Suspended sediment transport across a tidal flat

Abstract

Many investigations have shown that most of the suspended sediment supplied to the Wadden Sea originates from the North Sea. Based on measurements of current velocity, depths and suspended sediment concentrations in the Danish part of the Wadden Sea, computations indicate, however, that suspended sediment is in fact exported to the North Sea. The reason for this discrepancy is twofold: Firstly, measurements have been carried out mainly in tidal channels where the largest amounts of water and suspended sediment are in the main transported during ebb periods. Secondly, computation of net transport in an estuarine environment such as the Wadden Sea always suffers from the difficulty that net transport within each single tidal period is of the same order as the uncertainty of the transport rates during ebb and flood periods respectively. The first problem can be dealt with if measurements are carried out in both tidal channels and over the tidal flats. The second problem can be solved by collecting time series instead of discrete measurements because accumulation of the transport rates found for successive tidal periods will neutralise the relatively large random error. In this paper, time series of suspended sediment flux across a tidal flat covering a full spring neap tidal cycle are presented. They showed net transport of suspended sediment in a landward direction which is mainly caused by flocculation of fine-grained sediment during periods of high concentration level. During such periods the field settling velocities of suspended sediment are so large that the settling- and scour-lag processes can be effective.