As part of the Mangrove Living Lab project, this research examines how multiple drivers influence coastal erosion in Bạc Liêu, Vietnam. Currently, observed erosion in this region cannot be explained by natural sediment redistribution, and literature suggests that tidal flows are more cross-shore dominated in Bạc Liêu compared to adjacent provinces. Consequently, four key anthropogenic drivers that primarily act in the cross-shore direction are explored: (1) land subsidence due to increased groundwater extraction, (2) reduced sediment availability, (3) foreshore degradation leading to higher waves, both induced by fluvial sediment deficit, and (4) the construction of sea dikes and fishpond dams in the intertidal zone, resulting in tidal flow restriction, wave reflection and coastal squeeze.
Two main methods are applied: a cross-shore elevation level analysis to assess spatial differences in subsidence and sediment supply, and a numerical model to assess the influence of each driver individually and combined. The elevation analysis revealed that subsidence varies spatially along the coast. Moreover, historic satellite images show that a site without subsidence has been accreting in the past, while a site experiencing subsidence remained stable during the same period. After 2004, they both began eroding. This suggests that differences in subsidence rates largely explain spatial variations in current coastline positions, while the erosion itself is likely driven by additional drivers. However, the limited spatial and temporal coverage of elevation transects highlights the need for more extensive data collection. Numerical modeling indicates that reduced wave heights and increased suspended sediment greatly reduce erosion rates, and can even overshadow the effect of subsidence when combined. This demonstrates that historic and ongoing fluvial deficit is likely the main driver of the current erosion. The role of intertidal structures remains inconclusive due to model limitations and differing theoretical interpretations. Refining the current model or developing more advanced alternatives will help improve understanding of these coastal erosion processes. This is needed to support the development of integrated solutions that protect both the communities and ecosystems of Bạc Liêu. ... Read more

Tidal flats play a critical role in coastal systems. They serve as essential buffers that protect inland communities from flooding by storm surges, contribute significantly to the maintenance of coastal biodiversity, and function as crucial sites for carbon sequestration. Moreover, they are indispensable for land reclamation projects, supporting economic activities such as agriculture and aquaculture. However, rapid coastal economic development and assertive human interventions have intensified conflicts of interest between socio-economic demands and the preservation of these fragile ecosystems. This increasing tension underscores the necessity for a detailed understanding of tidal flat evolution and the complex interplay between natural processes and anthropogenic activities.

This research aims to deepen our understanding of unvegetated tidal flat morphodynamics by examining both natural processes and the impacts of human interventions. The insights gained will provide valuable theoretical guidance for the sustainable and effective management of tidal flat resources in the future. To achieve this, the study focuses on tidal flats along the Jiangsu Coast in China as a case study. Because it is a typical example famous for its extensive and diverse tidal flat systems. At the same time, it has a long history of coastal land reclamations. A multi-faceted approach is adopted, combining field dataset analysis, a process-based Delft3D model, and a hybrid DET-ESTMORF model, which together provide a robust foundation for theoretical insights in sustainable coastal management.

Based on a unique field measurement data along the Jiangsu Coast, the morphology and sediment characteristics of the unvegetated intertidal flats along this coast are analysed. Both cross-shore and alongshore variations are observed. In the cross-shore direction, sediments exhibit a pronounced coarsening from the landward to the seaward side, indicating tide-dominated forcing. In the alongshore direction, the coast is divided into two parts depending on its morphological state. The northern coast, which is predominantly eroding, exhibits steeper intertidal slopes and a complex mix of sediment types, ranging from extremely fine to coarse deposits. The variation in sediment composition is attributed to natural processes such as self-weight consolidation and surface armouring. In the accreting southern coast, while tidal flat slopes are generally becoming milder towards the south, the corresponding bed surface sediment grain size is becoming coarser southward. This relationship between slope and sediment grain size in the southern coast is opposite to the findings on other muddy tidal flats.

In order to investigate the mechanism behind the region-specific alongshore pattern of the southern Jiangsu Coast, we formulate a conceptual model with special focus on describing the alongshore variations in hydrodynamics and shoreline evolution. Subsequently, a highly schematized Delft3D numerical model is employed in diagnostic mode. By integrating these approaches, the sediment provenance is found to be the sole factor capable of explaining the observed pattern.

Apart from natural processes, anthropogenic activities can also induce significant changes in tidal flat morphology. Available information on the tidal flat morphological responses to human interventions is limited based on the single-time measurements data. We therefore evaluate the resilience of tidal flats to anthropogenic disturbances (specifically, upper flat enclosure reclamation) using an extended DET-ESTMORF model. A series of sensitivity tests under various environmental conditions is conducted with this model. Our findings indicate that the concave or convex shape of tidal flat profiles is primarily governed by dominant hydrodynamic forces (whether tidal or wave-driven), while other variables modulate the profile slope. For a tidal flat to revert to its pre-reclamation configuration, it must exhibit continuous seaward progradation under natural conditions. Such restoration typically occurs in environments with abundant sediment supply, subdued wave activity, and the absence of cross-shore constraints imposed by shore-parallel tidal channels.

