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

Developing an approximation of a natural, rough gravel riverbed both physically and numerically

Near-bed and pore space turbulent flows are beginning to be understood using new technologies and advances in direct numerical simulation (DNS) and large-eddy simulation (LES) techniques. However, the riverbed geometry that is used in many computational studies remains overly simplistic. Thus, this study presents the development of an artificial representation of a gravel riverbed matrix, and the assessment of how well it approximates a natural riverbed. A physical model of a gravel riverbed matrix that was 120 mm deep, 300 mm wide, and 2.048 m long was manufactured from cast acrylic. Additionally, a numerical approximation of the physical model was created and used for analysis. The pore matrix of the artificial riverbed was found to be comparable to that of a natural gravel riverbed in terms of its porosity and void ratio. The diameters of the artificial riverbed’s surface particles were found to vary less, with fewer irregularities, than those found for natural gravel riverbeds; yet, they were normally distributed similarly to natural riverbeds. A power spectral density function showed that the artificial riverbed exhibited a degree of roughness that was much lower than that found in nature. Thus, the hydraulic resistance and friction factor will both be lower than desired. These findings suggest that the novel methods that have been developed in this study can offer both the physical and numerical approximation of a gravel bed surface that is comparable to a natural gravel riverbed with low surface roughness, reduced particle size variance, and typical particle distribution and porosit

Numerical modelling of colmation and decolmation processes for gravel-bed river restoration schemes

It is well established that man has greatly influenced river sediment loading, which has had a detrimental effect on the aquatic ecosystem, in particular on salmonid spawning in gravel-bed rivers. Successful spawning relies upon a balance between colmation and decolmation processes. Excessive colmation results in juvenile fish being injured through abrasion and adhesion. Without decolmation, juvenile fish trying to emerge from the riverbed, following their incubation period, become trapped. Sediment oxygen demand and intragravel flows can also be influenced by colmation and decolmation resulting in changes in dissolved oxygen levels in the riverbed. Therefore, river restoration schemes often aim to emulate the balance between these processes. However, though conceptually well understood, the physical processes of colmation and decolmation are at best poorly described. This makes the design of restoration schemes challenging and as a result many have had little effect on salmonid spawning whilst some have even been detrimental. It is only with recent advances in technology that it has been possible to understand the complexities of the processes, in particular the influence of microscopic turbulent flows within the near-bed region and within a riverbed’s pore matrix. This research aims to further understanding of colmation and decolmation by focusing on the quantification of turbulence close to and within the riverbed facilitating the modelling of these processes. By enhancing the capability of the 2D numerical hydraulic modelling package DIVAST (Depth Integrated Velocities And Solute Transport), this research ultimately aims to improve the design and assessment of gravel-bed river restoration schemes