Shallow bays in the Caribbean, like Baie Orientale and Baie de L'Embouchure in Saint Martin, are often sheltered by coral reefs and covered by seagrass meadows. They provide valuable services as tourism and coastal protection. The ecosystems are linked through biological, chemical and physical processes. But they are under pressure due to sea-level rise. The response of one of the ecosystems to climate change could impact the other ecosystem. In order to predict the impact of sea-level rise on the biogeomorphology in Baie Orientale and Baie de L'Embouchure, the hydrodynamic model Delft3D Flexible Mesh is applied. The effect of seagrass meadows and coral reefs on both flow and waves are captured with this model. In this way, the long term change in average hydrodynamic conditions due to sea-level rise is determined depending on the response of the ecosystems. A wave-driven circulation is found in both bays with flows of 0.5 m/s over the reefs and currents of 0.2 m/s inside the bays. The hydrodynamic conditions are mainly determined by the reef height. Depending on the response of coral reefs to climate change and the amount of sea-level rise, the wave height inside the bays and the wave-induced currents increase. Under the worst-case scenario, where coral reefs degrade and seagrass meadows die, flow velocities increase by more than 100% in Baie de L'Embouchure and by 200% in Baie Orientale under a sea-level rise of 0.87 m. The significant wave height rises to 300% in Baie Orientale and doubles in Baie de L'Embouchure. But this increase of hydrodynamic stresses is not expected to lead to devastating damage to coral reefs and seagrass meadows. Instead, the response of coral reefs will be determined by changing water temperatures and ocean acidification. A shift in seagrass occurrence due to the changed hydrodynamics is expected. The long term impact of sea-level rise on the biogeomorphology of Baie de L'Embouchure and Baie Orientale seems to be limited. The ability to mitigate the impact of sea-level rise is shown and the resilience of the ecosystems proved, which is very promising for other shallow Caribbean bays that are threatened by sea-level rise.

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 main objective of this thesis is to increase the understanding of the effect of shellfish presence on the stability of loose rock revetments and to investigate the possibilities in the design. Pacific oyster (Crassostrea gigas) and blue mussel (Mytilus edulis) presence on loose rock revetments was quantified. C. gigas and M. edulis are common shellfish species in the port area and act as ecosystem engineers. They require a certain salinity level, water temperature, phytoplankton ratio, and sufficient submersion time for daily feeding and respiration. The current presence of shellfish in the port area was predicted based on salinity levels generated by an OSR model. Outcomes were compared to qualitative measurements of C. gigas and M. edulis presence performed in the port area. C. gigas is present on revetments where salinity levels exceed 16‰, which is the case at Maasvlakte 2, Beerkanaal, and Calandkanaal. M. edulis were hardly observed in the port area on revetments. M. edulis can move relatively easily and detach from the surface after mortality occurs in contrast to C. gigas. They are, therefore, not considered a reliable structural addition, so only the effect from C. gigas was studied. C. gigas presence leads to binding of stones in revetments. This binding of stones will increase the stability of stones in the revetment. The relationship between the presence of oysters and stability upgrading is described using a connectivity model. The relationship between the presence of oysters and binding of stones is measured in the port area, these measurements are used to validate the model. Measurements showed that the cumulative coverage ratio of dead and living C. gigas decreases as one moves up to the intertidal zone from the mean low water level (MLW). The binding of stones increases when the coverage ratio of C. gigas is larger. This shows that there is a relationship between the coverage ratio of oysters and the binding of stones, and this assumes that there is a relationship between the coverage ratio of oysters and stability upgrading. The presence of oysters comes with structural, environmental, and ecological impacts. The conditions in the Port of Rotterdam as a system itself can pose various opportunities and threats for oyster coverage, such as global warming, pollution, and future port plans. Complementary measures can be implemented to mitigate the limitations and threats. The maintenance demand, and associated costs, decreases when oysters are present. Oysters are usually present naturally; their presence can be stimulated by enhancing C. gigas populations at locations with conditions that would allow potential oyster growth. Enhancement strategies depend on local conditions. Application is most promising when bow thruster or ship wave impact is normative because oysters are mainly found up to NAP +0.5 m. Additional enhancement costs are approximately equal to the expected reduction in maintenance costs. Possible ways of application in the Port of Rotterdam are summarized in a flowchart.

