Mechanism of tidal driven vertical suspended sediment transport in Dumai estuary

In tide-dominated estuaries, the peak of suspended sediment concentration (SSC) is closely linked to estuarine processes. The response of SSC to current velocity is not instantaneous but exhibits a phase lag, and the SSC amplitude decreases with height above the estuary bottom. This paper demonstrates this phenomenon using an analytical model of vertical sediment dispersion in the Dumai estuary. Initially, we found a low correlation between the observed SSC and the 2D horizontal transport model (MIKE 21 Mud Transport). We then employed a vertical sediment deposition one-dimensional water column model, incorporating settling velocity and tidal effects in the boundary value problem. The analytical solution, derived through the separation of variables and the eigen function expansion, revealed that horizontal SSC dispersion at low tide is longer than at high tide, indicating stronger tidal effects at the estuary entrance, where sediment accumulation is likely. By considering the transport of the simulation results from the vertical SSC to the horizontal SSC at high tide conditions, the correlation increased. These observations suggest increased surface sediment concentration due to re-suspension and strong mixing conditions. Additionally, we found that the phase lag effect indicates that SSC concentrations are depth-dependent, with sediment accumulation following the tidal cycle in deeper waters and occurring after one tidal phase in shallower waters.