Coastal areas are highly dynamic systems sensitive to natural and anthropogenic change. The range of social, economic, and environmental functions served by diverse coastal environments makes understanding their geomorphology valuable. Parametric shape functions have historically been used to represent the cross shore profile of specific coastal environments. Geometric (consequential) parameters of these functions are often related to parameters representing key morphological drivers (causal parameters) to provide first order predictive capabilities. Advances in the application of data-driven modelling techniques presents opportunity to develop a unified characterization of cross shore morphology in diverse coastal environments. However, the ability to represent diverse profile geometries using a single parametric representation has not been observed. The aim of this study is therefore to evaluate the ability of parametric expressions to effectively characterize diverse coastal profile geometries. Here, a new parametric function is presented that can effectively represent key geometric attributes of diverse cross shore profiles. This function demonstrated high performance in the representation of both theoretical profile morphotypes (mean SSE = 0.04) and measured profile data from Narrabeen-Collaroy Beach, Australia (mean RMSE = 0.05 m) compared to eight existing parametric functions. Parametric values were effectively applied to group profiles with similar geometry using manual labelling and K-means clustering. The average profile of each cluster (i.e., the cluster centroid) provided a good representation of the grouped profiles, particularly when parametric grouping was used in place of empirical grouping. Correlation analyses between causal and consequential parameters demonstrated an ability to identify expected geomorphological trends using the new parametric function. Application of cluster centroids for correlation analysis provided amplified correlation strengths for expected geomorphological relationships, particularly when clustering with fitted parametric values. These results suggest that a parametric representation of the coastal profile has potential to characterize and group diverse profile geometries, and that causal-consequential parameter relationships have potential to identify geomorphological trends. The application of these results provide a foundation for the development of a predictive coastal profile model. This model could be trained using data from diverse coastal environments to predict coastal profile response to changes in the metocean climate related to human interventions and climate change.

Sand wave is one of the large-scale seabed morphological features with a typical wave height of metres and a wavelength of hundreds of metres. Its mobile nature causes a nuisance to offshore infrastructure and marine transport. Pre-sweeping is a construction procedure commonly involved in submarine utilities installation to dredge existing sand waves but recovery of sand waves is usually observed after dredging. The underlying processes of sand wave recovery remain relatively underexplored. Given the potential implications for improved dredging planning, this thesis aims to investigate the drivers of sand wave recovery and examine the modelling performance of Delft3D FM on the recovery process through a case study. The numerical model is the first 3D morphological model specifically for simulating sand wave recovery after pre-sweeping. With the incorporation of DelftBlue and MORFAC, computational efficiency has been significantly enhanced. The results show that the transverse slope effect and the oblique current flow supply sediment into the dredged area causing sand wave recovery. This study also elaborates on the two stages in the recovery: an adaption period referring to the morphological adaption of the dredged profile in response to the forcing and a subsequent regeneration period referring to the growth of a feature developed during the adaption period. Although the precise estimation of the recovery duration by Delft3D FM remains challenging, this pioneering research provides an explanation on how tidal flow drives the recovery and reveals the accomplishment of Delft3D FM in modelling the phenomenon. Further exploration of this subject, such as the impact due to storm and the influence of trench configuration, is suggested to further enrich our understanding of sand wave recovery.

Flood defense will have to be strengthened all over the world as sea level rise poses great challenges for the safety of coastal settlements. Strengthening traditional hard flood defenses is becoming increasingly more difficult due to limited space in the coastal zone and large costs inflicted with renovation. Adaptive ‘soft solutions’ pose an alternative as they make use of natural processes that are able to adapt to changing boundary conditions. The Prins Hendrik Sand Dike is a pilot project of such a soft solution placed in front of an existing hard sea dike. The Prins Hendrik Sand Dike is the subject of this research.
This research strives to identify the role of waves, currents, and water levels on morphological development of the Prins Hendrik Sand Dike. Which requires an identification of the morphological development during the 3.5 years of service lifetime. This is followed by a characterization of the forcing climate based on measured on-site data. This knowledge is used to hindcast the full forcing climate during the PHSD lifetime and calculate morphological development. Lastly, a comparison of calculated and observed morphological development reveals suitability of engineering formulae on low-energy beaches.
Results indicate the spit head develops at a faster rate than predicted in model studies. In addition, constant erosion in the form of coastline retreat is observed in a section that serves as primary sea defense. These findings hamper successful development of habitat and possibly decrease the longevity of the sea defense if maintenance is not performed. Furthermore, waves have been determined to be the dominant forcing mechanism resulting in sediment transport and morphological development. In addition, systematic water level changes that occur under specific wave conditions are identified causing increased Northeastward transport of sediment. These findings likely explain discrepancies between the previously modelled and observed morphological development.
Implications of this research contribute to an understanding of the interconnected nature of forcing scenarios at low-energy beaches and can be used to improve modelling efforts of low-energy beaches. Furthermore, it provides a basis for Hoogheemraadschap Noorderkwartier to make decisions on maintenance activities.

