Vertical land motions near the coast, stemming from land subsidence or uplift often exceed the rates of local sea-level rise. Coastline changes are especially significant in beaches with shallow slopes, where significant loss (or gain) of land is experienced from landward shoreline retreat (or seaward shoreline advance). A novel global dataset; Shoreline-Monitor, derived from satellite images between 1984 and 2024 contains annual shoreline positions every 100 meters along the coast. This has been supplemented with local land subsidence or uplift measurements from diverse remote sensing techniques including GNSS & InSAR, as well as in-situ measurements. Combining these measurements with knowledge of beach geometries lets us estimate the contribution of land subsidence or uplift to coastline changes. In Hudson Bay, Canada, estimates of coastline changes stemming from post-glacial rebound of bedrock since the previous ice age closely match observations of more than 3 meters of shoreline advance per year over a stretch of 400 km. Estimations of coastline changes caused by earthquakes such as in Kaikoura, New Zealand, are consistent with observations, with some areas experiencing over 300 meters of sudden horizontal shift of the coastline. Applying such spatio-temporal analyses in other coastal regions susceptible to vertical land motions will provide a more representative assessment for long-term trends in land loss and flood risks associated with coastline changes.

The Netherlands are prone to flooding, with 30% of the country lying below sea level. To protect its densely populated and economically important coastal zone, Rijkswaterstaat (RWS) applies a maintenance strategy based on regular sand nourishments. These nourishments involve the placement of large volumes of sand, typically dredged from the North Sea seabed, to compensate for structural erosion. While this process is effective for maintaining coastal safety, it is resource-intensive and increasingly unsustainable in light of climate change, sea-level rise, and stricter environmental ambitions. With national coastal programmes such as Kustgenese 2.0 and the Dutch Coastline Challenge calling for more sustainable and adaptive maintenance strategies, complementary alternative concepts are under consideration. One such concept is sand recirculation: the artificial redistribution of sand within the coastal system, without the need for additional offshore extraction. Internationally, this principle is already applied through various forms such as sand bypass systems, which transport sediment along the natural littoral drift to counteract obstructions like jetties; sand backpass systems, which move sediment against the dominant transport direction to replenish eroding areas; and periodic dredging campaigns, where accumulated sediment is relocated within the same littoral cell. These systems respond to sediment imbalances caused by human interventions and offer a sustainable alternative in settings where offshore resources are limited or environmental impacts must be reduced. This thesis explores whether such a recirculation strategy can be a technically feasible addition to the nourishment policy at Maasvlakte 2. This protruding coastline, located on the western boundary of the port of Rotterdam, experiences a structural morphological imbalance: persistent erosion in the westfacing bend and visible accretion in the southern section (in the form of a spit), primarily due to strong gradients in alongshore sediment transport. Currently, large-scale nourishments from offshore sand sources are periodically used to reinforce the eroding zones. However, the simultaneous accumulation of sand in the south offers potential for a circular approach to sediment management. This study aims to evaluate whether this locally available sediment can be redistributed to erosion-prone areas through artificial means, thereby reducing the demand for traditional offshore sand sources. The research begins with a morphodynamic analysis of Maasvlakte 2, using various datasets including bathymetric surveys, coastal profile data, and shoreline extractions. Accretion and erosion patterns were quantified and compared to predictions made using the one-line ShorelineS model, which simulates coastal evolution based on alongshore transport driven by transformed offshore wave conditions. This comparison confirmed that the largest erosion hotspot coincides with the western bend of the coastline, while the southern section consistently accumulates sediment. An additional finding, which was not initially anticipated, is the identification of a secondary accretion hotspot that may serve as a viable source area for sand recirculation. This results in the presence of both a southern source (the spit) and a northern source, each subjected to distinctly different hydrodynamic conditions and therefore requiring a distinct approach to extract sediment from these respective hotspots. Annual redistribution volumes between 300,000 and 415,000 m³ were identified as potentially technically recoverable, equating to a reduction of approximately 42-59% in the need for offshore-originating sand nourishments. Several operational strategies were explored, including the use of trailing suction hopper dredgers (TSHDs), cutter suction dredgers (CSDs), and sand backpass systems, in which sediment is transported continuously through pipelines. For each strategy, design parameters such as dredging volume, frequency, location, and required vessel or system dimensions were assessed. A Multi-Criteria Analysis (MCA) was used to evaluate each option based on technical feasibility. The evaluation revealed that a TSHD is the most suitable option for the northern source, while a CSD is preferred for the southern source. This distinction arises from the deeper location and greater flexibility requirements in the north, which suit the mobile operation of a TSHD. In contrast, the southern site allows for more precise, ii iii stationary excavation nearshore, favouring a CSD in combination with barge transport. In a subsequent multi-criteria analysis that considered operational feasibility, costs, and other relevant factors, the TSHD and the sand backpass system emerged as the most favourable options for their respective locations. The greatest technical complexity lies in the redistribution of sediment within the western bend of Maasvlakte 2, where bi-directional sediment transport dominates. This area requires nourishments to be placed across a wide spatial range and over varying water depths, posing significant operational and morphological challenges for any recirculation strategy. Although the research is limited to a pre-feasibility level, the results indicate that artificial sand recirculation could reduce reliance on offshore resources while maintaining coastal safety at the Second Maasvlakte. However, legal and regulatory barriers remain, as current Dutch legislation restricts the reuse of dredged material in water depths smaller than 20 meters for nourishment purposes. Moreover, the influence of cross-shore processes and seasonal transport variability, especially under tidal forcing, remains uncertain and would require further investigation. This thesis concludes that sand recirculation at the Second Maasvlakte is potentially technically feasible under the studied conditions and has the potential to serve as a sustainable addition to the Dutch coastal maintenance strategy. Its implementation depends on careful system design, regulatory reform, and alignment with broader environmental goals. Further research is recommended to optimise operational parameters and develop integrated system concepts that satisfy the demands of both coastal managers and other stakeholders.