Overall, the insights derived from this study enhance our understanding of the interactions between natural hydrodynamic processes, sediment dynamics, morphology change and human-induced alterations. The findings highlight the importance of continuous monitoring and the adoption of adaptive management strategies to ensure that coastal development preserves the ecological integrity and long-term stability of tidal flat systems.
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Salt marshes are complex and highly productive ecosystems in coastal areas. They play crucial roles in providing habitats for diverse species, sequestering carbon, and serving as natural buffers against storms. However, salt marshes have been threatened by human activities (e.g. reduced sediment supply) and climate change (e.g. sea-level rise), leading to a potential loss. This loss of salt marshes highlights the importance of an influx of sediment to salt marshes for maintaining their structure and resilience. Mudflats, located adjacent to salt marshes, facilitate sediment transport to these marshes under certain conditions. Therefore, advancing our understanding of sediment transport between mudflats and salt marshes is important, as it can provide valuable insights into effective salt marsh management.

This research aims to unravel the varying sediment transport processes between mudflats and salt marshes under different hydrodynamic and sediment dynamic conditions. Chongming Saltmarsh from the Yangtze Estuary and Paulina Saltmarsh from the Western Scheldt Estuary have been selected as study cases. The distinct differences in hydrodynamic forcing and sediment availability between these two estuaries contribute to differing environments and states of their intertidal systems. These differences enable us to compare the sediment transport processes across divergent systems and explore the mechanisms governing the long-term evolution of salt marshes.

Saltmarsh creeks are recognized as efficient conduits that actively facilitate the exchange of water and sediment between mudflats and salt marshes. To identify the role of marsh creeks in sediment transport between two different intertidal systems, the sediment transport processes in a main creek and on the adjacent mudflat in Chongming Saltmarsh (China) and Paulina Saltmarsh (the Netherlands) have been investigated (Chapter 2). Our findings revealed notable differences and common patterns in sediment transport between the two systems. In Chongming, SSC exhibited significant asymmetry between flood and ebb tides, with large SSC peaks occurring during most flood periods. This asymmetry in SSC caused the marsh creek in Chongming to function as a conduit for sediment import. Furthermore, distinct overbank and underbank tides were observed in Chongming. During underbank tides, sediment was trapped and retained within the creeks, only to be eroded and transported to the marsh during subsequent overbank tides. Additionally, the mudflats in Chongming showed a relatively rapid recovery after erosion events. These mechanisms were not observed in Paulina Saltmarsh, where a net export of sediment through the marsh creek was recorded during calm weather. In both systems, the SSC in marsh creeks showed a slight increase due to local erosion of the creek bed but responded more significantly to the erosion of mudflats, indicating that the main sediment sources of the high SSC result from the sediment advection rather than local erosion. These comparative findings suggest that the role of marsh creeks in sediment import and export is closely linked to the availability of sediment from adjacent mudflats, highlighting the importance of mudflats for the growth of salt marshes.

After recognizing the role of main creeks in sediment transport within turbid systems, the role of creek tributaries in sediment delivery still remains poorly understood. Therefore, field measurements were conducted in a main creek and in a secondary creek within Chongming Saltmarsh. These measurements revealed the dual roles of saltmarsh creek systems in drainage and sediment transport, as well as the mechanisms driving residual sediment flux within saltmarsh creeks (Chapter 3). The results indicated that the main creek played a dominant role in sediment delivery, while the secondary creek, influenced by the presence of vegetation, was more effective as a drainage conduit and contributes less to sediment transport. Additionally, the direction and magnitude of residual sediment flux are influenced by the relative importance of asymmetries in net discharge and sediment concentration. Overbank tides primarily result in an ebb-dominant flow asymmetry, which tends to drive sediment export along with the net outflow. However, the abundance of sediment during flood tides can occasionally counteract this export tendency, mitigating the impact of flow asymmetry on sediment export.

Sediment can be imported from mudflats to salt marshes through marsh creeks and marsh edges. To address how varying tidal and wave conditions affect sediment transport within marsh creeks and over marsh edges (Chapter 4), a two-month field campaign was conducted in Paulina Saltmarsh. Field data revealed that tidal ranges determine the direction of residual sediment flux in the marsh creek, while wave intensity determines its magnitude. Conversely, wave intensity determines the direction of residual sediment flux over the marsh edge, whereas tidal ranges determine the magnitude. Specifically, sediment was imported through the marsh creek during tidal cycles with small tidal ranges and strong waves, whereas sediment was imported through the marsh edge during tidal cycles with large tidal ranges and weak waves. These findings offer deeper insights into sediment transport through marsh creeks and marsh edges under different tidal and wave conditions, which is crucial for effective salt marsh management.

This dissertation explores sediment transport between mudflats and salt marshes in two different systems, providing insights into the roles of marsh creeks and marsh edges in facilitating or impeding sediment import to salt marshes under varying conditions. The findings offer guidance for developing conservation and management strategies to support salt marsh growth in response to decreasing sediment supply and accelerating sea-level rise. ... Read more

