Analysis of the prototype sediment collected from the Three Gorges reservoir (TGR) indicates roughly 80% of the accumulated sediment is less than 20 μm in size

but it remains unclear whether such sediment mixture could form flocs while settling. This study has designed a coaxial double-cylinder rotational flocculation device for non-intrusive observation of the flocculation and settling of sediment collected from TGR

and conducted two series of tests

modelling the water-sediment conditions of the reservoir in the laboratory and adopting the image technology to identify settling particles. The results show that the measured settling velocity of particles is significantly different from those of single sediment particles of the same size

and a significant portion of settling particles exceeds the maximum size of the input sediment

indicating the flocs of TGR sediment would form in the prototype reservoir. Then

we develop a relationship between Reynolds number and drag coefficient for the flocculated sediments

and derive the density of the flocs based on the fractal theory

and a relationship between flocs’ effective density and size. Finally

we develop two methods for calculating the settling velocity of sediment-induced flocs

and verify them against measurements

showing a satisfactory agreement.