TIDAL VARIATION IN THE SETTLING DIAMETERS OF SUSPENDED MATTER ON A TIDAL MUD FLAT

During one tidal period, measurements of the variation of current velo cities, suspended sediment concentration and settling velocities of th e 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 diamete rs. Current velocities follow a variation pattern well-known from othe r parts of the Wadden Sea. Variations in current velocities are respon sible for the variation in suspended sediment concentration and thereb y indirectly control the equivalent particle sizes, because high suspe nded sediment concentration favours the formation of flocs. Maximum co ncentrations of up to 532 mg/l were recorded at the beginning of the f lood period and at the end of the ebb period, when current velocities are high. This is reflected in the median equivalent settling diameter s, which show corresponding high values of 83 mu m at the beginning of the flood period and 96 mu m at the end of the ebb period.