Coastal regions, particularly beaches, are vital and dynamic areas for human activities. They serve various functions and are crucial barriers safeguarding coastal cities. However, the beach faces an increase in population and natural hazards caused by climate change are creating new challenges that coastal engineers are working to address.

This study introduces a new technique, SHORECAST (Satellite-derived Historical and future Orientation-based Relation for Estimating Coastline Adjustments and Sediment Transport), to enhance the Satellite-derived Shorelines (SDS). This technique is designed to estimate the shoreline dynamics in front of the shoreline and predict shoreline positions globally, offering a quick and accessible alternative to the existing models. The primary objective of this study is to employ shoreline position data obtained from SDS to estimate Longshore Sediment Transport (LST) and predict shoreline position quickly and effortlessly.

To achieve the study's objective, SHORECAST is developed and adapted into a multi-step framework using the annual dataset from the Shoreline Monitor of Luijendijk et al. (2018). The dataset assesses sandy and non-sandy beaches and their historical shoreline position at transects every 500 meters along the coast for the last 37 years. Only sandy shoreline evolutions are considered for the development of this new tool. First, a routine is developed to find coastal cells to which the research's aim could be applied. Secondly, multiple algorithms were deployed on the coastal cells to obtain the historical shoreline orientation and the LST. Combining these values leads to a correlation for predicting shorelines.

Out of all the coastal cells studies, three have been chosen: Nouakchott (Mauritania), Aveiro (Portugal) and Delfland (the Netherlands). The Nouakchott cell was split into a north and south side. These cells have in common that they all have a littoral barrier at one of the boundaries of the coastal cell with the assumption that there is no sediment transport. Due to this assumption, the LST could be calculated for each coastal cell. Nouakchott North experienced an annual sediment transport of 0.66 million m3 between 1985 and 2020, a volume increase of 23.01 million m3. Conversely, Nouakchott South experienced an erosion rate of 0.92 million m3 per year, resulting in a total loss of 32.36 million m3 over 35 years. In this same period, the Aveiro shoreline has accumulated a volume of 17.83 million m3 of sediment. In addition, the Aveiro shoreline displayed significant fluctuations compared to Nouakchott.

The Delfland coastal cell is the most complex coastal area among the three selected cases due to two littoral barriers at the boundaries and the anthropogenic measures in the past 37 years. This results in a volume increase of 45.55 million m3, similar to the beach nourishment volume of 46.51 million m3. As a result, it can be concluded that only the beach nourishment is visible in the SDS data, even though shoreface and dune nourishments have also been carried out during this period.

The findings indicate that the SHORECAST model, which incorporates the "Single-line theory" and specific boundary conditions, can generate multiple predictions. This makes it a universal tool for estimating sediment transport over time, even with future anthropogenic measures. It is important to note that not all assumed zero boundaries in sediment transport are zero in reality. Apart from SDS data, other shoreline position data can be integrated into the model to achieve similar results. Further research is needed to explore the possibilities of improving the understanding of different boundary conditions, thereby enhancing the obtained outcomes and the practicality of this study. An important suggestion is to explore the feasibility of identifying littoral barriers and other boundary conditions. This would make the developed model more robust and widely applicable.

