The Rhine-Meuse estuary is part of the Southwestern Delta area in the Netherlands. The nature present in the estuary suffered a great impact due to urbanization, as a natural delta transformed into an industrialized river, mainly due to port activities. Various parties decided to collaborate in order to return the tidal nature in the Rhine Meuse estuary through constructing tidal parks. Although the idea of reintroducing tidal nature into an industrialized estuary seems promising, problems arise when the projects are being executed. Human interventions necessary to complete the design of the tidal parks often lead to undesired morphological changes. These either consisted of an excessive amount of cohesive sediment import, or hardly any import. Consequently, maintenance in form of dredging or nourishing activities is required. When introducing tidal nature, little maintenance is desired, both from a financial perspective and from an ecological perspective. The undesired morphological changes result from limited knowledge on the impact of human interventions on the hydro and morphodynamics in tidal parks. This research therefore primarily focuses on gaining new knowledge on the hydrodynamics and morphodynamics inside tidal parks and assessing the effect of different elements such as groynes and longitudinal walls on the latter. This is done through a case study, the Groene Poort, a collective name for the tidal parks located in the Nieuwe Waterweg.

A depth-averaged (2DH) Delft3D model is used to research the hydro and morphodynamics of the tidal park. A constant tidal forcing and a constant fresh water discharge are used as boundary conditions. The effects of wind, waves and density driven currents are neglected. The effects of changes in hydrodynamic forcing, including a morphological tide, a spring tide and a spring tide in combination with a surge on the morphodynamics are determined. This research showed that the tidal flow results in relatively small velocities inside the studied tidal park with mean velocities around 5 cm/s. The critical velocities for transport of noncohesive sediment transport are only reached at the west entrance of the studied tidal park. At this location the sediment deposits, further transport towards the river bank is not possible resulting from the magnitude of velocities. A spring tide is responsible for an increased import of noncohesive sediment at the river bank area.

This research provides a guide on the effect of relatively small adaptations in geometry which will enhance or decrease the natural processes of sedimentation and erosion in the tidal parks. Tables are included providing the important parameters both with respect to the hydro and morphodynamics of various adaptations in geometry. These tables can be used in future design and adaptations of already constructed tidal parks. However, the effect of the surrounding area including the location in the river greatly impact the hydrodynamics and morphological development of the tidal parks.

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.

KEY POINTS:
• While the formation of the intertidal shoal Hinderplaat was a direct consequence of a human intervention, its degradation was a natural response of the system to frequent hydrodynamic forcing conditions.

• Tidal currents push the Hinderplaat in offshore direction, but this is counteracted by waves: these generate cross-shore and longshore currents on the shoal, inducing its landward migration and southward spreading.

• Extreme discharge events provide a significant contribution to channel formation on the Hinderplaat by generating initial local breaches that are enhanced by regular tidal flows.
EXECUTIVE SUMMARY:Ebb-tidal deltas (ETDs) evolve constantly under the influence of natural processes and anthropogenic activities. The Haringvliet ETD in the Southwest of the Netherlands is an extreme example of the latter: closing off the estuary in 1970 triggered a regime shift, completely altering the evolution of the ETD. Initially, the coastnormal sandy shoals evolved towards a narrow coast-parallel intertidal spit: the Hinderplaat. Subsequently, this tidal flat breached around 1995. Thereafter, the flat eroded continuously while sediment was transported from the flat towards the coast.
Previous research with the aim to understand the morphological development of the Haringvliet ETD has provided insights into the processes that are responsible for the large morphological changes directly after the construction of the Haringvliet Barrier. The processes driving the observed ongoing erosion and flattening of the Hinderplaat are however still poorly studied and understood. Therefore in this research, the underlying mechanisms are investigated and linked with anthropogenic interferences and meteorologicalevents.
To this end, a combination of data analysis and numerical simulations is applied. In an analysis of singlebeam bathymetry measurements (gathered in the Vaklodingen dataset), the development of the subtidal shoreface and the intertidal area of the Hinderplaat was explored. In general, the degradation of the Hinderplaat can be divided into three developments: (1) a landward migration since its formation, (2) a lowering since 1992 after a period of significant heightening and (3) a breach and channel formation in 1995, after which the shoal spread and merged with other shoals.
A depth-averaged (2DH) Delft3D model is used to explore the mechanisms behind these developments and to reveal the relative importance of the tidal-, river discharge-, wind-, surge- and wave-forcing driving the flow and sediment transport in the ETD. The first two developments (landward migration and lowering) occurred gradually, hence they were possibly a response of the system to the regular hydrodynamic forcing. Various simulations with a wide range of forcing-scenarios are performed to evaluate this. The third development(breaching) was observed in the same year an extreme discharge had occurred. A morphodynamic hindcast is performed to study the link between the two.
Model results indicate that the landward migration of the Hinderplaat was induced by regular wave action from WNW to NNE direction. Such wave conditions generate cross-shore transport over the shoal in eastern direction. This counteracts the effect of the tide, that enhances (much smaller) residual transport rates in seaward direction. Furthermore, breaking waves generate longshore transport along the Hinderplaat, with the highest transport rates at its southern end. This resulted in its spreading in southern direction and lowering of the highest part of the shoal. In addition, a correlation is found between the sudden increase in height of the Hinderplaat before the degradation and the construction of the Slufter. Considering the abundant transport rates from the Slufterdam towards the shoal, it is concluded that the port extension served as an important sediment source directly after construction. However, the sediment supply might have decreased in subsequent years. The hindcast simulation confirms that breaching of the shoal was a direct consequence of the high discharge event of February 1995: the strong offshore directed currents created an initial channel which was later on maintained by the tide.
Wind-driven currents are very effective in generating residual flow patterns at the shoreface in front of the Hinderplaat. Besides, both the wind and surge act as important amplifiers for the sediment transport rates on top of the shoal. However, waves are indispensable for the morphological changes of the study area as the other hydrodynamic drivers are hardly capable of generating sediment transport by themselves.
Concluding, it is undoubted that the formation and thereby existence of the Hinderplaat is a direct consequence of the man-made closure of the Haringvliet estuary. However, its degradation (lowering, landward migration, breaching and further spreading) is a response of the system to the natural hydrodynamic forcing. Therefore, this evolution can be considered as part of the intrinsic behaviour of the shoal that is independent of human interventions in the former estuary.