This dissertation is triggered by the prevalent observation of conflicting stakeholder interests in highly urbanised deltas. These are coastal areas containing estuaries - i.e., water bodies where river water meets and mixes with seawater - which can form open or closed systems, and which can be natural or (highly) modified (e.g., trained, dredged or closed-off channels and canals). In these deltas, many stakeholders, including local communities, agriculture, shipping, and ecology, rely on estuarine services such as water safety, freshwater availability, deep and calm waters, and natural dynamics. Due to climate change and socioeconomic developments, many of these estuaries have become increasingly pressured, meaning that during extreme conditions, such as droughts, the variety of stakeholder interests can no longer be simultaneously satisfied. Consequently, system modifications to improve one stakeholder's interest may come at the expense of other stakeholders' interests, requiring complex trade-off decisions.

Here, waterborne transport is a key player. To facilitate the movement of goods and people, water transport entrepreneurs employ ever-larger vessels that call at ports ever more frequently. To accommodate this trend, waterways have been deepened and new lock complexes have been constructed. While such system modifications have brought beneficial effects to regional and (inter)national economies, they have also exacerbated saltwater intrusion, which negatively impacts freshwater availability during droughts. A framework to quantify a fair trade-off between the interests of waterborne transport and other estuarine user functions is currently absent. Instead, policy and decision-makers rely on qualitative analyses based on oversimplified models, hindering rational policy and decision-making. This problem particularly holds for the impact of physical system changes on waterborne transport performance, which are often not quantified.

Consequently, interventions aimed at improving waterborne transport often neglect the potential negative impacts on other stakeholders, leading to suboptimal and non-integrated solutions that may be ineffective in the long run.

The objective of this dissertation is, therefore, to assemble a methodological framework that can rationally quantify the trade-offs between impacted stakeholder interests for interventions in systems where waterborne transport plays a significant role. To achieve this, a two-step approach is followed. First, a method is developed to quantify the impact of system modifications in estuaries on waterborne transport performance. Second, this quantification method is included in a framework that can evaluate multi-stakeholder interests for an intervention in the estuary. This framework results in a trade-off curve between the impacts on the stakeholders' key performance indicators.

As a result of the first step, this dissertation found that vessel waiting times are the key performance indicator for quantifying the impact of system modifications on water transport. These waiting times are primarily caused by the cascading effects of downtime and congestion, which are currently not quantified by any existing method. To include these effects, this dissertation identified the open-source simulation library OpenTNSim to be the most suitable for further development. Additional modules were developed and added to this library to resolve the aforementioned `cascading effects of downtime and congestion' for both open and closed estuarine systems. The proposed quantification method was validated by its implementation as a nautical traffic model in a real-world case study of seagoing vessels calling at a liquid bulk terminal in the Port of Rotterdam. In this case, the nautical traffic model was considered valid when a sufficient part of observed waiting times could be reproduced and explained. One year of AIS data was analysed to obtain a representative fleet in the model with realistic origins, destinations, speeds, turning times, and laytimes at the terminal and anchorage areas. In addition, geospatial data and one year of hydrodynamic data were used to derive model input. Together with the actual maintained bed levels, port layouts, and tidal accessibility policies, the model resolves tidal downtime and infrastructure congestion. Analysis of the model results reveals that the nautical traffic model was able to unravel 73.4\% of the observed non-excessive vessel waiting times. Moreover, the unresolved excessive waiting times are believed to be caused by other processes that are not related to the system's state. Hence, the implemented method is considered valid to quantify waterborne transport performance as a function of the physical system.

In the second step, the now-validated quantification method for waterborne transport performance was included in a developed framework to evaluate trade-offs between the stakeholders' interests. The framework entails a train of models that link state indicators of the physical system to performance metrics for stakeholder interests. The model results are used to quantify the trade-off between port performance and freshwater availability as a function of bed-level variations in the open system of the Nieuwe Waterweg (NWW) in the Rhine-Meuse Delta. The resulting impact curves are insightful; they reveal how waterborne transport performance and freshwater availability compete as the bed level of the NWW changes. Freshwater availability improves when the bed level is raised, albeit at the expense of water transport performance, while water transport performance improves when the NWW bed level is lowered, albeit at the expense of freshwater availability elsewhere in the estuary. By adding valuation functions to each of the performance curves, stakeholders can express how important they find certain levels of performance loss. Ultimately, the framework leads to an optimal depth, although this decision remains a political process.

In conclusion, this dissertation enables a more rational trade-off between stakeholder interests in estuaries where water transport plays an important role. The underlying agent-based nautical traffic modelling method, implemented in the OpenTNSim library, and trade-off framework can be further expanded and applied. For this, further validation of the modules is recommended, particularly for closed systems, as they were only validated for open estuaries, with a specific focus on tidal windows and salt intrusion effects. Furthermore, to extend the applicability of the proposed quantification method for water transport performance, this dissertation recommends considering the incorporation of additional physical factors that affect downtime and additional sources that contribute to congestion. This may require the incorporation of additional datasets and the involvement of additional computing power. Moreover, to extend the applicability of the trade-off framework, it is advised to incorporate additional stakeholder interests and to involve stakeholders in constructing realistic valuation functions. With these additions, the proposed approach becomes more widely applicable, opening the door to its application to other estuaries around the world. @en

Mangrove forests are highly valuable environments that offer numerous ecosystem services, including carbon storage, serving as nurseries for fish and shrimp, and providing timber. Additionally, these ecosystems offer coastline protection against erosion and flooding by attenuating waves and trapping sediment. Despite the crucial role of mangrove ecosystems, global mangrove cover has significantly declined in recent decades, with deforestation and coastal erosion being one of the contributing factors, launching a variety of initiatives for mangrove rehabilitation. To rehabilitate mangroves along eroding coastlines, permeable structures are designed as a solution. These structures dissipate incoming waves and reduce current strength, creating low-energy areas behind them that are conducive to sediment deposition. However, these structures frequently prove ineffective in the rehabilitation of mangroves, possibly due to the absence of clear design guidelines, and a lack of understanding regarding the influence of design parameters on the performance of these structures.

This study, therefore, aimed to develop an integrated design guideline for permeable structures aimed at rehabilitating mangrove habitat along eroding coastlines by combining scientific knowledge with insights gleaned from pilot projects conducted globally. Furthermore, the research aimed to deepen understanding of the influence of design parameters on the performance of these permeable structures by conducting simulations of various structure configurations in a numerical, process-based model, and evaluating the performance of each configuration in reducing bed shear stress across the sheltered areas behind them. A nested model was employed in Delft3D-4, configuring the flow and wave modules to be coupled in order to incorporate the interaction between waves and currents.

The developed conceptual framework and design guideline as well as the identified correlations between design parameters and structure performance in this study contribute to enhancing the design process of permeable structures aimed at rehabilitating mangrove habitat. It provides preliminary insights, highlights significant knowledge gaps and provides recommendations for further research, thereby opening up opportunities to expand knowledge surrounding the design of permeable structures aimed at morphodynamic restoration of mangrove habitat.

Worldwide, estuaries are centres of life. Their connection with the open seas while providing fertile lands and freshwater has resulted in these regions becoming major hubs of human settlement and activities, which is expected to continue growing in magnitude. However economically useful, the open connection to the seas also poses a challenge: freshwater availability. Via this open connection saline water can enter the estuary and contaminate the freshwater reserves - so-called (estuarine) salt intrusion. This may cause freshwater shortages and damage to all living beings that depend on this invaluable resource.

