Author(s): Federico Maggi; Johan C. Winterwerp
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Keywords: Cohesive suspension; Floc structure; Growth process
Abstract: Early investigations (Wells and Goldberg, 1993, Winterwerp, 1998, Manning and Dyer, 2002, De Boer et al. 2000) on suspended cohesive sediments in natural waters showed the role of aggregate (floc) shape and structure on the overall balance of sediment (e. g. transport, sedimentation, resuspension, etc. ). In particular, the time evolution of size and settling velocity distributions has been widely studied in the laboratory and field. Those studies suggest that attention is to be paid to the features of flocculated sediment at small length scales in order to improve understanding and modelling of the behaviour of natural sediment. In this paper we present a study aimed at characterising the structure of flocs consisting of kaolinite. Our investigation is based on laboratory measurements of aggregates formed in the settling column described in Maggi et al. (2002). Images of flocs are recorded through imaging techniques (PIV) and analysed within the framework of (multi) fractality as proposed in Maggi et al. (2003b). These quantities enable us to establish to which extent a cohesive suspension subject to flocculation processes (aggregation and breakup) produces flocs with complex structure. We use these results to characterise the dynamical features of cohesive sedimentary flocs, by considering their behaviour in time at small length scales. The main result of this experimental work is that the geometry of cohesive sedimentary flocs are (1) scale-dependent (i. e. fractal dimension, porosity and relative density change with floc size) and (2) timeindependent (i. e. flocs of a given size show similar characteristics at different times).
Year: 2004