Threatened by rising sea levels and other climate change induced hazards, there is an increased need for coastal flood protection, such as storm surge barriers. However, the construction of such barriers may lead to unwanted coastal changes. This thesis examines the effect of the Eastern Scheldt storm surge barrier on sediment pathways and the implications for coastal erosion. In addition, possible sediment-based interventions to redirect the current with the aim of reducing coastal erosion were investigated. First, a literature study was performed to provide historical context on the natural and human induced development of the ebb tidal delta. A data analysis on the sediment budget of the ebb tidal delta was performed, which showed that from 1960 to 1987 the ebb tidal delta gained sediment, from 1987 to 2010 it lost sediment, and from 2010 to 2019 it gained sediment again. A numerical model in Delft3D FM was used to produce sediment transport vector fields over a morphologically representative tidal cycle, which were then visualized using SedTRAILS. The research found that the barrier decreased the strength of the ebb current on the entire ebb tidal delta. The Roompot Zuid has a predominantly ebb-dominant character before the barrier in 1976 but changes to a flood-dominant character due to the construction of the barrier, which weakened the ebb-current. In 2019, part of the Roompot Zuid regained an ebb-dominant character. The Schaar van Onrust also became flood-dominant due to the barrier, eliminating the sediment-retaining effect the ebb dominated current in this channel. This shift indicates that sediments that previously stayed in front of the coast of Noord-Beveland are now redistributed further along the coast in south western direction, which led to a sediment deficit and erosion at the coast. The tested interventions seem to induce little structural changes in the tidal currents. The removal of sediment causes the sediment transport patterns to converge into the dredged area, suggesting deposition. Therefore the tested interventions do not seem effective from a coastal management perspective. The approach used here may also be useful for the assessment of sediment transport impacts at other sites where the construction of similar barriers is being considered.

A managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. In a managed realignment, the newly formed intertidal area acts together with the realigned dike as a nature-based flood defence system. In this thesis, the focus is on the managed realignment of Perkpolder, a former polder located in the south of Zeeland (The Netherlands). This realignment serves as nature compensation for the dredging activities in the Western Scheldt. The realigned area contains an intertidal flat which facilitates ecological services.

A combination of different data sources (sediment samples, flow velocities, turbidity measurements and bathymetric data), together with a hydrodynamic model of the Western Scheldt and a morphodynamic model of Perkpolder are used to interpret the hydrodynamic and morphodynamic response of Perkpolder after the managed realignment.

Bathymetric data composed of LIDAR, multibeam and single-beam measurements shows three main developments in Perkpolder: (1) erosion of the frontal entrance area, the inlet and the seaward side of the creeks, (2) infilling of the creeks landward of the first bifurcation and (3) sedimentation on the intertidal area and even more sedimentation of the pond. By comparing the temporal evolution of the bed levels of the frontal entrance area with measured concentrations at the inlet of Perkpolder, it can be concluded that the frontal entrance area acted as a finite source of sediment that eroded quickly in the first years and increased sedimentation within Perkpolder during these years.

New realignment projects can profit from the research at Perkpolder. Bed shear stresses of the initial proposed layout of Perkpolder gives a clear indication if the creeks are setup correctly. Furthermore, an estimate of the initial sedimentation can be based on the accretion rate of other intertidal areas within the estuary. Since realigned areas are often more sheltered and have a lower elevation, accretion rates may be higher. Moreover, not only the forcing of the estuary, but at the same time the availability of different sediment sources and the initial bathymetry shape the morphodynamic response of the realignment site. Therefore, it is recommended to measure the bathymetry for a sufficient long period (e.g. 10 years after the realignment) to get grip on the mechanisms shaping the morphodynamics at these areas.