Lake Bardawil is a hypersaline shallow micro-tidal double-inlet lagoon on the northern coast of the Sinai Peninsula in Egypt. The inlets, named Boughaz 1 and Boughaz 2, connect Lake Bardawil to the Mediterranean Sea. The local population relies on the fishing yields from the lagoon, however, in the current situation, the lagoon’s inlets are unstable, needing constant maintenance dredging to stay open. Additionally, due to the hypersaline situation (limiting fish attraction) and high fish catch demand, the current fishing situation is unsustainable. The Weather Makers propose a solution to the problem using dredging interventions. Enlarging the inlets of Lake Bardawil is expected to result in increased fish migration, decreased salinity in the lagoon and increased stability of the inlets. This research aims to investigate the short-term response of Lake Bardawil and specifically its inlets to the dredging interventions proposed. To investigate the response to the interventions, two numerical models are used; a hydrodynamic model in D-flow FM and a coupled morphological model using D-flow FM and D-waves. The model results are analysed in terms of the hydrodynamic tidal response, combined hydrodynamic response to tide and weather and finally the morphological response to tidal and weather effects. From the hydrodynamic model, it can be concluded that the interventions result in a larger tidal prism for the lagoon by a factor of 1.6. This is expected to result in increased fish migration and reduced salinity in the lagoon. As the tidal prism increases, so does the discharge through the inlets. The increased discharge through the inlets results in inlet velocity amplitudes that exceed the theoretical critical value for convergence to a dynamically stable inlet cross-section for both inlets. The interventions affect more than just tidal currents in Lake Bardawil. Weather effects in the lagoon are amplified by the interventions. This is demonstrated by increased interaction between the inlets in terms of net import and export of water. During winter storms, the wind-induced flow from Boughaz 1 to Boughaz 2 is greater in magnitude. The wind waves caused by these winter storms result in sediment resuspension. This results in larger sediment transport from the basin to Boughaz 2. Using the coupled model, the morphological response in both inlets can be predicted. In Boughaz 1, the present sediment-importing character is amplified. Whereas the similar summertime sediment-importing character of Boughaz 2 is reduced substantially. The sediment export of Boughaz 2 during winter is larger in the new situation. The interventions result in increased sedimentation in the dredged channel of Boughaz 1. This does not hinder the increased tidal discharge at Boughaz 1, which is the main intended function of the interventions. This means the increased sedimentation does not influence the intended functioning of the interventions in the short term. Due to the increased sedimentation in the inner dredged channel, there is no indication of increased stability in Boughaz 1. Boughaz 2 shows less morphological activity in the situation after interventions. As is the case with Boughaz 1, there is no hindrance to the functioning of the interventions during the short simulation period. There is reduced deposition of sediment in the inner flood delta region of the inlet and the updrift side of the inlet channel compared to the known unstable situation during both winter and summer simulations. This indicates improved stability for Boughaz 2. The short-term response to the interventions in the inlets of the Lake Bardawil system is as intended, the tidal prism is increased and while there is only an indication of increased stability in Boughaz 2, there are no short-term impediments to the intended primary functioning of the interventions in either of the inlets. ... Read more

A rise in the global mean temperature induced by climate change is expected to have a large impact on ecosystems in all regions of the world. One of the threats is accelerated sea level rise (SLR). This may induce the loss of intertidal areas in tidal inlet systems. The long-term morphological response of tidal inlet systems can be modelled using reduced complexity model ASMITA (Aggregated Scale Morphological Interaction between Tidal inlets and the Adjacent coast). The ASMITA model simulates morphological development on an aggregated spatial and temporal scale by imposing a morphological equilibrium condition. As such the model is fast, allowing for multiple long-term simulations. The model is physics-based and the parameters can be related to field values.

Currently, salt marshes are not implemented in ASMITA. However, salt marshes could be of importance to the morphological development in tidal inlet systems. Moreover, it is relevant to assess the resilience of the ecologically important salt marshes by themselves. The aim of this research is to implement salt marshes in ASMITA to assess their influence on the rest of the tidal inlet system and to gain insight into the long-term morphological response of salt marshes to accelerated SLR.

Salt marsh development is governed by horizontal and vertical processes. The marsh height increases by capturing mineral sediment and by the accumulation of plant biomass. Autocompaction and deep subsidence lead to a decrease in marsh height. The implementation of salt marshes in ASMITA relies solely on the input of mineral sediment. At the marsh edge, generally, a cyclic behaviour of sedimentation and cliff erosion occurs. Due to the high degree of spatial aggregation, cliff erosion is excluded from the model extension. The governing processes for salt marsh development in the ASMITA model extension are mineral sedimentation, sediment availability and relative SLR.

The spatial and temporal aggregation of governing processes for salt marsh development are included in the aggregated advection-diffusion equation and model parameters for the horizontal & vertical exchange of sediment, and sediment availability. Data analysis on hydrodynamic conditions and salt marsh development was conducted for the derivation and calibration of these model parameters. To verify the salt marsh implementation, three ASMITA models were created. A one-element salt marsh model consisting of only a salt marsh element, and two different multiple elements models, which contain the ebb-tidal delta, channels, tidal flats and salt marshes.

It can be concluded that ASMITA can model the mineral sedimentation on a salt marsh but depicts a large sensitivity to the parameter setting, particularly for the sediment concentration. Based on the chosen parameter configuration, the Oosterkwelder salt marsh is preserved when subjected to SLR rates below 16 mm/year.

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

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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In an era of rising seas and other challenges posed by climate change, coastal regions like the Netherlands are facing ever graver threats. Strategic sand nourishments could mitigate the threat of coastal erosion and sea level rise on barrier island coasts while limiting ecological impacts. However, insufficient knowledge of sediment transport pathways at tidal inlets and ebb-tidal deltas prevents an informed response in these areas.

The main goal of this project was to describe and quantify the pathways that sediment takes on an ebb-tidal delta. To reach this goal, we focused our analyses on Ameland ebb-tidal delta in the Netherlands. Before we could begin to tackle this challenge, we needed to develop new tools and techniques for analyzing a combination of field measurements and numerical models. These include a method for analyzing the stratigraphy and mapping the morphodynamic evolution of ebb-tidal deltas, a new metric for characterizing suspended sediment composition, and innovative use of sediment tracers. We also established a quantitative approach for looking at and thinking about sediment pathways via the sediment connectivity framework, and developed a Lagrangian model to visualize and predict these pathways efficiently.

