As ongoing erosion in the Waal is causing multiple challenges for the Netherlands, the Room for the River 2.0 programme is looking at solutions to halt this erosion. One of the proposed solutions uses Longitudinal Training Walls (LTWs) to create a multi-channel system that reduces flow velocities while maintaining sufficient water levels in the main navigation channel. An LTW can narrow the main channel during low flow, but widen the total system during high flow. In 2015, three LTWs were constructed in the Waal as part of a pilot for potential application along the rest of the river. This research focuses on the one near Wamel.

The auxiliary bank channel near Wamel showed rapid sedimentation in its deeper sections during the first years. This appears to be linked to an initial response and bank erosion, rather than indicating an unstable system progressively filling up. After the inlet sill was heightened, the channel shifted to erosion over large parts of its reach. The remaining deposition along the stretch where the bank retreated seems to be slowing down as the bank stabilises.

Against the steep slope of the inlet sill and wall, gravity redirects near-bed flow and sediment back downslope towards the deeper main channel, forming a vortex with flow spiralling against the slope, reducing sediment transport into the auxiliary channel. Large-scale two-dimensional models are not able to explicitly resolve this local vertical vortex that plays a key part in the sediment distribution between the main and auxiliary channel and therefore in the stability of the LTW system.

Research into LTWs has grown considerably, but questions remain regarding the morphological stability of the auxiliary bank channel and how to schematise LTWs reliably in the large-scale coarse grids typically used for long-term river morphodynamic modelling. This research investigated different schematisation methods for LTWs in the coarse grid of the DVR model using ten-year simulations of the Waal from 2016 to 2025. A refined model, based on the same formulations and parameters as the DVR model, was developed and used as a benchmark to evaluate the coarse-grid schematisation variants. Representing the wall and inlet sill solely as a 2D weir with a small increase in bed level, as currently done in the DVR model's Baseline schematisation, proved insufficient, since this does not form a physical barrier able to block and redirect sediment once overtopped.
Incorporating the structure directly into the bathymetry instead allows the model's transverse bed slope adjustment to act on the slope of the structure, activating the deflection needed to redirect sediment back into the main channel.

Even with the structure incorporated into the bathymetry, the coarse model struggled to reproduce the lower transport rates into the auxiliary bank channel seen in the refined model, for two reasons. First, the wide grid cells fail to represent the steep slope of the wall and inlet sill, weakening the gravitational deflection. Second, the structure was placed too far into the main channel, misaligning with the upstream bank, allowing longitudinal inflow that degraded the side-weir entrance geometry, and further increased the sediment transport into the auxiliary bank channel.

Two ways of compensating for the missing slope were tested: increasing the strength of the transverse bed slope deflection based on the Koch & Flokstra formulation, and locally reducing the transport magnitude through a calibration factor in the Meyer-Peter–Müller transport formula. However, both resulted in further problems in the main channel. The increased Koch & Flokstra deflection required to match the transport rates observed in the refined model or to correct the missing factor of local steep slopes unrealistically flattened the point bars in the main channel bends. Reducing the transport magnitude instead of the direction resulted in deposition in front of the inlet and an increase in transport rates again over time.

This research recommends incorporating LTW structures directly into the bathymetry, with careful attention to grid alignment relative to the crest and the effect of misrepresenting steep slopes. Local grid refinement, or a sub-grid formulation for the sediment deflection along a steep-sloped longitudinal structure, offers a future solution for representing the local geometry and the sediment distribution between the two channels.As ongoing erosion in the Waal is causing multiple challenges for the Netherlands, the Room for the River 2.0 programme is looking at solutions to halt this erosion. One of the proposed solutions uses Longitudinal Training Walls (LTWs) to create a multi-channel system that reduces flow velocities while maintaining sufficient water levels in the main navigation channel. An LTW can narrow the main channel during low flow, but widen the total system during high flow. In 2015, three LTWs were constructed in the Waal as part of a pilot for potential application along the rest of the river. This research focuses on the one near Wamel.

The auxiliary bank channel near Wamel showed rapid sedimentation in its deeper sections during the first years. This appears to be linked to an initial response and bank erosion, rather than indicating an unstable system progressively filling up. After the inlet sill was heightened, the channel shifted to erosion over large parts of its reach. The remaining deposition along the stretch where the bank retreated seems to be slowing down as the bank stabilises.

