Longitudinal transport of net suspended sediment in the river-dominated Modaomen Estuary of the Pearl River: Effects of river, tide, and mouth bar

The suspended sediment dynamics in estuaries are strongly affected by the interaction of rivers, tides, and morphological factors. However, little is known about the combined impact of these driving factors on longitudinal net sediment transport. A field investigation of current velocity, salinity, and suspended sediment concentration (SSC) was conducted in the Modaomen Estuary of the Pearl River. Hydrological data were simultaneously measured at three mooring stations in the longitudinal direction on July 31, and August 8, 2017, covering a neap tide and a spring tide. Net suspended sediment transport patterns were also analyzed. Current velocity, SSC, and salinity exhibited spatial and temporal variations during the spring–neap tides. Seaward net sediment transport was dominant during the observation period and, vertically, double patterns of net sediment transport inside and outside the mouth bar appeared to occur in the longitudinal profile. The net sediment flux can be divided into three major components—advection sediment transport, tidal-pumping, and vertical circulation. Of these, seaward sediment advection was the dominant component, which generally conformed to the dynamic characteristics in a river-dominated estuary. Salinity intrusion into the bottom layer caused stratification, inhibited vertical diffusion, and enhanced sediment settling, resulting in an elevated SSC in the bottom layer at the mouth bar. Eulerian residual flow primarily contributed to the seaward advection transport. Tidal asymmetries during the ebb–flood tidal cycle enhanced tidal-pumping sediment transport, and the main driving factor, SSC asymmetry, corresponded well with sediment flux. The mouth bar exerted a significant morphological effect on salinity intrusion and promoted longitudinal estuarine circulation, thus affecting longitudinal sediment transport. Our results contribute to a better understanding of the underlying mechanism of suspended sediment transport in complex dynamic environments as well as the feedback between the hydrodynamic structure and morphology in estuaries, facilitating the development of evidence-based guidelines for estuarine and coastal engineering and management.