The techniques developed here are useful in a wider range of coastal settings beyond Ameland, and are already being applied in practice. We foresee that the main impacts of this project will be to improve nourishment strategies, numerical modelling, and field data analysis. This dissertation also points forward to numerous opportunities for further investigation, including the continued development of the connectivity framework and SedTRAILS. By managing our coastal sediment more effectively, we will set the stage for a more sustainable future, in spite of the challenges that lie ahead. ... Read more

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

The tidal channels in the Eastern Scheldt basin, are out of equilibrium due to the reduced tidal prism as a consequence of the construction of the Storm Surge Barrier in 1986. An estimated 500 million cubic meters of sediment is required in order to reduce the cross-sectional areas of the tidal channels such that the system reaches a new equilibrium. This phenomenon is referred to as sediment starvation. The sediment starvation has been causing severe erosion of the intertidal areas which are the only available sources of sediment to feed the tidal channels because the storm surge barrier practically blocks sediment import into the estuary. The intertidal areas form the habitats for the benthic commu- nity, foraging ground for wader birds, and rest area for aquatic animals. In addition to their ecological value, the intertidal areas are valuable wave dampers and therefore important for flood protection of the hinterland. Their erosion thus harms the ecosystems as well as the flood safety of the hinterland. Mitigation measures have been carried out in the form of directly nourishing the intertidal areas (Roggen- plaat, Galgenplaat, and Oesterdam). These nourishments have had detrimental effects on the ecology that remained for a period of 5 years after the implementation. Nourishing the channels, hence reduc- ing the cross-sectional area of the channels, can feed the intertidal areas gradually, thus preserving or increasing the ecological value of the intertidal area. Nourishing the channels can succeed according to the theory on channel-shoal interaction and the experience from the Western Scheldt; however, this has not been studied yet for the Eastern Scheldt. In this study, the effects of a nourishment in a tidal channel have been evaluated for different nourishing methods, volumes and locations in the Eastern Scheldt Estuary. The objective of this research is to answer the following research question: Is a tidal channel nourishment in the Eastern Scheldt a feasible way of supplying the channel’s surrounding intertidal areas? To answer this question, we applied a 2DH numerical model (ScalOost) that runs in the Delft 3D soft- ware. For this research, the forcing consists out of tidal elevations as well as a wind climate. For reasons of simplicity and to limit larger computational time, wind waves are excluded from the model’s forcing. The numerical model is capable of simulating the hydrodynamic effects and the morphody- namic evolution of a tidal channel nourishment. For four channels, the effect of nourishing on the velocity magnitude is studied for different ways of nourishing (elevating or narrowing the channel). The model results show that the considered nourishments cause a local increase of the velocity magnitude and an additional flow on the surrounding intertidal area during flood. According to the analyses of the computed hydrodynamics, the Krabbenkreek and the Brabantsche Vaarwater channels show the most potential considering the velocity magnitude increases and velocity direction changes. There- fore, these two cases were analysed separately in form of two case studies with an in-depth hydro- and morphodynamic analysis for various nourishment designs. For the Krabbenkreek, a nourishment of 2 million cubic meters increases the maximal flow velocities in the order of 0.15 m/s, such that the critical velocity for sand transport (0.45 m/s) was exceeded over a larger part of the channel; to approximately 750 meters further landwards. The period in which the critical velocity is exceeded, increased by 15 to 60 minutes per tidal cycle. The results of the morphodynamic simulations indicate that 2.5% of the initial nourishment erodes over the first year, of which 80% settles above the MLW-line. Bearing in mind the model’s limitations, it is concluded that a tidal channel nourishment in the Krabbenkreek feeds the intertidal area at a slow pace such that the ecology is not adversely affected. The Brabantsche Vaarwater and its two main bends were used to study the effect of secondary flow on sediment transport and eventually on the behavior of a nourishment. Historic data, as well as theoretical analysis, indicate that in both bends, the centrifugal effect is dominant over the Coriolis effect for gen- erating secondary flow. Model results confirmed this observation, yet the dominance of the centrifugal effect is larger in the second bend. As outer bends tend to erode and inner bends to accrete, the outer bends were nourished (750 & 920 ∗ 103 cubic meters) in order to use the secondary flow to transport sediment towards the inner bend and eventually onto the intertidal area. The simulation results show that for both bends, 3% of the initial nourishment erodes after the first year of which 80% accreted on both the inner and outer bend. The morphodynamic simulation results do not confirm the dominance of the centrifugal forces on sediment transport, as larger accretion rates than those simulated on the inner bends were expected. Although the results were not as expected, the nourishments did increase the velocities which increased the suspended sediment concentration in the channel and as simulation results show, sedimentation in sheltered areas. A tidal channel nourishment in the Eastern Scheldt has been proven to be a potentially successful way of indirectly nourishing the channel’s surrounding intertidal areas. However, the accretion rates were predicted in the order of 2% per year, whereas this would be 100% if directly nourished. Furthermore, the impact of a tidal channel nourishment on the sediment starvation in the whole basin is small con- sidering the proposed volumes in this research only represent 0.4% of the actual sediment demand. Nourishing a tidal channel should be considered in view of maintaining ecological values. ... Read more

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

Estuarine intertidal flats comprise valuable ecosystems and act as an important sediment source for the adjacent salt marsh systems. However, accelerating sea-level rise threatens the mudflats and associated ecosystems, where the mudflat accretion lag behind sea level rise. A reliable forecast on the morphological developments of the mudflat under sea-level rise scenarios is of vital importance to assess sea level rise impact on the estuarine system.