A large part of the Dutch coast, the barrier islands in the Wadden Sea included, would be eroding if the deficit in the total sediment budget was not compensated for through nourishments. Ebb-tidal deltas have an important function within the coastal system that make them of interest for coastal defence as a source and transport path for sand to the back-barrier basins and the island coastlines. As part of the Kustgenese 2.0 project, a pilot nourishment of 5.5 million m3 has been placed on the Ameland ebb-tidal delta. Improved understanding of sediment bypassing processes and the interaction with a nourishment is needed for strategic placement of nourishments in the future. In this thesis we analyse the high-frequency bathymetric dataset available at Ameland inlet between 2005 and 2020 and model the tide- and wave-driven transport pathways using to SedTRAILS to determine how sediment bypassing works under natural circumstances and under the influence of a nourishment. A new series of conceptual models has been proposed. These show that the formation and growth of a series of ebb-shields on the western side of the ebb-tidal delta plays an important role in sediment bypassing. Their development gradually results in transport pathways connecting the western side of the ebb-tidal delta to the major transport pathway along the ebb-delta front, which forms a direct connection to the Ameland coast in 2017. The influence of the pilot nourishment on sediment bypassing processes is limited, adding volume to the system but not altering the existing transport pathways. Transport pathways show that sediment from the nourishment eventually reaches the Ameland coast and is unlikely to feed the Terschelling coast or the tidal basin. This is valuable knowledge for the future sustainable coastal management of Ameland inlet, which can also be extended to other inlets.

Due to climate change, temperatures are rising worldwide resulting in sea level rise, higher evaporation and possible changes in precipitation patterns. These effects of climate change influence salt intrusion, particularly by affecting the freshwater supply. In The Gambia also a planned hydropower dam and possibly future irrigation development will affect the freshwater supply. The combined effects of climate change, the hydropower dam and irrigation development on salt intrusion are unknown. A river south of The Gambia, the Casamance, turned strongly hypersaline during the Sahelian droughts, which strongly affected the rivers discharge in the early 80s. The freshwater supply in the Casamance after these droughts was insufficient to flush away the salt and the estuary never restored. This led to the decay of the entire ecosystem that existed in and around the Casamance. The Gambia river is of major importance to the country, the agricultural sector relies on the freshwater it supplies for growing crops, the fishing industry relies on sufficient numbers of fish to catch and the forestry industry makes use of the mangroves growing around the river. All these industries combined account for ±75% of the working force in The Gambia. If the Gambia estuary turns strongly hypersaline such as the Casamance, this would have disastrous results. This thesis aims to clarify the effect that climate change, the hydropower dam and irrigation development have on salt intrusion in the Gambia river. A 1D model, SALNST, is used to model salt concentrations over the river length up to the year 2100. The effects of climate change, the hydropower dam and irrigation development are represented by four model parameters: climate projections, sea level rise, dam operations and irrigation development. Multiple parameter values, ranging from mild to strong conditions or impact are implemented, creating a set of 27 scenarios. These scenarios treat the parameters independently to distinguish the individual parameters effect on salt intrusion. It can be concluded that The Gambia is prone to climate change when considering salt intrusion. Due to the importance of the river, monitoring sea level rise and the development of climate change will be essential for planning and taking counter measures. Understanding the effects that climate change can have regarding salt intrusion can also be used in negotiations to reserve some freedom in the hydropower dam operations. In addition, solutions that limit the increase in evaporative surface for strong sea level rise (e.g., dikes along a part of the Gambia river) prevent extreme salt intrusion lengths and hyper salinity. The hydropower dam causes a reduced range of salt intrusion lengths, this creates easier to predict salt concentrations which is beneficial for aquaculture. Furthermore, the reduced maximum salt intrusion is also beneficial for agriculture that makes use of irrigation.