Sea level rise and land subsidence increasingly put more pressure on Dutch flood protections. Additional to regular dike reinforcements, new, more nature-based, solutions are considered, such as a double dike system (DDS). Such system, consisting of two parallel dikes in combination with an interdike area in the form of a salt marsh, offers the opportunity to adapt via hydro-morphological processes to sea level changes. An increased bed level of the interdike area of a double dike system potentially positively affects wave attenuation. Furthermore, if the second (landward) dike breaches, the elevated salt marsh of the interdike limits breach growth and discharge through the breach. However, previous studies show that it is hard to prove the feasibility and effectiveness regarding flood safety of a DDS relative to a single dike and an in-depth hydraulic engineering substantiation for flood risk reduction is lacking. The objective of this research was to investigate which factors determine the effectiveness of a DDS, with the focus on the dynamic development of the system. This leads to the following research question: Under which circumstances can a double dike system be an effective alternative to a regular dike reinforcement? It is possible to distinguish three distinct systems based on the function of the first (seaward) dike: dynamic open DDS, dynamic closable DDS, and static closed DDS. This distinction is primarily based on the degree to which water is permitted to pass through the first dike under both normal and extreme conditions via an opening in the dike. This interaction between the outerdike area and interdike is unobstructed for a dynamic open DDS and almost nonexistent for a static closed DDS. To achieve a more hybrid approach, dynamic closable DDS utilizes adjustable structures like tidal culverts to regulate the inflow. Under normal tidal conditions, the permitted inflow governs the sediment supply and the potential for accretion of the bed level in the interdike area. Depending on the design of the system, during extreme storm conditions, the first dike can act as primary flood defense or as breakwater that provides shelter for the second (landward) dike. This research focused on an open dynamic DDS system, allowing for maximum potential bed level accretion of the interdike area. In doing so, the first dike functions as a breakwater and failure of the system occurs when the second dike is breached. Due to the unrestricted in- and outflow, the assumption is made that water levels are uniform and equal to the hydraulic conditions outside the system. The double dike systems are analysed with the help of two approaches: failure probability approach and local individual risk (LIR) approach. The first approach compares systems based on equal probability of failure. The second approach compares systems based on an equal LIR of 10−5 per year and includes the expected mortality in case of flooding. Systems have been compared to each other with the help of calculated dike heights and total dike volumes. Dikes were tested for failure due to overtopping or overflow. Therefore, a transmission model was applied to describe the transmission of waves through the system: wave transmission over the first dike, wave propagation in the interdike area and wave overtopping over the second dike. The probabilistic calculation method FORM was used to find the failure probability and design point. For the LIR approach, this model was extended with a 1D flood model to determine the flood characteristics and subsequent mortality fraction if failure of the dike occurs. Firstly, a simplified case study was performed based on conditions in the Western Scheldt. Opening the existing dike in the current hydraulic conditions necessitates the construction of a second dike that is only 1.7 meter lower than the single dike system. A sea level rise (SLR) of 0.4 meter in 2070 and 1.0 meter in 2120 was implemented. In addition, morphology scenarios were applied, ranging from no accretion to maximum accretion. In the scenario with maximum accretion, rapid accretion to Mean High water takes place, after which the bed level accretion follows SLR. Comparing systems based on equal failure probability, a single dike should be raised identically to the SLR. In the case of a double dike system and 1.0 meter SLR, this increase in height of the second dike should be approximately 1.15 meter to maintain equal failure probability. Sedimentation in the interdike area has a negligible impact on failure probabilities due to the dominant influence of water depth, meaning the water depth is too high for waves to be affected by the bottom of the interdike area. However, this comparison changes when based on equal LIR. With 1.0 meter SLR, the additional required height of a single dike increases to 1.7 meters. Applying this approach to a double dike system, the effect of the height of bed level of the interdike area on the breach flow does have significant influence. Analysing the effectiveness of a double dike system in comparison to a single dike system based on volume increase, these differences are magnified. However, generally, it can be noted that the extreme storm surge in the Western Scheldt is dominant and a second dike with substantial dimensions is needed to comply to its retaining function. Secondly, a sensitivity analysis was performed on four different parameters and system configurations: 1) height of the first dike in combination with the length of the interdike area, 2) hydraulic conditions with lower water levels, 3) varying area size of the hinterland and 4) implementation of vegetation. The effect of the first dike on the failure probability is not dominant, since the required increase of the height of the second dike is not proportional to the applied decrease in height of the first dike, increasing its effectiveness in terms of dike volume. This effectiveness increases also for hydraulic conditions with lower extreme water levels. In addition, there is a positive relation between the effectiveness and the area size of the flooded hinterland. Vegetation does not have any influence on the dike height due to the high water depths under design conditions. In conclusion, the following three location specific circumstances are important for an open DDS to be an effective alternative to a regular single dike reinforcement: • Sufficient sediments present to allow for a higher rate of sediment accretion compared to sea level rise. • Hydraulic conditions in which waves are more important than storm surge. • Large hinterland area. It is more advantageous to employ the local individual risk approach in comparative studies assessing the effectiveness of double dike systems. The significant positive impact of bed level accretion on flood impact is disregarded when only considering failure probabilities. It is recommended to improve the flood model, implementing a 2D model and considering the effect of the foreshore on breach growth, to gain a more comprehensive understanding of the potential of an elevated bed level of the interdike area on the flood impact, so it can be effectively incorporated into the Dutch flood risk approach. In addition, it is recommended to consider the implementation of an open dynamic DDS in coastal areas with high sediment concentrations and suitable hydraulic conditions (sufficient tidal range and mild storm surge). Furthermore, the wave damping function of the first dike is not critical for interdike areas with sufficient length and reduced depth. It is more efficient to prioritize the flood safety of the second dike. This allows for more effective design and management decisions concerning the height and safety requirements of the first dike. Finally, it is advisable for areas with high water levels during extreme conditions to consider a closable system. This entails the ability to close the gaps in the first dike under specific conditions, such as utilizing tidal culverts, limiting the hydraulic loads on the second dike. However, implementing a closable system necessitates larger investments in modifications to the first dike. Additionally, the maintenance of the first dike becomes more crucial due to its retaining function first. Therefore, it is recommended to conduct a comprehensive cost-benefit analysis to compare the open dynamic double dike system with the closable double dike system.