Different tools exist that can predict the long-term evolution of the mudflats, viz. Delft3D, ASMITA and the hybrid model (Delft3D-ASMITA). Since the hybrid model is newly developed, the comparison among the various approaches has not yet been available. However, it is significant to know if they can produce the same results.

The research aims to compare the three modeling approaches (Delft3D, ASMITA, and the hybrid model) based on a case study in South San Francisco Bay. This comparison will reveal the strengths and weaknesses of the three approaches as well as indications where the approaches may strengthen each other.

The research is conducted in three main phases. Phase 1 consists of the sensitivity analyses in Delft3D for the case of South San Francisco Bay; Phase 2 contains the calibration of one-element and multi-element ASMITA models to reproduce the Delft3D model; Phase 3 focuses on the potential to improve the simulation efficiency of Delft3D in the hybrid model.

Model result comparison shows that, after the calibration, the ASMITA and hybrid model can efficiently simulate the same cases as in Delft3D. However, the upper part (landwards end) of the mudflat is more sensitive to the water level changes in the hybrid model due to the different sediment transport computation modules. The power indicating the relation between the equilibrium and actual morphology as well as the reference level is the important calibration coefficient to adjust the steepness of a mudflat. It can be concluded that the Delft3D model is used as the foundation to calibrate ASMITA and the hybrid model, both of which can improve the simulation efficiency with simplifications, especially in the long-term morphological development.

The study provides clear insights into the comparisons among different modeling approaches in the case of the long-term morphological developments of mudflats by the impacts of sea-level rise. It is recommended to do further research on the configuration of a 2D model and the conduction of the combination of different modeling approaches in other similar cases to confirm the validation. ... Read more

Ebb-tidal deltas play a key role in the morphology of barrier coastlines and tidal inlet systems as they serve as a natural source of sediment. They are typified by a dynamic morphology that interacts with the adjacent coastlines and sponsors a unique ecological habitat. Under increasing socio-economic, ecological and climate-induced constraints, it becomes imperative to obtain a better understanding of the evolution of ebb-tidal deltas in order to maintain and preserve these morphological features in the near-future time horizon. An increased interest has therefore been raised to understand, quantify and predict the development of ebb-tidal deltas.

In the context of developing a future-proof coastal management and maintenance strategy, the efficiency of ebb-tidal delta nourishments has been further investigated in research programmes such as Coastal Genesis 2.0. For the Ameland inlet, this entailed the construction of a 5 million m3 pilot nourishment over the course of March 2018 to February 2019 and subsequent monitoring in the years following. This research investigates the impact of this pilot nourishment on the natural behaviour of the Ameland ebb-tidal delta. The second goal is to build further knowledge on the modelling capabilities of present state-of-the-art models for the Ameland ebb-tidal delta.

To this end, a process-based Delft3D model is applied to hindcast the morphological development of the ebb-tidal delta with a particular interest in the evolution of ebb-shields and -chutes over the course of 2005 to 2020. It is identified that the representation of the wave-induced processes is key for capturing the development of ebb-shields and -chutes in the model predictions. Therefore, we applied and experimented with an updated nonlinear wave orbital velocity parameterisation and assessed its contribution to the modelling performance.
This thesis demonstrates that the evolution of ebb-shields and chutes on the outer delta is contingent on its initial presence in the initial bathymetry. Hence, the initiation of ebb-shield development is not inherited in the model response. These insights have been integrated and synthesised to assess the application of the present state-of-the-art model as a forecasting tool for the morphological development of ebb-tidal delta nourishments.

Ultimately, it is shown that a model using schematised boundary conditions and an efficient morphological updating scheme is able the predict the yearly-averaged development of the pilot nourishment on the ebb-tidal delta. It is demonstrated that the 2019 pilot nourishment only locally influences the behaviour of the Ameland ebb-tidal delta. Nourished sediment is likely to be redistributed along the ebb-shields, contributing on the long-term to the sediment exchange process with the downdrift coast of Ameland. The location of the ebb-tidal delta nourishment is thereby important for the sediment exchange process and the development of local features. Placing ebb-tidal delta nourishments to the south of the Westgat invokes a primary sediment exchange between the coast of Terschelling and surrounding morphological features. A secondary readjustment of the Westgat thereby influences the development of the ebb-shields and -chutes. Constructing an ebb-tidal delta nourishment north of the Westgat results in a sediment exchange between the local ebb-shields and the downdrift coast of Ameland. Our results also demonstrate that an increase in the pilot nourishment construction height enhances the redistribution of sediment along the ebb-shields.

Lastly, this study identifies further opportunities for improving medium-term morphodynamic models for the Ameland inlet by incorporating time-dependent boundary conditions including new transport formulations such as SANTOSS. ... Read more

Burrard Inlet (Vancouver, Canada) has been the home of the Tsleil-Waututh Nation (TWN) for thousands of years. Over the past decades, ongoing erosion has been observed along the shores of Burrard Inlet and the TWN reserve specifically. This leads to loss of land for the TWN community, damage to infrastructure, and exposure of historic sites with cultural value. Currently, there is insufficient knowledge concerning both the governing processes for sediment transport and transport pathways into, within, and out of Burrard Inlet. This knowledge is needed to propose and evaluate effective measures to prevent further erosion. This study aims to investigate the transport pathways in Burrard Inlet and give more insight into the mechanisms governing sediment transport in this inlet.