Against the steep slope of the inlet sill and wall, gravity redirects near-bed flow and sediment back downslope towards the deeper main channel, forming a vortex with flow spiralling against the slope, reducing sediment transport into the auxiliary channel. Large-scale two-dimensional models are not able to explicitly resolve this local vertical vortex that plays a key part in the sediment distribution between the main and auxiliary channel and therefore in the stability of the LTW system.

Research into LTWs has grown considerably, but questions remain regarding the morphological stability of the auxiliary bank channel and how to schematise LTWs reliably in the large-scale coarse grids typically used for long-term river morphodynamic modelling. This research investigated different schematisation methods for LTWs in the coarse grid of the DVR model using ten-year simulations of the Waal from 2016 to 2025. A refined model, based on the same formulations and parameters as the DVR model, was developed and used as a benchmark to evaluate the coarse-grid schematisation variants. Representing the wall and inlet sill solely as a 2D weir with a small increase in bed level, as currently done in the DVR model's Baseline schematisation, proved insufficient, since this does not form a physical barrier able to block and redirect sediment once overtopped.
Incorporating the structure directly into the bathymetry instead allows the model's transverse bed slope adjustment to act on the slope of the structure, activating the deflection needed to redirect sediment back into the main channel.

Even with the structure incorporated into the bathymetry, the coarse model struggled to reproduce the lower transport rates into the auxiliary bank channel seen in the refined model, for two reasons. First, the wide grid cells fail to represent the steep slope of the wall and inlet sill, weakening the gravitational deflection. Second, the structure was placed too far into the main channel, misaligning with the upstream bank, allowing longitudinal inflow that degraded the side-weir entrance geometry, and further increased the sediment transport into the auxiliary bank channel.

Two ways of compensating for the missing slope were tested: increasing the strength of the transverse bed slope deflection based on the Koch & Flokstra formulation, and locally reducing the transport magnitude through a calibration factor in the Meyer-Peter–Müller transport formula. However, both resulted in further problems in the main channel. The increased Koch & Flokstra deflection required to match the transport rates observed in the refined model or to correct the missing factor of local steep slopes unrealistically flattened the point bars in the main channel bends. Reducing the transport magnitude instead of the direction resulted in deposition in front of the inlet and an increase in transport rates again over time.

This research recommends incorporating LTW structures directly into the bathymetry, with careful attention to grid alignment relative to the crest and the effect of misrepresenting steep slopes. Local grid refinement, or a sub-grid formulation for the sediment deflection along a steep-sloped longitudinal structure, offers a future solution for representing the local geometry and the sediment distribution between the two channels. ... Read more

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

Coastal dunes play a crucial role in protecting sandy coastlines while supporting valuable ecological functions. Their development reflects interactions between wind-driven sediment transport, marine hydrodynamics, and vegetation. In particular, early-stage dune vegetation of marram grass, strongly influences whether bare sand surfaces evolve into patchy embryonic dunes and, ultimately, continuous foredune ridges. Despite its importance, vegetation is often represented in coastal models in a highly simplified or prescribed manner, limiting the ability to reproduce the spatially heterogeneous and stochastic nature of early dune development observed in reality.

This thesis investigates how the dominant environmental drivers of embryonic dune vegetation can be translated into a reduced-complexity modelling framework that reproduces observed development tendencies at coastal-system scales. The focus is behavioural realism rather than detailed plant physiology, capturing when and where vegetation establishes, persists, densifies, or is destroyed as a result of interactions between physical forcing and feedbacks.

A probabilistic vegetation model is developed in which patterns emerge from the balance between establishment and decay. Three controls govern this balance: limitation by water-level exposure, morphodynamic suitability based on burial and erosion, and spreading from existing vegetation. These controls jointly determine establishment and survival probabilities, allowing vegetation to arise dynamically rather than being imposed. The model is implemented in a one-way forced configuration using spatial bed-level change fields and a representative marine water-level signal extracted from an existing coupled marine–aeolian morphodynamic simulation. Hence, vegetation, responds to, but does not modify, the hydrodynamic, aeolian, or morphodynamic forcing.

