The Oosterschelde has been an area of morphological change for centuries. Both floodings and human influences have caused the Oosterschelde to have its current shape. During the last century before implementation of the Delta plan (1953) the estuary was still expanding as the channels deepened and the tidal flats increased. With the construction of the Delta works the tidal range decreased with 12 % whereas the tidal prism reduced with 31 %. As a consequence of this reduced tidal motion, the tide is not able anymore to counteract the erosion of the tidal flats which is caused by waves/wind. Therefore, the surface area and the height of the tidal flats reduces (‘Zandhonger’). This decrease of tidal area is an undesirable situation as the unique tidal nature of the Oosterschelde provides a lot of functions for both economy (oysters, mussels), ecology and recreation. Furthermore, the Oosterschelde is used as a transport route by cargo vessels.

This research studies the morphological development of a model of the Oosterschelde with two hypothetical interventions: removal of the storm surge barrier (SSB) and applying 2 metre sea level rise (SLR) in 50 years. This is done with four model scenarios: a run with SSB in place, without SLR (1), a run without SSB, without SLR (2), a run without SSB, with SLR (3) and a run with SSB, with SLR (4). This last run was used as sensitivity run in order to see which hypothetical interventions has more impact.

The model results led to conclusions which parts of the model are represented well and which parts of the model need to be improved. The results made clear that current model is promising as the tidal range was modelled correctly for the majority of the Oosterschelde. Also the model represented the tidal prism well compared to the calculated tidal prism (tidal prism = tidal range * wet surface area of the basin – the sediment volume of the tidal flats). It was found too that SLR can be modelled in a correct way by forcing a water level at the boundaries of the model.

It was found that wave activity is an important process with respect to the development of the tidal flats. Therefore, the frequency of the wave computations should be chosen in such way that reliable wave heights are present for each water level during the tidal cycle at all locations in the Oosterschelde. Another key factor which should be improved is the availability of sediment in the model as this determines the (desired) growth of the tidal flats with SLR. Processes/indicators which give information about this availability (ebb/flood dominance, sediment characteristics) can give insight in the development of the tidal flats. With improvement of modelling these two processes it may be possible to improve model capabilities.

A managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. In a managed realignment, the newly formed intertidal area acts together with the realigned dike as a nature-based flood defence system. In this thesis, the focus is on the managed realignment of Perkpolder, a former polder located in the south of Zeeland (The Netherlands). This realignment serves as nature compensation for the dredging activities in the Western Scheldt. The realigned area contains an intertidal flat which facilitates ecological services.

A combination of different data sources (sediment samples, flow velocities, turbidity measurements and bathymetric data), together with a hydrodynamic model of the Western Scheldt and a morphodynamic model of Perkpolder are used to interpret the hydrodynamic and morphodynamic response of Perkpolder after the managed realignment.

Bathymetric data composed of LIDAR, multibeam and single-beam measurements shows three main developments in Perkpolder: (1) erosion of the frontal entrance area, the inlet and the seaward side of the creeks, (2) infilling of the creeks landward of the first bifurcation and (3) sedimentation on the intertidal area and even more sedimentation of the pond. By comparing the temporal evolution of the bed levels of the frontal entrance area with measured concentrations at the inlet of Perkpolder, it can be concluded that the frontal entrance area acted as a finite source of sediment that eroded quickly in the first years and increased sedimentation within Perkpolder during these years.

New realignment projects can profit from the research at Perkpolder. Bed shear stresses of the initial proposed layout of Perkpolder gives a clear indication if the creeks are setup correctly. Furthermore, an estimate of the initial sedimentation can be based on the accretion rate of other intertidal areas within the estuary. Since realigned areas are often more sheltered and have a lower elevation, accretion rates may be higher. Moreover, not only the forcing of the estuary, but at the same time the availability of different sediment sources and the initial bathymetry shape the morphodynamic response of the realignment site. Therefore, it is recommended to measure the bathymetry for a sufficient long period (e.g. 10 years after the realignment) to get grip on the mechanisms shaping the morphodynamics at these areas.

Intertidal flats and salt marshes can protect hinterland against wave energy and flooding by tides or storm surges. Thus, the development of tidal flats is important to foresee. The presence of specific small scale features could indicate a certain development and large scale morphological shape. The shape is a predictor for development and it is expected to influence small scale feature formation. Tidal flat management could benefit from technological process, e.g. in drones, if these indicators can be used. Hence, the aim of this research is to find indicators and gather sufficient knowledge to make them applicable.

The main research question is: "Can small scale morphological features on estuarine tidal flats be used as indicators for large scale morphological shape and development?”. The main question contains two aspects. The first facet is whether the small scale morphological features are indicators for large scale morphological shape and development. It focuses on detecting indicators. The second part is about gathering knowledge to properly use the indicators. It is investigated which mechanisms explain the found indicator roles of the small scale morphological features. The focus is on the identification of the mechanisms. Two sub-questions can be extracted from the above:
1. Are the small scale morphological features on estuarine tidal flats indicators for large scale morphological shape and development?
2. Which mechanisms explain the found indicator roles of small scale morphological features?

Indicators are detected with the analysis of aerial photographs and elevation measurements at transects in the Eastern and Western Scheldt. New and already in literature known features are identified and classified in this process. The mechanism that form and erase the small scale morphological features and their dependence on large scale morphological shape are investigated to explain the found indicators. The mechanisms are determined in various ways: Two 1D models are created, a field campaign is conducted, (subsequent) aerial photographs are looked into, elevation measurements are analyzed in detail and literature research is done.

Small scale morphological features on estuarine tidal flats can be used as indicators in the Western Scheldt. This research detects and explains features that indicate large scale morphological shape, development, hydrodynamic conditions and ecological activity of an area. However, the applicability of the indicators in other systems should be proven by extending this research to other situations. A specific sequence of small scale morphological feature categories between salt marsh and waterfront, a build-up, has been identified as indicator for convex large scale morphological shape and indirectly for accreting behavior. All the features in the build-up are indicators on their own. Megaripples, seen within build-up, indicate (future) convex profiles. Furthermore, all of the features indicate a certain relative height and slope when they are seen within the build-up. These individual indicator roles are clarified by elaborating on the hydrodynamic conditions that form and erase the features. Therefore, the prevailing conditions can also be determined with the indicators.