The flow velocities in tidal channels are already rather complex by the presence of various tidal components, wind driven flow and estuarine circulations. An extra level of complexity is introduced when the flow on top of an intertidal flat is considered (Le Hir, 2000). This research aims at understanding the complex flow patterns on top of a large-scale intertidal flat and on assessing the morphological consequences. The focus of this study is on the Roggenplaat, which is with an intertidal area of 14.6 km2 the largest intertidal flat fully surrounded by channels of the Eastern Scheldt (The Netherlands, see Figure 1). The flat is subject to a mean tidal range of 2.6 m and is characterized by a typical sediment grain size of 0.25 mm. Two large tidal creeks in the Northwest are the remainder of the merging of separate flats 80-150 years ago. Since the late 1980s, the flats in the Eastern Scheldt have been eroding severely because of the construction of a storm surge barrier and various compartment dams (Louters, 1998). A nourishment of 1.65 million m3 is planned on this flat for 2017, to compensate for its lowering. This study combines the results of an Acoustic Doppler Current Profiler (ADCP) measurement campaign with the results of a numerical model. Apart from validation material for the numerical model, the ADCP data is also analysed individually. The focus of this study is on the present-day hydrodynamics and morphodynamics of the Roggenplaat, which is essential knowledge for the design of appropriate nourishment strategies. Furthermore, physical insights achieved in this study are relevant for the understanding of other large-scale intertidal flats around the world.@en

Three key factors that caused the erosion at the lower part of the intertidal area are identified: (1) the peak of the storm (with the highest waves) occurred during low water leaving the high elevations unexposed; (2) the storm surge induced unusually high flow velocities during low water; (3) the water depth at the lower part of the flat was small (±1 m) for several consecutive hours causing the waves to be highly effective. A low-water storm is therefore very effective in eroding the bed and should be considered as an important morphological event for specific parts of the tidal flats. Despite the fast recovery of the bed, the initial bed level was not reached. This implies that the storm has a longterm effect as well. Although long-term morphological consequences seem limited by the fast recovery, ecological consequences are expected to be substantial. @en