For this purpose, a Delft3D FM model of the area is set up and calibrated. This model is used to analyze sediment transport in the inlet under various forcing conditions. Transport pathways are visualized using SedTRAILS.

The model shows that flows and sediment transport in Burrard Inlet are tide-dominated and governed by the topography. Flows are strongly accelerated in constricted areas (First Narrows and Second Narrows), which leads to large velocity differences. Following the velocity field, sediment transport patterns are correspondingly dominated by these topographical restrictions. In the wider basins, flows slow down and form eddies. The model results suggest that these eddies act as sediment sinks. Additionally, sediment is lost into Indian Arm, a deep fjord with low flow velocities at the eastern end of Burrard Inlet. The possible pathways for sediment originating from the eroding shorelines at the TWN reserve are visualized. As soon as sediment from these banks is mobilized, it tends to move away from the shore with a final destination either in one of the eddies or in Indian Arm. The impact of wind and waves on the sediment transport patterns is limited.

Since first European contact in 1792, the shoreline of Burrard Inlet has changed significantly due to dredging activities, land developments, and industrial development as the city of Vancouver was built. Reconstructed historic shorelines are implemented in the model to assess the consequences of these shoreline changes on the sediment transport. Model results show that the tidal prism and the velocities in the Narrows have decreased since 1792, while the tidal range has increased. Moreover, sediment mobilized along the eroding shorelines showed greater potential for deposition along these same shores in 1792, compared to the present-day situation.
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The Gulf of Mottama, located in the southwest of Myanmar, is home to morphologically interesting processes. An entrance of around 100 km wide narrows into a funnel­shaped bay towards the Sittaung River in the north. The entire estuary region is subject to strong dynamic morphological activity which is alleged to be driven by the large tidal energy and sediment inputs. The dynamics of the tidal channels in the Sittaung estuary have resulted in rapid bank erosion of up to 3 km/y. The Sittaung estuary harbours a unique set of characteristics. This has made the question of which processes and mechanisms cause the bank erosion a complex one for which no clear cut answer is currently available. The knowledge of the main processes and mechanisms is valuable to improve the understanding of morphological functioning in dynamic estuaries. The main research question has thus been formulated as: • What is the effect of the large incoming tide and storm events on the bank erosion of the Sittaung estuary and to what extent can this erosion be simulated with a numerical model? The research objectives have been achieved through a literature analysis, system analysis, satellite analysis and modelling simulations. Main conclusions of the research have been as follows: The satellite analysis has shown little to no correlation between the wet season discharge increase and the bank erosion rates. It also showed large differences between erosion rates at different heights of the estuary. Modelling results indicate a prominent role but cannot substantiate a quantitative impact of the tidal forcing and associated tidal bore. River discharge fluctuations were shown to have little effect on the bank erosion. • The results of both the satellite and numerical modelling analysis have shown no additional implications on the dynamic morphology caused by large incidental storm events. Long­term simulations with respect to the bank erosion resulted in several hindrances. Heightened levels of channel incision occurred through a fault in the numerical schematization. The representation of bank erosion in the model has also been subject to stalling. ... Read more

Estuaries are partially enclosed water bodies where river water mixes with sea water. Estuaries provide important ecological functions which are strongly regulated by estuarine hydrodynamics and sediment dynamics, and also by human interventions. Sustainable management of such systems therefore requires a thorough understanding of the interplay between hydrodynamics, sediment dynamics, and human interventions. However, estuaries are often complex systems influenced by river runoff and coastal hydrodynamics (tide, wind, and wave), which all interact with human interventions on various time and spatial scales. Our understanding of estuaries is still insufficient to understand the response of strongly engineered systems to both human interventions and to natural fluctuations. Many estuaries worldwide are strongly influenced by a wide range of human interventions, including engineering constructions, deepening, and land reclamations. An example of highly engineered estuaries is the Changjiang Estuary (CE), China. The upstream river discharge and sediment load is strongly influenced by the Three Gorges Dam (TGD), a multi-purpose dam in the Changjiang River aiming at optimizing flood control and irrigation, and generate hydropower. In the North Passage (NP), an outlet and the main navigation channel of the CE, the Deepwater Navigation Channel (DNC) has been constructed to improve channel navigability. The DNC project includes constructions of dikes and groynes, and regular dredging work. These various interventions strongly influence estuarine hydro- and sediment dynamics but take place concurrently, and therefore their individual impact is not straightforward to assess. A better understanding of the impact of these interventions requires systematic analysis of hydrodynamic and sediment transport processes in relation to the interventions. This dissertation aims to unravel the effect of groynes on lateral flows and sediment transport in a tidal channel-shoal system (i.e. the NP). Groyne fields provide buffer zones, with a salinity lagging behind that in the main navigation channel. The resulting lateral salinity gradients drive lateral density currents, which in turn modify longitudinal salinity gradients in the main channel. These salinity-driven currents also impact the lateral sediment exchange between the main channel and the groyne fields. The effects of groynes on lateral flows and lateral sediment exchange are analyzed using numerical simulations in combination with in-situ observations. Water-bed sediment exchange processes are investigated in more detail using measurements collected with two tripods deployed in the CE. Measured bed level changes are analyzed by semi-automatically fitting the Krone-Partheniades equations to the bed level data using observations of velocity and sediment concentration. This method provides continuous timeseries of sediment properties related to erosion and deposition. It is demonstrated that the erosion parameters are strongly fluctuating, and not constant as typically assumed in numerical models. Such a variability needs to be reflected in a model, either by time-varying parameters or including more detailed processes (for example, consolidation). This dissertation introduces a method to obtain a parameter space that includes the values and accuracies of all potential combinations of input parameters, which is important input for morphodynamic models. To further quantify effects of groynes on hydrodynamics and sediment dynamics, an idealized hydrodynamic model with a single channel with groynes is developed and analyzed. The idealized system has geometric features comparable to the NP, but is set up in such a way that the groyne field aspect ratios (the ratio of the distance between contiguous groynes to the length of groynes) can be systematically investigated. Model results reveal that groynes can influence channel hydrodynamics and local mixing conditions, which influence lateral flows and the longitudinal salt intrusion. Salt intrusion is highest for intermediate aspect ratios, but weaker for very wide or narrow groyne fields. These results highlight the complexity of the hydrodynamics in salt fresh-water transition zones, and specifically the role of human intervention thereon. ... Read more