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.

This study assesses the application of graph theory to examine the connectivity of aquatic habitat in the Sliedrechtse Biesbosch and preserve or improve the area’s ecological value. The study addresses the relation between hydrodynamics and ecology, and evaluates different definitions of connectivity. Graph theory provides a novel and promising approach to assess connectivity in aquatic environments. With increasing availability of data, more accurate and informative analyses can be carried out, resulting in more effective designs of restoration measures. The application of graph theory in such research has, as one of its main strengths, its visual accessibility to laypersons or others without expertise in interpreting numerical model results. Furthermore, graph theory can offer both a holistic system view of a water network by evaluating the system as a whole, as well as a local view which is possible by looking at the single nodes and edges in the graph. This enables investigation of the role of individual channels within the system and the identification of vulnerable spots within the network, limiting the availability of aquatic habitat.

Graph theory is applied in various fields of research. As numerous metrics exist to examine the properties of a graph, an introduction to the most important metrics is given in this study. Adequacy of metrics is dependent on the questions posed and parameters relevant for the specific topic to be investigated. It is shown that in aquatic habitat connectivity, metrics such as betweenness centrality and bridges are indicative to obtain a general view of the network. When temporal variations play a role, as is the case in a tidal area, metrics as the number of components (NOC), the order of the largest component and the length of connected pathways (LOCOP) of the largest component are suitable to determine the connectivity. Whereas these metrics show useful in studying aquatic habitat connectivity, they may be less appropriate in connectivity studies of the same water system aiming at other fields of application, e.g. sediment connectivity.