A rise in the global mean temperature induced by climate change is expected to have a large impact on ecosystems in all regions of the world. One of the threats is accelerated sea level rise (SLR). This may induce the loss of intertidal areas in tidal inlet systems. The long-term morphological response of tidal inlet systems can be modelled using reduced complexity model ASMITA (Aggregated Scale Morphological Interaction between Tidal inlets and the Adjacent coast). The ASMITA model simulates morphological development on an aggregated spatial and temporal scale by imposing a morphological equilibrium condition. As such the model is fast, allowing for multiple long-term simulations. The model is physics-based and the parameters can be related to field values. Currently, salt marshes are not implemented in ASMITA. However, salt marshes could be of importance to the morphological development in tidal inlet systems. Moreover, it is relevant to assess the resilience of the ecologically important salt marshes by themselves. The aim of this research is to implement salt marshes in ASMITA to assess their influence on the rest of the tidal inlet system and to gain insight into the long-term morphological response of salt marshes to accelerated SLR. Salt marsh development is governed by horizontal and vertical processes. The marsh height increases by capturing mineral sediment and by the accumulation of plant biomass. Autocompaction and deep subsidence lead to a decrease in marsh height. The implementation of salt marshes in ASMITA relies solely on the input of mineral sediment. At the marsh edge, generally, a cyclic behaviour of sedimentation and cliff erosion occurs. Due to the high degree of spatial aggregation, cliff erosion is excluded from the model extension. The governing processes for salt marsh development in the ASMITA model extension are mineral sedimentation, sediment availability and relative SLR. The spatial and temporal aggregation of governing processes for salt marsh development are included in the aggregated advection-diffusion equation and model parameters for the horizontal & vertical exchange of sediment, and sediment availability. Data analysis on hydrodynamic conditions and salt marsh development was conducted for the derivation and calibration of these model parameters. To verify the salt marsh implementation, three ASMITA models were created. A one-element salt marsh model consisting of only a salt marsh element, and two different multiple elements models, which contain the ebb-tidal delta, channels, tidal flats and salt marshes. It can be concluded that ASMITA can model the mineral sedimentation on a salt marsh but depicts a large sensitivity to the parameter setting, particularly for the sediment concentration. Based on the chosen parameter configuration, the Oosterkwelder salt marsh is preserved when subjected to SLR rates below 16 mm/year. The ASMITA salt marsh extension can be employed to obtain an expeditious first impression of long-term morphological salt marsh development. However, due to the lack of incorporation of detailed processes, the model should not be employed for in-depth analyses of salt marsh development. The interaction between the salt marsh element and the remaining tidal inlet system components requires further model improvements.

TheAmazon-Orinoco river plume is a buoyant freshwater lens of 1.2 × 106km2, which has been traced over 2000 km from the Amazon river mouthinto the Caribbean Sea and along the Lesser Antilles. The river plume iswarmer than the surrounding open-ocean waters, with temperaturedifferences up to 1.5 ∘C caused by a stratification-induced barrier layerinhibiting vertical mixing and coloured matter increasing solar energyabsorption. Due to its magnitude, the river plume affects thehydrodynamics and the oceanic conditions in the Western Tropical NorthAtlantic (WTNA) substantially, but its variations on interannual time scales andthe corresponding relation to local sea-surface temperature (SST) are notwell understood. The Caribbean Sea is a region of high ecological value as itis home to extensive coral reefs, which are especially sensitive topersistent high SST. Therefore, this study investigates the interannualvariability of the Amazon-Orinoco river plume and its relationship to SSTsin the Caribbean Sea and the WTNA. It is hypothesised that fresh anomaliesof the river plume salinity pattern are indicative of a more extensivetransportation of the heat contained in the river plume. As a result, itis expected that interannual variations of dominant river plume pathwaysaffect the magnitude and location of anomalous SSTs. To test thishypothesis, model reanalysis fields of oceanic conditions from 1993 to 2017are used to conduct statistical analyses. In this context, the river plumevariability is determined using specific regions of freshwater influenceestablished