A study of five European estuaries by Winterwerp and Wang (2013) showed that tidal ranges increased significantly along with the ongoing narrowing and deepening of these estuaries. In addition to an increase in tidal range, a dramatic increase in suspended sediment concentrations has been observed in the Ems (Germany) and Loire (France) estuaries, often referred to as a regime shift towards hyperturbid state. While it is generally accepted that the increase in tidal range is a consequence of deepening and narrowing, it is not generally true that deepening and narrowing leads to a dramatic increase in sediment concentrations. Whether deepening is a potential cause for the high sediment concentrations observed in the Loire, is still unclear. Although very little observations have been reported over the 20th century, literature clearly shows that sediment concentrations increased drastically over the years. The scarcity of data not only applied for the sediment concentrations over the years but also for bathymetry and hydrodynamics, resulting in uncertainty about the historical states of the estuary. To cope with the large range of uncertainty and variability in the reported observations and model parameters, an idealised model called iFlow has been used. The strength of the iFlow model for this research was twofold. Firstly, iFlow allowed to identify the essential physical processes driving the estuarine sediment dynamics. Secondly, iFlow is computationally efficient, allowing to perform extensive sensitivity studies. In this way, we could determine which processes play a role in the current state of the Loire estuary and how they are affected by deepening. Furthermore, by mapping the effects of the uncertainty on the different physical processes, robust conclusions could be drawn about the effects of deepening despite the given uncertainty. The effects of deepening on the physical processes has been assessed by only varying the depth in the model. Deepening generally resulted in enhanced sediment import or decreased sediment export. Hence, the transport capacity in the Loire became much more favourable for the import of sediment. In order to determine the robustness of these findings, an extensive sensitivity study was performed of 14.157 simulations (1287 conditions for 11 bottom profiles over time) to capture the influence of uncertainties in multiple input parameters. For the bottom profile representing the bathymetry in the year 1900, none of the conditions resulted in a hyperturbid state, whereas independent of the conditions, the likelihood to find a hyperturbid state always increased for increasing depth. From this we can conclude, that the observed regime shift towards high sediment concentrations would not have happened without the deepening of the Loire estuary. ... Read more

To exploit its socio-economic functions, engineering measures are regularly applied in estuaries. Estuaries are, however, known to be very complex systems. Stemming from this complexity is the generation of a so-called estuarine turbidity maximum (ETM), which poses great siltation problems to the engineering measures. An engineering measure, which is looked upon in this thesis, is a trench accommodating for the construction of a submerged tunnel. Despite the complexity of estuaries, trench siltation rates are predicted in practice by simple empirical engineering tools. However, a lot of uncertainty is associated with the predicted trench siltation rates, as such engineering tools do not capture the complex estuarine mechanisms. These mechanisms are found to dominate the sediment supply to the trench and the subsequent trapping of sediment in the trench. Therefore, in this thesis, it is investigated to which degree of certainty trench siltation rates in estuaries can be predicted, based on a process-oriented and engineering-oriented viewpoint. This is researched according to the implementation of a detailed process-based numerical model, which is considered to be highly accurate. For this purpose, the following case study is adopted: a trench near the ETM of the well-mixed Scheldt Estuary, at Oosterweel, Belgium. The uncertainty of the numerical model is estimated based on a sensitivity analysis, which maps the epistemic uncertainty, and a scenario analysis, which approximates the intrinsic uncertainty. For verification purposes, modelling results are compared with the state-of-the-art theory on trench siltation mechanisms, and estuarine sediment transport and trapping mechanisms. These are thoroughly analyzed in this thesis based on an extensive literature study. Additionally, a link to practice is made by comparison of the degree of certainty and practicality of the numerical model with the engineering tools. It could be estimated, based on the parameter uncertainty, that the epistemic uncertainty of the numerical model equals approximately 1.5 times the expected siltation volumes. A similar uncertainty due to intrinsic uncertainty was estimated, as the trench siltation rates showed a strong dependency on the variation in forcing regarding tide, river and storms at sea. In total, this led to a quantifiable uncertainty in the order of 2.5 times the expected siltation volumes.
Regardless of this uncertainty, great confidence is put in the performance of the numerical model, as in accordance with existing literature of the study area and theory on well-mixed estuary, important mechanisms in the supply of sediment to the trench were found to be: salinity-induced circulation, tidal rectification, Stokes’ drift and river discharge. Additionally, the governing sediment trapping mechanisms of the trench, found by the model, are in line with state-of-the-art literature. In contrast, yet unidentified by literature, the longitudinal salinity gradient over the estuary also seem to dominantly influence the trapping efficiency of the trench, in particular during flood in which it induces a strong decrease in sediment trapping. In comparison, it is believed that engineering tools, applied for trenches in estuaries, are prone to very high epistemic uncertainty caused by model inadequacy. This is because stand-alone application of the engineering tool on the problem gave a significant over-estimation of the siltation volumes, as predicted by the numerical model. Furthermore, the engineering tool was found to behave differently within a tidal cycle, and on changing environmental conditions. Above epistemic uncertainties due to model inadequacy could, however, not be quantifiably supported, as the total quantifiable uncertainty was in the same order of the numerical model. In conclusion, the degree of certainty of the trench siltation rates is believed to be improved significantly using a detailed numerical model instead of engineering tools. However, a huge drawback of the application of detailed numerical models, is the complexity and the impracticality of the numerical model. Therefore, this thesis opts for the development/use of a more sophisticated semi-empirical tool for engineering measures in estuaries. Though, more research is recommended on trenches in both similar and different type of estuaries in order to generalize and confirm the findings of this thesis. ... Read more