In recent decades, a growing number of shoreface nourishments have been carried out along the Dutch coast to mitigate coastal erosion. Designing shoreface nourishments requires a comprehensive understanding of their complex morphological evolution to develop optimal solutions. However, accurately predicting this evolution in Delft3D-FM has proven challenging. Excessive flattening of nourishments in morphological process-based models leads to an inaccurate representation of the cross-shore profile, and the implications of this inaccuracy on longshore predictions remain unclear. To address this issue, a hybrid model has been developed that seamlessly integrates a data-driven component into the process-based numerical model Delft3D-FM through Basic Model Interface. Contrary to previous study efforts, the focus is not on improving the representation of physical processes, but on incorporating a data-driven component into Delft3D-FM to more accurately represent observed behavior in cross-shore modeling. The presented modelling framework facilitates the prevention of excessive flattening of shoreface nourishments in Delft3D-FM by enabling user-defined manipulation of bed level changes during simulation. The hybrid model is then applied to reproduce a shoreface nourishment at the coast of Ter Heijde for two main objectives: (1) to demonstrate the proof-of-concept of a hybrid model approach and (2) to examine how inaccuracies in cross-shore modelling affect longshore predictions, thereby highlighting the added value of a hybrid model approach.
The results demonstrate the proof-of-concept of the hybrid model. Unlike the standard model, which showed excessive flattening of the nourishment, the hybrid model maintained its nourishment shape and followed observed cross-shore evolution over three months of morphological modelling. We found that a hybrid model approach has the potential to more accurately represent the nourishment’s lee effect, which plays a crucial role in the morphological response to a shoreface nourishment. By preventing excessive nourishment flattening, larger waves break earlier and/or more frequently in the hybrid model, causing a calmer wave climate in the lee of the nourishment compared to the standard model. As a result of increased wave sheltering, the flow velocity in the lee reduces which causes sediment supplied supplied by longshore currents to settle at a higher rate compared to the standard model, leading to increased sedimentation in the nourishment’s lee.
As a result of the enhanced representation of the lee effect, the alongshore redistribution of sediment differs between the hybrid model and the standard model. After three months of morphological modeling, differences of 10-25% between the models attributed to the nourishment sediment are observed in the lee of the nourishment. Additionally, the hybrid model shows a trend of increasing divergence from the standard model over time. This indicates a sustained added value of the hybrid model over time.
This thesis represents a contribution towards a data-integrated approach in process-based modelling of the complex evolution of shoreface nourishments, highlighting the potential added value of such an approach. Therefore, we anticipate this thesis to be a starting point for more sophisticated ways to incorporate accurate cross-shore evolution in numerical models in the context of shoreface nourishments.

This thesis explores the potential of a fixed artificial bypass system for a sustainable and eco-friendly approach to coastal management in IJmuiden. The presence of the IJmuiden Port disrupts the natural processes, causing significant morphological changes to the coastal area. Current management practices require frequent dredging and nourishment to mitigate the erosion of the downdrift coast and maintain the depth necessary for navigation in the channel and port. These activities result in significant emissions and adverse ecological impacts. Particularly affecting benthic life, organisms living in or on the seabed.


Implementing a fixed artificial bypass system, already proven successful in similar projects globally, emerges as a potential solution to reduce dredging and nourishment activities. This system aims to restore the natural sediment transport by pumping sediment from the updrift to the downdrift side. In addition, its more continuous discharge of sediment is anticipated to be less disruptive for benthic life compared to traditional nourishment methods.

The best-case scenario calculation performed in this thesis presents that an artificial bypass system at IJmuiden could potentially reduce dredging activity by 3.5% and nourishment activity by 37%. To assess whether these reductions can be achieved, this thesis introduces a newly developed framework for assessing the effectiveness of sediment bypass concepts based on four performance indicators: (1) Dredging Activity of Channel and Port, (2) Sediment Demand of Downdrift Coast, (3) Impact on Benthic Community and (4) Feasibility.



This method includes simulating the response of the coastal system after implementing varying bypass concepts using a Delft3D model. The Delft3D model’s applicability and predictive skill are assessed via hydrodynamic and morphodynamic validation. Concluding that the model can reproduce the general trends but introduces numerical errors in the exact quantification of the morphological development. Despite this limitation, the output from these Delft3D simulations was used to evaluate the response of different artificial bypass concepts based on the four performance indicators. The first two indicators are based on the simulated sediment transport values and assessments of the development of the bed. The evaluation of the third indicator is based on a calculation performed using the developed benthic evaluation tool, named the 'Benthimeter'. This newly developed tool provides a method that intends to visualize and quantify the impact on the benthic community induced by nourishment activity. Although the Benthimeter requires further calibration and validation, it marks a good first step towards integrating ecology into coastal management.


The results of this thesis demonstrate that the coastal system of IJmuiden allows for sediment withdrawal, where allready 10% of the required annual sediment trap was observed within one simulated month. Also northward sediment dispersal towards the downdrift coast was observed at simulations, indicating that such a system could reduce the sediment demand. These findings provide confidence that the principles of bypassing sediment around the port of IJmuiden hold. Consequently, it is anticipated that an artificial bypass system would, to some amount, reduce the need for dredging and nourishment activity. Also, the calculated impact on the benthic community confirms the hypothesis that a more continuous nourishment approach reduces the impact on the benthos. While the findings of this study provide an initial indication of the potential effectiveness of an artificial bypass system at IJmuiden, they do not provide long-term effect estimates. Further research is suggested to examine the primary drivers of dredging and nourishment activity, along with efforts to simulate the equilibrium state to evaluate the long-term effects.


The most important contribution of this thesis is the introduction of innovative tools, guidelines, and effective methods. This framework can be used in future research to improve our knowledge of sustainability and ecology in coastal practices.

