Modelling graded sediment transport and bed evolution in a tidal harbour

This paper presents the development of a sediment transport model to predict the bed evolution processes in estuarine and coastal waters. The model is based on an existing hydrodynamic and sediment transport model, with significant refinements being made to enhance its capability for simulating the transport of graded sediments under nonequilibrium conditions. A multifraction sediment transport model has been developed to replace the existing single-fraction model. The sediment mixture is divided into several fractions according to the grain size. For each fraction, the particle size is considered to be uniform, and its transport form, either as suspended load or bed load, is determined by the magnitude of a suspension index. A bed evolution model is developed to simulate the processes of bed level change and sediment grain size sorting. The model is applied to a laboratory model harbour, for which measurements of tidal currents and bed level inside the harbour are available. The water level, velocity distributions, and bed level changes predicted by the numerical model are compared with the laboratory data. Comparisons are also made between predictions made by the fractional model and the single-size model. The effect of bed sediment size change on the erosion process has been investigated