Determining the Existence of the Fine Sediment Flocculation in the Three Gorges Reservoir

Abstract

Several proofs for the existence of flocculation in the Three Gorges Reservoir (TGR) are found through field measurements and laboratory experiments. The suspended particles and bed sediments are fine, and the particle diameters stay the same along the water depth, indicating that flocculation may occur. Both the vertical distributions of sediment concentrations (at low flow velocities) and measured settling velocities suggest that the effective settling velocities are larger than those of the primary particles, providing strong evidence for flocculation. The flow velocity zones of deposition and transportation are found, and the demarcation velocity of the two zones is approximately 0.5–0.6 m/s. It is thought that flocculation occurs when the flow velocity is smaller than the demarcation velocity; otherwise, the flocs will be disrupted by turbulent shear. Laboratory experiments are conducted by using the sampled bed sediments, and floc structures are also observed, confirming the existence of flocculation in the TGR.