Model studies for flocculation of sand-clay mixtures

Results are presented from a combined experimental and numerical study aimed at comparing the flocculation behaviour of purely-cohesive (clay) and mixed (sand-clay) sediment suspensions under equivalent controlled hydrodynamic conditions. The experiments were conducted in a grid-stirred settling column and focussed on measuring the parametric influences of grid-generated shear rate and local suspended sediment concentrations on the time-evolution of the micro- and macrofloc size distributions generated in the column, as well as representative maximal and root-mean-square floc sizes. The results indicate that for kaolin clay suspensions under low-medium shear rates, initial aggregation rates and the peak or quasi-equilibrium floc sizes attained increase with the clay input concentration; this latter effect due to the larger proportion of macroflocs generated within these runs. By contrast, under high shear rates, representative floc sizes for kaolin clay suspensions remain largely unchanged over the experimental duration, with little influence from clay input or in-situ concentrations, and no macroflocs present in the resulting floc size distributions. The addition of the fine sand fraction to the kaolin clay suspensions is shown to reduce both initial aggregation rates and the representative floc sizes attained in the column for runs under low-medium shear rates, whilst having negligible effect on the flocculation behaviour for the sand-clay mixtures under high shear rates. These results suggest that the sand fraction inhibits flocculation at lower shear rates due to an additional floc break-up mechanism resulting from direct sand-clay interactions (e.g. particle-floc collisions). The importance of these inter-fractional (sand-clay) interactions diminishes, in comparison to shear-induced floc break-up, under higher shear conditions. A one dimensional vertical (1DV) model incorporating a population balance equation (PBE) that includes new representation of these multi-fractional (sand-clay) collisions is applied to simulate the kaolin clay and sand-clay settling column tests. In general, the 1DV PBE model predictions provide good agreement with the measured in-situ concentrations and quasi-equilibrium floc sizes attained, but under-predict floc sizes during the initial aggregation phase due to uncertainty with the upper boundary condition in the 1DV model domain. Furthermore, the reliance of the 1DV PBE model predictions on empirical floc break-up rates associated with shear-induced floc fragmentation and multi-fractional (sand-clay) collisions warrants further attention to better define the microscale dynamics of these processes for their improved representation in the PBE model. It is anticipated that this multi-fractional approach represents an improved basis for modelling flocculation processes within natural sedimentary environments, such as estuaries and tidal inlets, where bed sediments often consist of interacting cohesive (i.e. muds) and non-cohesive (i.e. silts, sands) fractions

Monitoring and characterisation of sand-mud sedimentation processes

Estuaries and tidal inlets are often characterised by the presence of both cohesive and non-cohesive sediments. Knowledge of the sedimentation behaviour of sand-mud mixtures is therefore crucial to the understanding and prediction of the time-dependent structure (i.e. mixed or segregated), composition and erodibility of sediment bed deposits developing within these environments. In the current study, a series of settling column tests are conducted to investigate the hindered settling and initial bed consolidation phases of a range of sand-clay mixtures to determine the parametric conditions under which bed segregation occurs. A new, non-invasive, electrical resistivity measurement technique is employed to capture both temporal and spatial changes in the density, porosity and composition of the evolving sand-clay bed deposits, complimented by time-lapse images of the sedimentation process within the column. The results show that the formation of segregated (sand-clay) bed layers with bed deposits is largely controlled by the initial fractional composition (i.e. relative sand and clay concentrations). Specifically, mixtures with low clay contents are shown to form well-defined (sand-clay) layer segregation within the resulting deposits, while higher clay contents result in more transitional segregation patterns or no layer segregation (for very high clay concentrations). The physical mechanisms under which these different segregation types can be generated are illustrated through predictions from an existing polydisperse hindered settling model. This model indicates that the degree of bed segregation, and time scale over which this occurs, correlates well with the difference in predicted hindered settling characteristics and upward displacements associated with the sand and clay fractions, respectively. In this regard, the new experimental dataset provides validation for the polydisperse model (for the first time), with the combined data and model predictions providing new insight into mixed (sand-clay) sedimentation processes

The motion of fine sand particles in turbulent open channel shear flows over porous bed conditions

