Roles of advection and sediment resuspension-settling in the turbidity maximum zone of the Changjiang Estuary, China

Abstract

Understanding mechanisms of suspended sediment transport in estuaries is critical for predicting estuarine geomorphic evolution, managing navigation and fisheries, and monitoring the health of estuarine ecosystems. Yet monitoring suspended sediment transport in estuaries is challenging due to the influences of both riverine and marine processes, and traditional in situ observations cannot directly differentiate contributions of multiple physical processes (e.g., advection and resuspension/settling). Here we investigated the effects of advection and sediment resuspension/settling on suspended sediment transport within the turbidity maximum zone at the South Passage of the Changjiang Estuary. The Changjiang catchment is notable for its long history of intensive human activities including dam construction, which has caused significant geomorphic changes along the river and its estuary. We conducted synchronized hydrodynamic measurements at three monitoring stations along the river channel and imported time series of current velocity and suspended sediment concentration (SSC) at these stations into a simplified, one-dimensional box model to separate in situ SSC into advection- and resuspension/settling-induced components. Our results show that high SSC consisted of a background SSC of 0.93 kg m−3 maintained by advection that contributes to 97.9% of the total average SSC, and a fluctuating SSC controlled by sediment resuspension and settling. The background SSC was higher than the fluctuating SSC at all times, indicating that sediment advection was stronger than local sediment resuspension in the river channel. We compared time series of SSC components, bed-level change and suspended sediment transport rates in the South Passage with previous observations in the nearby subaqueous delta, and found that the South Passage of the Changjiang Estuary experienced net accretion during summer because of the relatively high background SSC maintained by advection of fluvial sediment. In contrast, the subaqueous delta experienced erosion due to strong local sediment resuspension and weak sediment delivery by advection. Our results highlight the spatial variability in the magnitudes of sediment advection and resuspension/settling as key drivers of geomorphic changes of deltas and estuarine channels in response to sediment reduction caused by upstream dam construction.