Tidal flats are ecologically valuable habitats that support benthos and birds while providing natural flood resilience. In the Western Scheldt estuary in the Netherlands, centuries of land reclamation and other anthropogenic interventions have reduced the tidal flat footprint. At two locations, Bath and Zimmerman, the shoreline retreat poses a flood risk that affects flood resilience and tidal flat ecology.

A countermeasure is the installation of rubble mound groynes, which shelter areas between them from waves and currents, creating low-velocity zones that promote sedimentation. The impacts of groynes vary with hydrodynamic processes (tides, wind, waves) and autonomous morphological processes, such as migratory channels and shoals. Bed responses to groynes are variable, differing across tidal flats within the same estuary and even within a single site.

This study evaluates the impact of groynes on tidal flat morphology, quantifies bed response times, and defines the hydrodynamic extent of groynes. It also examines how design aspects of groynes (length, position, layout) drive sedimentation, erosion, and low dynamic area evolution.

This is achieved via a combination of data analysis (topography, water level, velocity) and a hydrodynamic model. The three-dimensional hydrodynamic model refines an existing Delft3D-Flexible Mesh (D3D-FM) model by Deltares to locally refine, calibrate, and validate for the tidal flats at Bath and Zimmerman. While the model is morphostatic, it describes velocities that drive morphodynamics.

The results demonstrate that groynes are capable of reducing velocity magnitudes and producing low dynamic area: 13.1 hectares at Bath and 10.4 hectares at Waarde/Zimmerman. The greatest impact is within the footprint of the groyne field; however, velocity reduction effects can extend laterally up to 2 km on tidal flats with existing groynes. The magnitude of impact is strongly moderated by autonomous processes; natural channel migration and shoal dynamics at Zimmerman dampen the relative impact of the structures. Since the tidal prism remains mostly unaffected by groynes, a reduction in flow velocities on tidal flats is counterbalanced by an acceleration in velocities in other areas. Detached groynes cause flow contraction and localized erosion across the tidal flat, whereas groynes attached to the salt marsh shift high-velocity contraction zones seaward into the main channel and toward opposing channel banks. Despite this redistribution, groynes achieve a net gain in velocity reduction when evaluating area and magnitudes impacted. Two autonomous processes continue to erode tidal flats in the short-term: channel migration and overbank flow at channel bends lower the bed and facilitate higher velocities. However, tidal flats in the Western Scheldt have been recorded rising for half a century; this long-term sedimentation can lead to an expansion in low dynamic area.

In practice, groynes should be paired with complementary interventions where site conditions warrant, requiring a detailed understanding of near-field and far-field dynamics so that interventions are matched to the site-specific problem.

Future work can strengthen the robustness of the hydrodynamic model's quantification of low dynamic area by testing it under various wind conditions and waves. Additionally, the upper tidal flats should be monitored to assess their risk of developing into salt marshes. ... Read more

The Houston–Galveston region is one of the most economically important coastal areas in the United States but is highly vulnerable to hurricane-induced storm surges. To reduce flood risk, the U.S. Army Corps of Engineers (USACE) has proposed a storm surge barrier across Bolivar Roads, the primary tidal inlet connecting Galveston Bay to the Gulf. The current reference design includes three permanent barrier islands that reduce the available flow area by approximately 30%, potentially affecting tidal exchange and navigation under normal operating conditions.

This study investigates how the hydraulic and navigational impacts of these barrier islands can be minimised through conceptual design optimisation. An analytical modelling framework was developed to evaluate alternative barrier configurations by representing the hydraulic resistance of the barrier as a function of island geometry, surface roughness, obstruction, and barrier location. The model was used to assess the sensitivity of key design parameters and identify the dominant factors governing tidal exchange.

The analyses show that inlet obstruction is the primary parameter controlling the hydraulic impact of the barrier, whereas detailed island nose and tail geometries have only a minor influence. The USACE reference configuration reduces the tidal amplitude from 31 cm to 24 cm, corresponding to a 22% reduction relative to the existing inlet and exceeding the adopted design objective of limiting tidal amplitude changes to approximately 10%.