In this study, the effects of extreme flood events on vegetation and morphological patterns in a natural river were investigated. For this purpose a new vegetation-development model was introduced and linked to a hydro-morphodynamic Delft3D Flexible Mesh model. Based on the results in this study it can be concluded that the direct effects of an extreme flood are relatively low, with almost no vegetation removed during a flood event. However, the extreme floods lead to an increased area suitable for seedling colonization within the river floodplain, which is partly caused by the formation of a secondary channel in the extreme flood scenarios. This causes an increase in the seedling recruitment rate in the years directly after an extreme flood and in a higher vegetation coverage. ... Read more

Estuaries are dynamic partially enclosed water bodies that are constantly or periodically influenced by the ocean and at least occasionally impacted by river discharge. This creates unique but fragile ecosystems that have to be managed with care in order to be recreationally, economically and ecologically valuable. One of the management issues is water quality, which is mainly influenced by hydrodynamic processes and other processes affecting the transport of dissolved or suspended materials. The understanding of the hydrodynamic processes of an estuary and its physical drivers is crucial for management. One of these fragile ecosystems that have been prone to many human interventions in the past, is the Leschenault Estuary. In this thesis, a 3D-numerical model is developed in D-Flow FM to unravel the governing hydrodynamics of the Leschenault Estuary. Besides, field measurements provided data used as input for the numerical model and more information about the dynamics of the Leschenault Estuary. Most importantly, a methodology is proposed to improve the efficiency in drawing relevant conclusions from observed and modelled data. Efficient and easy-to-apply classification methods are therefore considered that could be powerful management tools. To validate this methodology, a scenario has been considered where the Preston River is aligned to the Bunbury Port. The governing hydrodynamic processes in the Leschenault Estuary are internal circulation, stratification and turbulence, which are predominantly driven by the freshwater discharge and the tides. However, the dominant physical processes are highly dependent on the seasonal conditions and the specific locations. In general, three different seasonal conditions were distinguished: normal summer, normal winter and peak river discharge conditions. In summer, freshwater discharge is reduced, which increased the impact of tidal stirring and vertical mixing. In winter, the high river flow generated more stratified flows and under peak discharge conditions some areas of the estuary presented salt-wedge regimes. The Leschenault Estuary can be spatially subdivided in four distinct regions (southern, central, northern and riverine basin), based on the governing hydrodynamic processes. The southern basin is the most dynamic and can not be specified by a single regime due to the influence of the ocean, the Preston River, the Collie River and the northern regions. The central and northern basins were classified as partially-mixed throughout the year and showed weakly to strongly stratified water bodies, depending on the seasonal conditions. Furthermore, a classical estuarine circulation develops under normal winter conditions. In summer, the northern regions become hypersaline, generating an inverse circulation in the central and northern basins. The Collie River is characterized by a partially-mixed water body, with high stratification. Under peak discharge conditions, the salt wedge can be temporally forced out of the river and partially out of the estuary. Winds, waves and Coriolis have a significant influence on the hydrodynamics of the central and northern basin, due to their shallow and stagnant waters. The driving forces of the salt transport were obtained by the decomposition of the salt flux. The main physical processes affecting the salt transport between the estuary and the ocean along the transect of the 'Cut' over the whole year and in winter were freshwater discharge, topographic trapping and Stokes drift. Stokes drift was dominant in summer, followed by freshwater discharge and topographic trapping. This indicates that regardless the season the advective terms were dominant drivers of the salt transport. It also indicates the seasonality of the salt flux at the 'Cut'. The dominant salt flux components at the central basin was the Stokes drift, followed by freshwater discharge and topographic trapping. The Preston River alignment was compared with its current location to provide useful information for management and to evaluate the efficacy of the adopted classification methods. The scenario results were significantly different than the initial model results and the Preston River alignment had a substantial impact on the hydrodynamics of the Bunbury Port and the southern basin. Bunbury Port circulation became more stratified and its seasonal variation was increased, while the southern basin became less dynamic and partially-mixed, with low to high stratification. The Collie River and central and northern basins however remained almost unaffected. Further, the physical drivers of the salt transport did not vary much but the role of the Stokes drift became relatively more pronounced at the `Cut', due to the decreased influence of freshwater discharge and topographic trapping. At the central basin, the Preston River alignment had a negligible effect on the physical drivers of the salt transport. However, larger spatial and temporal variability of salinity and temperature distributions were observed. It is therefore recommended to conduct an additional ecological valuation of this intervention. Valuable insights were presented in this thesis that have been critically validated. The used methods have proven to be efficient and valuable tools for management. ... Read more