Morphodynamics of the south Baltic seabed in the remote nearshore zone in the light of field measurements

Abstract

The paper presents results of bathymetric surveys in the remote nearshore zone of the south Baltic (c.a. 1–2 Nm off the shoreline at depths of 16–20 m). Measurements were collected twice in the vicinity of the Coastal Research Station (CRS) in Lubiatowo (Poland), first on November 2017 and then on December 2018. In addition the wave and wind parameters were collected during the “in-between” period of bathymetric measurements. The wind velocities and directions were measured at CRS Lubiatowo while the wave data were collected near CRS Lubiatowo, 1.5 Nm from the shoreline, at the depth of 18 m. The analysis is based on a differential map determined from the bathymetric data, as well as on hydrodynamic conditions between the measurements. The results show changes in the seabed elevation ranging from a few to 70 cm in a year. Sonar measurements were also made in 2017. The images revealed bottom ripples with an approximate height of 5–20 cm and length of 100–200 cm. Theoretical studies focus on the dimensionless shear stresses (Shields parameter) computed from wave and wind data. The uniqueness of this research lies in the fact that at such depths sediment movement has recently been proven and it brings more light to this yet not well-known topic. This paper proves the validity and shows the evidence of recent assumptions of the appearance and migration of rippled forms at the bottom of non-tidal or micro-tidal basins.