Guided by these findings, successive design modifications were applied to reduce hydraulic resistance while maintaining navigational functionality and flood protection performance. The resulting optimised concept combines a single shortened permanent barrier island with convex geometry, vertical concrete walls, and an alternative barrier location. This configuration increases the tidal amplitude from 31 cm to 32 cm relative to the calibrated existing inlet, corresponding to a negligible increase of approximately 1%, while simultaneously improving navigational conditions through a wider navigation opening, larger vessel clearances, and improved channel alignment.

The results demonstrate that a storm surge barrier requiring permanent support islands can be hydraulically feasible at Bolivar Roads, provided that island obstruction is explicitly minimised during conceptual design. The developed analytical framework offers an efficient tool for evaluating alternative barrier concepts before more computationally intensive numerical modelling or physical scale experiments are undertaken.
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The Dutch Wadden Sea is a large intertidal area where complex morphodynamics govern the transport of water and sediment around the channels and mudflats. The behaviour of sediment becomes increasingly unpredictable due to anthropogenic influences and climate change. Research and monitoring on sediment and the dynamic mudflat are crucial in understanding and projecting the effects of these influences on the Wadden Sea.

This thesis focuses on a GoPro action camera installed on the mudflat near Holwerd. Over a three-month period, the camera took a picture every 15 minutes. The images show the changing mudflat, slowly transforming from a rolling landscape into a defined terrain. Analysis of the changing mudflat supports the research into morphology and sediment behaviour.

A Multilayer Perceptron (MLP) was constructed to perform semantic segmentation, where every pixel of the GoPro images is classified as either mud or water. A comprehensive model was set up, where manually labelled data and multiple image features were used to train the model and process the large image dataset. Model evaluation returned a macro-IoU of 0.71 and a macro-F1-score of 0.82. However, a detailed look at the classification results indicated critical model shortcomings and unnatural proportions of water and mud. Filtering of incorrect predictions resulted in a small dataset appropriate for further analysis.

The results indicate a strong correlation between predicted mud percentage and potential evaporation, revealing that this relationship can be observed from pictures. Spaghetti plots illustrated no patterns of change during single low-water periods. Finally, analysis of weekly average predictions reveals regions of growth and decrease on the mudflat. The channels and shallow pools are observed in particular, since these areas show divergent behaviour. It is theorised that the flow velocity influences the erosive and settling capacities of sediment. The shape of channels and shallow pools influences this velocity and, by extension, the morphological development of the mudflat.

Overall, the thesis demonstrates how an action camera on a mudflat can be used for observing both morphological changes and the forces that define this change. While the MLP does not deliver optimal results, it lays the foundation for future work on more advanced machine learning techniques. Finally, practical recommendations for future camera monitoring projects are given. ... Read more

The Port of Rotterdam is the dominant maritime gateway of the North Sea region and has repeatedly expanded seaward to accommodate increasing maritime, industrial, and logistical demand. Following large-scale developments such as Maasvlakte 2, the Port of Rotterdam is now assessing new spatial expansion options. One option is a direct seaward extension of Maasvlakte 2, referred to as a Seaward Port Expansion (SPE). Five design alternatives have been developed for this concept, denoted A1, A2, B1, B2, and C, representing progressively larger seaward extensions.

An important concern for the Port of Rotterdam is the potential effect that a future seaward expansion may have on fine sediment dynamics and, consequently, on maintenance dredging. The Port of Rotterdam depends on continuous dredging to maintain nautical depths in its access channels and port basins. After the construction of Maasvlakte 2, annual maintenance dredging volumes increased substantially in subsequent years. This raises the question of whether a future SPE could trigger an increase in siltation and dredging demand. The central hypothesis is that the modified coastal geometry may alter flow patterns, mobilise offshore fine sediment, increase fine sediment import through the Maasmond, and thereby increase sediment accumulation within the port.

