In this study, we focused on the effect of the degree of floc growth on the moving velocity of the interface between sediment of flocculated montmorillonite and supernatant in the semi-dilute suspension, which is characterized by the start of an extremely slow movement of interface followed by an abrupt settling and ending in consolidation. Na-montmorillonite suspension coagulated under different ionic strengths, ranging from 0.5 M to 1.5 M of NaCl, was placed in a settling cylinder. The initial height of suspension was varied from 13 cm to 50 cm. The flocculated suspension was left to settle in the cylinder after the manual mixing of end-over-end. Changes in the height of the interface between the flocculated sediment and the transparent supernatant were measured as a function of elapsed time. It was confirmed that the maximum settling velocity increased with an increase in the height of cylinder within the range of our measurement. This tendency was found to be more significantly pronounced by the growth of flocs. This result indicates the presence of a feed-forward mechanism to enhance the upward motion of fluid or downward motion of flocculated sediment, or both. That means, sedimentation is accelerated by the growth of big flocs, and the growth of big flocs is accelerated by the sedimentation. These motions will eventually induce the generation of an upward plume or channel of water. The formed plume flutters slowly with rather a large scale. We term this phenomenon the "sedimentation turbulence."