The current study aims to investigate the physical mechanisms controlling fine sediment transport within open channel shear flows over porous beds, with particular emphasis on the role of flow turbulence in particle settling and deposition processes. Preliminary visualisation experiments used a VHS camera to observe the near-bed motion of sand particles and their behaviour within the surface layer of a rhombically-packed bed of uniform spheres. Measurement of near-bed particle trajectories indicate that turbulent particle fall velocities w's are generally larger than fall velocities measured in still water ws, most notably for finer sand grades. Distinctive modes of particle behaviour observed at the bed interface also suggest that flow-separation eddies, generated within surface interstices, have a primary influence on subsequent particle motion, i.e. deposition or re-entrainment. Similar particle behaviour is also displayed in a natural gravel bed. A more detailed analysis of sand particle motion in turbulent open channel flow was carried out employing a high-speed camera and particle-tracking technique to record and analyse particle trajectories within different flow regions. The non-dimensional ratio of measured particle fall velocity w's and still water fall velocity ws was used to indicate the relative enhancement of vertical particle motion within the turbulent flow conditions. Experiment-averaged values of this ratio reveal that particle fall velocities are generally enhanced (i.e. w's/ws > 1) in recorded near-bed and intermediate flow regions (z/H 0.5) and hindered (i.e. w's/ws < 1) in a recorded outer flow region (z/H 0.5). The ratio w's/ws also reveals a general tendency to increase with decreasing grain size di. Vertical profiles of the normalised particle fall velocity w's/u* are shown to be analogous to turbulence intensity distributions (u' rms/u* and w'rms/u*), with the highest values of w's/u* occurring in the near-bed region and coinciding approximately with the regions of highest turbulence activity. This clearly implies the existence of turbulence-enhanced particle fall velocities within the flow conditions considered. Application of a quadrant analysis technique reinforces this notion, revealing further similarities between conditioned turbulent fluid fluctuations and particle motions, in particle, the dominance of 'inrush' events (quadrant 4) in the near-bed flow and 'ejection' events (quadrant 2) away from the bed

Hydraulic modelling of interfacial processes for two-layer maximal exchange

The paper deals with the hydraulic modelling of two-layer maximal exchange; where two control sections are required for the stratified, bi-directional flow to be fully controlled. A novel mass flux transfer model is considered in the two-layer hydraulic exchange that includes a solution for the reversed-flow conditions of the two-layer system. This stratified-flow effect is associated with an internally-generated net-exchange barotropic flow components, which may be associated with the interfacial mixing processes. Similar recirculation-type effect in the stratified flow is present in salt-wedge estuaries. Predictions from the hydraulic model incorporating mass flux transfer between the counterflowing layers is compared to experimental data of exchange flows with and without net-barotropic forcing

Quantifying settling rates of particulate wastes from aquaculture cages

Ongoing expansion of aquaculture in Scottish coastal waters, by unlocking additional salmon farming capacity and supporting sustainable development within the Scottish salmon sector, is balanced by requirements from the Scottish Environment Protection Agency (SEPA) to minimise environmental risk from this expansion. In this context, the fish farm waste deposition model NewDEPOMOD (SAMS, 2021), developed by the Scottish Association for Marine Science (SAMS), is the preferred regulatory tool to assess both ongoing compliance of existing fish farms and for the appraisal and licencing of new aquaculture applications. The predictive capabilities of NewDEPOMOD clearly rely on accurate physical representation and parameterisation of the release and movement of waste materials from aquaculture cages, as well as the spatial extent to which the seabed beneath and adjacent to the cages is adversely affected by deposited particulate wastes, such as uneaten food and faecal materials. At present, the settling velocities for waste feed and faeces are represented in NewDEPOMOD by distributions around a mean settling rate for each type of particulate wastes, which have only limited verification against measured settling data.A series of laboratory experiments utilising a sophisticated grid-stirred settling column facility is conducted to better define the expected range of settling velocities for both types of particulate wastes and include, for the first time, the additional influence of turbulent flow conditions. Careful control of the turbulence intensity and shear rate generated in the column by varying the oscillation stroke and frequency of the grid array produces turbulent flow fields that can mimic both sheltered and exposed coastal marine environments in which aquaculture sites are deployed. Within the laboratory study, settling velocity data of both types of particulate wastes are captured through illumination and visualisation with digital CMOS cameras and application of a particle tracking technique using ImageJ software. A large and statistically-significant number of particles will be captured and measured, under different grid-generated turbulent flow conditions, to develop new improved distributions of settling rates for both particulate types (i.e. feed and faeces), as well as new relationships between settling velocity and turbulent shear rates. The parameterisations from this laboratory work will be evaluated in NewDEPOMOD to ensure it is optimised for current aquaculture management practices and environmental protection. The settling velocity results can also inform the evaluation of current field-based methodologies to understand, and quantify, the percentage of feed that remains uneaten at any given farm site. Accurate quantification of this value is of significant interest to the wider aquaculture sector, and particularly in relation to emerging and innovative farming techniques such as semi-enclosed fish farm systems.<br/