This research aims to develop mitigation measures to this estuarine salt intrusion following the Building with Nature-philosophy. Thus, the goal of this research is to develop nature-based solutions to mitigate salt intrusion. This is achieved by (1) exploring the potential of estuary-scale interventions to affect salt intrusion; (2) conceptualising various nature-based solutions that mitigate salt intrusion; (3) evaluating nature-based solutions in a multidisciplinary context; and, to conclude, (4) reflecting on the role of nature-based solutions.

The exploration of potential estuary-scale modifications is a computationally expensive endeavour for which a novel simulation strategy is developed. This strategy is presented in Chapter 2 and proposes the use of machine learning techniques to determine the input space - i.e., which model simulations to execute, and which to exclude. The aim of the strategy is to put more focus on exploring the output space instead of exploring the input space.

Subsequently, the model simulations are analysed in Chapter 3. Thus, Chapters 2 and 3 present respectively the method and the results of an extensive sensitivity analysis of estuarine salt intrusion to estuary-scale modifications. The end-result of Chapter 3 includes a shortlist of potential nature-based solutions to mitigate salt intrusion, including a ranking based on the sensitivity analysis.

The conceptualisation of nature-based solutions focuses mainly on two potential options: (1) a (temporary) sill, or submerged dam (Ch. 4); and (2) enhancement of intertidal area (Ch. 5). In addition to these novel nature-based solutions, Chapter 6 evaluates a third nature-based solution: shallowing of an estuary. From a physical perspective, this mitigation measure is well-known but mainly poses challenges in the socio-economic domain, which is why it is not extensively covered in the conceptualisation-phase of this dissertation but the evaluation-phase instead.

Both the sill and the intertidal area show a dependency on estuary class in how effectively the mitigation measures are. For the sill it holds that the weaker the tide, the more effective the sill mitigates landward salt transport - i.e., salt intrusion. When the tide is limited, salt intrusion is largely (if not fully) driven by gravitational circulation, for which the sill functions as a wall beyond which the formed salt wedge can hardly penetrate. However, with tidal energy increasing, the tidal momentum to push the saline water over the sill also increases resulting in more salt intrusion - the sill functions more like a speed-bump than a wall. All in all, a sill is most effective for estuaries with little tidal influence.

Enhancement of intertidal area increases the mixing in the estuary. When the dominant salt transport mechanism is related to the estuarine circulation, this enhanced mixing by increasing the intertidal area reduces the salt intrusion. However, the opposite holds for estuaries in which the salt transport is dominated by the tidal oscillation. This means that in case of salt wedge and partially mixed estuaries, enhancement of the intertidal area reduces salt intrusion; and in case of well-mixed estuaries, the intertidal area increase promotes salt intrusion. Thus, the additional (vertical) mixing caused by the intertidal areas reduces salt intrusion as long as there is something to mix - i.e., as long as there is a (sufficient) vertical salinity gradient.

The evaluation of nature-based solutions is inherently multidisciplinary. In this dissertation,
two different perspectives are considered: (1) socio-economy (Ch. 6), and (2) socio-ecology (Ch. 7). In both cases, the Rhine-Meuse Delta is considered as case study.

The socio-economic evaluation addresses the effect of water depth on two major stakeholders in an estuary: a port, and water boards. These stakeholders have opposing interests regarding the water depth: a port benefits from enhanced water depth to facilitate larger vessels, but the resulting contamination of freshwater reserves via salt intrusion is negatively affecting water intakes, and everyone that depends on them. Chapter 6 presents a multidisciplinary evaluation method based on Pareto-fronts, which are to inform decision- and policy-makers. The Pareto-front in Chapter 6 shows that the port performance remains relatively unaffected for shallowing until two metres with the current bed levels, while the freshwater availability improves. However, beyond this level of shallowing, port performance drops substantially against limited gains in freshwater availability.

The socio-ecological evaluation focuses on the effects of reopening a closed-off estuary on two opposing interests as well: freshwater availability, and estuarine ecosystem functioning. Reopening is beneficial for the ecological diversity in the former estuary, but comes at the costs of returning saline influences into a freshwater lake - i.e., freshwater availability is hampered. The ecological implications are mapped by translating hydrodynamic (model) data to an ecotopes-potential map due to which the ecological impact of interventions can be quantified. As in Chapter 6, Pareto-fronts are used as method to inform decision- and policy-makers. The Pareto-front in Chapter 7 shows an increased ecological diversity in the former-estuary without impeding the freshwater availability when partially opening the gates. However, once the salt intrusion reaches the most western water intakes, the freshwater availability drops without major gains in diversity.

This dissertation has shown the complexity of developing future-proof nature-based solutions to mitigate salt intrusion. Building on the lessons learned, the reflection of this dissertation proposes a next step for nature-based solutions: DARE (diverse, adaptive, and robust engineering; Ch. 8). DARE is based on nature's own approach to dealing with uncertainties. The three axes of DARE show how to deal with uncertainties in the three dimensions of engineering: (1) a diverse set of solutions to deal with uncertainty in forcing conditions; (2) an adaptive approach with room to change course in response to the uncertainty that comes with time; and (3) robust - or even antifragile - solutions regarding the uncertainty in performance. With DARE, the focus shifts from the input to the output: Which part of the output is desirable, and what are the options to get there?

The nature-based solutions presented in this dissertation form a starting point for further explorations, with work in this dissertation already being used as a stepping stone for other studies. Besides further exploring the opportunities of nature-based solutions to mitigate salt intrusion, next steps also include bringing these findings into practice. This includes reassessing past estuarine modifications from a new perspective, one that is less susceptible to the challenges that the future may have in store.
@en

Because of the large uncertainties associated with rising sea levels, present coastal management is inclined towards utilizing soft, nature-inclusive, and adaptive measures over traditional hard protection structures. This shift towards more sustainable and multi-functional solutions is often called Building with Nature. In the coastal zone, these nature-based solutions typically use sand as a building material, have a larger scale than traditional sand nourishments, and serve multiple purposes. The definition of effectiveness of these nature-based solutions can vary for the different project aims. This means that models that predict future states of these interventions must be able to forecast various coastal state indicators (CSIs), such as dune volume, beach width, or habitat area. Using loose-placed sand in coastal protection is intrinsically associated with increased uncertainties of the coastal state compared to more traditional hard protection measures. Therefore, this dissertation examines uncertainty in predicting largescale sandy interventions in the coastal zone and its effect on different CSIs. First, observations of a recent large-scale intervention, the Hondsbosse Dunes, were analyzed to illustrate the evolution of various CSIs in the first years after placement. This 35 million m3 nourishment included a shoreface, beach, and dune and was built in front of a sea dike to serve as flood protection while creating space for nature and recreation. The nourishment created a significant coastline curvature, leading to erosion in the central, most protruding part of the nourishment, bordered by zones with accretion. The artificial cross-shore profile rapidly mimics the surf zone slope and beach width of adjacent beaches. At the same time, the dune volume increased, and the dune foot migrated seaward along the entire nourished site, regardless of whether the subaqueous profile gained or lost sediment. These contrasting trends of different CSIs highlight the need to predict changes in CSIs individually to assess a project’s effectiveness Next, the balance between different sources of uncertainty in predicting the evolution of a large-scale sandy intervention was investigated for the Sand Engine mega-nourishment. The sources of uncertainty in such predictions can be either intrinsic or epistemic. Intrinsic sources are inherent in the system, whereas epistemic sources are related to limitations in knowledge (related to the model). The relative importance of intrinsic and epistemic uncertainty was investigated using a probabilistic framework in which sediment transport is considered a function of random wave forcing (intrinsic) and model (epistemic) uncertainty, calculating transport using a one-line model. The applied wave climate variability was obtained from long-term wave observations, whereas model uncertainty was quantified using Generalized Likelihood Uncertainty Estimation (GLUE) relying on monthly observations. ..@en

This research focuses on the implementation of the Building with Nature (BwN) approach in the design of artificial coral reefs. The goal is to explore environmental preferences and design tools for coral development, considering both coral habitat potential and coastal protection services. The research also aims to optimize artificial reef design for a case study in Addu City, Maldives.

