Observations of the Properties of Flocculated Cohesive Sediment in Three Western European Estuaries

• Published in: Journal of coastal research 2004-12, p.70-81
• Main Author: Manning, A. J.
• Format: Article
• Language: English
• Subjects: Density; Estuaries; Flocculation; Mud; Observational research; Particulate matter; Sediment transport; Sediments; Suspended solids; Turbidity
• ISSN: 0749-0208; 1551-5036

The inability to adequately measure floc properties, due to their fragility, has posed a distinct problem in the understanding of the flocculation process. Therefore, within the framework of three recent European Commission funded projects — COSINUS, SWAMIEE and INTRMUD - in-situ measurements of floc size and settling velocity were made using the video-based INSSEV (In-Situ SEttling Velocity) instrument. This acquisition technique enabled the calculation of floc effective density values by using a Stokes' Law relationship. This paper utilises in-situ data collected from experiments conducted in the Tamar (UK), Gironde (France) and Dollard (The Netherlands) estuaries, with the purpose of examining how floc properties vary in tidal waters and identifying the factors which influence their resultant characteristics. Floc populations were seen to exist as combinations of macroflocs (floc size > 160 mm) and microflocs, with the macroflocs tending to represent 38-70% of the suspended particulate matter (SPM) concentration during periods of low turbidity. For higher concentration suspensions in excess of 5 g 1- 1, the microflocs constituted less than 10% of the SPM. The distribution of particulate matter throughout the macro- and microfloc sub-populations showed a direct correlation with the suspended solids concentration. To quantify this inter-relationship, the dimensionless SPM ratio parameter is introduced and is calculated by dividing the percentage of macrofloc SPM, in a single floc population, by the remaining of percentage of microfloc SPM. An SPM ratio of unity would represent an equal apportioning of the floc mass between the two floc sub-populations. An empirical relationship representing the SPM ratio in terms of SPM (mg 1- 1) is: SPM ratio = 0.815 + 0.00318 SPM – 0.14×10-6 SPM2. Qualitatively, macroflocs are seen to exist as: "cluster", "stringer" or "comet" types, which collectively demonstrate effective densities of less than 90 kg m-3. Average macrofloc settling velocities ranged from 2 mm s-1 for the low SPM concentrations, such as those experienced in the Dollard estuary during neap tides, up to 4-6 mm s-1 for floc populations observed within a concentrated benthic suspension (CBS) layer in the Tamar estuary. In contrast the microflocs, from which the macroflocs are primarily composed, demonstrated effective densities ranging between 30 to 1000 kg m-3. From the data two main types of microfloc can be identified. The first is a high density microf