In this study, the behaviour of fine sediment in settlings basins has been investigated. The understanding and prediction of return water quality assessments is improved by a comparative modelling study. Settling basins are required to decrease the exhaust of fine sediment particles when dredging and reclaiming land. Fine sediment particles can cause turbidity, which damages underwater life and the environment. Turbidity should therefore be prevented.
The study focusses on the assessment on settling basins during a tender phase. Often, limited information is available and time pressure is high. Therefore, it is required to have a straight-forward but robust method to evaluate the effectiveness of a settling basin design in meeting outflow criteria, based on a solid process understanding.
With a comparative modelling study, the main assumptions of the current assessment by Van Oord (width-uniform conditions, neglectable density effects) are researched. The models compared are an inhouse tool of Van Oord (RWQ model) and Delft3D. Next to the main assumptions, different types of settling behaviour throughout the basin are researched.
The research shows the importance of density driven currents in distributing the sediment concentration over the settling basin. This leads to width-uniform sediment concentration. This explains why the Delft3D model and the RWQ model predict the same outflow concentration (order of magnitude) while having different assumptions. Wind causes additional mixing and therefore higher outflow concentrations. The settling velocity of particles has a very high influence on the outflow concentration.
For future assessments the RWQ model can be used. For better process understanding, a Delft3D model could be added. When outflow concentrations are near critical, mitigating measures should be implemented. Mitigating measures are often a fraction of the costs compared to cancelling dredging vessels.

The People's Republic of China experienced large socio-economic development over the last decades, which was mainly concentrated in coastal regions. Jiangsu Province is among these regions, which shows large socio-economic differences between north and south. The Chinese Central Government has allocated Binhai County as a new coastal development zone to lessen inequality. New infrastructure (e.g. Binhai Port) was constructed and industrialisation increased in order to boost local development. Binhai County is located at the Abandoned Yellow River Delta. In 1855, the Yellow River shifted its course which cut off sediment supply to the delta. Major land retreat was stopped by the construction of sea dikes from the 1930's. Ongoing erosion is undermining the present sea defence and endangers the hinterland. The erosion along the delta has engaged the disappearance of mudflat area. Furthermore, large development in southern Jiangsu has led to constraints in usable land for which large mudflat areas were reclaimed. Multiple research pointed the negative effects of reclamation activities on ecology (e.g. decline of shorebird populations). After recognition of the importance of mudflats by the Chinese Central Government, the national policy was set to protect and restore mudflats along the Chinese coast. The erosive features of the Abandoned Yellow River Delta (AYRD) needs counteracting measures. Within this research it is aimed to integrate a multi-purpose design in which engineering and ecology are mutually enhanced. Following the Building with Nature guidelines multiple innovative solutions can be presented. Within this research it is aimed to protect the delta by implementation of islands in front of the coast. Furthermore, it is aimed to enhance mudflat growth which adds ecological value to the delta area. It is expected that by implementation of islands a reduced energetic environment is created in which fine sediment can settle. The Jiangsu Regional Model (JRM) was made available for this research, which is based on Delft3D software. An outcrop has been made at the area of interest which was refined (3:1) and further referred to as the Abandoned Yellow River Delta Model (AYRDM). The hydrodynamical forcing was completed by the addition of waves. The AYRDM was set-up with three sediment fractions from which one added cohesive fraction. The production simulation was set-up for ten morphological years by using a morphological factor of 50. The better obtained understanding of the governing processes optimised island configuration design. Fifteen configurations were designed and subdivided over two series: the Basic- and Exploratory series of eight- and seven configurations respectively. The Basic series was set-up with a systematic approach in order to represent the concept of barrier islands. The Exploratory series comprehend designs based on the entrapment of sediment by using sediment transport patterns. The definition of a mudflat was set from -0.88 m to all above, relative to mean sea level in order to measure mudflat growth. The findings within the reference production simulation (without the implementation of islands) showed a flood dominant environment in which flood direction is south. Hence, south- and offshore directed residual sediment transport were observed along the AYRD. The outgoing sediment flux from the AYRDM increased by 2.2E7 ton/year when waves were included. The addition of waves increased the average grain size by 26 µm at siltation areas due to sediment stirring and cross-shore transport. The effect of island configurations showed most favourable sediment entrapment when elongated islands and small inlet widths were applied. The shallows induce larger friction on tidal movement for which current velocities and herewith sediment transport capacity is reduced over distance. Hence, depositional areas were found to be radially deflected to the south. Secondary flow, due to eddy formation, showed significant streaming induced sediment transport behind the islands ends. Small bed slopes induce larger distances between islands and mainland China, which holds for relatively larger mudflat areas. Wave penetration was reduced significantly when islands were implemented, which created a less energetic environment. The largest increase of mudflat relative to the reference simulation is 59.8 km2 after ten morphological years by implementing 16 km2 of island area. The findings within this study are representative for the Abandoned Yellow River Delta area and show the potential of islands in order to protect the delta and create additional ecological/recreational benefits alongside.

