Investigation of Flocculation Characteristics in the Yangtze Estuary

Abstract

The dynamic process of flocculation is crucial for the transport and fate of cohesive sediment and adherent nutrients, heavy metals and pollutants etc. In order to improve the understanding of sediment flocculation dynamics in the estuary, field surveys in the Yangtze Estuary and laboratory experiments with natural mixed sediment were conducted. In-situ observations showed that the mean floc diameters (DM) ranged from 14 to 95 μm, and there were significant temporal and vertical fluctuations within a tidal cycle and between spring-neap cycles. Both high and low slack waters had greater DM, and neap tide’s average floc size was determined to be 57% larger than spring tide. The fractal dimension of flocs in the YE was primarily between 1.5 and 2.1, and effective density of flocs declined as floc size increased. According to experimental findings, DM in the 7 PSU saltwater first increased with rising shear rate G below a threshold of G = 20–30 s−1, then decreased with additional increases in G. Turbulent shear rate plays a crucial role in altering floc structure. Flocs formed in large shear forces were more compacted and had higher fractal dimensions. These studies contribute to the understanding of fine sediment dynamics, and facilitate the researches on reservoir, port and waterway sedimentation and pollutant transportation etc.