The pattern of large-scale bed forms in the Langeland Belt (Baltic Sea)

Abstract

The Langeland Belt forms the southern portion of the main water-exchange channel (Great Belt) which connects the brackish Baltic Sea with the fully marine North Sea. Strong, pulsating aperiodic currents generated in this channel are governed entirely by sea-level and density differences between the adjacent basins; they are thus strictly meteorologic currents. Tidal currents are of some importance in superimposing their amplitudes on undirectional currents. Side-scan sonar surveying revealed the direct influence of these currents in molding bed forms. The rich inventory of large-scale bed forms detected in these surveys consists mainly of the following: 1. (1) Sand ribbons: thin streaks of sand migrating over a lag-sediment bottom; width between 1 and 60 m, length generally more than 100 m. 2. (2) Comet marks: erosional areas down-current from obstacles; lengths up to or exceeding 100 m. 3. (3) Sand shadows: tails of sand deposited behind larger obstacles such as isolated megaripples on a lag-sediment bottom. 4. (4) Megaripples with wavelengths between 10 and 100 m which occur in several large fields; smaller megaripples occur on sand ribbons. The bed forms (1)–(3) form new or revised contributions to the family of large-scale longitudinal bed forms already established by other authors using side-scan sonar. The regional distribution of these bed forms is related to sand availability, current strength and water depth. Direction and length of comet marks proved to be the most useful indicators of current direction and of relative current strength. Based on bed-form morphology, the maximum current velocity lies in the center of the channel and is responsible for the largest comet marks, sand ribbons and sand shadows, but not for the megaripples. The distribution of current directions as interpreted from bed-form morphology and orientation, allows a detailed reconstruction of the current paths and shows the domination of inflow conditions. The bed forms mapped represent rather extreme current conditions. This is also indicated by the fact that traverses rerun after 14 months showed essentially no change in the large-scale bed-form pattern.