using Empirical Orthogonal Function (EOF) analysis ofanomalous sea-surface salinity (SSS). Cross-correlations analysis relatingthese EOF modes of with atmospheric processes show that the interannualvariability of the river plume is dominated by wind-inducedadvective transport and -mixing. Strong winds along the Brazilian shelfare related locally increased SSS, while a weak southward component makesfor extensive spreading of the low-salinity plume waters. Additionally, weshow that high river discharge affects SSS east of the Lesser Antillesafter a lag of three months. Through its modulation of these atmosphericprocesses, there is a strong indication that the El Niño-SouthernOscillation affects SSS variability in the main along-shelf northwestplume pathway, with low SSS 1–9 months after a La Niña event. DecreasedSSS are found in phases 2 and 3 of the Madden-Julian Oscillation, whileincreased SSS was observed in phases 6 and 7. However, the evidence forthis relation is weak and should be investigated in further research. The results show that, opposed to the hypothesis,a more extensive river plume is not associated with higher SSTs in theCaribbean Sea. However, strong correlations are found between river plumesurface area and SSTs at a lag of 1 year. Based upon results of previousstudies, we argue that the river plume has the ability to pre-heat themixed layer in the WTNA leading to extreme temperatures in the followingyear. It is wise to conduct a Lagrangian parcel back-tracking experimentto verify this mechanism.

The Wadden Sea is a unique and ecologically important coastal region in the southeastern North Sea, stretching along the coasts of Denmark, Germany, and the Netherlands. It is known for its vast tidal flats, salt marshes, and barrier islands, forming a dynamic and constantly changing landscape, creating rich biodiversity. The Wadden Sea in the Netherlands is composed of the mainland coastal areas and the Wadden Islands (such as Texel, Vlieland, Terschelling, Ameland, and Schiermonnikoog). This area plays a crucial role in protecting the land and supporting the people who live there. It serves as a natural defense against storm surges and flooding, supports local businesses through fishing and tourism, and provides a vital ecosystem for numerous plant and animal species. However, infrastructure development in the Wadden Sea area, including offshore wind farms, shipping routes, and coastal engineering, has impacted some ecosystems. The development is necessary for economic growth, such as shipping infrastructure for tourism on the Wadden Islands and offshore wind energy due to the energy crisis. These developments will definitely shape the landscape in the coming years. In the thesis, data on infrastructure and ecology was collected and analyzed to find conflicts between them and how they affect the landscape. This research formed the basis for the design phase. Then, for the territorial scale, three strategies were put forward. Moreover, two sites are chosen to demonstrate how these strategies can be applied to site-specific design. Both sites share the idea of preserving and adapting positive landscape features and restoring the natural process, achieving minimal intervention.

In recent years, the interest in integrating shore protection with the local ecosystems has increased. In comparison with the traditional reinforced embankments, integrated shores form a gradual transition from land to water where shore vegetation can establish itself. These nature-friendly shores have a positive impact upon the water quality, flora and fauna and the amenities of the shore. Some of the benefits of vegetation are its capability of reducing the hydrodynamic loads on the shore and its ability to increase the soil strength against erosion. Both effects have been investigated in detail in previous research and have been implemented for coastal protection designs in sandy coastal settings. But a designing tool for cohesive shorelines in lake environments is still missing. In this research, the different aspects important for the erosion of a vegetated cohesive shoreline are studied and the first steps towards a process-based design tool are taken. The model results show that the dominant processes for erosion can be found at the point of wave breaking and at the start of the run-up, as both combinations give the highest induced bed shear stresses. This can be used for a strategic placement of vegetation to dampen the wave impact and reinforce the soil through the roots. There are situations in which a partially vegetated shore is sufficient to withstand the daily wave attack. This allows the vegetation to grow naturally to full strength, which is required in a storm situation. In this way a financial benefit can be reached as well as a healthier vegetated shore, as a large part of the vegetation has grown naturally.

