New insight into coastal processes in the southern Baltic Sea: relevance to modelling and future scenarios

Abstract

Understanding the relationships between shoreline changes and morpho-geological features of the seabed and hydrodynamics is important for predicting coastal dynamics. The southern Baltic barrier coast is distinguished by rhythmic shoreline features of different scales, forming an erosion-accumulation system. This study is an attempt to explain the relationships between shoreface-connected, obliquely oriented sand ridges, the distribution of wave energy, and the impact of waves on shoreline changes. A 28.5 km stretch of the Polish coast, up to 2 km offshore, was investigated using a multibeam echosounder, a sub-bottom profiler, a side-scan sonar, and sediment coring. Analyses of a DTM, aerial photographs and maps from the late 19th century show that the boundaries between accretion and erosion zones on the coast have shifted eastward at a rate of ~10–11 m/year. Hydrodynamic modeling shows that depressions between the ridges are “energy windows” through which higher energy waves reach the shore. The asymmetry of the ridges and their orientation relative to the prevailing direction of waves suggest that they have been moving eastward. Measurements of ¹³⁷Cs content in the sediment cores prove that the thickness of the dynamic layer exceeds 1.5–2.0 m. When large-scale sand ridges migrate, “energy windows” also migrate with them, as does the entire erosion-accretion system. In conclusion, it is highly likely that the average shoreline retreat measured on the scale of a few centuries and dozens of kilometres may be smaller than we suspect based on observations made in recent decades.