Coastal areas are appealing for human beings and are the most populated areas of the world. Therefore, coastal processes have large impacts on economy and on millions of people's lives. Not only processes such as erosion, extreme events, and flood events must be understood and monitored, but the uncertainty given by the rapid climate change on these processes and on coastal systems pose a bigger issue on these populated areas. One of the key parameters of coastal processes, which is notably required for modelling coastal floods, is the beach face slope. Unfortunately, there is a lack of a reliable global dataset of the steepness of the beach face, which has been indicated to be key limitation for developing operational coastal inundation forecasting systems, as well as for quantifying the run-up and set-up contribution at the shoreline relative to global sea-level-rise.

In this report, a methodology for providing this parameter at a global level by using a LiDAR-based satellite (ICESat-2) is presented. Here, the accuracy of the beach face slope estimation, its dependency on the angle and distance between ICESat-2 measurements (passes) and the transect perpendicular to the coastline they are projected into (transect), a spatial and temporal exploration of ICESat-2 measurements on world's coasts, as well as the limitations and opportunities of this methodology are presented.

The findings of this study reveal strong visual agreement between the beach profiles and beach face profiles obtained from the validation datasets and ICESat-2. Moreover, the computed metrics, including the Mean Absolute Error (MAE), Mean Squared Error (MSE), and Root Mean Squared Error (RMSE), present low values, indicating a high level of agreement. The MAE values, ranging from 9 to 34 cm, not only demonstrate good agreement within the beach and beach face profiles but also indicate that the errors fall within the inherent variability of the beach. For assessing the impact of the angle and distance between passes and transect on the beach face slope extraction, synthetic data is generated using Digital Elevation Models (DEMs). The results demonstrate that long and spatially homogeneous beaches withstand larger α, whereas smaller, more variable, and pocket-shaped beaches are more sensitive to large angles due to the rapid decrease of spatial information alongshore. Nevertheless, regardless of the beach type, the closer the synthetic ICESat-2 measurements intersect over the beach face, the more likely the extracted beach face slope is to match the actual one. For both analysed cases, the beach face slope can be extracted with a tolerable level of uncertainty up to 60 degrees, and if the synthetic data is close to the beach face, the angle that it bears can be increased up to 70-80 degrees.

The novel methodology for upscaling the beach face slope extraction is validated using the Dutch yearly coastal dataset, JARKUS. The results reaffirm the dependence of the beach face slope on the angle between pass and transect. It is concluded that even though further refinement of the algorithm for demarcating the beach face is necessary, the extraction of beach face slopes can be implemented on a global scale, if some challenges are overcome. Moreover, this methodology yields a valuable byproduct in the form of cross-shore profiles, which can be extracted globally with high accuracy. This additional output enhances the utility and versatility of the methodology. Additionally, a spatial and temporal exploration of ICESat-2 ATL03 data on the world's coasts, by globally sampling boxes of 20 km², is implemented. Then, by analysing the preliminary results from boxes already processed, it is observed that these boxes can encompass anywhere from 0 to 160 passes over the course of the available 5-year data. On average, approximately 63 passes are recorded within each box. From this analysis, it is concluded that particular coastal stretches, depending on their geometry and cloud coverage, can be effectively monitored for beach face slope changes using ICESat-2. Moreover, from the information extracted from these boxes, a map is being constructed for the purpose of revealing potential areas where the extraction of beach face slopes are possible given the beach geometry and availability of data. In summary, this study provides a novel methodology that sets the basis for a new era of coastal management and monitoring using LiDAR-based satellites at a global level.

While waves are propagating towards the shore and are interrupted by an obstacle like a groyne, they will turn around the tip into the sheltered region of the groyne. This sheltered region is called the shadow zone and contains a reduced wave climate. The turning of the waves is based on the lateral transfer of wave energy along the wave crest, caused by a gradient in wave height. This process is called diffraction. Breaking wave heights and angles inside the shadow zone will be influenced significantly because of wave diffraction. Since variations in breaking wave height and/or angle are responsible for gradients in the alongshore sediment transport, should the process of wave diffraction be taken into account while simulating the shoreline evolution in the vicinity of a groyne. Different methods were found to incorporate the effects of wave diffraction inside the coastline model ShorelineS.

The improved model performance of ShorelineS regarding a real-world case study is addressed by using the shoreline of Constanta, Romania. The consequence of incorporating wave diffraction effects onto the shoreline evolution of Constanta is demonstrated in detail. The accretion close to the Southern groyne and erosion near the Northern groyne is visible in the numerical result of the improved model. Therefore, matching the observed anti-clockwise rotation of the coastal cell. Without accounting for diffraction effects, this matching result was not achievable. The transition zone width is found to be an important factor in determining the coastline shape affected by diffraction. After calibration of this parameter, the numerical result demonstrated to be in almost perfect agreement with the observed coastline shape. The root mean square error and bias reduced with factors of 5.5 and 5 compared to the numerical result excluding diffraction effects.