The model is applied to the Sand Engine mega-nourishment in the Netherlands, which provides a unique natural laboratory due to its unprecedented scale, initially bare sandy surface, rapid morphological evolution, and extensive research and monitoring record. Model results are evaluated at multiple levels, including domain-scale development trends, sub-area specific indicators derived from observations both in time and space, and process-based diagnostics using selected points of interest. This multi-level evaluation allows assessment of both emergent vegetation patterns and the underlying mechanisms driving them.

The results indicate that the model is able to reproduce several characteristics of embryonic dune vegetation development observed at the Sand Engine. Vegetation expansion occurs within realistic spatial and temporal scales, exhibits asymptotic mean densification behaviour, and responds coherently to geographical steering by features such as the dune lake. The analysis highlights the importance of stochastic establishment, facilitation through nearby vegetation, and spatially variable bed-level change in generating realistic heterogeneity. Limitations include sensitivity to water-level thresholds in flat nearshore areas, overestimation of vegetation merging rates in some locations, and constraints imposed by spatial resolution and one-way forcing.

These findings demonstrate that simplified behavioural representations can capture essential dune–vegetation dynamics when dominant environmental controls and stochasticity are included explicitly. The framework is suited for exploratory and comparative studies of dune development and for scenario-based assessments in coastal management. Recommendations are made for future integration of the vegetation model into fully coupled marine-aeolian models, where vegetation feedbacks on sediment transport and dune initiation can be explicitly represented, enabling more comprehensive simulation of dune–vegetation co-evolution. ... Read more

The Pannerdense Kop is a bifurcation point where the Bovenrijn splits into the Waal and the Pannerden Canal. In recent decades, the discharge partitioning changed to more discharge in the Waal at the expense of the Pannerden Canal, which negatively impacts navigation, flood safety and freshwater availability in the downstream river area. This recent change of the river system is linked to changes in bed level that may have started as a result of peak flows in the 1990s. An overview of peak flow impacts on the bed is missing whilst peak flows will occur more frequent and increase in magnitude due to climate change.

This thesis investigates the morphological response of peak flows on the river bed at the Pannerdense Kop, using field measurements and numerical models. Five sources of field data are used, all based on bed level measurements. A 1D Sobek model and a 2D Delft3D model are analysed to determine the peak flow response in these existing morphological models. The model results are compared with the field data.

Results indicate that peak flows seem to cause deposition at the upstream end of the Waal and to a lesser extent at the Pannerden Canal. Additionally, several patches of erosion and deposition over peak flows are related to floodplain outflows, groyne fields, and bends which are not captured in the 1D model because of width smoothing. Changes on this scale are captured in the 2D model although improvement is possible. On a smaller scale, results indicate that river dunes are present during peak flows and disappear in the weeks after the peak. These small-scale changes are not in the models as a result of the grid size exceeding the dune length. One source of field data shows that peak flows may also lead to a large-scale erosion adjustment wave in the Waal, although further research is required to determine whether such large-scale morphological changes occur, to characterize the nature of these changes, and to assess the extent to which they are represented in existing numerical models. These insights improve the understanding on the morphological response to peak flows at the Pannerdense Kop, and the possibilities and limitations of current morphological models.
... Read more

In times when the Dutch coastline faces pressure from coastal erosion due to the growing challenges of sea-level rise, sustainable coastal policies are essential. Increasingly large and more voluminous maintenance works are required. New sand nourishment strategies, such as mega-nourishments, are being tested along the Dutch coast. Due to the enormous quantities of sand, local safety standards increase. One example of such a project is the Hondsbossche Dunes (HBD). Although its primary function is the realization of local flood safety near the former sea dike, it also feeds nearby coast and dunes caused by natural forces like waves, currents, tides, and wind. Understanding rates and quantities of sediment dispersion to see how coastal state indicator evolve (e.g. coastal volume, beach width and depth contours) is crucial for policymaker. They must maintain the coastline as its main function of flood defense for the future, while also accommodating recreation and nature.

