Suspended sediment transport on a temperate, microtidal mudflat, the Danish Wadden Sea

Abstract

Time series measurements of suspended sediment transport and bed level change on a microtidal mudflat of the Danish Wadden Sea were carried out between February 1997 and September 2000. The data show that the mudflat is generally accreting except for periods with onshore winds when wave-induced erosion takes place during periods of low water levels at the site. In general, a continuous net input of mud was recorded during periods of weak or offshore winds, i.e. the import is not event controlled, and this was interrupted by episodic loss of mud during a few tidal periods with onshore winds. The net landward transport of suspended sediment during calm weather or periods of offshore wind is caused by settling and scour lag and tidal asymmetry. A very large input of suspended material was observed in the period following a strong storm, the landward directed net transport during the five tidal periods following the storm being equivalent to approximately 40% of the net annual accumulation in the area. The typical depth of sediment reworking at the intertidal study site is 2–5 cm. Some seasonal variation can be deduced from the data set with a tendency for deposition during spring and summer and erosion in the winter period. The seasonality was especially clear at the most landward station where accretion following periods of ice-formation in the area is due to grounding of sediment transported by ice-floes but otherwise deposition is mainly taking place in the warmer seasons. Net accretion over the three-year measuring period along the transect varies between 0.6 and 1.9 cm a−1. Deposition is largest at the most landward station and the accretion found on the basis of repeated bed level measurements compares well with 210Pb and 137Cs dating.