Effects Of Various Factors On The Characteristics Of Cohesive Sediment Flocculation

Flocculation processes of cohesive sediment strongly affect the sedimenttransport, erosion and deposition in harbor basin and channel, and evolution ofestuaries and coasts. However, there are various factors responsible for cohesivesediment flocculation itself. Therefore, it is so significant to study the various factorson the impact of cohesive sediment flocculation and settling processes, in order tobetter understand the mechanism of cohesive sediment flocculation. Based on theLattice Boltzmann (LB) model of cohesive sediment flocculation combined with theXDLVO theory, influence factors of cohesive sediment flocculation are investigated.The main results are summarized as follows:1. The XDLVO theory is introduced into LB model. The XDLVO forcecalculated by the LB model agrees well with that by formula. The parametersensitivity is investigated to theoretically analyze the effects of XDLVO forces oncohesive sediment flocculation. It is shown that LB model based on the XDLVOtheory can describe reasonably inter-particle forces and can be further applied into thesimulation of cohesive sediment flocculation.2. The LB model based on the XDLVO theory is applied to study theflocculation characteristics of various clay minerals. The results show that Illite is theeasiest to form flocs, followed by Kaolin Montmorillonite in order, which agree withthe previous experimental results,3. The effects of pH value of water, hardness, salinity and temperature oncohesive sediment flocculation are simulated and analyzed. It is shown that the effectsof pH value on sediment flocculation have the same tendency with its impact on thesediment surface potential. When the pH value is larger than6, it exerts few effects onsediment flocculation. The influence of hardness is notable in low salinity whileignorable in high salinity. In estuarine and coastal areas, the effects of pH value andhardness can be nearly ignored. For the results of salinity, it is shown that increasingsalinity will promote flocculation firstly and then lightly inhibit it. When the salinityis5.0ppt, it is most likely to form flocs. It is indicated that the mechanism of salinityeffects on flocculation is reflected by the sticking probability of particles. Cohesivesediment is more likely to form flocs in higher temperature and result in largeraverage settling velocity. This phenomenon is attriibuted to the combined influencesof water viscosity and interaction between sediment particles due to temperature. 4. Effects of turbulence on cohesive sediment flocculation are investigated by theoscillating-grid experiments and LB numerical simulation respectively. It is shownthat time histories of sediment concentration, sediment size distribution, floc size,settling velocity of floc, effective density of floc and fractal dimension are affected byturbulent shear rates during settling and flocculaton of particles. In general, the shearrates promote flocculation firstly and then inhibit it. The optimum turbulence shearrates for flocculation varies with the water salinity and sediment concentration.5. An LB model describing the settling and flocculation behaviors of oblatespheroid paticles is developed based on the spatial motion equation of oblate spheroid.The model is well validated by settling velocity and drag coefficient of single oblatespheroid particle under various Reynolds number conditions. And the differentmotion patterns of single oblate spheroid particles are explained from the point ofinteraction between solid particle and its surrounding fluids. Furthermore, theinfluence of initial relative positions of two particles on sediment flocculation isindicated from the aspect of hydrodynamic and interaction forces between particles.