Some of the sediment which is eroded from the Dutch coast ends up in the Wadden Sea. Because of this the morphological development of the Wadden Sea is important, not only for the Wadden Sea itself but also for the maintenance programs of the adjacent coastlines. This is one of the reasons that the morphological development of the Wadden Sea is extensively studied. Some of these studies use the ASMITA model to make predictions of the morphological development with accelerating sea level rise. The Groningerwad is a part of the Wadden Sea consisting of a number of smaller tidal basins which has not been modelled with ASMITA. It has not yet been necessary to structurally nourish the coastlines surrounding the Groningerwad. However, as sea level rise increases it might well be possible that the coastal profiles surrounding the Groningerwad require nourishment. Therefore this thesis aims to study the Groningerwad with ASMITA to make a prediction of how the area will develop with accelerating sea level rise. To do this a morphological study is performed to determine the current morphological developments. This morphological study, based on available literature and bathymetry measurements of the area, finds that the Groningerwad is a highly dynamic area. It also determined the area and characterizing volume of each of the tidal basins, which have been used to set up the ASMITA model. For each basin in the Groningerwad an ASMITA model is set up using the information from the morphological study. The ASMITA model is used to make predictions for the development of the intertidal, channel and delta volumes of each of the Groningerwad its basins. The required parameters for the model have been derived from relevant formulas and the assumption that the Groningerwad is currently in a morphodynamic equilibrium. This was done because the time period for which bathymetrical measurements are available are to short to allow for a proper calibration procedure for these parameters. With this setup the ASMITA models show that all basins will lose intertidal sediment volume with rising sea levels. The larger basins of the Groningerwad also will not reach a new dynamic equilibrium state with large levels of sea level rise rate increase. When comparing these results to other basins in the Wadden Sea, it appears that the basins in the Groningerwad respond a lot slower than other Wadden Sea basins. Given the difference between the Groningerwad and the Wadden Sea and the fact that the time period over which bathymetrical data is available was to short to fully calibrate the model the recommendation is made to revisit this study when more data is available and it is possible to calibrate the relevant parameters.

Bed roughness is an important parameter for the prediction of sediment transport as well as calculating flow conditions near the bed. Numerical models are extensively used to make these predictions. In these models it is fundamental to have proper values for bed roughness. The bed roughness is being estimated using a dataset from an ongoing pilot project in the Dutch western Wadden Sea. This dataset consists of 40 days of velocity measurements using ADV (8 Hz) and ADCP (1 Hz) instruments and concentration of SPM measurements using OBS instruments. To estimate the bed roughness, the dataset is analyzed using four methods for calculating the bed shear stress. The logarithmic profile, turbulent kinetic energy, vertical turbulent kinetic energy and the Reynolds stress method. The data has been processed and averaged per tidal phase, so statistical analysis can be applied to it. From this analysis, it is found that the concentration of SPM increases at 6 cm with increasing wind speed. As a result of this increasing of SPM the bed roughness also increases. This leads to the hypothesis that suspended sediment makes the bed rougher, and is not primarily governed by horizontal advection but also local resuspension. A 1DV numerical model is used in which horizontal advection is excluded to test this hypothesis. Simulations are performed with stationary boundary conditions, using combinations of water depth (0.2 to 2.8 m) and velocities (5 cm/s to 70 cm/s). These simulations are imposed with an initial homogeneous concentration. For every combination, the concentration is increased gradually until the concentration profile becomes L-shaped. Besides simulations with stationary conditions, timeseries of water depth and velocity are used to simulate one tidal cycle. All simulations performed with this numerical model do not take wind and waves into account and water-bed exchange is excluded. From the simulations with stationary boundary conditions the roughness is calculated using the LP method. It is found that bed roughness increases with increasing initial homogeneous concentration. From the simulation of one tidal cycle, it is found that the roughness increases towards the turn of the tide. After the turning of the tide, the concentration profile becomes L-shaped, and the roughness is decreased. The concentration profile becomes homegeneous again after a certain threshold of velocity and waterdepth and at the same time the roughness increases again. During a tidal cycle, it is possible to have a collapsed concentration profile, which indicates a lower bed roughness and thus a smoother bed.

