Sandwaves and other large-scale bedforms as indicators of non-tidal surge currents in the Skagerrak off Northern Denmark

Abstract

Results from side-scan sonar surveys made at two sites in the Skagerrak off Hanstholm and Hirtshals show that transverse and longitudinal current-induced bedforms are widespread in both areas down to a water depth of 70 m. A maximum sandwave height of 7 m is found off Hanstholm. Comet marks have a maximum length of near 50 m. Off Hanstholm wave-action markedly affects the seabed at depths of less than 25 m. This can cause significant deformation of current-induced bedforms. The bedforms are not relict structures. They demonstrate that maximum bottom flow velocities associated with the Jutland Current can locally reach a magnitude of at least 70–100 cm/s. Bedform configuration and sediment distribution off Hansholm indicate continuation of the Jutland Current bottom flow axis in a northeast direction, diverging from the coastline. This may favour sand transport in greater water depths than is usually found elsewhere in the area. As an exception, at one occasion at the site off Hirtshals megaripples were found to have southwest-facing leeside slopes and small comet marks were also directed towards the southwest. This must have resulted from exceptional outflow conditions, with bottom current speed in excess of 60 cm/s.