A conceptual model is developed based on an extensive literature review, incorporating critical engineering and ecological variables relevant to coral habitat potential and coastal protection services. This conceptual model serves as a practical design tool, fulfilling three primary purposes: identifying variables that require further examination to explore coral habitat potential, identifying variables that require analysis to explore coastal protection services, and identifying design variables that influence physical-chemical and biological variables for potential integral solutions.

The research also utilizes OpenFOAM, a numerical modeling tool commonly used in coastal engineering, to design artificial reefs. OpenFOAM accurately models flow and turbulence regimes, crucial for the ecological and biological functioning of coral reefs. An OpenFOAM numerical model is set up, calibrated, and validated for the case study in Addu City. Three design alternatives with different slopes are evaluated using a multi-criteria analysis (MCA), considering criteria such as coastal protection, coral habitat potential, and costs. The numerical model is applied to assess flow regimes and wave transmission for different design variables.

Based on the MCA results and the evaluation of criteria, an artificial reef with a mild slope of 1:3 is selected as the optimal integral solution for the case study. This design performs well in terms of coastal protection, coral habitat potential, and costs. The research demonstrates that the conceptual model and OpenFOAM numerical model are valuable design tools for assessing and optimizing artificial reef design, adhering to the principles of the BwN approach.

In conclusion, this research contributes to expanding knowledge and developing tools for implementing the BwN approach in artificial reef design. By considering both coral habitat potential and coastal protection services, the design of artificial reefs can be optimized to provide multiple benefits, including environmental and socio-economic values. The conceptual model and numerical modeling tool offer practical assistance in the design process, promoting the integration of nature and engineering for sustainable coastal solutions.

This research focuses on a new system for nourishing the Dutch coast with sand, which utilizes semi-autonomous sailing barges and an electrical crawler to continuously replenish the beach. Yearly, the Dutch government grants contracts to nourish the Dutch coast with volumes around 10.000.000 m$^3$. The Dutch dune coast protects the low part of the Netherlands against inundation from the North sea. This sand is dredged by Trailer Suction Hopper Dredgers (TSHD) on a depth of 20 m and transported to the coast. This transport causes a CO2 emission of about 190 kilo tonne a year. As emissions are expected to grow with increasing sea levels the Dutch government looks for ways to improve the high-energy demanding strategy that is currently used. Sweco introduced the Slow Sailing, and believes in the energy reducing concept and therefore initiates this further research. The general idea of this concept is to reduce the energy demand and emissions associated with traditional sand nourishment practices by using multiple barges that sail slowly and continuously nourish the Dutch coast. The aim of this research is to concretize this concept, in which feasibility of the system compared with energy, emission and cost calculations are central. To achieve this goal, the Slow Sailing system is compared to the traditional trailer suction hopper dredger, currently used for sand nourishment in the Netherlands. The comparison is based on a scenario in the Northsea. This leads to the main research question: What is the gain in energy consumption, emissions and cost for the Slow Sailing concept compared to classical nourishment strategies? In order to achieve this goal a review of current sand nourishment strategies was conducted. Next, the OpenTNSim model was adapted to perform energy calculations and enable a comparison of energy demands and emissions between the new system and the traditional trailer suction hopper dredger. Finally, a cost evaluation was conducted to compare the cost per cubic meter of sand for a 1.000.000 cubic meter nourishment project at the Egmond aan Zee nearshore in the Netherlands.

Compared to the traditional trailer suction hopper dredger, the Slow Sailing system showed a significant factor 15 decrease in energy demand in the sailing component of the dredging cycle. The Slow Sailing system with barges of 1500 m$^3$ was found to be the most energy-efficient and cost-effective option, with an estimated project cost of 3.00 euro/m$^3$, while the TSHD with 4280 m$^3$ was found to be the cheapest option, with an estimated project cost of 2.55 euro/m$^3$. In terms of CO2 emissions, the Slow Sailing system emitted significantly less CO2 compared to the TSHD, with a difference of 1000 tonnes CO2+ emission in a one million cubic meter dredging project. Overall, the results suggest that the Slow Sailing system is a more energy-efficient and environmentally-friendly option for sand nourishment of the Dutch coast.

Storm surges cause elevated water levels, potentially leading to dune erosion, which is crucial to understand due to the protective function of dunes and sandy coastal defences against flooding. Dunes are dynamic systems, extensively studied through flumes and field observations. Collecting hydrodynamic and morphological data from erosion-prone land reclamations can inform dune research and design optimisations. Loss of volume in land reclamation projects can result in setbacks, increased costs, and emissions. While dune erosion models estimate volume changes, they may deviate significantly under annual storm conditions due to calibration with normative storm conditions. Dune erosion typically occurs when the initial dune foot elevation is exceeded, resulting in a post-storm dune foot elevation at the maximum water level within the storm. Therefore, the (initial) dune foot might play an important role in the resulting erosion volume. The current knowledge on the dune foot behaviour of sandy coastal defences due to annual storm conditions is too limited and therefore the main research question of this thesis is: ‘How does the dune foot of sandy coastal defences behave due to annual storm conditions and what is the influence on dune erosion?’

This thesis performs an analysis of two field sites with hydrodynamic and morphological measurements, RealDune-REFLEX on the Holland coast and Land Reclamation Philippines in Manila Bay in the Philippines. For Land Reclamation Philippines, the hydrodynamics are transformed using two hydrodynamic instruments and validated using an ERA5 wind-driven SWAN model. The definition of the dune foot position is based on the maxima of curvature of the dune profile above a certain threshold following the second derivative method. The maximum total water level elevation is approximated with the measured water level elevation and an empirical parametrisation of the wave runup based on offshore wave conditions and the foreshore slope.

For RealDune-REFLEX, negligible erosion occurred for maximum total water level elevations far below the dune foot. For elevations just below and exceeding the dune foot, it shows that the post-storm dune foot correlates with the maximum total water level elevations. The dune foot could translate upwards by dune erosion and downward by bed level lowering and occasional avalanching. At Land Reclamation Philippines, it was found that pre-storm, more alongshore variability was observed in the vertical and horizontal position of the dune foot compared to post-storm. Minor coastal features eroded and resulted in larger dune foot retreats on those transects. Major coastal features remained present and influenced the post-storm dune foot position.

