Flocculation Measured By Video Based Instruments in the Gironde Estuary During the European Commission SWAMIEE Project

• Published in: Journal of coastal research pp. 58 - 69
• Main Authors: Manning, A. J., Dyer, K. R., Lafite, R., Mikes, D.
• Format: Journal Article
• Language: English
• Published: Coastal Education & Research Foundation (CERF) 01.12.2004
• Subjects: Average linear density; Density; Estuaries; Flocculation; Flow velocity; Mud; Observational research; Particulate matter; Sediments; Shear stress
• ISSN: 0749-0208; 1551-5036

Acquisition of in-situ images of real estuarine mud flocs is a recent (during the past ten years) development. Furthermore, images coupled with the corresponding floc characteristics offer distinct advantage over previous studies. Therefore, an improved understanding of floc structure (e.g. size and shape) and composition (e.g. porosity and mass) could provide scientists with a greater insight into the flocculation of mud particles and their characteristics. Using simultaneous measurements of the distribution of floc sizes and settling velocities collected during the European Commission funded SWAMIEE project, this paper draws on a selection of visual floc images and illustrates the types of aggregates (i.e. flocs), together with their relevant floc properties, which typically comprise a floc population at various points in the tidal cycle. Measurements were carried out in the lower reaches of the Gironde estuary during neap tides, where the near bed region turbulent shear stress ranged between 0.06-0.6 N m⁻², but the near bed suspended particulate matter concentration did not generally exceed 200 mg l⁻¹. The floc size and settling velocity were measured primarily by the low intrusive INSSEV (In Situ Settling Velocity) instrument, with additional optical floc data provided by the Braystoke tube based VIL (Video In Laboratory) system. The study found that either very low turbidity or very high turbulence tended to produce slow settling macroflocs (floc size > 160 μm) with settling velocities of about 1-1.4 mm s⁻¹. Flow periods which generated moderate rates of turbulent shear stress between 0.4-0.47 N m⁻² were found to produce the most productive environment for flocculation. These ambient conditions resulted in the formation of low density "comet-shaped" macroflocs about 0.5-1 mm in length and settling at a rate of 2-3 mm s⁻¹. These macroflocs displayed settling velocities up to 5-6 times greater than those typically applied in flocculation parameterisations for inclusion in vertical mass settling flux simulation modelling. When the optimum shear stress was combined with a sampling run suspended particulate matter concentration peak of 139 mg l⁻¹, the macrofloc fraction only represented 35% of the ambient suspended particulate matter. However, the resultant high macrofloc settling velocity meant that this fraction actually contributed 66% of the mass settling flux. Representation of the Gironde estuary floc characteristics in terms of a constant fractal dimension, did not adequately represent the varied distribution of flocs observed in-situ. The faster settling macroflocs and microflocs had respective fractal dimension values of 2.3 and 2.6, whilst the very porous slower settling macrofloc displayed a fractal dimension of only 2. The sample mean fractal dimensions ranged from 2.1-2.5 for all Gironde estuary floc populations.