Observations of fine suspended sediment processes in the turbidity maximum at the north passage of the Changjiang Estuary, China

Shipborne vertical profiles of horizontal current speed/direction, salinity and suspended sediment concentration were measured on two separate spring tides (30-31 Oct. 1993; 28-29 May 1994) at two ends (head and mouth) of the North Passage, respectively, in the Changjiang Estuary, East China Sea. Fine sediment suspension concentrations were monitored by using 0.5 and 1.5 MHz acoustic monitors. These data reveal bottom boundary layer fine sediment processes which are important to the formation of the turbidity maximum. The dynamic nature of fine sediment processes in the turbidity maximum were also observed by acoustic imaging. Tidal resuspension and estuarine baroclinic circulation are dominant processes for the formation of the turbidity maximum at the head (landward end) of the North Passage. The former is due to tidal asymmetry while the latter is caused by gravitational circulation due to density difference between the river runoff and the intruded seawater. The dominant processes are resuspension of the cohesive mud bed, and re-entrainment of the near-bed high concentration suspensions by tidal shear flow at the mouth (seaward end) of the North Passage. Mid-water high turbidity suspensions were present following both maximum flood and ebb tides, especially at the mouth of the North Passage. It is unclear how these suspensions are formed by estuarine processes, but sediment flocculation effects are important in this region. They are also important to the formation of the turbidity maximum there.