Continuous sea level rise and growing environmental awareness have led to increasing implementation of nature-based solutions to counter coastal erosion. An example in the Netherlands is the Sand Engine---a mega-scale sand nourishment designed to feed the Dutch coast over a period of 20 years. For such designs to work, a good understanding of the governing natural processes is paramount. The behaviour of the sandy coast, however, is subject to natural variability in future weather and uncertainties in the interactions between sand dynamics and hydrodynamic forcing. To predict the evolution of coastal systems, engineers apply numerical models. Next to uncertainty due to variability in natural forcing, such models introduce a series of model-related uncertainties, which are often not consistently included in predictions. With limited knowledge of the magnitude of these uncertainties, the long-term strategy development and design of projects are impeded. Parameter uncertainty denotes our limited knowledge of the values of free model parameters and is an important source of overall model uncertainty. This study aims to quantify parameter uncertainty in process-based coastal area predictions by analysing uncertainty bounds for morphodynamic predictions of the Sand Engine. The analysis of parameter uncertainty is carried out for a study period of 14 months, from August 2011 (directly after construction of the Sand Engine) until October 2012. Using advanced numerical acceleration techniques, a synthetic dataset of 1024 morphological Delft3D predictions is generated, each with an identical hindcast period but different model parameter settings. First, a sensitivity analysis (elementary effects method) is performed to find the most influential parameters on three morphological indicators: cumulative volume change, shoreline position, and bed level change. Based on the sensitivity analysis, five parameters are selected for an uncertainty analysis. Subsequently, parameter uncertainty is quantified by the Generalised Likelihood Uncertainty Estimation (GLUE). To increase the convergence speed of the samples, quasi-random Sobol sampling is applied. To validate the applied parameter ranges and assess model performance, the predictions are compared to observations. Using GLUE, uncertainty bounds for the morphological indicators and posterior likelihood distributions of the five parameters are derived from the 1024 model runs. Finally, through a simplified uncertainty comparison, the relative influence of parameter uncertainty and wave climate variability is examined. A first key conclusion is that uncertainty in input parameters translates to significant uncertainty in predictions: cumulative erosion volumes for the peninsula show a spread of 1.3 -0.4/+0.7 million m3 after 14 months. Extended to bed level changes in 2D, the uncertainty bounds result in spatial uncertainty maps, which reveal that uncertainty is highest (5--6 $m$ spread) in the northern area of the peninsula, where a sand spit develops. Most uncertainty (90% for volume changes) develops in the first seven months, during which uncertainty growth correlates strongly to periods of high morphological activity (r > 0.95 for volume changes). Further, posterior likelihood distributions of the parameters indicate that, of the five selected parameters based on the sensitivity analysis, only three (f_sus, gamma, and d_50) significantly contribute to parameter induced uncertainty, while the other two (alpha_rol and theta_sd) are considerably less influential. This contrast in results is attributed to a resolution problem in the sensitivity analysis. Optimised parameter sets derived from the posterior likelihood distributions result in similar model skill as the best GLUE simulations (BSS = 0.8), implying that they may be close to the maximum achievable model skill within the examined parameter ranges. Finally, a simplified estimate of the variation due to wave climate variability is in the same order of magnitude as the parameter induced uncertainty bounds, implying that both uncertainty sources form significant contributions to overall prediction uncertainty. The presented results have an impact on two key levels. First, they can be used to communicate and address uncertainty in predictions of coastal change. For example, the spatial uncertainty maps can let stakeholders understand the potential range of outcomes for a certain design. Second, the results, combined with the created dataset, can provide valuable information for future morphodynamic studies. This work contributes to justifying the need for stochastic simulations, which are expected to be increasingly used for many coastal engineering and management purposes in the years to come.

In recent years, nature-based solutions have gained recognition for their capacity to reduce flood hazard on coastal communities. These coastal threats range from waves and tides, to future storm variability and predicted sea level rise. Vegetated foreshores are one of the measures that fall under this palette of solutions, and are the focus of this thesis. While vegetated foreshores can be colonized by several plant species (mainly mangroves or marshes), this thesis presents the study of phenomena occurring at salt marshes in an attempt to determine which are relevant indicators that can predict the presence or absence of this ecosystem at a given location. Moreover, the wave height attenuation capacity of such vegetated foreshores under different conditions is also assessed for different effectiveness indicators, in a way that provides insight into whether this habitat is not just feasible, but also effective as a coastal protection measure, with respect to wave height attenuation.

Results suggest that within a water system with similar sediment composition, inundation-free period and daily-occurring hydrodynamics can explain the presence/absence of vegetation at most sites, while storm conditions can the determining condition for vegetation absence in other locations. For comparison between systems, the dimensionless wave-induced excess bed-shear stress factor may serve as a relevnat proxy. These conclusions indicate the occurrence of sediment re-suspension during approximately 20% of the assessed time, at the most.