Concluding, an increase in the maximum total water level elevations above the initial dune foot height led to the highest erosion quantities of the dune front. However, no significant linear relation was found between the initial dune foot position relative to the maximum water level elevation and the resulting dune erosion. The post-storm dune foot elevation was found to correlate significantly to the maximum total water level elevation, approximated with an empirical runup formula, for upward and downward dune foot translation due to an out-of-equilibrium upper foreshore slope. Change of the horizontal dune foot position was found to relate to some extent to the storm duration and intensity of the total water level elevation exceeding the dune foot. Dune foot retreat magnitudes can be of the same order for small and large values of this quantification method and therefore exceedance of the dune foot does not provide a proper estimation of the dune erosion volume. It is found that the mismatched volume of the initial profile with the equilibrium condition, defined as an equilibrium slope reaching from the intersection of the initial profile with the maximum total water level elevation, is a more accurate estimation of the dune erosion volume. This methodology provides a practical way of predicting erosion volumes in sandy coastal defences using simple initial conditions, making it applicable to engineering practice.

For further research, it is recommended to improve the mismatch method based on the shape and size of the equilibrium profile. Second, study the dune foot dynamics within a storm using continuous LiDAR laser measurement and therewith validate dune erosion models. Third, civil contractors should enhance the quality of surveys in periods where dune erosion is expected to establish valuable datasets. Lastly, numerical modelling of complex typhoon-induced nearshore hydrodynamics could lead to more accurate insights into the alongshore behaviour of typhoon-induced dune erosion and the validity of the methods used.

Many coasts around the world experience coastal erosion as a result of climate change, lack of sediment supply or human interventions. In the coming decades, coasts will be exposed to increased natural forcing because of climate change and sea level rise, leading to increased risks of coastal flooding and erosion. As sandy coasts are highly dynamic systems, it is difficult to assess the impact of these risks. Long-term coastal evolution is evaluated on large temporal and spatial scales, which introduce uncertainty. Hence, probabilistic estimates are required rather than single, deterministic values. Complex, process-based models have been developed to simulate morphological changes in coastal zones, but these come at large computational expense. As probabilistic approaches require a large number of simulations, it is necessary to reduce computational times as much as possible.

Variability in coastline position is induced by both cross-shore and longshore processes, which occur perpendicular and parallel to the coastline, respectively. Extreme storm events and sea level rise are responsible for the long-term cross-shore coastal dynamics, while gradients in longshore sediment transport act as the main driver for the longshore coastline changes. This study focuses on developing a framework to combine predictions of long-term cross-shore and longshore processes in a probabilistic way. The framework combines probabilistic predictions of cross-shore effects with deterministic estimates of longshore-induced coastline change. Two separate models are studied for the cross-shore and longshore component, namely the Probabilistic Coastline Recession (PCR) model and the ShorelineS model. Both models are computationally efficient due to the use of reduced complexity sediment transport formulations and a simplified representation of the beach profile.

The developed framework is validated by a case study of the IJmuiden coastline, the Netherlands, to assess the effects of the blockage of longshore sediment transport by the breakwaters between 1967 and 2007. Consequently, projections of coastline development are determined for the period of 2023 to 2100, for a range of IPCC sea level rise scenarios. To model storm erosion with the PCR model, a synthetic time series of storm events is simulated by establishing joint probabilities of wave forcing variates. Moreover, the PCR model provides stochastic estimates of coastline recession, enabled by the large number of simulations. Regarding ShorelineS, a representative wave climate based on data of local wave conditions was used to compute the longshore sediment transport.

The ShorelineS simulation over the hindcasted period clearly resulted in accretion on both sides of the breakwaters in the order of 100 to 900 meters, in accordance with field observations. This outcome indicates that limited threats of coastal erosion are currently present. However, when analysing the projections for 2023-2100, the PCR model predicted that a combination of storm impact and sea level rise can definitely lead to coastline recession, with a median retreat of 18 meters and 32 meters for the RCP2.6 and RCP8.5 sea level rise scenario, respectively. Moreover, the PCR model indicated that at some point the coastline erosion might be larger than 100 meters with a probability of 1.5% for RCP2.6 and 4% for RCP8.5. The combined projections confirm the importance of accounting for both cross-shore and longshore impacts on long-term coastline evolution and show that the developed framework is a useful method for this purpose.

For further research it is recommended to study long-term coastline evolution while including interaction between cross-shore and longshore processes, to analyse its contribution to long-term coastline changes. It is also advised to further investigate how to obtain stochastic projections of coastline development induced by longshore processes, as this study only presents the cross-shore component in a probabilistic way. Furthermore, stochastic projections of sea level rise and beach recovery can be studied as well.

The Ems Dollart estuary is facing high levels of turbidity, which hinders the growth of algae and impacts the entire ecosystem. In recent years, turbidity has increased here with 0.5 to 3% per year between 1990 and 2010, corresponding to a doubling of suspended sediment concentration (SSC) within 24 years for the upper bound of 3%. This trend is mainly attributed to a decrease in the amount of sediment sinks. The Ems-Dollart 2050 program aims to address this issue by exploring various solutions, aiming to extract one million tons of dry matter from the estuary per year. This study investigates the potential of creating additional sediment sinks by scaling up managed realignment polders. The aim of the research is:

To investigate the important factors of influence for sediment capturing efficiency in a managed realignment polder, to make the mud extraction from a turbid estuary as efficient as possible while creating new natural and agricultural value.

Using the Delft3D FM numerical model, the study focuses on two key factors that influence sediment capturing, namely the effects of the surface area and the effect of the tidal prism (i.e. the volume of water that flows into an area during a tidal cycle), with the goal of making sediment extraction from the turbid estuary as efficient as possible. The study finds that the general concept of using a managed realignment polder to extract sediment is viable. However, the calibration process using Polder Breebaart revealed that the model struggled to accurately replicate some of the accumulation patterns in the low-dynamic system in this polder. While the model seems more appropriate for the project area, significant uncertainty remains due to the sensitivity of the results to sediment properties and calibration parameters.

The study's results suggest that a smaller tidal range can result in a more uniform sediment distribution across the area. This can be achieved by implementing a submerged weir behind the open entrance to the area. An even distribution of sediment could enhance the natural values in the intertidal area, which is a key goal of the Ems-Dollart 2050 program. In addition, wind-generated waves appear to have a major influence on the distribution of sediment over the area. Due to the extra shear stresses, waves provide more resuspension, which transports the sediment further.

Furthermore, the study observes a significant accumulation of sediment in a side-basin near the entrance. The high sediment accumulation in this area, compared to adjacent areas without accumulation, may be attributed to local low energetic conditions. This finding has potential implications for raising agricultural land using transitional polders.

In conclusion, this study provides insights into the effectiveness of using managed realignment as a method for extracting sediment from a turbid estuary. While it highlights some important aspects and influences, it also underscores the need for further research to confirm the findings and reduce the uncertainty in the results. Nevertheless, the potential benefits for both natural and agricultural values make this an intriguing avenue for future exploration.

Due to accelerating climate change, which entails more extreme storm conditions and exponential sea level rise, the protective function of our dikes and levees is under growing pressure. The presence of so-called forelands (e.g., beach, mudflat, and tidal marsh) are promising Nature-based Solutions to reduce the probability of dike failure and to mitigate the consequences after a dike breach. However, the positive effects of forelands depend, for instance, on sediment composition, geometry, and the presence of vegetation. These effects have not been studied in detail and have not been incorporated into reliable physical and validated mathematical models yet.

This report describes an experimental study on the stability of different foreland types. Particularly, the following aspects were considered: investigating whether surface and headcut erosion are indeed the dominant foreland erosion mechanisms, plus providing experimental data for validation purposes for models such as the extended BRES model.

