound infragravity waves and free infragravity waves. Bound infragravity waves are generated by short-wave group forcings and are bound to the propagation of the wave groups. Conversely, free waves are infragravity waves that are released from the wave-group in the surfzone, reflect at the shoreline, and propagate back out of the surf zone. In a semi-enclosed basin with mild bathymetric changes such as the North Sea, free long waves that reflect off one coast can propagate large distances and impact other coastlines.

Modelling the response of a coastline during a storm event requires an offshore boundary condition that represents the incident short and long waves. The boundary condition for the incident infragravity waves is often derived by assuming a local equilibrium between the directionally spread sea-swell waves and bound infragravity waves following the K. Hasselmann (1962) method. This approach has proven to be problematic for two main reasons. First, the method tends to overestimate the incident infragravity wave height by assuming a local equilibrium is achieved at the model boundary; however, observations have shown that the transfer of energy from the short-wave groups to the underlying bound wave is gradual on a sloping bed. Second, by applying the equilibrium K. Hasselmann (1962) method it is implied that only bound infragravity waves are present at the boundary underestimating the total incident infragravity wave energy.

An analysis of XBeach in surfbeat mode revealed that the model can confidently predict infragravity wave behaviour on a natural beach slope with two well-developed bars during a storm event by calibrating the roller breaker slope coefficient and wave breaker coefficient. Infragravity wave heights were reasonably accurately predicted in the surfzone, but the model generally overpredicted the most energetic infragravity waves.

Further, the study investigated the impact of free infragravity waves on dune erosion predictions by simulating the behaviour of two 1D planar beach slopes to the inclusion of free long waves at the boundary. The study revealed that on a beach with a 1:35 slope the relative increase in dune erosion volume was 44.1% when the bound infragravity energy is equal to the free infragravity energy relative to the case excluding free waves. For the same conditions, a 20.6% increase was observed in the maximum runup. For a milder 1:70 slope a 36.4% and 6.1% increase in dune erosion and maximum runup was observed.

The results demonstrate that the dune response of a coastline is sensitive to the inclusion of incident free infragravity waves at the boundary. Moreover, neglecting the presence of free infragravity waves at the boundary may underpredict the dune response during a storm event.

Rapid climate change and the corresponding estimated sea level rise can affect the performance of the coastal defense structures such as breakwaters, seawalls, and dikes. In order to improve these coastal defenses, a detailed understanding of the processes which contribute to wave run-up and overtopping over the coastal defenses needs to be established. Following the exponential growth of computing capacity around 1970’s, a wide variety of computational models were developed to study fluid flow. Traditionally, three computational paradigms have existed in order to study wave transformation and surf zone hydrodynamics: phase averaged models, phase resolving models, and Computational Fluid Dynamics (CFD) models. Limitations posed by the underlying linear wave theory in phase averaged and other simplifications in the phase resolving models, may not provide sufficient detail in wave breaking, wave energy dissipation, wave run-up, wave overtopping, and potentially other detailed hydrodynamic processes. This lack of resolution in depth averaged models for wave-breaking, wave run-up, and wave overtopping processes motivates a detailed investigation using CFD based models, which can correctly mimic wave-breaking and other hydrodynamic processes.

The recent growth in available computational capacity has greatly improved the applicability of CFD based models for large scale transient flows such as waves near a coast. Additionally, the developments in wave generation and wave absorption boundary conditions by Jacobsen et al. [2012] in the open-source CFD toolbox OpenFOAM, have facilitated the use of OpenFOAM in coastal engineering applications. This encourages investigating the coastal environment using relatively complex models, thus providing insights into fundamental processes which contribute to coastal safety. To that end, this thesis focuses on investigating wave overtopping and the underlying processes which contribute to the aforementioned hydrodynamic aspects.

Overtopping demands accurate capture of the free surface (interface between water and air). The waveFoam solver suffers from numerical diffusion of the interface, consequently requiring a different approach to mimic the sharp interface. In order to cater to this deficiency, a new solver which combines the capabilities of waveFoam [Jacobsen et al., 2012] and isoAdvection [Røenby et al., 2016] which has the ability to capture sharp interfaces by means of a sub-grid approach has been integrated (waveFlow) and used in this study. In addition to the new solver, a new set of Reynolds Averaged Navier-Stokes (RANS) closures developed by Larsen and Fuhrman [2018] for wave modeling applications have been employed to correctly capture turbulence levels under breaking waves. The preliminary steps include calibrating and assessment of the newly integrated waveFlow solver. Using a relatively simple conceptual test case, a comparison of the free surface behavior and overtopping discharge was carried out. This calibration test was followed by a comparison of numerical results with the experimental investigations carried out by Ting and Kirby [1994]. Following this benchmarking study, experimental studies carried out by Flanders Hydraulics investigating wave overtopping over dikes in shallow foreshore environments was validated. A comparison of waveFlow and waveFoam was made to assess the qualitative and quantitative differences between the two interface capture methods on overtopping. Using this new solver, OpenFOAM was able to reproduce the surface elevation and significant improvement in the overtopping results were obtained for identical model setup in comparison to the waveFoam solver. A coupled approach using a potential flow solver named OceanWave3D aided simulation of large domain wave propagation and helped to cut down the computational time.