From the wave attenuation effectiveness assessment, considerations for tidal regime, surge height, and type of wave forcing were used to determine the degree of effectiveness of salt marshes in the studied water systems, while defining effectiveness in terms relative to a mudflat located at MSL. Results suggest that salt marshes located in macro tidal sites, like in the Western Scheldt, can dissipate more wave energy than habitats located in micro or meso tidal areas, such as the Wadden Sea. On the other hand, results treating the different wave loads further indicates that greater attenuation may be achieved in sheltered coastal locations such as the Western Scheldt estuary, indicating that this system may be more effective in sheltered than in exposed coastal areas.

Finally, the set of conclusions provided by both sections may serve to determine whether an arbitrary location is capable of hosting salt marsh vegetation, as well as the effectiveness of such nature-based solution with respect to wave height attenuation. The elaboration of an assessment tool was not achieved in this thesis; however, in order to perform a quick check for salt marsh introduction as an effective flood defense measure at a given site, results indicate the gathering of necessary data such as bed level elevations - for inundation-free period analysis - as well as occurring hydrodynamic forcing and sediment properties at such arbitrary site (excess bed shear stress determination, evaluated with the previously proposed limits). With respect to the degree of wave height reduction effectiveness, the determination of the occurring tidal regime, design surge heights, and considerations of sheltered/exposed coastal conditions may serve as a first estimate for this purpose.

The last couple of years, the interest in large nourishments, that also feed the adjacent coast, has increased. A feeder nourishment is suggested to be cost effective as well as ecological more beneficial than traditional nourishments. However, their behaviour is more complex and the need for better morphological predictions has increased. Earlier research has given insight in the morphological behaviour and the driving forces of a feeder nourishment in the Netherlands (the Sand Engine). A new feeder nourishment was constructed in October 2019 at the coast of Bacton (UK). This provides a new opportunity to evaluate the feeder nourishment behaviour outside the Netherlands. This thesis describes the results of a numerical morphological model of this feeder nourishment called Bacton Sandscaping (hereafter: BSS) A full bathymetric survey was conducted three times with echo-sounder, LiDAR scanner and photogrammetry during the first half year of the BSS. The surveys showed that the erosion of the first two months is four times higher than the erosion volumes of the months following, indicating a strong initial response. Moreover, a clear link between the morphological response of the sub-aerial beach and submerged beach was found. The numerical model has been calibrated and validated extensively with the bathymetric data of the first half year of this nourishment. The model is very well capable of representing beach volume changes and scores excellent on the Brier skill score. To obtain this excellent score, similar calibration parameters for waves were used as in the Sand Engine case in the Netherlands. The strength of this model was then used to evaluate the effect of offshore wave heights on the erosion volumes of the nourishment and showed that significant wave heights lower than 1 meter are causing 57% of the observed erosion. As the model is well capable of simulating the first half year it is then assumed that the model is also capable of simulating beach volume changes for a different grain size as well as different wave event intensities. The model predicted that sediment grain size and wave event intensities do not have a significant effect on the morphological behaviour of the BSS, therefore the initial state of the nourishment is more important in how the nourishment will evolve. The BSS model is a step forward in the understanding of the morphological behaviour of feeder nourishments and can be used for future modelling purposes of the BSS.
For future feeder nourishments it is advised to focus more on the design shape of the nourishment and not so much on the wave event intensity and grain size of the nourishment. In addition, future calibration of feeder nourishments can further point out if feeder nourishments can use similar calibration parameters for waves.

KEY POINTS (I) A biophysical model framework (BMF) for corals is developed in which four environmental factors are included: (1) light; (2) hydrodynamics; (3) temperature; and (4) acidity. (II) The full feedback loop between corals and their environment forms the core of this model framework, where the morphological development is new and closes the feedback loop. (III) The developed BMF predicts the coral response to environmental input via (mainly) process-based relations within the accuracy of climate projections. (IV)- The BMF supports both the deep reef refugia hypothesis and the turbid reef refugia hypothesis. (V) The BMF contributes to the development of protection and recovery programs and is not site-specific. (VI) The BMF is developed for long-term predictions - in the order of decades to centuries - but runs on daily averages and is therefore applicable for assessing the response of corals on shorter time-scales; such as months to years. SUMMARY The increasing pressure on Earth’s ecosystems due to climate change becomes more and more evident. These pressures are especially visible at coral reefs. Therefore, a good understanding of the biophysical mechanisms controlling these ecosystems is needed, so that accurate predictions of their survival can be made. Such an understanding is also needed to develop efficient recovery and protection programs vital to the maintenance of these ecosystems. Because the research on marine ecosystems is relatively young and the phenomenon of coral bleaching is yet to be fully understood, there is no comprehensive framework in which the complex interactions between corals and their environment are combined. In this study, a biophysical model is developed in which four environmental factors are included in a feedback loop with the coral’s biology: (1) light; (2) hydrodynamics; (3) temperature; and (4) acidity. Literature from multiple disciplines is combined to find the interdependencies between the corals and their environment. These relations include coral growth, coral bleaching, storm damage, and recruitment/recolonization of corals. For the connection with the hydrodynamics, a coupling is made between the biological model developed here and Delft3D-FM. The composed biophysical model is a big leap forward in understanding the world of coral reefs, as it is the first construction of a model framework including four environmental factors in which the hydrodynamics are included in the feedback loop. Furthermore, it creates the ability to assess recovery and protection programs based on the four aforementioned environmental factors; e.g. the susceptibility of coral bleaching can be reduced by increasing the attenuation of light through the water column. Because more environmental factors have a role to play in the coral dynamics, the framework is constructed such that these can be added relatively easily.

