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.