A fine-grained sediment budget for the Sylt-Rømø tidal basin

Abstract

A budget for net accumulation of fine-grained sediment (< 63 μm) has been set up for the Sylt-Rømø tidal basin. Net accumulation within the basin was computed from 210Pb core dating and mapping of the intertidal and supratidal surface sediments. It was found that a yearly mean value of 58 · 103 tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64% to the net budget; the fluvial input and primary production contribute 14% and 15%, respectively. Local salt marsh erosion accounts for about 5% of the budget and atmospheric deposition for only 2%. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Rømø tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged water volume between the tidal basin and the sea. This index shows that in the Sylt-Rømø tidal basin, fine-grained suspended sediment "filters" out of the exchanged sea water at a rate that is 12 times lower than in the Grådyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical and hydrodynamical parameters and to a lesser degree of sediment availability.