Twelve experiments were performed in the sediment flume facility of the Hydraulic Laboratory of the faculty of Civil Engineering and Geosciences at the Delft University of Technology. A foreland during breaching conditions (i.e. high flow velocities) was simulated by an experimental setup, a so-called broad-crested weir. This weir was developed, constructed, and built within the facility to simulate and study erosion processes of two foreland sediment types, sand and clay. Geotechnical properties were obtained by performing sediment analyses in the Geoscience and Engineering Laboratory. In order to try to exclude the influence of cracks, a special adjustment of the facility was made by covering the sediment package with plywood panels; these experiments are referred to as ‘Tompouce’. For collecting data, the facility was fully instrumented with hydraulic sensors (discharge, flow velocities, and water levels) and synchronized cameras. The erosion process was analyzed and quantified from the visual data.

The experiments enabled us to answer the main research question, “Which erosion mechanisms are important to consider for forelands under dike breach conditions?”. For non-cohesive sediments, surface erosion should be considered the dominant erosion mechanism. In the case of cohesive sediments, however, headcut block erosion should be considered the dominant erosion mechanism. Here, when cracks are present, the erosion rate increases considerably. Especially the latter effect is a new finding; this mechanism should be included in foreland-dike breach development modeling. More research is needed into the quantitative effect of cracks on the stability of a vegetated foreland when it is subject to (sudden) high-water events.

San Andrés is a Colombian-Caribbean Island located 800 km from the Colombian coast. On its Eastern side there is a barrier reef formation protecting the island from offshore incident waves. Due to the protected environment created by the coral reef, sandy beaches can be formed on the East side of the island.
During normal conditions, waves come predominantly form the East (90% of the wave climate), have significant wave height of 2 m and period of 8 s. Besides, San Andrés is situated on the Caribbean hurricane route, which can cause an enormous damage to the island. The storm season at San Andrés is between October and December, which is also when major erosion events take place.
The economy of San Andrés is mostly built upon tourism, specially related to its biodiverse ecosystems and Caribbean beaches. The island’s ecological environment is composed by mangroves, seagrasses, and coral reefs, attracting a wide spectrum of fauna and flora to its ecosystems.
During the Masterplan for Coastal Erosion (PMEC), San Andrés was pointed out as a location in which coastal erosion is problematic. In a follow-up of this Masterplan, the island was elected to be part of a program in which solutions against coastal erosion would be presented. This research is part of this project, as a parallel trajectory to get a more profound understanding of the system and the possible mitigation measures that could be applied on the island.
With increasing urbanization and frequency of extreme weather events, erosion is becoming a problem with which San Andrés and its residents are repeatedly having to deal. Erosion is specially problematic for the Northern part of the island, called Spratt Bight. This region is not only the most densely populated area of the island, but also economically and touristically very important. Its beach presents periodically eroding patterns during storm seasons, when wave action drives the sediment towards the East, decreasing its beach width almost to none. A decreasing beach width has a direct negative impact on tourism, making coastal erosion in Spratt Bight not only a coastal safety problem, but also an economic issue.
This study aims to look into the main hydro- and morphological processes driving coastal erosion in Spratt Bight and, using the Building with Nature philosophy, propose a set of solutions to mitigate this problem. To reach this objective data analysis and literature research has been carried out, after which different environmental conditions were modelled using the numerical model Delft3D.
During these activities it was found that independently of its direction, waves approaching San Andrés break upon the coral reef and induce a water level set up inside the coral lagoon. The difference in water level in- and outside the lagoon generates a current and sediment transport, which is directed towards the western opening in the coral reef.
When the Northern waves approach the island (1.5% of the wave climate), the same water level set-up phenomenon is observed. However, as waves are approaching form the North, they not only break upon the reef, but are also able to enter the sheltered lagoon through the western opening in the coral reef. These waves are able to bend around the reef reaching the shore and the headland on the Northern part of the island, inducing a longshore current and a sediment transport that is southeastward directed. The result is that Northern waves are mostly responsible for a strong westward and erosive sediment transport pattern. These waves are mostly observed between October and March, which coincides with the storm season in San Andrés.
Besides, it was found that the Eastern waves are responsible for restoring the (dynamic) equilibrium profile of Spratt Bight Beach. However, this restoring force has a less strong intensity, taking more time to restore the beach than to disrupt its equilibrium.
The solutions proposed include seagrass restoration to enhance ecology, restrain sediment transport and attenuate wave heights; the beneficial reuse of dredged material, to nourish Spratt Bight Beach; and finally, the implementation of artificial coral reefs as breakwaters to prevent the newly nourished sediment to be lost from the system. Besides, artificial coral reefs enhance the ecosystem by attracting fauna and flora increasing biodiversity. All proposed solutions have a positive impact on the beaches and therefore on tourism and the economy of the island. This makes them multifunctional solutions, serving the main goal of protecting the beach while at the same time creating benefits for other functions and values in the area. Following in this way the prescriptions of the Building with Nature design approach by van Eekelen and Bouw (2020).

This thesis analyses the importance and application of considering multiple coexisting wave trains in sediment transport predictions.

The wave trains at offshore and nearshore locations are analyzed by wave spectral partitions (Portilla at el., 2009). The temporal variability of consistently occurring wave trains can be analyzed through so-called wave families (Portilla et al., 2015). The wave partitions show that a unimodal wave spectrum occurs 35% of the time in front of the coast of the province of North Holland in the Netherlands. This means an error is introduced 65% of the time as unimodal spectra are assumed in traditional methods that use generalized wave parameters to estimate sediment transport. When the wave field is not unimodal, the angle between wave trains is considerable. Only 39% of the time is the angle between a wave train and the most energetic wave train smaller than 60°. For 43% of the time, the angle is between 60° and 120°. The remaining 17% are wave trains propagating in opposite directions. The difference in wave angle is half of the time larger than 50° when a threshold of one meter of deepwater significant wave height over the whole spectrum, defined by Holthuijsen (2007), is used.

Unsupervised machine learning techniques Principal Component Analysis, Maximum Dissimilarity Analysis, and the kmean clustering algorithm are used to group similar wave spectrums over time, spatially group wave families, analyze the wave climate's spatial-temporal variability, relate wave - and weather conditions, and assess their impact on wave-driven potential sediment transport. The Principal Component Analysis is used as a dimensionality reduction technique. This study shows the physical meaning of the Principal Components and their temporal variability in the wave frequency-directional spectrums, where Principal Components describe wave energy in each frequency-directional bin. The kmean clustering algorithm groups similar wave spectrums over time and gives insight into wave conditions, like calm and stormy conditions, called Sea States. The Maximum Dissimilarity Technique changes the focus between common conditions or extreme events. The wave partitions, wave families, and Principal Components give additional insight into the Sea States.

Potential sediment transport estimations are made by traditional methods that use a representative wave frequency and direction and through vectorizing and superpositioning the estimations by wave partitions and wave families. The outcomes show that it is challenging to predict sediment transport for multimodal wave conditions with state-of-the-art sediment transport formulations and models. The results show that the wave period significantly impacts sediment transport. The sediment transport for coexisting wave trains causes about 58% of the time more sediment transport than traditional methods. Here the not considered interaction between wave trains on bedload and sediment suspension will enhance sediment transport. Furthermore, the sediment transport estimations give insight into the wavefield and weather conditions that contribute most to the yearly wind-wave induced potential sediment transport caused by high energetic or consistently occurring conditions.

