| Abstract: | The nearshore region of the German Bight is characterised by a large tidal flat area of 7500km2, which is a wedge of alluvial material lying in front of the coast. The elevation of a flat depends on the long-term water-level and currents. As a result of the rising water-level, the flats are migrating over an inclined Pleistocene basement. This migration depends on the balance of sedimentation versus erosion which is a function of water depth. Regular measurements of the water-level have been made since the middle of the 19th century. From the observations of the MHW it is estimated that the water-level has risen to approximately 30cm in the past hundred years. Recent investigations show that the change of the half-tide water-level is less than that of MHW. The increasing tidal range also changes the sedimentation processes over the tidal flats. The quantitative influence of the altered tidal range on the sedimentation and the morphology of the Wadden Sea is as important as that of the influence of mean water-level elevation. Changes in the elevation of the coastal plain caused by tectonic forces or subsidence cannot be determined. Extensive investigations were conducted to determine quantitatively the sedimentation processes which have taken place since 1936. The sediment budget of the Flackstrom basin (low macrotidal coast) is presented. With respect to the MHW datum plane, sedimentation of 20 × 106m3 has taken place in this area. This is equivalent to a mean silting-up of 30cm over the past 46 years, an average 0.7cm/year. The sedimentation process is dependent upon the mean elevation of the flat. Most of the sedimentation has taken place above the half-tide water-level. Therefore, the flanks of the flats have risen more than the tops and the sand-ridges, which certainly reflects the very small long-term water-level elevation change of only 3cm which occurred during the period investigated (1936 to 1982). Recent investigations of the present state show a relationship between the tidal range and the height of the main sedimentation-zone. Comparisons of the Flackstrom (tidal range approximately 3.40m) and parts of the Dutch Wadden Sea (tidal range approximately 1.80m) indicate that the main sedimentation-zone moves down-shore from half-tide water-level to mean-low-water with the decrease of the tidal range, due to differences in the general course of the hypsometric curves over the tidal flat areas. |
