Modelling of cohesive sediment transport in a tidal lagoon-an annual budget

The annual net sediment flux for an enclosed estuary is usually calculated as a residual value in sediment budget studies based mainly on sediment core dating. In this study, the numerical model MIKE 21 MT was used to directly model the annual net transport of cohesive sediment for the Lister Dyb tidal area in the northern part of the European Wadden Sea. The model was calibrated by use of a combination of field data and calibration parameters derived from existing literature. The model reproduced the hydrodynamics satisfactorily as well as the concentration level and variation of suspended cohesive sediment concentration. Net import for the year 2002 was found to be 45 000 t of cohesive sediment, corresponding to a mean tidal period import of 64 t which compares to an approximate deposition of 0.1 mg l−1 of the tidal prism. Gross annual transport through a control cross section of the tidal inlet was 1 million ton meaning that 4-5% of the sediment transported in suspension was on average deposited within the tidal lagoon. This net transport was found to be rather constant from tidal period to tidal period thus supporting the theory that the processes of settling and scour lag, tidal velocity asymmetry together with sediment floc formation are the controlling processes for import of fine-grained sediment to shallow coastal plain estuaries. Temporary export of sediment from the area occurred during two major wind events in January and February when the maximum loss of sediment during one single tidal period was computed to be 40 000 t which is of the same order as the yearly net import of sediment. This indicates that net annual import of cohesive sediment to shallow estuaries may be much more variable than inferred from core dating studies found in literature.