An experimental and numerical investigation on dynamics of cohesive sediment

• Main Authors: Wen-YangHsu, 許文陽
• Other Authors: Hwung-Hweng Hwung
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/40014859289822550091

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 101 === This dissertation aims at employing laboratory experiments and numerical modeling to investigate behavior of cohesive sediment, including sedimentation in static water and wave-mud interaction in a two layer system under different wave loadings. The first process is part of fine sediment transport, while the other belongs to wave hydrodynamic problem and usually accompanied with significant wave damping. Used sediments are in-situ sediment and pure kaoline, respectively. For sedimentation, the main focusing is to evaluate how diffusion affects the settling of cohesive sediments. We calculate the settling velocity and diffusion coefficient using the complete advection-diffusion (A-D) equation, where concentration-dependent settling velocity and diffusion coefficients are introduced. In order to solve the A-D equation via least-squares method, a series of experiments were conducted with different suspended concentration. The experimental results indicate that the competition between settling flux and diffusive flux at various suspended sediment concentrations (SSC) can dramatically change the settling process. The diffusion coefficient can reach 102~104 times the molecular diffusion, which depends on the local concentration. Finally, the results reveal that suspensions of cohesive sediment can settle with one interface or no interface when diffusion is introduced, depending on the magnitude of the diffusive flux. The generation of internal waves induced by surface wave in the fluid mud environment is examined via laboratory experiment in order to study its importance on surface wave damping. As wave propagates over soft muddy bottom, both viscous shear stress and interfacial motion contribute to energy dissipation. Moreover, identifying oscillatory mode of internal wave is critical on selecting suitable rheological model when study wave-mud interaction. A fully fluidized mud layer with a homogeneous concentration distribution is created to investigate the resulting energy dissipation for different wave heights, periods and mud densities. Visualization technique and wave gages were applied to measure interfacial motion and surface wave damping, respectively. Experimental results suggest that the induced interfacial amplitude depends mainly on the relative water depth, and the mode of internal oscillation was identified. Finally, based on the derived energy equation for two-layer system, we find that the surface wave damping caused by mud motion is minor. On the other hand, wave attenuation caused by the work done by shear stress is investigated via laboratory experiments and numerical modeling. Rheological behavior of kaoline exhibits hybrid properties of Bingham and pseudo-plastic fluid. Moreover, the measured time-dependent velocity profiles in the mud layer reveal that the shear rate under wave loading is highly phase-dependent. Measured shear rate and rheological data allow us to back-calculate the time-dependent viscosity of the mud layer under various wave loading, which is also shown to fluctuate up to one order of magnitude during one wave period. However, the resulting time-dependent bottom stress is shown to only fluctuate within 25% of its mean. Measured wave-averaged bottom stress is well-correlated with wave damping rate in the intermediate wave energy condition. Commonly adopted constant viscosity assumption is then evaluated via linear and nonlinear wave-mud interaction models. When driving the models with measured wave-averaged mud viscosity (forward modeling), wave damping rate is generally over-predicted for low wave energy condition. On the other hand, when a constant viscosity is chosen to match the observed wave damping rate (backward modeling), the predicted velocity profiles in the mud layer are not satisfactory and the corresponding viscosity is lower than the measured value. These discrepancies are less pronounced when waves become more energetic. Differences between the linear and nonlinear model results become significant in low energy condition, suggesting an amplification of wave nonlinear effect due to non-Newtonian rheology. In general, the constant viscosity assumption for modeling wave-mud interaction is only appropriate for more energetic wave condition