Predictions of the morphology of coastal areas are used to make decisions on coastal defence and nature conservation. To predict this morphology, simulations made by morphological models are used. To base decisions on this morphology predictions, we want the uncertainties in these predictions to be small. The goal of this research is to get a better insight in the uncertainties in a morphological model. By having a better understanding of these uncertainties, the decisions made using modelled predictions are stronger substantiated. The specific area focused on in this research is the Frisian Inlet, which is located between Ameland and Schiermonnikoog in the Wadden Sea. To achieve the goal of this research, data assimilation is used. Data assimilation combines data with prior knowledge of a model to find the distribution of probabilities of estimates of a true state. In this research, the used data are bed level measurements and the estimates are simulations for the bed level height made by Delft3D. Data assimilation methods make use of a distribution of model outcomes which should cover all possible outcomes. So, to set up data assimilation successfully, we want to use a parameter that induces a significant change in the bathymetry outcomes of Delft3D. Therefore, a sensitivity study is performed which considers the following six parameters: current related roughness, wave related roughness, wave-related suspended load sediment transport factor, wave-related bedload sediment transport factor, the transverse bed slope and the tidal amplitude. For each parameter, ten values are chosen to simulate. Which of the parameters induces the most change in bathymetry is assessed, using the difference between simulation result and the observation and the mean squared error skill score. The transverse bed slope shows the most variation and is further used in the data assimilation method, which is a particle filter. Hundred uniformly distributed values of the transverse bed slope, between 0.5-100, are used to create different Delft3D simulations that give a bed level prediction. This initial distribution is used for three different periods: 1970-1975, 1975-1979 and 1979-1982. These epochs are defined by the availability of bed level measurements. After one iteration of the particle filter, a new distribution of the parameter values is found. This is used in the next iteration in the same epoch. In each epoch, three iterations are made. The methodology as described in this thesis leads to a convergence of the initial distribution. In epoch 1 and 3, the distribution focuses on values of the transverse bed slope in the high range of the initial distribution. The resulting distribution of epoch 2 focuses on values around a transverse bed slope value of 70. This study shows that it is possible to get a better understanding of the distribution of \gls{alfa} values that leads to probable bedlevel predictions by applying a particle filter. The application of this method brings the simulations closer to each other, but the simulations did not get closer to the observations. The contribution to our understanding of data assimilation for morphodynamic models is that it can be used as a calibration tool for a specific parameter.

