Influence of the density stratification on the vertical distribution of suspended sediment in a partially mixed estuary

The North Passage of the Changjiang Estuary, also known as the Yangtze Estuary, is a typical partially mixed estuary exhibiting significant variations in density stratification across different reaches. These stratification-induced variations in the vertical profiles of suspended sediment concentration (SSC) significantly impact sediment transport and the trapping of sediment at the estuarine turbidity maximum. This study analyzes multi-site hydrological and sediment data collected in the North Passage during the wet season of 2016, using mathematical and statistical methods to quantitatively assess how density stratification suppresses turbulence and influences SSC profiles, with the hypothesis that density is controlled by salinity. Results indicate that under non-stratified conditions, SSC profiles generally exhibit a linear distribution pattern with minimal variation in vertical gradients. While under stratified conditions, SSC becomes more concentrated near the bottom, resulting in exponential SSC profiles. Under stratified conditions, models assuming vertically uniform eddy diffusivity, which account for the suppression of turbulence by stratification, yield SSC profiles that agree well with measured SSC profiles in the North Passage, highlighting the critical role of stratification in influencing sediment dynamics in estuarine environments.