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.