In recent years, there has been an increasing need for solutions against the threats that SLR and climate change represent for coastal systems, especially in low lying areas like the SW Dutch coast. Simultaneously, awareness of the impacts that human activities have on natural environments has surged. Integrated and sustainable solutions are necessary. The Delta21 plan proposes an integrated plan for flood-protection, energy storage, and natural ecosystem restoration for The Netherlands, specifically for the Haringvliet estuary. This large-scale intervention will produce large disturbances in the system. Despite the resilient character of tidal basin systems \parencite{Wang2009}, the evaluation of the intervention's effects, in terms of the initial response and the variations in the mechanisms driving morphological changes is compulsory. Assessing the bed level changes is especially important, considering the valuable intertidal and subtidal ecosystems present in the Haringvliet mouth. It is essential to guarantee their preservation despite the large disturbances. The evaluation of the Haringvliet mouth response is performed employing a long-term morphodynamic 2DH computational simulation to forecast the response of the system. The models implemented for the simulation are the Delft3D-FLOW and Delft3D-WAVE. To reduce computational times, the modeling approach applies input reduction and acceleration techniques. The hydraulic forcing is schematized (morphological tide, waves, and discharges) while maintaining the seasonal character of the input. A variable morphological acceleration factor (MF), depending on the wave condition is applied. The variable MF allows us to employ low values during strong-episodic conditions and larger during more regular conditions. The approach is especially beneficial in the presence of oscillating forcings that are also enhanced by the MF. Finally, the impact of the D21 plan on the Haringvliet mouth is assessed by studying the alterations to the hydrodynamic regime, the related net sediment transport pathways, and the observed bed level changes. The analysis of the resulting morphological evolution focuses on the variations of the intertidal and subtidal zones and the mechanisms behind them. We showed that the tidal regime shifts again towards a long-basin regime once the D21 is implemented. The intervention also causes the emergence of new subtidal and intertidal areas, mostly within the Tidal Lake, resulting from the redistribution of the material from the main channel. The original intertidal and subtidal areas, formed by the Hinderplaat and Slikken van Voorne, are almost undisturbed. This behavior guarantees the preservation of the intertidal and subtidal ecosystems. The results also show that despite the initial dredging activity necessary for the D21 plan implementation, which decreases the extent of shallow areas, the emergence of intertidal and subtidal features balances the negative effect as long as no further dredging is performed. The TL shows a net sediment export of material at both control cross-sections (the Haringvliet Dam and the new TL inlet). The findings of the morphological development agree with the known morphology and processes of mixed-energy tidal inlet systems, despite the large-scale intervention. This study provided the opportunity to evaluate the emergence of typical tidal features in a highly disturbed system, where the tidal inlet is shifted further offshore. To achieve an accurate forecast, anthropogenic forcing signals also had to be considered and extrapolated to long-term morphodynamic simulations. The forecasting of large-scale anthropogenic interventions in sensible systems, such as tidal estuaries, is performed successfully by adapting an existing model. The development of this type of study will be increasingly relevant in the future for the evaluation of measures against SLR during conceptual project stages.

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

Like many rivers around the world, the Loire river in France has a history of human interventions in order to facilitate navigation and port development. Next to affecting bed levels directly, the heavy modification of the river-estuary has induced significant changes in the hydrodynamic and morphodynamic behaviour of the Loire. As a result, the Loire river is experiencing an ongoing bed degradation. In the estuary, the interventions have led to an amplification of the tidal amplitude, a shift of the tidal and salt intrusion limit in upstream direction and a larger tidal asymmetry. According to Winterwerp and Wang (2013), this tidal deformation leads to an increased import of fine sediment in the estuary and a reduced hydraulic drag. This in turn enhances tidal deformation further. Due to this positive feedback mechanism the Loire estuary has evolved into a hyper-turbid state, associated with large suspended sediment concentrations and the formation of fluid mud.

The behaviour of the river-estuary on large temporal and spatial scales and its response to historical and possible future interventions is not yet sufficiently understood. Therefore, an idealized, computationally efficient, three-dimensional morphodynamic model of the river-estuary is developed with the FLOW module of the Delft3D software suite, in which the main processes influencing the morphology of the Loire river-estuary are included.

The current estuarine geometry, lacking the presence of large intertidal areas, induces a flood-dominant tidal signal. In the mouth of the estuary, the baroclinic pressure gradient introduces a mean landward-directed velocity near the bed and amplifies the mean seaward-directed velocity near the surface. This gravitational circulation causes the formation of an Estuarine Turbidity Maximum at the tip of the salt wedge, which is strengthened further by the flood-dominance of the tide. Model results confirm the feedback mechanism between tidal deformation, the import of fine sediment and the effective hydraulic drag as described above. However, next to tidal deformation, strengthening of the gravitational circulation also plays a large role in this mechanism for the Loire.

