Abstract:

Water and suspended sediment transport is a key process related to the erosion-deposition evolution of an estuary. The previous studies were mostly based on single fixed-point observations in flood season. However, understanding of the situations in the dry season and the distribution along the cross section is insufficient. Based on navigational observations along a cross section during spring and neap tides in January 2021, this study investigated the cross-sectional distributions of current velocity, suspended sediment concentration (SSC), water and sediment fluxes, as well as the characteristics of water and sediment transports during the dry season in the upper reach of the North Branch of the Yangtze Estuary. Results indicate that there are differences in start/end times of flood and ebb tides between different locations in the cross section: earliest at shallow waters on the south side, next at deep trough on the north side, and latest near the deep trough in the northern section, inducing differences in cross-sectional distribution of current velocity at different times. The average current velocities during flood and ebb tides both obviously decrease from deep waters on the north side to shallow waters on the south side. The cross-sectional distribution of average SSC during flood and ebb tides is related to the magnitude of SSC: higher at shallow waters on the middle and south sides than at deep waters on the north side when SSC is low; however, with minor difference in the cross section when SSC is high during spring tides. The water and unit-width sediment fluxes indicate a decreasing trend from deep waters on the north side to shallow waters on the south side during both spring and neap tides, consistent with the cross-sectional distribution of current velocity and water depth. The net sediment transport in a tidal cycle reveals a pattern of moving seaward at deep waters on the north side, and landward at shallow waters on the middle and south sides during neap tides. During spring tides, the extent of landward sediment transport in the cross section is larger, and the volume of landward transport is significantly larger than seaward transport, as well as landward transport during neap tides. This study facilitates a deeper understanding of water and sediment transport characteristics in the upper reach of the North Branch and provides a foundation for further exploration of the control factors of geomorphological erosion and deposition.

Key words: current velocity, suspended sediment concentration, water and sediment transport, cross-sectional distribution, North Branch of the Yangtze Estuary