Tidal basins are highly dynamic environments with a complex behavior that is often disturbed by human activities. Considering that tidal dynamics have a direct impact on surrounding engineering infrastructures, adjacent coastlines, nature environment and socio-economic human activities, it is crucial to know the impact of human interventions on these dynamics. This research focuses on the closure of the Gulf of Khambhat in India, which aims at the creation of a fresh water reservoir in the gulf by partly closing the current estuary with the Kalpasar dam. The most recent design of the closure concerns a 30 km dam from the eastern bank near Aladar to the western bank near Bhavnagar. This closure will significantly reduce the total basin area. Consequently, major and morphodynamic changes are expected in the basin. The Kalpasar project has been on the Indian Government's agenda since 1986. Royal HaskoningDHV was involved in the pre-feasibility study, which was presented in the late 1990s. Since then, the existence of a detailed report study has not been confirmed and the status of the announced feasibility studies by the Indian government is unknown. The main objective of this research is to investigate the morphodynamic response at the seaward side of the dam after the closure. A process-based morphodynamic model has been developed to study this response. Deltares and NIOT have provided a two-dimensional (2DH) numerical model of the Gulf of Khambhat to study the tidal propagation in the basin. After calibration and validation of the hydrodynamic predictions, the model has been extended to a morphodynamic model in order to perform morphological calculations. Several adaptations have been made to improve the hydrodynamic simulations of the model; the main contributing factor was the new initial bathymetry. An extensive spin-up simulation has been performed to gather this. To deal with introduced model artefacts, the model results are compared to the reference case. Therefore, two almost identical simulations are performed: the only difference is that one run contains the Kalpasar dam while the other does not. This way, the relative effect of the dam is determined. Moreover, the morphological results have to be interpreted qualitatively, since the predictive skill of the model has not yet been determined. Model results show an overall increase of the tidal range in the basin after the closure, which will have effect up to 100 km from the dam. Close to the dam, the range will initially increase from 7.88 to 10.25 m, and up to 10.50 m after 96 years. Besides, the tidal signal switches from being ebb-dominant to flood-dominant. The velocities around the dam become negligible and the velocities at the main western channel significantly decrease. The eastern channel remains the main channel of the gulf, although its maximum flood- and ebb-velocities also decrease. As a consequence of the hydrodynamic changes, the basin will start importing sediment, directly becoming a sink. On the long term, the area up to 40 km southward of the dam partly fills in. The eastern side of the main channel will also accrete, although its western side will erode over the years. The implementation of the dam will have negative impacts on several locations at the study area. Four vulnerable locations are identified, namely Dahej (India's largest LNG-terminal), Hazira (container terminal), Surat (> 6 million inhabitants) and Alang (largest ship wrecking worldwide). All of these locations will become prone to flooding because of the increased maximum water levels. Moreover, sedimentation at the approach channels of the ports of Dahej and Hazira may hinder their accessibility. The increased water levels at Alang may lead to the suspension of heavily contaminated sediments from these beaches. Furthermore, coastal erosion might be a problem for the entire study area, if the ebb-tidal delta is not large enough to balance the sand hunger of the basin. Relocating the dam is not an effective measure to prevent the negative impacts. Maintenance dredging activities will probably be required to maintain the accessibility of the ports. Moreover, major infrastructural changes will be needed to prevent floods. It is recommended to execute follow up studies with more detailed tools to determine the exact response at these locations. More accurate and up-to-date hydraulic and bathymetric data is required to develop these tools. To this end, it is highly recommended to partner with local parties like the Government of Gujarat, the EAG and local universities.

The growth of the offshore wind industry results in intensive usage of the sandy seabed in the North Sea, currently and in the coming decades. Large-scale bed forms are present in shallow seas with sandy beds such as the North Sea. The most dynamic bed forms are sand waves. Due to their dynamic behaviour, sand waves can interact within offshore human developments and together with their dimensions pose a threat; e.g. decrease in navigation depth, exposure of submarine cables, interaction with foundations of offshore wind turbines and destabilization of bed protections. A thorough understanding of the dynamics can result in less risks for the offshore wind sector and therefore bring down the levelized cost of electricity from offshore wind.
Currently, sand wave field dynamics are investigated by data-driven analyses. These analyses are based on seabed surveys over preferable more than 10 years and are considered most reliable at the moment. However, these surveys are very costly and/or often not available. Complex numerical models may provide an approach to analyse sand wave dynamics in a cost and time efficient way, though two aspects have to be considered. Not all relevant processes regarding sand wave dynamics are yet understood. Furthermore, due to the large scale of sand wave fields in combination with the fine grid resolution required to model sand waves, large computational efforts form a difficulty for numerical modelling of sand wave fields. Previous numerical studies focused on reproducing the length and height of sand waves. The migration direction is the next step towards the full prediction of sand wave fields and the subject of this research.
Recent data-driven analyses showed migration directions of sand waves in opposite direction over small spatial scale, possibly related to underlying seabed topography. Understanding the governing processes of the migration direction of sand waves including underlying seabed topography is the focus of this research using the numerical process-based model Delft3D.
To this end, an idealized model is used in which underlying seabed topography (tidal sand bank) is included. For a symmetrical tidal velocity signal, it is shown that the presence of the tidal sand bank influences the hydrodynamics on the scale of sand waves. Horizontal tide-averaged flow towards the top of the tidal sand bank on both flanks is observed. This results in sediment transports and migration directions on both flanks of the tidal sand bank towards the top of the tidal sand bank. The horizontal tide-averaged flow pattern around the tidal sand bank is disturbed by the inclusion of a residual current. Sand wave migration on both flanks in the direction of the residual current is the result. Including the S4-tide constituent does not disturb the tide-averaged horizontal flow pattern around the tidal sand bank. However, the asymmetry of the tidal velocity signal enhances migration in the direction of the asymmetry.
Finally, it is shown that also for a more realistic model the transition in migration direction can be explained due to the presence of the tidal sand bank. The tidal sand bank influences the hydrodynamics by creating areas in which the tide-averaged sediment transport in the ebb direction are enhanced and areas in which the tide-averaged sediment transports in the flood direction are enhanced. In this way a transition in the migration direction over the tidal sand bank is observed. The migration directions from the model results and migration direction from data-driven analyses show a comparable transition over the tidal sand bank.

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.