Deposition of fine-grained suspended sediment from tidal currents

Reliance on the commonly expressed assumption that fine-grained sediment is deposited from tidal systems only at slack tide is found to give rates of deposition much lower than those actually found in the German Bight. A model of quasi-continuous deposition is formulated with the assumption that the viscous sublayer of the turbulent boundary layer accepts sediment by settling but does not eject it back into the main flow. This model is only valid at low shear values (U*<1.2 cm sec−1) and accounts for rates of deposition in the German Bight where these values are obtained. Recent studies of the nature of the viscous sublayer have shown that it is periodically disrupted with fluid ejection as the result. It is argued that there is a slight excess of ejection over input for sediment and that rates of deposition by the quasi-continuous model will be somewhat too high. Application of these models to mud layers in tidally deposited sands and in flaser bedding shows that these layers cannot be produced during slack tide. The time is too short, and suspended sediment concentrations are too low. These layers must represent some periodicity other than that of tides, possibly that of storm and calm conditions.