In the present study, we aim to parameterize a flocculation model, based on a logistic growth equation, by conducting laboratory experiments. The flocculation experiments are performed using two types of natural sediments and different flocculating agents: salt (monovalent and divalent), extracellular polymeric substances, and living and dead microalgae Skeletonema costatum. It was found that the median size of flocs (D50) did not exceed the Kolmogorov microscale when salt-induced flocculation was performed (in the absence of organic matter), which is in line with previous studies. Flocs with organic matter reach sizes that are larger than the Kolmogorov microscale, and both their growth and steady-state size are salinity-dependent. In particular, divalent salts are shown to promote flocculation of sediment to organic matter. The logistic growth model can be used to study either the evolution of a class volume concentration as function of time or the change in size of a given class as function of time. The fine particle volume concentration decreases in time, whereas the coarse particle volume concentration increases, during the flocculation process. The mass balance between the two classes as defined by Chassagne and Safar (Modelling flocculation: Towards an integration in large-scale sediment transport models. Marine Geology. 2020 Dec 1;430:106361) is estimated.@en

Coastal areas are subjected to major anthropogenic influences, as they are traditionally economically important regions, which is reflected by the presence of harbours, especially at river mouths. The Dutch coastal area is influenced by the discharge of fresh water from the Rhine river that creates the Rhine Region Of Freshwater Influence (Rhine -ROFI). There is also an additional sediment supply by alongshore transport resulting from seabed or coastal erosion. The transport of sediment is primarily driven by hydrodynamics. River plumes that pass the estuaries reaching the coastal areas play an important role in terms of Suspended Particulate Matter (SPM) formation and transport, especially in ROFI regions. SPM is defined as a suspension of microscopic particles consisting of clay minerals (sediment) aggregated or not with organic matter. Aggregated particles (flocs) are composed of different fractions of inorganic and organic parts. Flocculation and aggregation is greatly promoted in saline environment, and sediment particles are thereby more prone to flocculate in coastal regions, leading to different transport, settling, deposition and erosion dynamics as compared to freshwater conditions. The general aim of this thesis is to present a flocculation model that properly predicts SPM formation by flocculation in space and time that can easily be implemented in numerical sediment transport models. @en

An 11 hours survey was performed on the 17th of September 2014 in the Rhine Region Of Freshwater Influence (Rhine-ROFI) about 10 km downstream of the mouth of the Rotterdam Waterway during calm weather conditions. Suspended Particle Matter (SPM) measurements were performed during a full tidal cycle, near the seabed, at neap tide, and samples were taken at 0.6 meter above bed for on-board analysis. The measurements were performed with (a) LISST 100X, a submersible particle size analyzer, (b) LISST-HOLO, a submersible digital holographic camera, (c) a home-made underwater camera and (d) an on-board LabSFLOC2 video microscopy equipment that used in-situ collected samples. The first aim of the present study was to compare the results obtained from the different monitoring techniques and to characterize the different types of suspended particles found in-situ. It was found that that the highly anisotropic particles present in the water column lead to multiple peaks in the Particle Size Distributions (PSD) found using the LISST 100X. Using the LISST-HOLO, underwater camera and LabSFLOC2 camera these particles could properly be imaged and meaningful PSD’s were obtained using these techniques in the size range > 20 μm. LabSFLOC2, LISST-HOLO and the underwater camera moreover provide information on the size and aspect ratio of particles. On the other hand, LISST 100X can be used to detect the fine fraction (<20 μm), a size range that is not accessible for the other techniques. From the analysis of the data on the survey day, three classes of particles were identified, based on composition rather than size (the sizes given here are purely indicative): (1) mineral (inorganic) sediment particles in the range size 5 - 20 μm, (2) organic/inorganic aggregates in the size range 20 - 200 μm and (3) organic particles in the size range > 200 μm. A large range of settling velocities (0.1 - 10 mm/s) and aspect ratios between 1 and 10 were recorded by video microscopy (LabSFLOC2). This spreading in settling velocities and aspect ratio was due to the different properties (shape, effective density and size) of the particles in the water column. The second aim of the study was to reproduce the flocs found in-situ in the lab and investigate the kinetics of flocculation between inorganic and living organic matter. Laboratory experiments were conducted with grab samples obtained from Port of Rotterdam harbour and living microalgae (Skeletonema costatum). The results of these experiments showed a shift in effective density upon addition of living algae to the sediment, which confirmed the flocculation ability between sediment and microalgae. The flocculation occured on a timescale of minutes and lead to flocs having a large spread in density for a given size, due to the heteregeous inorganic/organic composition of the flocs. This spread in density was at the origin of the large range of settling velocities for a given floc size observed in-situ, which leads to conclude that organic matter should be an important input parameter in sediment transport models.@en

The authors regret that three typos remained in the original published article: in Eqs. (6), (10) and (11) a minus sign should be inserted in front of the variable t in all the exponentials. In particular, the full solution of the change in population, Eq. (10), reads [Formula presented] The authors would like to apologise for any inconvenience caused.

