Sediment flux and sediment-induced stratification in the Changjiang Estuary

Abstract

Sediment deposition in the north passage of the Changjiang Estuary, where the Deep-water Navigation Channel (DNC) is located, has been a major concern in the past decades. To understand the suspended sediment dynamics and the effects of sediment-induced stratification on sediment flux in the navigational channel, field data on tidal flow and suspended sediment concentration (SSC) are collected and analyzed in this study. It is shown that net sediment transport is dominated by ebb currents in the study area. The net sediment flux is generally toward the ocean and the maximum value is found to be in the middle reach of the passage. In the lower reach of the passage, the net sediment flux is landward in the lower layer and seaward in the upper layer of the water column due to the two-layer feature of the estuarine circulation. Advective flux plays a significant role in transport of sediment in upper and middle reaches of the passage by carrying 70–100% of the suspended sediment. However, this amount is reduced to 30–60% in lower reach of the passage, where tidal effects become more important. The suspended sediment-induced stratification in the north passage is examined by calculating eddy viscosity. It is found that suspended sediment can reduce eddy viscosity by 10–30%. The highest depth-averaged SSC is located in the middle reach of the north passage, where the averaged SSC is 4–15 times higher than that in the upper reach. In this region, bed shear stress is larger at ebb, while SSC is higher at flood. It is inferred that suspended sediments in the DNC during flood are partially transported from a neighboring shoal, which plays an important role in sediment dynamics in the north passage.