Coastal bays in the Caribbean accommodate different marine ecosystems, including seagrass meadows and coral reefs, which provide important ecosystem services. However, these marine ecosystems are endangered. Seagrass ecosystems have a key role in coastal bays, but despite their alarming rates of loss, they receive little attention compared to other marine ecosystems. This study aims to provide a better understanding of the role of seagrass ecosystems in coastal bays by assessing their impact on the morphodynamic behavior. In order to achieve this, an existing hydrodynamic model has been extended to include morphodynamics. Two different bays have been investigated: Baie Orientale and Baie de l’Embouchure (St. Martin). These bays represent a partially exposed and fully sheltered bay, respectively. Both regular wave conditions, as well as extreme storm conditions, have been investigated to understand the consequences of seagrass loss on coastal erosion for different environmental climates. The coastal erosion has been found to be more prominent in the exposed region, whereas the more sheltered areas are more resilient. The erosion takes place in shallow waters but remains limited under regular swell conditions. The increased wave energy, during the extreme storm event, increases the erosion rates considerably. However, the regular swell conditions are normative in shaping the morphological development of these coastal bays when longer timescales are considered. The seagrass counters erosion most effectively in the foreshore and much less in deeper regions. Removal of seagrass in the foreshore (between 1-3m) halves the sediment stabilization. In contrast, the sediment stabilization increases if the meadows are able to spread especially towards the shore. In addition, the wave energy and waveform have a significant impact on the sediment stabilization. The stabilization typically decreases for higher and shorter (storm) waves, whereas it increases for smaller and longer (swell) waves. The long-distance interactions between seagrass meadows and coral reefs have led to their mutual coexistence, which is also found to be beneficial for the erosion control of the coastal bays. The dissipation of wave energy on top of the reefs fosters the sediment stabilization by seagrass. Moreover, the impact of the reefs on sediment stabilization also increases in the presence of seagrass meadows due to the additional drag exerted by the seagrass. The seagrass meadows are typically more effective under swell waves, whereas the reefs are more dominant in the dissipation of storm waves. Seagrass meadows and coral reefs form thus a synergy, in which the stabilization of sediments provided by the individual ecosystems is facilitated, reinforced, and complemented by the proximate presence of the other ecosystem. Finally, the role of seagrass on sediment stabilization has been explored under climate change. The coastal erosion increases for the considered sea-level rise scenarios. The impact of the seagrass on sediment stabilization decreases when the bays, seagrass and reef ecosystems are not able to keep up with sea-level rise. The synergy between seagrass and reefs emphasizes that the responses of both ecosystems are of importance for the future of tropical sheltered bays and should therefore not be managed in isolation.

To limit global warming to 2 °C or below, the IPCC emphasizes the need for large-scale carbon dioxide removal (CDR) alongside emissions reductions. There is a growing recognition of ocean-based CDR techniques and expanded research is needed. This thesis investigated the long-term (>100 years) carbon sequestration potential of enhancing the biological carbon pump by ocean nitrogen fertilization, a research domain which so far has received little attention. The objective was to make an initial rough quantification of the (cumulative) CDR potential of one-off ocean nitrogen fertilization achieved within 5 years after the fertilization, to evaluate its potential as a global CDR technique and for commercial implementation. An idealized one-dimensional vertical (1DV) model framework, coupling a marine ecosystem model with a vertical eddy diffusion physical model, was established to simulate ocean nitrogen fertilization, incorporating essential processes of ocean productivity and the biological carbon pump. Half of the ocean was excluded from the framework since these regions are incompatible with 1DV modelling. Additionally, regions where nitrogen is not the first limiting nutrient were excluded. Various fertilization simulations were performed in which the nitrogen limitation in the mixed layer of the remaining ocean areas was replenished by a one-off addition. Long-term carbon sequestration resulting from fertilization was assessed by comparing these simulations with baseline simulations of the biological carbon pump. An aggregated global outcome, forming the basis for feasibility assessment, was derived by averaging over simulations, while excluding hexagons with low sequestration efficiency as they contribute insignificantly to CDR. In principle, the one-off nitrogen fertilization shows an effective carbon sequestration efficiency (2.95 kg CO2 per kg N). However, the high and challenging-to-reduce carbon footprint of current urea production, coupled with challenges in collecting manure and the priority of utilising it in agriculture rather than directing it for carbon removal, leads to a low overall global carbon sequestration potential. Nevertheless, it is demonstrated that implementing one-off nitrogen fertilization at a location with high sequestration efficiency using manure can be commercially viable. The cost per net tonne of CO2 sequestered is lower than the current market price of a carbon credit, considering the transportation is conducted with vessels that would have navigated the route regardless. Based on the findings of this study, it is concluded that nitrogen fertilization has very limited potential to serve as a large-scale global carbon dioxide removal strategy, but can be effectively employed on company-level using surplus manure without agricultural purpose. The initial quantification, despite inherent limitations such as the use of a 1DV modelling approach, serves as a valuable addition to the scarcity of research in the domain of ocean nitrogen fertilization. Further more detailed research into strategic nitrogen fertilization for company-level CDR is justified, with this study providing valuable guidance on optimal research locations.