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Despite recent advancement in the field of sediment transport, the integration of cohesive sediment properties in large-scale transport models remain a challenging task. In order to model adequately the change in particle size that occurs in different environmental conditions, flocculation models based on the so-called Population Balance Equations (PBE) are often used. These models have to be efficient enough to be implemented in numerical transport models, and as full PBE's are time-expensive to run and depend on a huge amount of a-priori unknown parameters, simplifications have to be made. These simplifications comes unavoidably at the cost of properly accounting for the complex particle-particle and particle-fluid interactions. In order to stay as close as possible to the physical processes, we propose a different approach based on a logistic growth model that mimics the Particle Size Distribution (PSD) measured over time for all size classes. The parameters of the model can easily be found from laboratory measurements. In contrast to most models, the particle classes we propose are not defined by particle size, but in terms of mineral sediment composition. One class is composed of (unflocculated) mineral sediment particles, another of flocculated sediment particles and a third one of organic particles. The mass balance between classes and the way to obtain their corresponding average settling velocity are given. Mass balance and settling velocities are the required input parameters for all sediment transport models. The simplicity of the derived expressions, and their link with measurable variables, makes them good candidates for future implementation in such models.

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The characteristics of clayey suspensions, majorly composed of quartz microparticles, in the presence of anionic and cationic polyelectrolytes were investigated using different techniques. A wide range of clay concentrations was used, i.e., from 0.07 to 1000 g/L for different experimental techniques, based on the fact that the clay concentration possible to analyze with selected experimental methods was significantly different. The optimum flocculant to clay ratio was defined as the ratio that gives the fastest initial floc growth by static light scattering or fastest initial settling velocity by settling column experiments. In case of anionic polyelectrolyte, it was observed that the optimum flocculant dose depends on the amount of cations present in the system. For suspensions made with demi-water, a lower optimum flocculant dose (<1 mg/g) than for suspensions prepared in tap water (2.28 mg/g) was observed. At these lower salinities, the supernatant remained turbid in all the experiments and was, therefore, not a good measure for optimal anionic based flocculation. The equilibrium floc size at a given shear rate was found to be independent on the shear history of the floc and only dependent on the current applied shear. This was confirmed by both light scattering and rheological analysis. In case of cationic polyelectrolyte, the optimum flocculant ratio (5–6 mg/g) corresponded to the ratio that gives the lowest electrophoretic mobility for each clay concentration and to the ratio that gives the fastest settling velocity for the highest clay concentrations (12–15 g/L), where static light scattering measurements were not possible. All investigation techniques, therefore, proved to be good indicators for predicting the optimum flocculant to clay ratio. For the lowest concentrations (1.75–8.7 g/L) studied by settling column measurements, the optimum flocculant ratio was observed to increase with decreasing clay concentration, for fixed mixing conditions. The optimum flocculant to clay ratio was not always corresponding to the clearest supernatant and the size of flocs at optimum dosage was dependent on the mixing efficiency. The equilibrium floc size at a given shear rate was found to be dependent on the shear history of the floc and the current applied shear. This was confirmed by both light scattering and rheological analysis.

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The precise interactions between organic and inorganic particles in the context of flocculation is an on-going topic of research. The suspended particulate matter (SPM) found in estuaries is composed of both organic and inorganic particles with specific particle size distributions (PSD's). These PSD's are a function of the hydrodynamic conditions, suspended sediment concentration (SSC), organic matter composition, salinity and seasonal variations. A field campaign was carried out in August 2015 in the turbidity maximum zone of the Yangtze Estuary, where the SPM dynamics were recorded. The concentration of algae in the water column was indirectly measured through the chlorophyll-a concentration (CC). We show that there is a strong correlation between SSC and CC in the whole water column, for the whole tidal cycle. Additional flocculation experiments in the laboratory confirm that the largest observed flocs are predominantly organic-based, and that salinity alone could not induce the flocculation of the Yangtze mineral particles. A key parameter for the maximal floc size is the algae concentration to sediment concentration ratio. When this ratio is high, the D50 is high and vice-versa.@en