Driving forces of sandy sediment transport beyond the surf zone

The paper deals with experimental and theoretical investigations of forces that drive sediment motion beyond the surf zone of the southern Baltic Sea. The study site is located in the sandy coastal zone at Lubiatowo (Poland). Field surveys were carried out by the Institute of Hydro-Engineering of the Polish Academy of Sciences (IBW PAN) and the Maritime Institute in Gdańsk (IMG). The measurements comprise parameters of wind, waves and currents. The wind velocities and directions were recorded at the IBW PAN Coastal Research Station (CRS) in Lubiatowo, while the wave and current data were collected near CRS Lubiatowo, ca. 1.5 Nm from the shoreline, at a depth of 17 m. Theoretical investigations concern wind-induced currents, nearbed wave-induced oscillatory velocities and wave-current interactions. The concept of the apparent roughness related to the wave bed boundary layer is used in the description of wind-induced steady flow. A theoretical model of the wind-induced current is proposed in two variants, depending on the predominance of wave or current impact. The wind-induced flow model is successfully verified using measured current velocity profiles. Previously developed at IBW PAN, a three-layer sediment transport model is adapted to the study site and applied in calculations of sediment transport rates. Keywords: Wind-induced current, Wave–current interactions, Apparent roughness, Sediment transport rate