Experiment inspired numerical modeling of sediment concentration over sand–silt mixtures

•We carried out a series of flume experiments on two types of sand–silt mixtures.•Quasi-stationary high concentration layer was observed near bed under waves.•The vertical suspended sediment concentration profile of silt fraction is different from that of sand fraction.•A multi-fraction sediment model was developed and calibrated for silt classes.•The model provides promising agreements with the measurements in both wave only and wave–current conditions.

A series of flume experiments has been conducted to investigate sediment transport of sand–silt mixtures in both wave-only and wave-with-current conditions. Two types of sediments collected from a typical silty tidal flat were used: a silt-sized mixture with median grain size of 46 μm, and a very fine sand-sized mixture with median grain size of 88 μm. A high concentration layer (HCL) was observed near the bottom together with ripples under wave-only conditions. Sediment concentrations inside the HCL are quasi-stationary with the bulk Richardson number approaching a constant value. The thickness of the HCL can be scaled with approximately two times the damped wave boundary layer thickness. For the concentration profiles, we find that the vertical profile of the silt concentration appears different from the profile of the sand concentration, since the silt concentration decreases logarithmically within HCL, while homogeneously distributes outside the HCL. Finally, the reference concentration formulation of van Rijn (2007b) was recalibrated for the silt classes and applied in a multi-fraction model to predict the vertical concentration profile for silt and sand classes. The results show a promising agreement with the measurements, for both wave-only and wave-with-current conditions.