Results show that graph theory provides a useful instrument in analyzing the aquatic habitat connectivity of the Sliedrechtse Biesbosch, which is investigated in a case study. The ease at which key nodes and edges are identified offer great possibilities for the design of nature restoration measures. In the present layout of the study area, large variation of aquatic habitat connectivity occurs based on a flow velocity fragmentation threshold of 0.3 m/s, corresponding to the maximum tolerable flow velocity for the European flounder (Platichthys flesus). Due to tidal influences in the study area, flow velocities vary continuously and the threshold flow velocity is exceeded during part of the tidal cycle. Considering the available habitat of other species gives different results depending on the tolerable flow velocities of the specific species. As is shown in this research, a combination of graph theory and numerical modelling enables the design and simulation of different nature restoration measures and system layouts to improve the aquatic habitat connectivity of the area.

The method presented in this research can be particularly useful to ecologists investigating suitable habitats for specific fish species. Also for engineers and others involved in the design of nature restoration measures, the method can be helpful since the designs of restoration measures can be evaluated considering the effect on habitat availability. The research provides an informed basis for subsequent applications of graph theory and numerical modelling to aquatic habitat connectivity. By selecting the most suitable design parameters and improvements of the network schematization, a justified decision can be made on the most effective restoration measures concerning the improvement of available habitat. Especially considering a combination of habitat preferences, such as flow velocity, water depth and turbidity, can provide proper insight in the aquatic habitat connectivity of an area for specific species.

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.

The port of Rotterdam is one of the most important ports in Western Europe and via the 'Nieuwe Waterweg' it provides maritime trading between the large hinterland and the rest of the world. To provide port access for the large vessels, the channel and port basins need to be dredged regularly. At the same time, in other locations, sediment is required to fill erosion holes, develop natural habitats and improve water quality. Different stakeholders joined forces to acquire more knowledge about managing the sediment in the system. This resulted in the project called 'Proeftuin Sediment Rijnmond'(PSR). One of the pilot projects of PSR is located in the ’Nieuwe Waterweg’ where a longitudinal dam is constructed between groynes to study the effects on nourished sediment. The location used for this pilot is called Groyne Field 9. At the same time, the area around Groyne Field 9 is involved in another project called the 'Groene Poort'. This project focuses on natural development in the areas between groynes. The research in this thesis contributes to knowledge acquirement for the various projects. It studies the effect primary vessel waves have on the hydrodynamic behaviour in the semi-closed Groyne Field 9.

The study answers the research question: How can primary vessel waves impact the hydrodynamics in the semi-closed Groyne Field 9 in the 'Nieuwe Waterweg' and how can the hydrodynamic conditions be modified with different structural layout designs? Two different approaches are used to answer the question. First, a data analysis is conducted using 2 relevant data sets and secondly, a numerical model is developed to be able to study the topic in a more controlled environment.

The First part of the research analyses two data sets simultaneously. Water level and velocity measurement data obtained inside the groyne field is combined with Automated Identification System data (AIS data), which contains velocity, location and more information about sailing vessels. Combining these data sets enables the possibility to study the effect of the vessels' characteristics on the water level in Groyne Field 9. A regression analysis is carried out which results in an equation that shows the correlations of all parameters. The most significant parameter influencing the severity of the primary vessel wave is the sailing velocity. Other important parameters that cause water level drawdown are the length, width and draught of the vessel.

In the second part of the research, a numerical model of sailing vessels in the 'Nieuwe Waterweg' is developed in Delft3D Flexible Mesh software. This can be used to analyse the hydrodynamic effect on the groyne field in a controlled way. To simulate vessels passing through the waterway, a local atmospheric pressure field is used. This pressure field simulates the vessels’ hull and by moving the pressure field through the numerical domain, a primary wave is modelled. To research the effect of primary vessel waves on the semi-closed groyne field, various simulations are conducted and analysed which can be distinguished by two sets of simulations...