There is a demand by Rijkswaterstaat for CO2-neutral sustainable methods and technologies to maintain the Dutch coastline. The Zandwindmolen is a fixed CO2-neutral dredge, transport and nourishment concept that nourishes sediment continuously. The purpose of using a Zandwindmolen is to provide the entire surrounding area where the Zandwindmolen nourishes with sediment and allowing it to grow with the (accelerated) sea level rise. With the traditional nourishment method, it is often desired that the sand remains at the vulnerable site for as long as possible. Whereas, in a nourishment with the Zandwindmolen, it is often desirable for the sand to move to other nearby parts of the coast as quickly as possible.
Between 2011 and 2020, 27.5 million m3 of sand has been nourished at the Ameland Inlet. Therefore, it might be considered as an erosion hotspot. Because of the fixed nourishment method of the Zandwindmolen and the continuous sand shortage at the Ameland Inlet, there is a match. However, the usefulness of a Zandwindmolen depends on the extent to which the nourished sand volume is dispersed by natural processes in the short and long term.
In this study, both the short- and long-term effect of a continuous point nourishment in a tidal channel is investigated for four sediment fractions (100, 200, 300 and 400 μm). The goal is that the sediment is dispersed by nature in order to allow the entire Ameland Inlet (adjacent coastlines, coastal foundation, outer delta and Wadden Sea basin) to grow along with sea level rise.
For optimal dispersion, the nourishment must be carried out by means of a spreader pontoon with a sediment mixture concentration of 1-2%. By doing so, a mixing plume is created in which sediment settles according to their individual settling velocity. This makes the mixing plume prone to the tidal ebb and flood current and can therefore be steered towards either the North- or Wadden Sea.
In the short term (time scale: instantaneous to a few weeks), it appears that the dispersion of sediment is mainly determined by the initial sedimentation process of sediment and by re-suspension of settling sediment particles. The dispersion by natural sediment processes is minimal. In the longer term (timescale: half a year) the sediment spreads further after it has settled to the seabed. Based on half a year, the total dispersion is mainly determined by the short term dispersion.
It is found that a continuous nourishment with the Zandwindmolen is expected to ensure that the intertidal areas in the eastern Wadden Sea can continue to grow naturally with (accelerated) sea level rise. Moreover, the natural distribution of the nourished sand will, through sediment connectivity, compensate for the erosion (autonomous and due to sea level rise) of the inlet (especially the outer delta) and is also expected to contribute to keeping the North Sea coast of Terschelling and Ameland safe. Therefore, the Zandwindmolen could be a useful new sustainable nourishment method in a tidal inlet.

Coastal protection is required to keep coastal areas protected during storm conditions. Coastal dunes are a natural defense against flooding and erosion from the sea against storms. Many insights have been obtained regarding the positive effects of coastal dune vegetation on dune formation and growth in the past. Several recent studies have demonstrated the ability of both aboveground and belowground dune vegetation to reduce dune erosion during storms.

However, the impacts of dune vegetation during collision regime storm conditions are not taken into consideration in morphological numerical modeling. Consequently, it is unclear how to evaluate vegetation impacts during design assessments. Therefore, the goal of this research is to investigate the potential effect of dune vegetation on dune erosion during collision regime storm conditions and subsequently link this with dune rehabilitation projects. This research is divided into two parts.

First, the capability of the numerical model XBeach to simulate the potential effects of dune vegetation during the collision regime is investigated. Four vegetation approaches are identified in the model, which could possibly represent different dune vegetation effects. Using beach-dune profiles and erosion volumes obtained from two wave flume experiments, the performance and sensitivity of the vegetation approaches are tested in XBeach. Thereby, a distinction is made between the effect of aboveground vegetation: hydrodynamic altering and belowground vegetation: soil stabilization. The model results show that XBeach is capable of simulating dune erosion with vegetation during collision regime storm conditions. This can be primarily attributed to the increase of the critical slope in the avalanching module. This approach represents the soil stabilization effects of belowground vegetation. The application of the root model was added for even better simulations. This approach accounts for the additional root cohesion provided by belowground vegetation by increasing the critical velocity for sediment pickup. The values to be used for both vegetation approaches could not be defined systematically yet due to the small number of experiments assessed and little research found in the literature. It is recommended to obtain more information about the effect of dune vegetation on the stabilizing effects and the critical slope for avalanching. The application of a higher roughness value and the vegetation module, which both represent aboveground vegetation by altering the hydrodynamics, have shown to contribute insignificantly to erosion reduction in the examined cases.

The belowground soil stabilization approaches are applied in a case study to give a first indication of the applicability of XBeach at large-scale and in dune rehabilitation projects. An XBeach model of Beira, Mozambique was set up. A higher critical slope demonstrated profile evolution and erosion reduction in line with observations in the literature. During wave impact, the avalanching module is the most suitable approach to account for the erosion-reducing effect of belowground vegetation. The root model appears to be a more appropriate strategy for accounting for belowground vegetation during milder conditions and shorter storm duration. It was proven that XBeach has very good potential to evaluate the effectiveness of dune vegetation in the design phase. The case study illustrates that vegetation will significantly increase the erosion resistance of the flood dunes. The dunes with vegetation can handle an additional 20-30 centimeters design water level or 0.5-0.8 meter storm wave height without breaching compared to the same dune without vegetation. The design with vegetation has shown to be more robust. However, the approaches are based on different assumptions and limitations and therefore the results should be considered carefully. It is strongly advised to conduct more research in this relatively new study area.

This study investigates and proposes a method for quantifying the advantages of vegetation on dune erosion reduction. The relevance and added value of mature and robust dune vegetation for a resilient coastal dune system and protection against erosion and floods are confirmed. As a result, the incorporation of vegetation in dune rehabilitation projects is promoted.

Witteveen+Bos as part of construction consortium Nautilus, has prepared the design of a sea defence system in Middelkerke. The very shallow water conditions (h/Hm0 = 0.3) and complex geometry of the structure, i.e. a high berm and stepped revetment, caused uncertainties in the usage of the empirical equations from the EurOtop Manual (2018) for wave overtopping discharges. Therefore, small-scale experiments have been performed to asses the overtopping discharges.
Physical model experiments can be used to determine wave overtopping discharges, but can be expensive and time consuming. Recent studies show promising results for the use of SWASH as tool to predict overtopping discharges for relative simple geometries, but SWASH is suspected to be less accurate for more complex cross-sections. Two methods (SWASH, EurOtop) are used to predict wave overtopping discharges for the new boulevard Middelkerke and the results are compared with measurements from small-scale experiments conducted in Ghent. The goal is to asses the feasibility of using both methods for this complex structure with stepped revetment in shallow water conditions.

The EurOtop Manual (2018) uses influence factors to account for roughness at the slope and the presence of a berm in the structure. To use the empirical formula to determine the average overtopping discharge for this specific complex cross-section, a berm influence factor was added to the equation
for shallow water conditions developed by Altomare et al. (2016). Recent studies of Schoonees et al. (2021) found that the influence of the roughness of a stepped revetment on overtopping discharge mainly depends on: a characteristic step height, relative overtopping discharge and the wave period at the toe of the structure. Using their research as guideline, a roughness influence factor of γf =
0.75-0.9 was estimated for the stepped revetment for this case study. Although not validated for this specific configuration, using the (adjusted) equation resulted in very comparable average overtopping discharges compared to the physical experiments in Ghent.

