Experimental study on fall velocity of fine sediment in the Yangtze Estuary, China

• The variation of vertical position of flocs should not all attribute to the gravitational settling.
• An improved apparatus is presented for measuring setting velocity.
• SSC, salinity and temperature all affect fall velocity (FV), but to different extents.
• The relationships between FV and its various determinants are highly dependent on specific environmental conditions.

Fall velocity (FV) is such a fundamental parameter for sediment researchers that its accurate determination has been regarded as a top priority in improving numerical modelling and conceptual understanding of fine sediment dynamics. With their cohesive nature, fine sediments are prone to aggregation and form flocculation network structures (flocs). The rheological behaviour of fluid may complicate this problem. By means of a new apparatus, FV of fine Yangtze Estuary sediment can be studied in the laboratory. The experimental data show that (1) suspended sediment concentration (SSC), salinity and temperature all affect FV, but to different extents; (2) the relationships between the FV of estuarine fine sediments and its determinants are highly dependent upon specific environmental conditions; (3) the dependencies of various determinants (SSC, salinity and temperature) on FV in different flocculation stages are varied; and (4) for Yangtze estuarine mud, the FV peaks when the SSC is in the range 3–8 g/l, and the salinities for maximum flocculation settling are approximately 7 and 10 PSU in dry and wet seasons, respectively.