Echosounders are used to monitor suspended sediment concentrations (SSC) in coastal management studies and to monitor dredge plumes. Due to the dependency on sediment properties like particle size, shape and density, performance of single frequency measurements is limited when sediment properties are changing.

Using multi-frequency measurements, mean particle sizes can be derived and used to (partly) adapt to these variations. Based on field measurements in the coastal waters around Texel, performance of the single and multi-frequency methods was assessed under steady and varying measurement conditions.

While similarly high performance was obtained under steady conditions, the multi-frequency method outperformed the single frequency method significantly when measurements were taken over multiple tidal stages. Also, SSC estimations became less sensitive to selection of frequency. If further developed, the potential of SSC quantification using echosounders is significantly increased.

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.

The behaviour of tidal basins has been predicted using a modelling approach in which a morphological equilibrium for an entire basin was used frequently in the past. Because tidal basins have, to an certain extend, a fractal nature, it is expected that this approach could be used with a morphological equilibrium for subbasins as well.

In the first part of this thesis the morphological equilibrium for subbasins is examined. A numerical simulation using tracers is conducted in order to find the watersheds of the Ameland Inlet. The watersheds are used to divide the basin into subbasins of different scales. These subbasins are used to find a morphological equilibrium between the channel volume and tidal prism. The applicability limitations of the morphological equilibrium relation are also investigated in this part of the research.

In the second part of the research the newfound equilibrium is applied in an Asmita model. The modelling exercise is done in order to showcase a proof of concept of the multi element modelling approach. In this modelling approach both a 6 and 10-element model are used to look for the spatial differences in morphological behaviour within a basin.

In the first part of this thesis an equilibrium for the channel volume with respect to the tidal prism of subbasins in the Ameland inlet is found. This equation can be applied to basins (Channel and flat combined) with a minimum area of 40 km². This limitation is for both the equilibrium study as the Asmita modelling. The modelling exercise proofs that the multi element Asmita modelling is possible and gives a good insight into the spatial differences in morphological behaviour within the Ameland Inlet.

