CURRENTS IN TIDAL FLATS DURING STORM SURGES

Abstract

Wind action in the viewpoint of coastal engineering is mainly a topic for discussions or investigations of waves, breakers, storm surges, sand transport on beaches and so on. Offshore currents, generated by wind shear stress concern more the scientific field of oceanographers. But in shallow coastal water wind induced drift currents indeed may be important for coastal engineering problems, as sediment transport, sewage spreading, salinity and so on. For example, along the German North Sea Coast (Fig. 1) we have a rim of extended tidal flats, built up by sand and mud and covered only by a water layer of 1 to 2 m at high tide. At low tide sands and mud flats fall dry. Fig. 2, an aerial view, gives an impression of a typical tidal flat, the "Neuwerker Watt" at the south side of the Elbe Estuary. The distance between main land and the sea side border of the flat is here about 20 km. A lot of investigations have been carried out here - initiated by a harbour planning task - giving some interesting results about currents in the tidal flat area during strong winds and storm surges. For the current measurements a recording current meter (Fig. 3) has been used, fixed at a tripod aluminum frame in 40 cm above bottom. The propeller meter starts at velocities of 5 cm/s. Tests in a flume proved that orbital currents give no disturbance if unidirectional currents of a certain magnitude are superimposed.