To mitigate bed degradation, several measures are simulated that decrease the transport capacity of the flow, increase the sediment supply or do both. Over the first 10 years, sediment nourishments are the most effective according to the model. However, after 100 years of simulation the removal of all groynes present within the domain leads to the most sedimentation relative to the reference scenario. To decrease the import of mud into the estuary, two measures have been investigated. Increasing bed levels in the main channel of the estuary mainly leads to a decrease of the tidal range, whereas restoring tidal flats turns the tide from flood- into ebb-dominant. In both cases, a decrease of the salt intrusion length and a significant weakening of the gravitational circulation occurs, leading to a tidally averaged export of fine sediment and very low mud concentrations.

This graduation project investigates the main processes that are influencing the morphological development of the Outer Weser estuary by setting up a process-based model (Delft 3D) and applying it by means of simulating multiple scenarios, each driven by a different forcing. The aim of this project is to gain insight into the causes for the development of a two channel system in the Outer Weser and into the origin of the related alternation pattern of a dominant channel. Due to the construction of training walls and groynes at the end of the 19th century, the described characteristics disappeared and thus, they are a historical phenomenon but yet today not fully understood. However, to gain insight into the natural development in the Outer Weser estuary is beneficial for current and future engineering projects.
For the investigation a schematic model is generated in which only essential processes are considered and simplified. By applying a flat bed as initial bathymetry, using simplified boundary conditions, multiplying the morphological changes with a morphological factor and using relatively coarse grid, the model is able to predict long term morphodynamics. In the next step scenarios are composed by considering the most relevant processes that could be responsible for the generation of two channels and for the alternation, leading to six scenarios. These scenarios are compared to a base case.
In order to investigate the morphologic equilibrium, the development of two channels and their cyclic behavior, a number of methods are applied. These include time depended hypsometries, cumulative bed load transport, visual inspection and a special method for characterization of channel forms / geometries in a cross-section.
The six scenarios cover the influences of the Kelvin wave, the Coriolis effect, wind waves, extreme river discharge, the absence of river discharge and an increased tidal range. All simulations have a period of ten years of hydromechanics which in combination with the applied morphological factor of 400 results in 4000 years of morphodynamics. The scenarios as well as the reference simulation reach a morphologic equilibrium within the first fourth of the simulation time. Afterwards, all simulations reveal the development of two channels with different dimensions and locations with respect to the reference simulation: While the scenarios excluding the Kelvin wave and river discharge result in a stronger development of the western channel, the scenario excluding Coriolis and the scenario including wave action lead to a more dominant eastern channel. The simulation with extreme discharge and the simulation with an increased tidal range do not favor one of the two channels. For the alternation, the developed evaluation method indicates an alternation between western dominance and equal dominance for the no Kelvin wave and no river discharge scenarios. For the no Coriolis scenario the alternation is observed between eastern dominance and equal dominance. No alternation is found for the extreme river discharge and wave scenarios, while the only scenario indicating an alternation between eastern and western dominance is the scenario with an increased tidal range. For the scenarios in which an alternation is indicated the alternation period is determined and compared to the reference case. This shows a shorter alternation period for the no river discharge scenario and a similar alternation period for the no Kelvin wave and no Coriolis simulation, while the alternation period is raised in the increased tidal range scenario.
The conclusions are that the development of the two channel system is mainly caused by the tides and the basin geometry, since it evolves even when the Kelvin wave, Coriolis, the river discharge and waves are excluded. Furthermore, it is reasoned that the alternation pattern and period are dependent on the dominance of the tides and the depth of the two channels, due to a shortening of the alternation period in the scenario without river discharge (stronger domination of the tides) and an increased alternation period in the scenario containing an amplified tidal range (deeper channels). Additionally, it has been reasoned that the alternation might be introduced by the shape and geometry of the Weser estuary.