The existing hydrodynamic models consider full physics approaches to calculate storm surge at coastal regions. However, due to the complexity of the equations that model these processes, the computational time and power required to run them can be large, compared to models that consider simplified equations. By contrast, simplified hydraulic models lack physical background, what leads to a minor accuracy in the surge estimations with respect to hydrodynamic models. In this project, a stochastic model has been developed with the objective to estimate surge at the coast of Mississippi (United States) at a reasonable accuracy and time, without solving equations that represent complex physical processes. The stochastic model needs to be trained by means of hurricane data, including surge levels. This information must be generated beforehand, by simulating a limited number of hurricanes with an hydrodynamic model. In this project, the hydrodynamic model Delft3D Flexible Mesh has been used for this purpose. The approach followed to build the stochastic model has been based on three main steps. The first step has been setting up and validating the hydrodynamic model in Delft3D FM. Hurricane Katrina (2005) has been simulated to calibrate the input parameters of the model, by comparing the maximum simulated water levels at 41 stations along the shoreline of Mississippi to the high water marks observed at the same locations during the event. Tide, surge and wave setup have been considered in the validation. The results of the validation show a line best fit slope from the origin of 0.912 and an R-squared of 0.996. At Gulfport, the absolute error of the surge estimation is 19 centimeters, equivalent to a relative error of 2.5%. The second step in the construction of the stochastic model has been the generation of a historical hurricane data base. The hurricane best tracks have been retrieved from the HURDAT2 data base. The variables considered have been the forward speed and the forward direction of the hurricane at landfall, the wind speed at landfall, the distance from landfall location to a reference point (Galveston Bay) and the maximum storm surge during the hurricane. In this case, only the hurricane forcing is considered as external action. The storm surge has been recorded at Gulfport Harbour (central coast of Mississippi). The values of the surge have been obtained by using the validated model to simulate the historical hurricanes making landfall in a rectangular domain of 600 kilometers, being Gulfport Harbour the center of the rectangle. Due to the scarce number of hurricanes making landfall in this region, the tracks of the hurricanes making landfall in the North of the Gulf of Mexico but outside the rectangular domain have been shifted inside the domain, in order to generate a sufficiently large data base to train the stochastic model. A data base with 140 hurricanes has been built, from which the 85% (119 hurricanes) have been used for the training of the stochastic model and the other 15% (21 hurricanes) have been used for the validation of the stochastic model. The third and last step has been the setup and validation of the stochastic model, by comparing the storm surge obtained from the stochastic model to the surge obtained from the hydrodynamic simulations. The stochastic model used to estimate storm surge has been a Bayesian Network that assumes normal copulas to represent the joint distribution between nodes of the network. The calculated slope of the best fit line for the mean surge values has been 0.861, with an R-squared of 0.885. Moreover, the average standard deviation of the estimations is 1.16 meters. These results indicate a reasonable estimation of the surge by means of the Bayesian Network. This estimation can be made in the order of seconds.

Schiermonnikoog is a Dutch barrier island with wide beaches and dunes which are both of importance for recreation as for safety. However, after decades of coastal accretion, in the last years, a strong coastline retreat is observed at the island head in the northwest. In tidal inlets, various forcing conditions of tides, waves, winds and estuarine circulation can all influence the pathways and magnitude of sediment transport. The dominance of any one of these processes can have a controlling influence on the morphology of the delta, inlet and island coast. In the case of Schiermonnikoog it is unclear what drives the recently observed coastal erosion. Therefore, the relation between the ebb-tidal delta development and the coastal transgression has to be studied. With knowledge on the behavior of the system, well-founded decisions can be made about the policy and the management in the area. In this thesis, the newly developed Delft3D Flexible Mesh (FM) modeling suite has been used to set-up and calibrate a morphostatic model. This model consists of the coupled D-Flow FM (Kernkamp et al., 2011) and D-Waves modules (SWAN, Booij et al., 1999), and can adequately hindcast measured water levels, wave heights and the expected sediment transports in the investigated area at Schiermonnikoog. Model uncertainties are overcome by incorporating the entire Dutch Wadden Sea in the model schematization. The unstructured grid allows for a local increase of the resolution in the area of interest. By analyzing the influence of individual physical processes on the system, it is found that wave action is the main driver of the migration of the approaching sand shoal. With the present-day bathymetry, the flood channel in front of the island coast no longer experiences significant tidal currents due to its hydraulic inefficiency. This indicates its upcoming abandonment, which would allow for the attachment of the sand shoal to the island coast. However, wave-driven currents and related sediment transport through the channel obstruct a closure. Similar to the situation at a the neighboring barrier island Ameland in 2014, a divergence point in the channel is visible at the location where the largest waves reach over the approaching sand shoal. The resulting gradient in the wave heights and consequent gradient in the alongshore sediment transport can be linked to the observed coastal erosion. This shows the importance of the shape of the sand shoal for the coastal dynamics in the system. To better understand the final stage before the abandonment of a flood channel, a conceptual model is presented.