Due to river training in the past centuries, the bed level of the Waal started and is still decreasing by several centimetres a year. Erosion of the riverbed causes problems for objects underneath the riverbed like cables, pipelines and foundations of structures. Furthermore, some parts of the river are eroding at a higher velocity compared to other parts due to a difference in the composition of the bed material. This can result in a jump in the bed level while the water level cannot follow this jump. This results in less available draught for inland vessels. If no mitigation measures are applied and the bed level keeps decreasing, more problems are likely to occur resulting in less transport over the Waal. A possible solution to reduce bed erosion inside the Waal locally is the use of groyne field nourishments.
The objective of this research is to obtain more knowledge about the flow in a real-life groyne field and the effect of vessels on this flow. The research question formulated for this research is, therefore: \textit{what is the influence of vessels on the flow properties inside a groyne field and in what way do the characteristics of the vessels influence this?} To answer this research question a literature study is done first. Secondly, a measurement campaign is performed and finally, the results are analysed to investigate the effect of the characteristics of the vessels.
The literature study is done to obtain more knowledge about the flow properties inside a groyne field. The distinction is made between emerged and submerged groynes and the scenario with and without vessels. The situation where the groynes are emerged is dominant for the erosion inside the groyne field. In this scenario, the flow inside the groyne field consists out of one or two circulation patterns. The first circulation pattern is a large eddy in the downstream part of the groyne field, referred to as the primary eddy. When the groyne field is long enough, a second eddy is present in the upstream part of the groyne field, the secondary eddy. This eddy has a flow direction opposite to the primary eddy and a smaller flow velocity. When a vessel passes a groyne field, the flow pattern inside the groyne field changes due to the primary waves created by the vessel. Firstly, the water level inside the groyne field is raised due to the bow wave. Secondly, due to depression in the water level caused by the primary wave the water level inside the groyne field is lowered. Finally, the water level inside the groyne field is increased again due to the stern wave.
To investigate the effect of vessels on a groyne field, a measurement campaign is performed. The flow direction, flow velocity and water level inside the groyne field are measured with the use of several measurement instruments for two weeks. The obtained data by the measurements is analysed and visualised. The results show that the primary eddy remains partly intact when a vessel sails past the groyne field. Since the primary eddy remains partly intact, the water is guided towards the upstream part of the groyne field. In the upstream part of the groyne field, the secondary eddy does disappear and the flow is directed out of the groyne field. This results in the water, and sediment, flowing out of the groyne field mainly in the upstream part of the groyne field. This has resulted in a scour hole present in the upstream part of the groyne field.
In this research the effect of multiple vessel characteristics on the water level and flow velocity inside the groyne field is investigated. The vessel characteristics investigated are the draught, the sailing speed, the vessel length, the vessel width and the distance of the vessel towards the measurement instrument. The effect of vessels on the water level and flow velocity inside the groyne field differs for each vessel. Information about the vessels has been obtained by using AIS (Automatic Identification System) data. During the processing of the AIS-data, irregularities and errors were found and mostly removed from the AIS-data. According to the final results, the draught of a vessel does not influence the water level and flow velocity inside the groyne field. An increase in the sailing speed of a vessel and a decrease in the distance of a vessel towards the groyne field does have an enlarging effect on the water level difference and the flow velocity inside a groyne field. The combined effect of increasing the length and the width of a vessel also has an enlarging effect on the water level difference and the flow velocity inside the groyne field. It should be noted that the results show a large variance and it is impossible to predict the effect of a single vessel based on the vessel characteristics.
The measurement campaign showed that a constant water level fluctuation is present inside the groyne field even when no vessel is affecting the flow inside the groyne field. Multiple explanations for this fluctuation are given with transverse oscillations between both riverbanks being the most likely.
The measured flow pattern inside the groyne field without vessel is according to the literature. When the flow is affected by a vessel, the flow pattern differs from the flow pattern described in the literature. Furthermore, the effect of a vessel is likely not only depending on the characteristics of a vessel but also on the flow mechanism inside the river system such as the helical flow, the angle of the groyne field with respect to the main channel and the constant water level fluctuation inside the groyne field.
This research adds to the already existing knowledge about the flow in groyne fields. To investigate the effectiveness of groyne field nourishments, a pilot is planned where the actual nourishment will take place at the same location as the measurement campaign. Therefore, this research can be used to better prepare the planned nourishment pilot to investigate the effectiveness of groyne field nourishments. Furthermore, this research contains data about the flow pattern inside the groyne field without nourishment which can be compared to the situation during and after the nourishment pilot. In the end, this research can be a part of the answer to whether groyne field nourishments can reduce or stop the erosion inside the Waal river. 

The Oosterschelde has been an area of morphological change for centuries. Both floodings and human influences have caused the Oosterschelde to have its current shape. During the last century before implementation of the Delta plan (1953) the estuary was still expanding as the channels deepened and the tidal flats increased. With the construction of the Delta works the tidal range decreased with 12 % whereas the tidal prism reduced with 31 %. As a consequence of this reduced tidal motion, the tide is not able anymore to counteract the erosion of the tidal flats which is caused by waves/wind. Therefore, the surface area and the height of the tidal flats reduces (‘Zandhonger’). This decrease of tidal area is an undesirable situation as the unique tidal nature of the Oosterschelde provides a lot of functions for both economy (oysters, mussels), ecology and recreation. Furthermore, the Oosterschelde is used as a transport route by cargo vessels.

This research studies the morphological development of a model of the Oosterschelde with two hypothetical interventions: removal of the storm surge barrier (SSB) and applying 2 metre sea level rise (SLR) in 50 years. This is done with four model scenarios: a run with SSB in place, without SLR (1), a run without SSB, without SLR (2), a run without SSB, with SLR (3) and a run with SSB, with SLR (4). This last run was used as sensitivity run in order to see which hypothetical interventions has more impact.

The model results led to conclusions which parts of the model are represented well and which parts of the model need to be improved. The results made clear that current model is promising as the tidal range was modelled correctly for the majority of the Oosterschelde. Also the model represented the tidal prism well compared to the calculated tidal prism (tidal prism = tidal range * wet surface area of the basin – the sediment volume of the tidal flats). It was found too that SLR can be modelled in a correct way by forcing a water level at the boundaries of the model.