In this thesis the equilibrium cross-sectional areas of a tidal inlet are investigated. The most common used methods to study cross-sectional stability are the Escoffier method (Escoffier, 1940) and the Modified Escoffier method (Van de Kreeke, 2004). The Escoffier methods are mainly empirical and lack physical justification. Therefore, in this thesis a process-based model, as proposed by Hinwood et al. (2012), is used to show that in the long-term the inlet cross-sectional area approaches a stable equilibrium or closes. Additionally, the influence of higher harmonics on the equilibrium cross-sectional areas is studied. The stable equilibrium cross-sectional area of the Frisian inlet in the Dutch Wadden Sea is computed and the results are compared to earlier findings.

Over the last decades, the People’s Republic of China experience large economic development, which have led to severe shortages of usable land. To counteract this, the Chinese central government promoted the development of coastal provinces by reclaiming large sections of coastal zones. One of these regions is Nantong Binhai Park or Tongzhou Bay on the Jiangsu coast. This region is characterized with large intertidal mudflats and tidal channels and has a high potential for the development of agri-aquaculture as well as the construction of a deep-sea port. However, these intertidal mudflats are a vital part of ecosystems and support numerous species, such as several endangered migratory shorebirds within the East Asian–Australasian Flyway. Acknowledging the importance of both port development and existing endangered ecosystems, a more integrated coastal design is needed. Possible solution could be found through the Building with Nature approach, in which natural processes and ecological needs are included in the design. This requires a good understanding of the governing processes as well as the requirements from society, ecology and engineering perspectives.

In this thesis, a preliminary study was performed into a more sustainable alternative for the planned Tongzhou Bay Port, in which a basic connection was made between engineering and ecological point of view. A hydro-morphodynamic numerical model was made available and further developed for the modelling of the governing processes in the development of the intertidal mudflats at Tongzhou Bay. The model showed satisfactory results with respect to observations and net flow and transport features were closer analyzed. The different ecosystem types at Tongzhou Bay were quantified with ecotope classification and further validated with satellite-tracked data of two endangered shorebird species. Based on this validation, high-value ecotopes were determined based on these species and was used as an indication for the performance of alternative port configurations with respect to ecology.

With a better understanding of the governing processes for the development of the intertidal mudflats and ecotopes, a series of alternative port designs were conceived through several design cycle iterations. Guiding principles were the preservation of high-value ecotopes, the enhancement of these area’s development and the potential area created for future port development by each variant. Results showed that by understanding natural transport and siltation patterns, not only could alternative port configurations lead to the preservation of existing high-value ecotopes, but a stronger development of these areas with respect to the natural development could be realized as well. The preliminary results of this thesis show the potential of more sustainable alternative port designs through the Building-with-Nature principle, in creating benefits for both economic and ecological development of Tongzhou Bay.