In this case study, the usability of SWASH to predict the average wave overtopping discharges has been evaluated. First, the SWASH model was calibrated based on the incident wave conditions at the toe of the structure (Hm0, Tm-1,0) with the data the physical experiment in Ghent. This were the target wave conditions of this study since they are generally used to determine the average wave overtopping discharges (e.g. EurOtop) and to exclude the wave-structure interactions. Thereafter, the structure was added to the bathymetry with a smooth slope and local friction was added to represent the stepped revetment, as resolving the small steps would require an overly fine grid.

SWASH was able to reproduce the target incident wave conditions of the physical experiment very well (Hm0<3%,Tm-1,0<5%). However, when the structure was added to the bathymetry, larger differences were seen for the wave conditions at the toe compared to the physical experiment, for which no explicit explanation was found. Contrarily, Hm0 and Tm-1,0 include the full wave field, primary waves and infragravity waves, and the limited number of overtopping waves made it difficult to asses the influence of the difference in wave spectrum on the overtopping discharges.
The overtopping reduction due the added roughness/friction compared to a smooth slope reference test, was eventually related to a Nikuradse roughness height for the stepped revetment. A Nikuradse roughness of 1-1.5 times the characteristic step height of the stepped revetment, resulted in very similar average overtopping discharges compared to measurements from the Ghent experiment.

Both the EurOtop Manual (2018) and SWASH were able to reproduce the average wave overtopping discharges from physical experiments, but the results need a wide confidence band since the number of overtopping waves were limited and discharges small (q < 1 l/s/m). Each method has their own benefits and limitations. The EurOtop Manual (2018) is easy to use, but the validity of the empirical equations are uncertain for more complex geometries. SWASH gives more detailed information than only an average q: spatially and temporarily varying surface elevations and velocities along the domain, by which direct attack on the slope and structures (extreme values of u and F) can be calculated. SWASH could also be used as a ’numerical laboratory’ to further parameterize the influence of roughness on wave overtopping for a wide range of boundary conditions and structural configurations.

Mangroves are productive ecosystems. Degradation of mangroves has been observed since the 1970s, due to a variety of reasons. Successful rehabilitation focuses on restoring conditions suitable for mangroves. Sediment availability has been shown to positively impact mangrove growth. Therefore, it can likely play a role in mangrove rehabilitation. This research aims to explore if the rehabilitation of coastal mangroves is possible using a sediment nourishment.

A process analysis is performed, to identify the processes driving sedimentation on a mangrove coast. This analysis uses a schematised, cross-shore, one-dimensional model, based on global data of mangrove coasts. Convex, linear and concave bottom profiles are used. The forcing on the coasts consists of three different wave heights, tidal ranges, and sea level rise rates. A homogeneous mangrove forest is present above mean sea level.

Sedimentation within the mangrove forest is caused by a combination of processes. Waves pick up the sediment, which is transported by the tide. Tidal asymmetry then drives the actual sedimentation in the forest. An increase in asymmetry results in increased sedimentation. Mangroves increase the flood dominance of slack water asymmetry, but also cause an ebb dominant peak flow velocity and duration asymmetry.
Dependent on the tidal range, the influence of mangroves on net sedimentation can either be null, negative or positive.
Sea level rise increases sedimentation within the mangrove, by enhancing slack water asymmetry.

The main tipping point identified is the gradient of the bed slope in cross-shore direction. If the slope becomes milder towards the shore, i.e. a convex coast, this value is negative. If the slope becomes steeper, which is the case for concave coasts, it is positive. The tipping point is when the slope does not change towards the shore. The gradient of the slope then is zero. The net sedimentation is considerably less for concave coasts than for convex coasts.

Sediment nourishments can increase sedimentation in the mangrove forest. Four nourishment designs were simulated, using the same model as for the process analysis. The designs varied in location and achieved depth across the coast.
Three design considerations for a successful sediment nourishment have been identified, which are firstly the obtained bed level by the nourishment, secondly the slope of the original bed, and thirdly the location of the nourishment.
First, the obtained bed level by the nourishment influences the quantity of sediment pick-up. Higher bed levels increase the sedimentation within the mangrove, by increasing sediment mobilisation rates.
Second, the slope of the original bed largely influences the feasibility of a nourishment. Due to mild bed slopes, the construction of a nourishment is difficult and large volumes for a nourishment are required.
Finally, the location of the suppletion should be close to the mangrove. If the nourishment is located far offshore, the sediment is not transported towards the mangrove. Rather, it then is transported away from the coast.
This thesis has shown the rehabilitation of mangroves using a sediment nourishment is possible.

Coasts are constantly under the pressure of hydrodynamic conditions such as waves, tides, and storms. In the Netherlands, sand nourishments are executed every few years in order to maintain the country’s sandy beaches for purposes of safety, recreation, and ecology. In order to determine where to carry out these nourishments, the whole sandy coastline of the Netherlands is measured annually by Rijkswaterstaat. This annual survey, called JAarlijkse KUStmetingen (JARKUS), monitors where (increasing) erosive trends appear or persist and, due to its timespan (dating back to 1843), is a valuable dataset to understand the evolution of the Dutch coast. However, this measurement survey is restricted to its annual frequency and is costly. The use of optical satellite imagery for measuring land cover types and geographic features is rapidly becoming more popular due to their high temporal frequency and relatively low costs (due to the public availability of some satellite missions, such as NASA’s Landsat and ESA’s Sentinel-2 missions). This research studied the possibilities of using optical satellite imagery for measuring beach width dynamics in addition to the existing measurement campaigns.

In this study, we derived the Satellite-Derived Beach Width (SDBW) as the cross-shore distance between the Satellite-Derived Shoreline (SDS) and the Satellite-Derived Vegetation line (SDV). We adopted a widely used and validated method for SDS detection and adapted this method to establish the SDV detection method. The SDS and SDV are derived from optical satellites by deriving vectors from the border between two contrasting land cover types that are identified by differences in (sun)light reflectance values. The SDS and is derived from the contrast between water and land, the SDV from the contrast between and sand/sediment and vegetation. The SDBW data was measured from both composite and individual satellite images. The different techniques are suited for different applications, and both have their advantages and disadvantages. A composite image is an image that is composed of a sequence of individual satellite images available within a set window. E.g., a composite is the average image of all those images. Recent research showed that, at the cost of temporal resolution, composite images are suited for analysis of long-term (structural) shoreline trends since they mitigate certain factors influencing image quality (such as clouds and cloud shadows, waves and tides, and satellite instrument errors). Individual images are better suited for analyses of short-term dynamics since they provide instantaneous measurement data. However, they are hampered more by the factors mentioned above, and hence need to be screened before use...

Dredging is performed for various reasons, including port development, coastal defense and land reclamation. The economic and social importance of such projects is widely acknowledged, but there are growing concerns about the possible environmental impact of the dredge plumes that form as a temporary by-product of these activities. Suspended sediment blocks sunlight, and sediment that settles on the seabed buries coral and other organisms. These effects accumulate over the duration of a dredging project, and should be minimized.

An important step towards this goal is predicting plume formation and development. Here, we focus on an overflow plume from a trailer suction hopper dredger (TSHD). Although this subject has been studied for years, much is still unknown, due to the complex interaction of the processes involved. Because of this complexity, it is not uncommon in the industry to use rules of thumb, derived from empirical studies.

