Gratiot Nicolas, Manning A. J. (2004). An experimental investigation of floc characteristics in a diffusive turbulent flow. Journal of Coastal Research, Special Iss. 41, p. 105-113. ISSN 0749-0208.
Titre du document
An experimental investigation of floc characteristics in a diffusive turbulent flow
Journal of Coastal Research, 2004,
Special Iss. 41, p. 105-113 ISSN 0749-0208
The work presented aimed at investigating the influence of the physical processes effecting mud flocculation in a grid stirred experiment. Two natural muds from the Gironde estuary (organic and another with the organic content removed) and mud from the Tamar estuary (organic) were used to examine the influence of the biological and chemical compositions on interparticular collision efficiency (i.e. stickiness). For every experiment, the mean sediment concentration were deduced from extracted water samples and by optical methods. The turbulent shearing was estimated from high frequency measurements of the velocity profile. Floe size and settling velocity were obtained by high resolution video observational techniques. The study showed that particles are advected with the large scale turbulent eddies from low to high turbulent shearing regions. It does not imply an instantaneous response and floe properties remained predominantly homogeneous in the water column of the experimental tank. From the measurements made, it is therefore possible to conclude that time scales associated to the turbulent transport are much smaller than the ones associated to flocculation processes. As sediment concentrations increased up to 8 g(.)l(-1), the increase in particle collision frequency enhanced the flocculation process to such a high degree that only a small percentage of the total floe population fell within the microfloc (< 120 mu m) size range. Flocculation is especially significant for trials with organic muds and the effect of floe break-up by inter-particular collisions was not observed, at all shear and concentration levels, during our study. The Gironde Mud with the organic components neutralised produced only a minor number of flocs, at all shear and suspended concentration combinations, which exceeded the pre-sieving mesh size of 125 mu m. This highlights the important role organic matter plays in the flocculation of natural cohesive sediments. This paper compares the distribution of size and settling velocity of macroflocs and microflocs and mean floe properties for the different sediment types under the various experimental concentration and shear ranges. The findings highlight how floe properties change in response to their ambient environment.