The Eastern Scheldt basin is protected by a semi-open storm surge barrier that was completed in 1986. In 2015 a set of five turbines was installed in one of the barrier openings as a pilot for tidal energy extraction. Due to the construction of the barrier, the average tidal range and velocities have significantly decreased throughout the basin and therefore tidal and meteorological processes driving sediment transport over tidal flats are no longer in equilibrium with the bathymetry. Working towards a new equilibrium, tidal flats erode and channels fill up. This is expected to continue over a long period of time. Turbines installed in barrier openings will lead to an increase in effective resistance at a barrier opening and therefore a further drop in tidal range and velocities throughout the basin. An available two-dimensional Delft3D model (Pezij, 2015) covers relevant hydrodynamics and morphodynamics of the basin with barrier, openings, channels and tidal flats. This model was modified to account for tidal energy extraction. The tidal turbines were parameterized through a momentum sink that increases the flow resistance locally. This sink term was calibrated using a state-of-the-art three-dimensional model of one opening with five turbines. Upscaling scenarios were specified, in which different sets of the barrier openings are equipped with tidal turbines. Numerical simulations over one spring-neap cycle were performed for these scenarios. Results show small deviations in tidal range, volume and discharges throughout the basin due to tidal energy extraction, compared to changes that have occurred due to the construction of the barrier. Reductions in tidal range and volume appear to be near-linearly increasing with the number of turbines installed. Deviations in tidal range increase in landward direction. Between the barrier and mid-basin, peak discharges decrease in channels directly behind the barrier section with turbines and increase in channels behind sections with no turbines. From mid- to end-basin, discharges are not affected by positioning of turbines, only the amount of energy extraction. Simulation results indicate that the emergence time of tidal flats decreases due to tidal energy extraction as a result of an increase in mean low water level throughout the basin. The increase in low water level due to tidal energy extraction would only be a fraction of the increase in water level due to sea-level rise in coming years. Additionally, the reduction in acreage was estimated based on hypsometric curves of the three largest individual tidal flats. The reduction in acreage of these flats due to the rise in mean low water level resulting from tidal energy extraction is relatively small compared to the ongoing loss in acreage due to both sea-level rise and erosion resulting from the barrier construction. Further work is required to improve basin and turbine modeling and evaluate long-term morphodynamics (sediment transport). Separately a comprehensive socio-environmental evaluation is required to compare benefits from (renewable) tidal energy extraction to the (incremental) ecological impact resulting from further reduction in acreage of tidal flats that support animal and bird life.

Sea level rise poses challenges to the tidal inlet systems of the Wadden Sea, including the barrier islands. If no measures are taken, the shorelines of the islands will retreat and the basins may drown. In the Ameland Inlet, the Boschplaat at the eastern tip of Ameland and Bornrif at the western tip of Terschelling are currently shrinking.
This thesis evaluates different nourishment strategies to counteract the coastal retreat, with the ultimate goal of improving our understanding of how nourishment sand of different grain sizes is shared between the different morphological units of the Ameland Inlet.
The process-based model Delft3D is used as a means of predicting the transport pathways of nourishment sand on a time scale of 10 years. In this thesis an existing model of the Ameland Inlet is improved, in order to make it suitable for medium term nourishment calculations. The Mormerge (parallel online) approach is applied in combination with a phase shift in the tidal signal for the different wave conditions. As a result the computational time is significantly reduced. The bathymetry is updated based on the 2016 Vaklodingen and Bed Composition Generation is used to create a suitable bottom composition. The composition of the nourishments is based on the particle size distribution of the borrow area.
The model results suggest that different objectives, such as feeding the Terschelling or Ameland coast, feeding the basin, or reinforcing the ebb tidal delta, can be met by varying the nourishment location and composition.
The main findings regarding the transport pathways within the inlet system are as follows: Near the Terschelling beach import via Boschgat is dominant. Further offshore, bypassing via the ebb tidal delta is the most important transport pathway. Boschgat feeds the western part of the basin and Borndiep feeds the eastern part, but interaction between the two is limited. The model predicts some transport from Boschgat to Borndiep via Westgat under the influence of the tide, or driven by waves over the shallow area in between the channels. There is no connection in the other direction. Borndiep connects the basin to the ebb tidal delta, resulting in transport in and out of the system. Bornrif is the only location where a net westward transport into the inlet is observed.
Finer fractions are imported further into the basin by the tidal flow. When bypassing the inlet, they are transported further offshore, since they are more influenced by the tidal currents flowing out of the inlet. As a result coarser nourishment fractions end up closer to the Ameland shoreline.
The model has several limitations, mainly since insufficient data is currently available for a sound calibration and validation. However important phenomena observed in the field are reproduced by the model, such as the coastal retreat at Boschplaat and the bar migration at Bornrif. This gives confidence in the model performance regarding the predictions of the sediment transport patterns.