Large-scale bedforms, sediment distribution and sand mobility in the eastern North Sea off the Danish west coast

Two side-scan sonar surveys were carried out 9 months apart in a 24-km² off Thorsminde at the Danish west coast, were water depth is 12-18 m. Sediment samples were systematically collected in the same area using a 250-m sampling grid. The North Sea study area is exposed to strong wind-induced coastal currents and high waves, and thereby characterized by significant sediment transport. The seabed data were used to analyse the large-scale dynamical bedforms found in the area, the mobility of the bedforms, the areal variation of the seabed mean grain-size and the grain-size variation across the large bedforms. The compilation of side-scan sonar mosaics revealed very large bedforms, with a mean wavelength of approximately 500 m and heights of 1-3 m. These bedforms indicate a north-going net sediment transport, caused by the Jutland Coastal Current. Large dunes (L = 10-100 m) and smaller bedforms were also recorded, as well as large shoreface-connected ridges with a spacing of 3-4 km. The bedform configuration appears to be relatively stable, only small changes having been observed between the two surveys, even though a 100-yr cyclone and several storms occurred between the two surveys. The small changes observed, also indicate a net northerly sediment transport. The surface sediments reveal a remarkably high positive correlation between the mean grain size and the position across the very large flow-transverse bedforms, from coarse sand in the troughs to fine sand at the crest and lee-side slope of the bedforms.