The present work concerns coagulation-flocculation in drinking water treatment units. This study focuses on the link between hydrodynamics and floc size distribution. Experimental analysis of both floc size distributions and local hydrodynamics are performed in a jar-test vessel. In this study, water quality (pH) and coagulant (both type and dose) are fixed. Floe size distributions are analysed by image processing. The temporal evolution of the floc size distributions is determined from a statistical analysis of experimental data. It leads to basic information on floc agglomeration kinetics. The velocity field is analysed using Particle Image Velocimetry (PIV). The statistical analysis of the velocity field leads to the determination of the average velocity field and the average velocity gradients. In addition, instantaneous velocity fields and associated instantaneous velocity gradients are determined. It is shown to be important to locate their maximum values. The analysis highlights the close relationship between the characteristic size of the flocs and hydrodynamics, that can be expressed in terms of local dissipation of turbulence or in terms of velocity gradient.