Controlling factors in estuarine flocculation processes: experimental results with material from the Seine Estuary, Northwestern France

Flocculation is one of the main process of sedimentation which modifies suspended particulate matter (SPM) characteristics, i.e. size and settling velocity. A combination of physical, chemical and biological parameters control coagulation and fragmentation of natural samples. A newly developed instrument, the Video In Lab, was used to monitor the kinetics of flocculation. Flocculation tests were carried out on natural freshwater samples collected at multiples sites along the Seine estuary, spanning a variety of seasons. Testing was aimed at quantifying the influence of three main parameters: turbulence, SPM concentration and salinity. The tests show that weak turbulence favours flocculation, whereas strong turbulence limits the maximum floc size. Presence of available organic matter and high SPM concentration enhances kinetics efficiency and floc size. Salinity induces flocculation throughout a threshold action, controlled by SPM concentration, organic matter quality and floc density. These preliminary results emphasise the complexity involved in the understanding of natural flocculation. These processes are controlled by key parameters depending themselves upon the composition of the samples.