This research addresses the following research question: How do alternative SPE designs affect maintenance dredging demand in the Port of Rotterdam through changes in hydrodynamics and fine sediment transport? To answer this question, a process-based framework is combined with numerical modelling using Delft3D-FLOW and DELWAQ. The framework tracks the response of fine sediment to the SPE designs from the offshore domain, through the Maasmond transect, and into the port basins.

The model results show that the SPE designs strengthen tidal flow contraction around the seaward expansion, increasing flow velocities and bed shear stresses near the curvature of the designs. Clay and fine silt remain largely in suspension offshore and show only limited sensitivity to these hydrodynamic changes. Coarse silt/micro floc responds more clearly, with reduced bed mass in the scour area and increased accumulation potential in adjacent low-energy zones.

At the Maasmond transect, the SPE designs do not increase fine sediment import into the port. Designs B2 and C reduce the net import of coarse silt/micro floc by 4% and 9%, respectively, mainly due to lower near-bed sediment availability caused by increased offshore retention. Consistent with this, the SPE designs do not lead to a systematic increase in fine sediment accumulation within the port. Within the applied model set-up, and excluding storm events, morphodynamic feedback, and construction-phase effects, the SPE designs are therefore not expected to increase maintenance dredging demand through changes in hydrodynamics and fine sediment transport alone. ... Read more

Within the Rijkswaterstaat asset management strategy ProBO (Probabilistic Operations and Maintenance), quantitative reliability models are used to demonstrate compliance with legal safety standards of storm surge barriers and to support risk-based operations and maintenance (O&M) decisions. However, asset managers have raised concerns regarding the accuracy of current models and have experienced difficulties when addressing exploratory questions related to maintenance strategies and system changes.

To address these concerns, the internal review and support department for ProBO within Rijkswaterstaat (CSK) proposed an alternative reliability modelling process specifically intended for the development of new reliability models for complex storm surge barriers. In this process, modelling choices are explicitly guided by predefined model values, intended objectives, and progressive refinement. For this research, the selected model values were accuracy, usability, and transparency.

The suitability of this modelling process was investigated through a case study of the Haringvliet Sluices. A conceptual reliability model was developed and verified through expert sessions. The model itself was not the primary objective of the research, but rather served as a means to evaluate the modelling process in practice.

The results demonstrate that explicitly guiding modelling choices through predefined model values and intended objectives can support the development of reliability models that are better aligned with the needs of asset managers. Experts considered the resulting conceptual model capable of achieving its intended objectives. In addition, the iterative refinement approach, in which the model was progressively developed from a coarse to a more detailed level, proved valuable in maintaining transparency and supporting well-founded modelling decisions throughout the process. At the same time, the study shows that inherent trade-offs between the selected model values remain unavoidable.

The research further highlights the importance of extending the modelling process with a governance structure that supports both the reliability model and its users. This includes clearly defining modelling objectives, maintaining iterative review sessions between model developers and a predefined review body of stakeholders, and safeguarding model quality during the operational phase through disciplined documentation procedures.

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The Ramspol Barrier, an inflatable storm surge barrier, is an integral component of the flood protection system for the Zwarte Meer and its hinterland. However, its current operational procedure faces several challenges that compromise flood safety, cause disruptions for the shipping industry, and significantly burden the operation team. The current operational procedure activates specific steps based on predefined water level thresholds, often mobilizing the operation team and disrupting shipping without ultimately leading to a barrier closure. It is also questioned whether closures are always the best strategy for optimizing flood safety in the system.

This thesis aims to improve the operational procedure of the Ramspol Barrier by balancing flood safety, operational burden, and disruptions to vessel navigation.

