Dynamics and spatial variability of near-bottom sediment exchange in the Yangtze Estuary, China

Abstract

This study was conducted to examine the spatial variations in the exchange between near-bottom suspended and sea-bottom sediments in the Yangtze Estuary and adjacent region, as well as to explore the fate of suspended sediments in the study area. The relationship between the sand, silt, and clay contents of the sediments was analyzed by log-ratio analysis, which revealed a non-linear function for selective deposition and a wide range of grain-size distributions in the estuary. This finding does not conform to the nearly constant clay/silt ratios reported for other tidal basins around the world, due to non-linear sediment mixing under complex hydrodynamic conditions. The sediment exchange processes in the Yangtze Estuary were quantified based on the principle of mass balance. The average grain-size distribution of near-bottom suspended sediments from the estuary showed that approximately 49% of the riverine sediments accumulated in the mouth bar area, while the rest, which is primarily composed of fine-grained sediments, is transferred to the outer estuary and deposited in the form of flocs. The spatial distribution of the sediment exchange ratios demonstrated that small amounts of suspended sediment were deposited onto the seabed of the upper estuary (exchange ratio < 0.1), because the fine-grained suspended sediments in this region were transported to the mouth bar area by the ebb-dominated tidal flow. The sediment exchange ratios in the outer estuary also showed very low values (<0.1) due to the oceanic currents offshore that prevented the diffusion of riverine sediments further seaward. Intensive sediment exchange occurred in the inner estuary due to the sand-mud mixing which was controlled by bidirectional tidal flows. In addition, a high sediment exchange ratio occurred in the muddy area (>0.8) seaward of the river mouth, which implies that this is the present-day depocenter of Yangtze mud. The sediment exchange rates obtained by combining the dimensionless exchange ratios and bulk sediment accumulation rates, were found to be 2–3 cm/yr in the muddy depocenter, which extends to the south of the river mouth (from 122° E to 123° E longitude, at 31° N latitude).