Study On The Settling Velocity Of Fine Sediment In The Changjiang Estuary

The cohesive fine sediment widely exists in the area where the freshwater and saline water merge in the Changjiang Estuary, and takes place most easily flocculation. The increased settling velocity due to the sediment flocculation induces enhanced deposition in some sensitive areas, causing severe siltation in the navigation channel. As the one of key physical parameters of sediment dynamic characteristic, the settling velocity has been an important parameter for study of sediment movement and calculation of deposition amount. Thus, study of the settling velocity of cohesive fine sediment in the Changjiang Estuary has both the theoretical and practical significances.Settling velocity of sediment flocculation is affected by sediment particle size and concentration, as well as water salinity and turbulence. Direct in situ measurement of the settling velocity is difficult, while a reliable formula taking multiple factors into account is also rarely seen. We proposed a quantitative expression of settling velocity of flocculated sediment in the Changjiang Estuary based on a series of laboratory experiments, theoretical analysis, and numerical simulation in this study. The main results are as follows:1. An equipment platform was designed and assembled to measure the quiescent water settling velocity of sediment taken from the Changjiang Estuary under different salinity and sediment concentration. This equipment platform features high resolution, large measurement range, and small disturbance. The experiment samples were taken from the natural sediment and water in the Changjiang Estuary. A control volume method with a distinct physical meaning was adopted to determine the settling velocity. Two hundred experiments were conducted under different sediment concentration and salinity, which showed that the most favorable sediment concentration for flocculation is 4～5 kg/m3. The most favorable salinity for flocculation is 23 psu at sediment concentration of 4 kg/m3, and is 12 psu at sediment concentration of 5 kg/m3. Finally, the formula of quiet water settling velocity was obtained by correct of the settling velocity of single particle sediment by sediment concentration and salinity.2. Combined theoretical analysis, the impact of turbulence on the quiet water settling velocity was considered based on the laboratory experiments. At first, the vertical turbulent diffusivity and flocculation diameter were chosen as a basic parameter of turbulent impact function, and then the turbulent impact function was constructed by theoretical analysis and normalization. Then, based on the grain size distribution from multiple measurements in the Changjiang Estuary, the settling velocity of single particle was calculated. Finally, a quantitative expression of settling velocity in turbulent water was determined by considering sediment particle size, concentration, salinity, and turbulent diffusivity.3. The obtained formula of settling velocity of flocculation sediment was introduced the numerical model ECOM-si, which has been developed and used for a long period by the advisor’s research group. This model has been improved in calculation of wet/dry moving boundary, numerical scheme, and sediment pattern. The triple-nesting WRF was used to provide high resolution wind, and the SWAN was applied to simulate the wave parameters. Compared with the observed data, the validations of WRF and SWAN are well, indicating that the models can provide reliable wind field and wave parameters. Applied the three dimension hydrodynamic and sediment model, the parameter of turbulent impact on the settling velocity was calibrated with multiple observational data, The turbulent coefficient for settling velocity was calibrated based on simulations against multiple observational data, and the resultant expression of settling velocity was further validated. The results showed that the model performance has been improved considerably thus the resultant expression of settling velocity can be spread to application.4. The simulated longitudinal profile of sediment settling velocity in the North Passage of the Changjiang Estuary was presented, and its spatial and temporal characteristic and related impact factors were analyzed.