A comprehensive system and data analysis were conducted to understand the dynamics of high-water events, which are primarily driven by onshore winds, precipitation, and wind-induced water setups. These conditions, coupled with restricted drainage at the Afsluitdijk and increased discharges from the ZwarteWater and IJssel, result in rapid water level rises at the barrier. The current operation protocol triggers closure at +0.50 m NAP and an inland flow, protecting the Vecht Delta. However, this blocks outflow, delays vessel movements, and causes water accumulation in the Zwarte Meer. The analysis further revealed a strong correlation between higher water levels and wind setup. In comparison, the impact of discharges from the IJssel and ZwarteWater on higher water levels was minimal. The findings also highlighted the growing strain on the operation team and the disruptions faced by the shipping industry over recent years.

Nine scenarios were simulated using a developed reservoir model to assess the system’s sensitivity to wind setups and discharges, the effect of adjustments to the operation procedure, and the model’s predictive capabilities. The simulations revealed that closures during receding or stagnating water levels, or when wind setup was already developed, sometimes resulted in higher water levels than non-closure scenarios. In cases where water levels were between +0.40 m NAP and +0.50 m NAP, navigation could often be maintained, provided wind setups were reducing or stagnating, and no significant discharge peaks were predicted. Moreover, minor wind events at initial water levels above +0.50m NAP frequently triggered unnecessary closures, which could elevate water levels, highlighting the need for more robust closure criteria based on sustained flow rather than momentary fluctuations. When multiple peaks
occurred, the timing and proximity of these peaks played a crucial role in determining the impact on water levels. Earlier closures reduced water levels but extended operational disruptions, while higher discharges led to faster water level rises post-closure, requiring earlier openings. Lastly, enhancing the ability to predict critical water levels and closure and opening criteria can significantly benefit both operational teams and the shipping industry. Implementing 24- to 48-hour forecasts would enhance planning by ensuring teams are on-site when needed, minimizing unnecessary disruptions, predicting the timing and likelihood of Ramspol Barrier closures, and enabling the shipping industry to adjust schedules to reduce waiting times.

The results indicate that an adaptive, forecast-driven approach to barrier operation could potentially improve flood protection, reduce disruptions for the shipping industry, and alleviate the team’s operational burden. ... Read more

This report studies the ability of flood defences to reduce the risk of flooding. Storm surge barriers are a specific type of flood defence. Their function is to reduce the hydraulic loads, the water levels and the wave conditions on the dikes located behind the storm surge barrier. Safety standards for flood defences are established to ensure a minimum level of safety, accounting for the performance of flood defences, which is determined by their probability of failure.
The applicable requirements for storm surge barriers have developed over time. Various assumptions have been made in deriving these requirements, because the applicable standards for flood defences and the corresponding methods to assess their safety have developed. Specifically, assumptions have been made regarding the influence of storm surge barrier performance on the failure probability of dikes. The objective of this study is to build a framework that includes this relationship in a requirement for storm surge barriers.
The framework is built by analysing the relationships that define the flood protection system. It is considered that a reduction in storm surge barrier performance increases the water levels in front of dikes, which increases the dike failure probability and the risk of flooding. The framework built in this study calculates a requirement for structural failure of storm surge barriers based on an economic optimisation at flood protection system level. The dike failure probability is calculated by expressing storm surge barrier performance in terms of water levels in front of the dike and by representing the resistance of a dike to a certain water level using fragility curves. In the next step, investment functions are used to translate failure probabilities into costs to determine the minimum, and therefore optimum, costs at flood protection system level.
The framework is first applied in a generic context to outline the steps and potential applications of the framework. Second, the framework is applied to a schematised representation of the Eastern Scheldt, focusing on the case-specific aspects. The framework is applied to identify the criteria that should be considered in safety standards for storm surge barriers. A requirement for storm surge barriers considers the dike failure probability as a function of storm surge barrier performance. The maximum allowed dike failure probability relates to the maximum allowed consequences of flooding. Furthermore, the variation in dike investment costs compared to the variation in storm surge barrier investment costs is important to derive a requirement that corresponds to an economically optimum flood protection system.


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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. ... Read more

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. ... Read more

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.
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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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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... ... Read more

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.
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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.
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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.
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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.  ... Read more

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. ... Read more