Cohesive sediment is generally able to flocculate and create flocs, which are larger than individual particles and less dense. The phenomenon of flocculation has an important role in sediment transport processes of settling, deposition, and erosion. In this study, well controlled laboratory experiments were performed to investigate the effect of key hydrodynamic parameters: suspended sediment concentration (c) and salinity (S) on floc size and settling velocity. Experimental research was conducted in a 1L glass beaker of 11 cm diameter using suspended sediment samples from the Severn Estuary. A PIV system and image processing routine were used to measure the floc size distribution and settling velocity. This study found that the flocculation is enhanced with increasing sediment concentration but not with increasing salinity. The settling velocity was found to range from 0.2 and 1.2 mm/s. Settling velocity changed in the case of increasing suspended sediment concentration (SSC) was controlled by the value of salinity. The faster settling velocity rates occurred for the higher concentrations, as well as for low salinities of S 2.5 ppt. On the other hand, at high salinity of 20 ppt alongside increasing c, the situation was reversed, i.e., the lower the sediment concentration the faster the settling velocity, was found.