Assessing flocculation properties of cohesive sediments using optical settling measurements.
Supervisor: Hachem Kassem
Flocculation processes in coastal environments are determinant to the dynamics of cohesive sediments, which regulate, at some level, erosion and accretion patterns, dispersion of contaminants, carbon storage, and other phenomena that are linked to ecosystem health. The nature of floc size has been studied for decades due to its relevance to its settling, with multiples attempts of defining environmental variables that control floc behaviour, although no dominant influence has been attributed and accepted to an individual variable. This circumstance reinforces the need for studies in the field that can provide more certainty about the drivers to floc size and behaviour. This research aimed at investigating the relationship between floc size, concentration, and shear velocity of three distinct cohesive sediment samples, in an annular flume with salinity and temperature regulation. Purified powders of kaolinite and montmorillonite, and a sediment sample from the Southampton Water estuary were individually tested over progressively increasing concentrations in salt water, under different shear velocities. Suspended particulate matter (SPM) concentration was monitored by an Optical Backscatter Sensor and by filtration of water samples. Current velocity was monitored by an Acoustic Doppler Velocimeter. Water samples were taken from the annular flume and the properties of flocs in suspension were analysed in a laser diffraction particle sizer (Mastersizer 3000) and an automated image processing algorithm developed during the research (Floc Video Capturing – FVC). Results show an overall increase in floc size across all sediments under shear velocities ranging from 12.1 to 37.62 x10-3 m/s, suggesting a strong relationship between the variables. High floc size values were also associated with higher SPM concentration levels, primarily under shear velocities ranging from 3.7 to 6.66 x10-3 m/s. The biggest floc sizes were found within the Southampton Water sample experiment, in which values were greater than 100 μm. This behaviour is theorised to be linked to the presence of organic matter and biological activity, as the SPM concentration values were five to six times lower than those of the other two sediments. Both methods used in the study provided similar trends for floc size, although significant differences were found in between parts of the datasets, with the Mastersizer 3000 registering higher values. The kaolinite analysis using the Mastersizer 3000 resulted in a homogenisation of floc size values, indicating a possible destruction of the flocs during the mechanic stirring inherent to the method. The FVC provided more consistent patterns for all sediments, which is associated with the non-destructive nature of the method. The findings on the floc size relation with SPM concentration and shear velocity agree with other results in the literature, highlight the potential influence of extra non-measured variables, and provide a sample-preserving alternative tool for further studies.