It was found that wave activity is an important process with respect to the development of the tidal flats. Therefore, the frequency of the wave computations should be chosen in such way that reliable wave heights are present for each water level during the tidal cycle at all locations in the Oosterschelde. Another key factor which should be improved is the availability of sediment in the model as this determines the (desired) growth of the tidal flats with SLR. Processes/indicators which give information about this availability (ebb/flood dominance, sediment characteristics) can give insight in the development of the tidal flats. With improvement of modelling these two processes it may be possible to improve model capabilities.

The feasibility of installing a suction system, which consists of a pump and pipeline for transporting sediment from the sediment trap to the reallocation site to complement the water injection dredging (WID) method is accessed in this thesis. The combination of a suction system and a water injection dredger is called central suction-WID system. This thesis analyses the sediment return volume, necessary pump power, greenhouse gases production and necessary costs for implementing a central suction-WID system. Three different reallocation sites with different distances for central suction-WID are assessed: Inside of Nieuwe Waterweg, Outside of Nieuwe Waterweg, and Verdiepte Loswal..
Delft3D-Flow numerical model is used for modelling the hydrodynamic and the sediment return volume. The simulation is run for 14 days period in order to simulate spring and neap period. The numerical model result shows that discharging the sediment at the inside of Nieuwe Waterweg leads to the most sediment return, while discharging at the Verdiepte Loswal leads to the least sediment return volume. Increasing the sediment volume or reducing the particle size does not affect the sediment return rate.
The necessary pump power and energy consumption for reallocating the sediment to each location are assessed. The calculation shows that the major contributor to the energy consumption is the friction between the fluid mud and the pipe wall. Reallocating the sediment to the inside of Nieuwe Waterweg consumes the least energy while transporting it to Verdiepte Loswal needs more energy as it is the furthest reallocation site alternative.
Using a WID vessel produces fewer greenhouse gases than hopper dredger as it consumes much less fuel. Costs analysis shows that using a central suction-WID system is more expensive compared to hopper dredger. The costs of using a central suction-WID system increases as the distance between the sediment trap and the reallocation site is higher. However, combining it with another dredging method can be an alternative as it reduces the operational costs.

The Wadden Sea is a large system of tidal flats and barrier islands. Its individual features have been heavily researched, but sufficient knowledge on the sediment transport patterns on the scale of the Wadden Sea is not yet available. This thesis aims to indicate the dominant sediment transport patterns in the Dutch Wadden Sea and the hydrodynamic forcing mechanism the patterns can be attributed to. The Delft3D-FM model used to indicate these patterns is first adapted by implementing an improved Manning’s roughness field. To assess the patterns for each contributing hydrodynamic forcing mechanism, the tide, wind and waves are added to the model one by one. The results show that the Dutch Wadden Sea is a highly interconnected system, with the influence of each hydrodynamic forcing mechanism depending on the feature and inlet system.

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.