Literature demonstrates that one-line alongshore transport models can predict general coastline evolution trends at mega-nourishment sites well. However, their accuracy at other coastal state indicators can be limited due to, for example, the exclusion of cross-shore sediment redistribution. Recent efforts have focused on coupling such a one-line transport model (ShorelineS) with a new cross-shore redistribution model (Crocodile). Compared to other model types, the advantage of this coupling is its ability to calculate large spatial and temporal scales with minimal computational time. Additionally, the coupled model now accounts for a depth-dependent component by including this Crocodile model for the redistribution of sediment in cross-shore direction.

A data analysis of the coastal evolution at the HBD for the first 9 years was carried out. It shows that most volumetric changes occur in the initial 3 years. Furthermore, the data reveal that cross-shore activity varies over the depth. This becomes most evident from quantitative differences in volumetric changes for different vertical coastal cells (e.g. dune, beach, surfzone, shoreface). Also the differences in their correlation to the shoreline change suggest depth-dependent activity. For the models, this implies that the coupled model is likely to represent the magnitude and behavior of the volume (cells) and depth contours better.

The model comparison consist of the ShorelineS model, and ShorelineS+Crocodile (coupled model). An existing HBD ShorelineS model is extended with 4 years from 2020 to 2024, and includes a dune module, which enables the models to simulate the dunefoot and therefore provide insight in the beachwidth indicator. For the coupled model, Crocodile is used as a cross-shore module within ShorelineS that is configured with a dynamic equilibrium, and a alongshore varying and curved equilibrium grid, for its best performing run (SC5).

The results of the study suggest that the SC5 simulation improves ShorelineS on the volumetric evolution indicator; it halves the error to 250 m/m3 in 3 years time and it followings the temporal trend at the erosional zones very well. Additionally it adds new value and insight with the development of the cross-shore profile at every timestep. Although the magnitude is not yet always accurate, the SC5 simulation is able to represent independent evolution of depth contours. Despite its shown potential, the model results are constrained due to significant instabilities that appear in the model. These instabilities stem from a growing gradient-the difference in slope of the initial and equilibrium profile-which is posed by a bad translation of the equilibrium grid to a profile.

This research therefore recommends first and mostly to improve and solve the spatial and temporal stability as described above. This is necessary to make long-term predictions and contribute to future coastal maintenance strategies in the face of rising sea levels. Other relevant recommendations include creating a more realistic diffusion coefficient in Crocodile that balances subaerial and subaqueous changes better, integrating subaerial volume changes of Crocodile with ShorelineS' dunemodule in order to make a comprehensive comparison, and finally including location specific nourishments via Crocodile module and consequentially account for profile adjustment. ... Read more

The Lagos State coastline is increasingly vulnerable to erosion, driven by both a dominant eastward longshore sediment transport and ongoing human interventions. Climate change, through rising sea levels and a changing wave climate, is expected to further influence shoreline dynamics in the coming decades. Understanding how these factors interact is crucial for supporting long-term coastal management along this coastline of major economic and social significance. This study investigates the impact of climate change on shoreline evolution over a 50-year timescale, focusing on sea level rise, changes in significant wave height, and changes in wave direction. The onedimensional shoreline evolution model ShorelineS was applied and calibrated using satellite derived historical shoreline data. Simulations were performed for a range of climate scenarios, allowing for a separate evaluation of the individual contributions of sea level rise, changing wave height, and changing wave direction to coastal erosion and accretion patterns. Longshore sediment transport is projected to increase by up to 20% by 2075 under extreme RCP8.5 scenarios. This intensification directly drives changes in shoreline dynamics, leading to amplified erosion in some areas and enhanced accretion in others. Projected shoreline retreat can reach up to 700 meters locally under baseline conditions (i.e., without climate change) by 2075. Climate change can further amplify these patterns, with shoreline retreat increasing by up to 10% in erosion-prone areas near coastal structures. In contrast, accretive zones show a more moderated response due to counteracting effects between sea level rise and wave-driven accretion. Wave direction changes were found to have the strongest localized impact on shoreline retreat near coastal structures, while the influence of wave height changes was less consistent and more variable along the coast. Sea level rise causes relatively uniform retreat along the entire coastline. The results provide spatially explicit projections of shoreline retreat and accretion, offering insights into areas most at risk. These findings support the development of targeted, adaptive coastal management strategies for Lagos State in the face of a changing climate. ... Read more

