Multi-asymmetry on residual sediment transport in the branching channels of the Yangtze Estuary

Abstract

Tidal asymmetry plays a crucial role in sediment transport and morphological evolution in estuarine environments. While there is more than one tidal asymmetry, their individual contributions to residual sediment transport remain insufficiently quantified. In this study, we introduce a multi-asymmetry approach, utilizing short-term field data to quantify the contributions of flood-ebb asymmetries in water depth, flow velocity, duration, and suspended sediment concentration (SSC) on residual sediment transport. The approach is implemented in the branching channels-North Channel, North Passage, and South Passage, located in the turbidity maximum (TM) of the Yangtze Estuary. The results reveal that in the North Channel, seaward residual sediment transport is primarily driven by asymmetries in current velocity (44 %) and duration (33 %) due to strong river flow. In the North Passage, velocity asymmetry accounts for 39 % in the spring-neap tidal cycle, with a significant contribution from SSC asymmetry (33 %) at neap tides. The asymmetries result in the seaward residual sediment transport due to intensified ebb currents, amplified by human-induced channel modifications. Conversely, the South Passage exhibits landward residual sediment transport with a dominance of SSC asymmetry particularly at neap tides, suggesting offshore sediment supply. The residual sediment transport pattern indicates that the North Channel and North Passage primarily export water and sediment, respectively, whereas the South Passage mainly imports sediment. The inter-channel residual sediment transport circulation plays an important role in sediment trapping and tidal flat accretion in the mouth zone. The findings provide valuable insights into understanding sediment dynamics and help managing branching channels in estuarine systems.