One such rule of thumb, based on an influential publication by Becker et al. (2015), relates to the fraction of the overflow plume that is still (or again) in suspension once the developing plume moves from the dynamic near-field to the passive far-field. This source term fraction (STF) is important, because the large-scale area models that model plume development in the far-field require information about the characteristics of the plume entering the far-field zone. Becker et al. (2015) presented a realistic range of 0-20% for the source term fraction. This range is based on a combination of measurements and expert opinions. Although Becker et al. (2015) recommend characterization monitoring at the start of a dredging project to determine whether the 0-20% range needs adjusting, this is not always done in practice, due to time pressure, costs, complexity or lack of necessity.

Building on the work by Becker et al., this thesis aims to support a more informed implementation of TSHD source terms on the computational grid of far-field models. In order to explore the behavior of SHD dredge plumes, numerical simulations were carried out using computational fluid dynamics model TUDflow3D. In these simulations, the values of three model input parameters – the water depth, the overflow density and the crossflow velocity – were varied to gain insight into the behavior of the plume under the conditions studied. For each of the three parameters, three representative values were chosen on the basis of expert advice, and all combinations were tested, leading to 27 numerical simulations.

The results of the numerical simulations show that the amount of sediment in the plume has reduced to less than 20% of the sediment in the overflow plume at 11-42 minutes of plume development, corresponding to a distance of 0-1895 m from the dredging location (500-3800 m from the moving vessel), depending on the specific conditions. It is important to note that (only) three conditions were varied
in this study. These conditions were all steady state (e.g. no tidal flows). In real-life situations, many more (combinations of) conditions and hence a wider range of results are possible. Conditions where the plume was found to travel relatively long and far into the far-field, are a high ambient current, a low water depth and low overflow densities. The effect of these factors corresponds with existing studies.

The contribution of this work lies in the broad characterization of the developing plume under these various conditions: not only the magnitude of the plume is investigated, but also its vertical and lateral
distribution. Specifically, the contribution of the bottom 6-13 % of the water column to the total plume flux is highlighted. This is particularly of interest, as this zone is typically not included in ADCP
measurements. This increases insight in the processes underlying plume development and supports the appropriate determination and implementation of the far-field source term, including a more informed
use of the 0-20% source term fraction range proposed by Becker et al. (2015).

Sea turtles are a popular tourist attraction that offers travelers a unique nature experience. As a part of Saudi Arabia’s 2030 vision, an unprecedented amount of tourism development is being carried out under the ‘Red Sea Development Project’. The aim is to develop hyper-luxury islands in a sustainable and ecologically friendly way by closely following ‘Building with Nature’ principles. The tourism development site at Al Wajh Bank encompasses an archipelago of more than 90 islands and some of the sandy islands provide nesting grounds for Hawksbill and Green turtles. The thesis is a component of a broad study on investigating the preferential conditions for sea turtle nesting and conservation where TU Delft is collaborating with NIOZ and KAUST. The present study makes a preliminary assessment of the preferential conditions for sea turtle nesting of wave hydrodynamics and geomorphological conditions based on available data and numerical wave model simulations. The study is expected to provide information and guidance for future research for the conservation of sea turtle nesting at Al Wajh Bank.

The coral reefs provide breeding grounds for sea turtles in the gently sloping sandy beaches along the raised islands where the sand has ideal conditions of moisture, temperature, and distribution of sediment size for successful nesting and hatching of eggs at an optimum distance from the highwater line to avoid inundation by wave runup. In general, the foreshore at the turtle nesting site has a fore reef with a steep slope followed by a wide and shallow reef flat having high bottom roughness for dissipation of wave energy. The Al Wajh Bank spread over an area of 2880 km^2 has a very large and complex barrier reef system at the outer edge in the deep sea enclosing a massive lagoon with several islands. These islands have sandy beaches on the up wave (windward) side and mudflats with sand and vegetation in the sheltered zones. The vegetation is dominated by mangroves in the mudflats and seagrass in the shallow lagoon beds.

The study used the Delft3D WAVE stand-alone phase averaging spectral wave model for transforming waves from offshore to nearshore and to examine the energy dissipation characteristics of the reef system. The high-resolution bathymetry data for the Al Wajh Banks was obtained from GCC and relatively coarse resolution data for the offshore from GEBCO. The model simulated offshore wind and wave data validated using satellite altimetry, SAR, and Buoy measurements was obtained from BMT ARGOSS online services website “waveclimate.com”. The normal and extreme wave conditions were derived using this data. The data on GPS coordinates at the location of turtle nesting sites was provided by a survey undertaken by KAUST. A limited data on sediment size distribution at four islands south of Al Wajh Bank was made available by KAUST collected by ALS Arabia. The data on beach slopes, the distance of nests from HWL, geomorphology, vegetation was derived from secondary sources or in an indirect manner using bathymetry data.

The wave model results were analyzed for the nearshore wave heights and distribution of energy density at the fore reef and inside the lagoon. The wave model results revealed that the extensive barrier reef on the seaward side of the Al Wajh Bank is able to completely prevent and dissipate the energy of the offshore waves providing ideal conditions for turtle nesting at seaside islands. Further, the results show that the offshore waves do not have any role in the production and dissipation of wave energy inside the lagoon. The study confirmed that the waves inside the lagoon are exclusively local wind-generated waves. The wave climate inside the lagoon during storm conditions is only influenced by the prevailing strong winds and not dependent on the storm-generated extreme waves from offshore.

Most of the sea turtle nests have been found on the up wave or windward side of beaches with flat and wide reefs or fringing reefs between the reef crest and the high-water line. The wave model demonstrated the large energy dissipation rates under these geo-morphological conditions. In the absence of relevant data on sediments for the nesting and non-nesting beaches, no specific conclusions could be drawn. The beach slopes where sea turtle nests were abundant had slopes in the range of 1:10 and 1:20. The study indicated that the non-nesting sites inside the lagoon are in the sheltered zones of inner reef shelfs and behind the islands where mudflats with mangroves and other vegetation are abundant.

The wave runup is estimated using the HyCReWW metamodel at the nesting and non-nesting sites. The metamodel estimates indicated that the wave runup is of comparable magnitude both for the nesting and non-nesting sites. The comparison of runup distance along the beach indicated that the turtle nesting sites are located sufficiently away from the runup computed for 1 in 100-year wave height

A satellite imagery-based global shoreline data source (Shoreline Monitor, Deltares/TUDelft) was used to examine the erosion and accretion trends at Al Wajh Bank. The data identified two of the islands as having chronic erosion which have a small percentage of nesting sites. The analysis of the data identified that the turtle nesting sites are located on beaches with stable erosion/accretion rates.

The study confirmed the limitations of the Delft3D WAVE (SWAN) model used in simulating IG waves. Due to the very large domain and the course grid adopted in the present study the wave setup aspects have not been covered. The present study was useful in providing a range of information and guidance for further hydrodynamic and geomorphological data inputs at a regional scale and application of models like X-Beach or SWASH for a more comprehensive and detailed analysis on the influence of the surf and swash zone dynamics including IG waves on the sea turtle nesting sites. A systematic data acquisition on geomorphological and sediment characteristics will be useful for logistic regression analysis to study the dominant factors influencing the turtle nesting and non-nesting sites.