DYNAMICS AND MORPHOLOGY OF SAND BANKS IN THE SURF ZONE OF OUTER TIDAL FLATS

Extended sand banks ranging up to some hundred acres with a crest height in the MHW-level are typical structures of the outer tidal flats of the south-eastern North Sea coast (Fig. 1). Primary forms grow up in the surf zone at the sea-side tidal flat border. They are formed like bars and migrate towards the coast with a surprisingly high velocity, up to 150 m/year. The movement is caused by strong erosion on the sea-side slope - which is shaped beach-like - and deposition at the steeper land-side (lee-side) slope. It was found that surf action at the bar is linked with strong unidirectional currents across the crest, Up to 100 cm/s. The currents are most likely generated by wave set-up in front of the bank and by wind-drift water motion.