Study Of Fine Sediment Tranport And Scour Protection In Coastal Waters

Seabed change, the layout of the artifical islands and scour protection are associated with the movement of fine sediment. These problems can be solved by studying the fine sediment transport and the law of motion in coastal waters. Sediment transport in coastal waters affects the seabed change. In recent years, the seabed has been undergone the marked change as the decrease of the input of the river and the development construction in the coastal waters, the steady equilibrium of the seabed will be destroyed and the erosion could be occurred. Therefore, it is necessary to investigate the erosion of the seabed and scour protection around the local structure. Structured meshes models play the pioneering role in the sediment study, but the coastal boundary is generally irregular and complex, the unstructured meshes model can give the characteristics around the irregular boundary. Waves and tidal currents are the main driving forces of fine sediment transport, which can induce the complicated three-dimensional flow in the coastal zone. It plays an important role in the morphological change and the equilibrium profile over the seabed. The previous model did not account for the waves and the wave radiation stress is limited to apply to the coastal waters, and the silty sediment in the coastal zone is consisted of cohesive and non-cohesive fractions, thus a three-dimensional unstructured mesh sediment model is established to investigate the silty sediment transport under waves and currents. Meanwhile, a laboratory experiment is conducted to study the scour protection due to the fine sediment transport around the submarine pipeline over the silty seabed. The concents are shown as follows:(1) In the wave-current model, the depth-dependent wave radiation stresses and wave roller are added to the three-dimensional governing equation which accounts for the mixing coefficients and non-linear bottom stresses in wave-currents. The three-dimensional flow in the nearshore waters is simulated by the wave-current model. The circulation system of the nearshore waters is reproduced by simulating the wave-induced currents over the plane, irregular beaches and the artificial island from the experimental flume. The present model accounting for the depth-dependent wave radiation stresses with wave roller effect can give more accurate the velocities along the flume. The simulated results agree with the experimental data, and it shows that the model can predict the characteristics of the flow in the coastal zone, which give the hydrodynamics condition for the fine sediment transport.(2) In the mixed fine sediment model, the advection and diffusion equation is used to compute the silty sediment transport. The model accounts for the settling velocity, erosion and deposition fluxes related to cohesive and non-cohesive fractions. The morphological change of the seabed accounts for both non-cohesive and cohesive fractions. The present sediment model is applied to study the fine sediment transport over the silty seabed in the Liaodong Bay. During the validation between the simulated results and the measured data, the model accounting for both non-cohesive and cohesive fractions can give more accurate process of sediment concentration than cohesive fractions. Erosion and deposition fluxes of the non-cohesive fractions are larger than the flux of cohesive fractions, but the seabed change induced by the cohesive fractions is predominated. It is shown that the movement of cohesive sediment is complex and the transport of non-cohesive sediment is small. By comparing the previous results with sediment input from nearby rivers, the deposition of the silty seabed is decreased and the erosion is relatively obvious in the coastal waters.(3) Due to the long computational time in the calculation of the annual seabed change under waves and currents by the present three-dimensional model, the morphological speedup factor is employ by the model. It is applied to validate the model by computing the bed change from the previous experiment in a flume. The simulated sediment concentration and bed change have an agreement with the experimental data. In the practical application, the annual representative wave is calculated under the balance law of the longshore transport and the morphological change is predicted due to the construction of the artificial islands in the coastal waters. It is shown that the local scour is noticeable closed to artificial islands.(4) The scour around submarine pipeline with the prection of concrete bottom mattresses is carried out under the extreme waves and the currents over the silty seabed. The natural silty sediment from Huanghe Delta is used for the design of an experimental bed. The erosion process around the submarine pipeline is investigated. The stability of concrete bottom mattresses which cover the surface of submarine pipeline and the second scour induced by this protection are studied. Besides, the range of the parameter in the formulation of concrete bottom mattresses calculated the stable thickness is provided.