Cohesive sediment suspended in natural waters is subject not only to transport and deposition processes but also to reactions of flocculation, \textit{i.e.} aggregation of fine particles, and breakup of aggregates. Although aggregation and breakup occur at small and very small length scales compared to transport and deposition, flocculation can effectively have an impact on the large scale as well. Some phenomena that are correlated to (or find roots in) flocculation reactions are, for instance, coastal morphodynamics, pollutant and contaminant transport and deposition, and sedimentation in rivers, estuaries, lakes, canals, harbours and water basins in general. Water environments accommodate a variety of societal functions, like navigation and fish culture, provide the potable water supply and serve the industrial and waste water processes demand. At the same time, natural waters also bear several ecosystem functions that are essential for a healthy environment. The complicated balance between human need and ecosystem safeguard is one of the rationales for studying flocculation of suspended sediment in natural waters. The specific focus of this thesis is directed towards the genesis of flocs by flocculation of cohesive sediment and the behaviour of a population of flocs in aqueous environments. Floc size distribution, floc structure and flocculation models are investigated in this thesis by means of three methods: experiments, mathematical analysis and numerical modeling. The experimental activity has been carried out in the settling column of the Laboratory for Environmental Fluid Mechanics, aimed at collecting information on the population of flocs under different conditions of turbulence intensity. An optical system dedicated to collect images at scales comparable to the floc size has been designed and coupled to the settling column. This has enabled an innovative sight on the properties of a suspension of cohesive sediment, allowing the analysis of the floc size distribution and the structure of individual flocs. The images collected with the optical recording system are elaborated and analysed numerically to assess the properties of sediment flocs. First, the data have been processed to extract black-and-white images of individual flocs. Second, flocs have been characterised by means of several quantities, amongst which the size, fractal dimensions and disorder function. Third, statistics of the population have been calculated. This has been repeated for different experimental conditions. The result is an overall portrait of the time evolution of the floc size distribution, and of the response of a population of flocs to the turbulence field produced in the column at various intensities. In general we observe that flocs grow in time from a (nearly) monodisperse suspension of primary particles, developing a population distributed over a wider range of sizes. On the one hand (the large length scales), the population evolution appears to adapt to the forcing, the process that is coupled to a decrease in spatial entropy of the system and a reshaping of the floc size distributions into a more complex population. On the other hand (the small length scales), floc growth is accompanied by a decrease in fractal dimension and, at the same time, by an increase in geometrical complexity, \textit{i.e.} disorder. The analysis of floc structure and floc size distribution from the experimental results has given an innovative pulse to the modelling of flocculation of cohesive sediment suspensions by means of a new population balance equation. First, the floc structure has been implemented through a fractal model. Second, the population balance equation has been implemented for a full population of flocs. Third, different mechanisms of aggregation and breakup have been examined. The floc size distribution has been then compared with the experimental ones to evaluate the predictive skills of the model. The analysis of the results shows that the implementation of floc structure and kinematic processes at population scales enables a proper prediction capability of the model. The major innovative aspects of the present study are the characterisation of the geometrical structure of the flocs and the interaction among fractal aggregates within a population. This is the source of several considerations on the dynamics of cohesive sediment in particular and flocculating systems in general. However, this could not have been achieved without the experimental data obtained from the settling column, unique with respect to the length and time scales involved, the control parameters and the measuring techniques.