Tidal variation in the settling diameters of suspended matter on a tidal mud flat

Abstract

During one tidal period, measurements of the variation of current velocities, suspended sediment concentration and settling velocities of the suspended matter were carried out on a tidal mud flat 1.8 km west of Ballum Sluse in the northern part of the Lister Dyb tidal basin. The settling velocities have been converted to equivalent settling diameters. Current velocities follow a variation pattern well-known from other parts of the Wadden Sea. Variations in current velocities are responsible for the variation in suspended sediment concentration and thereby indirectly control the equivalent particle sizes, because high suspended sediment concentration favours the formation of flocs. Maximum concentrations of up to 532 mg/l were recorded at the beginning of the flood period and at the end of the ebb period, when current velocities are high. This is reflected in the median equivalent settling diameters, which show corresponding high values of 83 μm at the beginning of the flood period and 96 μm at the end of the ebb period.