Nature-based Solutions (NbS) have recently gained more interest in hydraulic engineering. It is based on the concept of using forces of nature rather than working against them. In addition, it focuses on using natural processes to fulfil co-benefits for the parties involved. One of the parties involved is nature. However, a quantitative analysis of ecological development is necessary to determine possible co-benefits for nature. This is still found challenging due to the dependency on many variables, the difference in spatial and temporal scales, the limitations in available information, and the non-linearity. Ecological development can be expressed with ecotopes, linking geomorphological and hydrological characteristics to abiotic characteristics. A Dutch Ecotope System for Coastal Waters (ZES.1) is a classification system of Rijkswaterstaat. It is a hierarchical system based on abiotic characteristics that classify ecotopes based on thresholds that are determined by ecological differences. In this thesis, the Ecotope Map Maker based on Abiotic characteristics (EMMA), is developed. It uses data from a validated hydrodynamic model as input and subsequently maps ecotopes based on the ZES.1. Ecotope labels are composed by combining labels that are given to values of salinity, inundation, flow velocity and substrate composition. The thresholds are calibrated using an ecotope-map of the Western Scheldt of RWS. This map is based on aerial photographs, laser altimetry, soundings, field measurements, and several models. The performance increased from 63 % to 84.6 % after the calibration. This increase is mainly due to (1) differences in the underlying data and (2) the application of deviating thresholds in the ecotope-map of RWS compared to the ZES.1. EMMA is developed for the preliminary design stage. How EMMA can be implemented is demonstrated by applying EMMA on an idealised estuary. Different ecotopes and varying acreages of ecotopes are found when the depth of the estuary is modified. In conclusion, EMMA creates many possibilities for ecotope-maps since it no longer depends on aerial photographs and other real-time data. A translation can be conducted between ecotopes and ecosystem services when a monetary value is preferred, or ecotopes can be broken down into eco-elements, which can subsequently be linked to biodiversity. With EMMA it is possible to predict ecological development, which contributes to the design of NbS.