All over the world, coastal protection measures are taken, which can be soft (e.g. sand nourishments on sandy beaches) or hard (e.g. seadikes or seawalls). Special care has to be taken to design the transition between these hard and soft flood defences, as they are often vulnerable components of a coastal defence system. This study therefore aims to give more insight into the morphological processes around hard-soft transitions, in order to make recommendations on design and nourishment plans for hard-soft transitions in the future. To this end, the hard-soft transition of Maasvlakte 2 (MV2) was studied. For the case of MV2 the limited knowledge of hard-soft transitions resulted in a conflict of the assessment method of the flood defence and the nourishment strategy according to the client requirements. The most important factors governing the morphological behaviour were studied using different numerical models (UNIBEST-LT/CL , SWAN and XBeach. From the results of the case study of Maasvlakte 2, it was found that the most severe erosion at the hard-soft transition is found during periods with strong southward longshore transports, which causes positive longshore transport gradients at the transition. Cross-shore processes such as storm erosion also indirectly contribute to this by transporting sediment from higher in the profile to lower in the profile. Regarding the influence of wave reflection, preliminary results were found. Using SWAN and UNIBEST, the effect of short waves was investigated, which showed that reflection of short waves is larger at the deeper located part of the hard flood defence. The preliminary results from XBeach showed that bound long wave reflection is larger closer to the hard soft-transition, and that these reflected waves also reach quite far offshore. As regards to the role of the tide, it was found that the dominant northward directed flood tidal current along the soft flood defence strengthens the supply of sediment from south during northward transport. On the other hand, the dominant southward directed ebb current along the hard flood defence hardly picks up any sediment and therefore it does not contribute to the morphological changes at the hard-soft transition.
Some generic observations in the morphological behaviour of MV2 are likely to be found at other hard-soft transitions: - A strong dynamic variability is usually present at the hard-soft transition, in the form of coastline retreat which gives a rotated coastline shape. - The reflection of short waves will likely be larger at the deeper part of the hard flood defence. Closer to the transition zone, where sediment from the soft flood defence is deposited, short wave reflection will be smaller, but long wave reflection will be larger. - In general the highest erosion will be observed in periods with oblique wave incidence where sediment is lost due to longshore sediment transport gradients. Based on these findings, several recommendations are proposed to design a hard-soft transition. Moreover this study presents some other design examples to minimise the nourishment frequency. Finally, recommendations are presented for further research.

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.

The St Lucia Estuary is Africa’s largest estuarine system and part of the iSimangaliso Wetland Park, which was listed as South Africa’s first World Heritage Site in December 1999. The St Lucia Estuary is connected upstream to Lake St Lucia and downstream it flows into the Indian Ocean. The St Lucia Estuary is located at a wave dominated coast and experiences a micro-tidal regime. Along the coast of South Africa about 70% of the estuaries do not have a permanently open link to the sea, i.e. they are temporarily open/closed estuaries, which are mainly regulated by the amount of river flow entering the estuary. Anthropogenic impacts and fresh water reductions have altered the natural system. The mouth of the St Lucia Estuary is unstable and has been closed for longer periods in the last century. The drought period ranging from 2002 has drastically affected the St Lucia System as a whole with the desiccation of 90% of the lakes, reaching the all-time low. The management authorities had to take action and research supported by the Global Environment Facility (GEF) which started in 2009 showed re-linkage of the Mfolozi River is the preferred solution to bring fresh water to the St Lucia System. This MSc study was initiated to investigate the effects of re-linkage of the Mfolozi River and dredge spoil removal from the estuary on the tidal inlet. The main focus is on the closure times of the inlet, the transition between an open – and closed inlet, and the multi-year morphological development of the inlet. The process-based model Delft3D is used to simulate various scenarios. The model is calibrated quantitatively on hydrodynamics and qualitatively regarding morphodynamics. The results from this study show that the re-linkage of the Mfolozi River leads to an inlet that remains open for majority of the time leading to a connection between the St Lucia Estuary and the Indian Ocean. This can make the St Lucia Estuary and Lake System a breeding ground for many marine species as it was before. The removal of dredge spoil however does not seem to be very effective. The closure times of the inlet barely change and the inlet re-imports most of the sediments. It is advised to limit the amount of dredge spoil removal, because the estuary will return to its equilibrium state.