A possible way of generating a stable and predictable base load of renewable energy, supplementing the unpredictable wind and solar energy, is dynamic tidal power (DTP). DTP is generated with the use of large dam, which is built in the sea, under an angle with the propagating tidal wave. A dam of sufficient length can create a phase difference between the propagating tidal waves on either side of the dam, creating head differences over the dam, which can be used to generate renewable energy using large turbines in the dam.
Previous studies considered a DTP dam built perpendicular to the coast and the direction of the propagating tidal wave. However, a structure of this size extending into the North Sea interferes with other functions of the area such as shipping. Instead of a perpendicular dam, an oblique dam off the Dutch delta coast has been proposed to reduce this interference. This design has the additional benefits that it could increase the coastal safety of the surrounding area and potentially increase the sediment budget of the Voordelta area in front of the coast.

In this study, a first-order assessment of an oblique DTP dam has been carried out. This assessment focuses on three aspects: the expected energy yield and how this compares to the expected energy yield of a perpendicular dam under the same conditions; the impact this dam has on the coastal safety of the surrounding area. To this end, an analysis of the change in hydrodynamic and morphological processes in the Voordelta as a result of the construction of such a dam has been carried out.

The propagation of the tidal wave around an oblique DTP dam in the North Sea has been modelled using the FINEL modeling software, applied as a two-dimensional flow model. The reference layout of the DTP dam has been determined beforehand based on several requirements concerning the location and length. Using a turbine module integrated into FINEL, the discharge through each turbine has been determined, base on which the energy output of the DTP dam has been calculated. A comparison was made of the energy output of the oblique dam and that of a perpendicular dam, as used in previous studies. To do this, the same model has been run with a dam positioned perpendicular to the Dutch coast.

A DTP dam with a length of 62.5 km and a southwest orientation starting at the Maasvlakte 2 was found to have a maximum power output in the order of 8 * 10^2 MW and a yearly generated energy yield in the order of 2 TWh. Both of these values were approximately a factor five lower than the power output of the perpendicular dam with the same length also starting at Maasvlakte 2. This is because, in the case of the perpendicular dam, a phase difference in the tidal wave over the dam was created, creating a large head difference over the dam. In the case of the oblique dam, the phase difference was lower, significantly reducing the water head over the dam. As opposed to the perpendicular dam, the head difference was created as a result of amplification of the tidal wave within the estuary.

Subsequently, a SWAN wave model has been coupled to the FINEL flow model. In this coupled model, a design storm with a return period of 10000 years has been simulated. Of this storm, the wave characteristics and water level in the area were compared between the scenarios with and without oblique dam. A large decrease in both significant wave height and peak period behind the dam was found across nearly the whole Voordelta area, with the largest decrease offshore immediately behind the dam. However, the significant wave height increased on the outside of the dam and near Westkapelle. The same result was found for scenarios where 25 cm and 80 cm mean sea level rise has been applied. This is because the incoming waves from a northwestern direction are blocked by the dam, resulting in locally wind-generated waves behind the dam becoming dominant in the area. On the offshore side of the dam, significant wave heights are increased as a result of reflection of incoming waves. The maximum water level during the storm decreases in the nearshore area.

As the area behind the dam is transformed into an area resembling an estuary, the dominant hydrodynamic and, consequently, the morphodynamic processes are changed. The dam blocks all incoming waves from the west and northwest, which are the dominant wave directions in the area. Because of this, the wave energy within the area decreases significantly under storm conditions. At the same time, the tidal amplitude is expected to be amplified within the area. These changes cause the area to become more tide-dominated. As a result, a decrease in the onshore directed sediment transport into the area is expected. The entrance of the estuary is expected to become more flood-dominant, as a result of which more inflow of sediment into the created estuary is expected. Dominant sediment transport mechanisms with the estuary are also changed, but for a more accurate view of the extent to which this happens, a complete morphological model is necessary, which has not been applied in this study.

In summary, a DTP dam oblique to the Dutch coast can provide a substantial amount of energy, although it is significantly less when compared to a perpendicular dam and to previous studies. A further advantage of this dam is that the area behind is sheltered against storm wave conditions and extreme water levels, thus increasing the coastal safety. It is foreseen that the area attracts more sediment by the creation of the dam, but this needs to be confirmed by a future morphological model assessment. ... Read more