Bardawil Lagoon is a tidal lagoon situated at the northern coast of the Sinai Peninsula in Egypt. It’s two artificial inlets, Boughaz 1 in the West and Boughaz 2 in the East, provide a connection to the Mediterranean Sea. They enable the water bodies to interact and support fish migration. Currently, regular maintenance dredging works are necessary to keep the two inlets open. The objective of this thesis is to analyse the effect of interventions applied to the two inlets on the lagoon-sea interaction, with the goal of transforming the present, unstable inlet system towards a stable tidal inlet lagoon by adapting one or both of the present inlets. This study is conducted on three system phases, being Phase 0, Phase 1 and Phase 2. Phase 0 consists of the initial situation without any interventions; Phase 1 contains the effect of adaptations to the Boughaz 1 inlet, and Phase 2 includes adaptations to Boughaz 2 in addition to the changes made in Phase 1. The new design in Phase 1 and Phase 2 consists of a deeper inlet cross-sectional area, the dredging of an approach channel, the addition of a nourishment, and the removal of the present breakwaters. Design elements are processed using a 2D-H Delft3D Flexible Mesh model and analysed under tide-only conditions with and without a prevailing wind climate added. Evaporation effects are included after the model calculations are made. The results are mainly assessed are the interaction with the Mediterranean Sea, the sediment transport character, and the inlet stability according to the Escoffier curve. Moreover, an analysis is made on the flushing of the lagoon and the effect of a prevailing wind pattern on the system. It is clear from both literature and the initial model results of Phase 0 that Bardawil Lagoon currently does not function as a morphologically stable tidal inlet system, as sedimentation occurs in both inlets. The water exchange between the Mediterranean Sea and Bardawil Lagoon is restricted by the inlets, which is indicated by the difference in tidal elevation on both sides of the inlet. Both inlets are positioned near the unstable equilibrium point on the Escoffier curve, indicating possible closure of the inlets in the future. Hence, interventions are required to establish a morphologically stable lagoon inlet system. By applying the proposed designs in Phase 1 and Phase 2, the limitations on the incoming tide shift from the inlets to the inner basin induces friction, thus removing the inlets as limiting factor. Moreover, taking into account both the prevailing winds and high evaporation effects, the total system is classified as having a sediment exporting character after Phase 2. High evaporation rates have a significant importing effect on the sediment transport character of the inlets. However, after Phase 2, these effects are reduced by a factor 3-5 compared to Phase 0, depending on the wind. The new cross-sectional area design also results in both inlets being positioned near the stable equilibrium point on the Escoffier curve after Phase 2, which is supported by the sensitivity analysis. Hence, it is concluded that the proposed adaptations achieve the goal of developing Bardawil Lagoon into a morphologically stable inlet system. The study provides good insight into the effect of system interventions on the morphodynamic stability of the inlets as well as the flow dominance regarding those inlets. It is recommended to construct a validated morphological 3D model which can provide insight in the long term response of the system to those adaptations.

Over the recent years, the Dutch Continental Shelf has seen the erection of various wind farms in an effort to counter climate change by replacing fossil-based power plants. Since the majority of these sites use scour protections which have a substantial impact on the surroundings, the Dutch government increasingly stimulates and requires ecological enhancement of such structures in order to restore the natural habitat and maintain the marine ecosystem of the North Sea. Oyster enhancement has been one of the methods used for the ecological enhancement of offshore wind farm scour protections. However, studies report that a successful structure for the harbouring of an oyster population is yet to be developed. Existing structures were found to be unstable or unable to provide a suitable habitat for the oysters (Bouma, Belze, et al., 2013; Stokes et al., 2012). The study at hand aims to develop a successful design for oyster broodstock structures for nature enhancement in offshore wind farms. It was conducted for Van Oord as part of project Ecoscour. To establish design criteria for the oyster broodstock structures, oyster habitats and the Borssele V site were analysed and a brainstorm session with partners from project Ecoscour was set up. The final criteria concerned: ecology; structural integrity; commissioning & construction; and monitoring. The design criteria were used to develop several viable structure designs. In order to rapidly assess the strength and stability of potential designs, a Python-based open source tool was built named Econstruct. With this tool, geometries can easily be evaluated under various influences, accounting for the effect of marine fouling. This could be used for quick investigations of loads, stability and structural integrity. The Econstruct tool was validated and analysed for sensitivity for the input parameters, of which the input radius and current velocity had the highest influence on stability. Using the environmental input parameters that hold for the Borssele V wind farm, a terraced design was proposed. After consultation with the manufacturer of the structure, D & M engineers, this proposal was altered to account for manufacturing limitations. The definitive design was evaluated using the Econstruct tool and found to be stable. It was then manufactured, and ultimately placed in Borssele V in October 2020. Several recommendations are made for future research and the design of ecological enhancement structures for offshore wind farms. Most importantly, the Econstruct open source tool can be used in the future for the design of structures for other ecological enhancement projects such as coral reef restorations.