Mudflats are coastal features present in numerous locations around the planet. Depending on the latitude and local conditions, mudflats can be vegetated by salt marshes or by mangroves and play an important role in coastal evolution. Vegetated mudflats can create a remarkable sea defense and their high ecosystem value has been proven by various studies worldwide. In this study, the response of a vegetated mudflat under extreme hydrodynamic forcing is analysed using, as case study, a specific part of the Guyana coast at South America. The sediment dynamics of the Guyana coast are dominated by Amazon River plume that shifts northwards and travels along the coast of South America till the Orinoco River delta in Venezuela. Mudflats are quite wide reaching tens of kilometres in width because of the high supply of sediment by Amazon and the formation of mudbanks along the coast travelling northwards and then westwards. The hydrodynamic environment is relatively mild as Guyana is located far from the track of tropical hurricanes; however swell events can occur caused by the North Atlantic cyclones. Despite of its mild hydrodynamic environment, there are often cases of overtopping of the seawall along Guyana coast every year with a severe and characteristic one been held on October 2005. This was the motive for the mangrove restoration project of the study area in 2011 and for the initiation of this study. The purpose of this study is to analyze the response of a vegetated mudflat under extreme swell and storm events and investigate the impact of vegetation on this response. Furthermore, the impact of the extreme hydrodynamic event on probable damage of vegetation is tested presenting the complete interaction between hydrodynamics and vegetation. The study area is the area of Chateau Margot, southern of Georgetown, where a mangrove restoration project took place in 2011 by planting Avicennia Germinans mangroves and field measurements were applied in the end of 2019 by the researcher Üwe Best. Using the 1-D process based model Mflat, based on an open source Matlab code, a 3.5km portion of the mudflat extending offshore is modelled. Vegetation dynamics and its influence on the flow is part of the model, so a full interaction between hydrodynamics, sediment dynamics, vegetation and morphological evolution can be simulated, establishing a complete bio-geomorphological model. Using field measurements and literature data, the model is calibrated for hydrodynamic conditions and for the vegetation parameters giving a profile in equilibrium. A sensitivity analysis is applied to understand the behaviour of the model and the interaction between various hydrodynamic and sediment parameters and morphodynamics. After calibration, two extreme cases are tested in the model: an extreme storm that could be expected in Guyana coast and an extreme swell event based on the real case occured in October 2005. The Mflat model, after calibration, is able to reproduce a stable profile with mangrove vegetation similar to the observations. Vegetation appears to be quite effective in reducing the wave energy and thereby protecting the coast behind it during the extreme events, while in long term results in accretion of the landward part of the mudflat. The impact of the extreme hydrodynamic conditions on vegetation is difficult to be quantified, but for the extreme cases corresponding to the hydrodynamics of Guyana coast, vegetation is expected to be resilient enough having minor damages that could be restored in a short time period. The very mild slope of the coast is a factor that prevents the bed level of being highly dynamic contributing to the stability of the profile.

The Wadden Sea is the largest system of tidal flats and barrier islands in the world extending from the northern Dutch coast to the coast of Denmark. Such a large natural area is ofgreat importance to bothhuman beings and ecosystems. Over 10,000 species of flora and fauna are found in theWadden Sea being a perfect habitat due to the relatively calm environment and high food availability. However, accelerating sea-level rise and human interventions, such as gas extraction, may induce an unwanted morphological change in theWadden Sea posing a threat to the natural habitats. This study investigates the morphological response of the Wadden Sea to SLR and subsidence induced by gas mining. A new hybridmodel whose aggregation level is between a process-based model
(Delft3D) and a aggregatedmodel (ASMITA) is applied in this thesis. The two-dimensional hybrid model applies depth integrated shallow water formulations and the advection diffusion equation for the transport ofsediments like Delft3D, but calculates the exchange ofsediment betweenbed andwater columnbymeans ofan equilibriumbathymetry concept under scenarios disturbing these equilibrium conditions. The Ameland inlet is chosen as the research area because it is a relatively autonomous
and undisturbed basin. The high robustness of the hybrid model makes it possible to apply a high morphological scale factor and a coarser grid compared to the processbased model, which shortens the computation time by orders of magnitude. However, the equilibrium concept also fixes the shoal-channel structure and suppresses channel migrations. Three different scenarios of sea-level rise rate (4, 6, 8 mm/year) are applied over a simulation period of 100 years. The general morphological response simulated by the hybrid model shows that the channels and the ebb-tidal delta erode acting as the main source ofsediment for accretion of the intertidal flats. The erosion/sedimentation is more pronounced with a higher sea-level rise rate. Sensitivity analysis shows a significant influence of the sediment diameter on the
channel erosion and sediment supply to the intertidal flats. Diffusivity plays an important role in the horizontal sediment exchange between the channel and the flat but only slightly influences the sediment import. Global equilibrium concentration and power n are similar to diffusivity in affecting morphological activity. The adaptation time scale is inversely proportional to both of these two parameters. The possibility of using more than one sediment fraction is proved and it can reproduce a more realistic sediment distribution as the observation. The hybrid model is also applied to simulate the morphological response to local
subsidence and the restoration after subsidence stops. The center of the subsidence circle dosen’t lower as much as the subsidence rate, which indicates that sediment is transported to the center. Sediment is supplied to the area ofsubsidence by the adjacent main channel. The result proves the sediment transport principle underlying the hybrid model that sediment is always transported along the gradient of the sediment demand.

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

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

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

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

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