Laboratory and numerical modelling studies of sediment flocculation processes in estuaries

Surface water is one of the fundamental parts of the environment and needs to be protected from all pollution sources for human survival. Urban development and human activities have increased the contamination of coastal and estuarine water due to insufficiently treated sewage, runoff from fertilised agricultural area and lawns and releasing industrial pollutant directly into river and estuaries. Cohesive sediment in estuaries can act as either a source or a sink of many pollutants, such as nutrients and heavy metals. Understanding the interactions between sediment and nutrients in water bodies is important because high input rates of nutrients can negatively affect water quality. The prediction of both deposition and the resuspension of cohesive sediment in estuaries supports the understanding of their turbidity, which is important in terms of the biomass of these water bodies and many of the occurring biochemical processes and the morphological processes, which determine the suitability and maintenance of fairways and harbour basins. The complex ways in which hydrodynamic and biochemical parameters affect cohesive sediment are primary reasons for the poor representation of the deposition, erosion and settling of cohesive sediment processes in modelling tools within water estuaries. However, our current understanding regarding the accurate prediction of cohesive sediment transport processes is insufficient because of flocculation processes which occur under certain circumstances (e.g. the increase of salinity in the brackish zone of rivers, which leads to the formation of flocs that are both larger and less dense than individual particles). The phenomenon of flocculation is known to play a significant role in the sediment transport processes of settling, deposition and erosion of cohesive sediment. There is no unique equation that can be universally used to predict the deposition and suspension rates of cohesive sediment because each estuary is dynamically and physically different from another and this is particularly true for the highly dynamic estuary e.g. Severn Estuary. Therefore, this study focuses on gaining a better understanding of the transport processes of cohesive sediment, including a better inclusion of the flocculation processes by developing a new settling velocity equation for cohesive sediment including flocculation processes as a function of hydrodynamic parameters. It also aims to apply this equation to a numerical model IV and to test this refined model by simulating the flocculation phenomenon in the highly dynamic Severn Estuary. This study employed an extensive experimental setup using a small scale particle image velocimetry (PIV) system. The experimental research was carried out using suspended sediment samples from the Severn Estuary in the UK. A PIV system and an image processing routine were used to measure both floc size distribution and settling velocity. The experimental results indicate that both the floc size and the settling velocity are controlled by the interaction between turbulence and salinity at salinities of less than 10 ppt. At a salinity either equal to or more than 10 ppt, both the floc size and the settling velocity were functions of only turbulent shear stress. The new equations were successfully applied in the Delft3D model; the model results show that they aptly were able to match with observed suspended sediment distributions throughout the estuary. Overall, the developed model can be regarded as a basic tool for being applied to help manage the suspended sediment processes in the Severn Estuary and for assessing the potential impact of climate change and human interference such as tidal renewable energy schemes in this water body

Effect of hydrodynamics factors on sediment flocculation processes in estuaries

Purpose Cohesive sediment is able to flocculate and create flocs, which are larger than individual particles and less dense. The phenomenon of flocculation has an important role in sediment transport processes such as settling, deposition and erosion. In this study, laboratory experiments were performed to investigate the effect of key hydrodynamic parameters such as suspended sediment concentration and salinity on floc size and settling velocity. Results were compared with previous laboratory and field studies at different estuaries. Materials and methods Experimental tests were conducted in a 1-L glass beaker of 11-cm diameter using suspended sediment samples from the Severn Estuary. A particle image velocimetry system and image processing routine were used to measure the floc size distribution and settling velocity. Results and discussion The settling velocity was found to range from 0.2 to 1.2 mm s−1. Settling velocity changed in the case of increasing suspended sediment concentration and was controlled by the salinity. The faster settling velocity occurred when sediment concentration is higher or the salinity is lower than 2.5. On the other hand, at salinities higher than 20, in addition to increasing SSC, it was found that the situation was reversed, i.e. the lower the sediment concentration, the faster the settling velocity. Conclusions Sediment flocculation is enhanced with increasing sediment concentration but not with increasing salinity