Evolution of the floc size distribution of cohesive sediments
2010
Mietta, F.
This thesis focuses on the flocculation process of cohesive sediment (mud) and in particular on the time evolution of the floc size distribution. Mud is mainly observed in estuarine environment and its settling velocity is strongly affected by its floc size distribution. Small flocs (1-50 micrometers) have a very small settling velocity and tend to remain in suspension while large flocs (50 micrometers-centimeters) are characterized by a large settling velocity and deposit rapidly inducing bed siltation. Moreover, small suspended flocs reduce light transmission through the water column and may carry pollutants through large distances. The investigation of the evolution of the floc size distribution is therefore relevant in the fields of civil engineering and ecology. Flocculation is the combination of the processes of aggregation and breakup and is influenced by the sediment and water properties, sediment concentration and turbulent motion. The relation between small scale particle-particles interactions and large-scale flocculation behavior is investigated in this thesis through both experimental and modeling work. In particular a series of coupled experiments where both the surface charge of the particles, quantified by their zeta-potential, and the flocculation behavior are observed. Flocculation is then modeled by means of a population balance equation where flocs are distributed in size classes and aggregation and breakup represent respectively birth and death of flocs in each class.
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