Wave Action Around The Building Sand Erosion And Sea-bed Evolution

The local scour and topographical change around offshore structures are studied numerically in this thesis. The numerical models are established for simulating the topographical change around structures. Because the computation using 3D numerical models is time-consuming, the depth-averaged 2D numerical models are applied in the calculation of wave and current fields for saving computer memory and increasing the computing speed. The shear stress at the sea bed is calculated by solving the depth-integrated momentum equation over the thickness of the boundary layer near the bed. The numerical models are implemented by finite element method.The numerical models proposed in this thesis are applied to simulate the local scour around the large-scale structures and the wave reflection and diffraction around structures can be computed. The finite element models for solving the Mild-slope equation and Boussinesq equations are firstly established in this thesis. The methods of coupling with an analytical solver, inner wave generation and wave absorption using spongy layer are applied to the boundaries in calculation of the wave field for reducing the numerical errors. The wave-induced current is calculated by solving a set of 2D shallow water equations in the horizontal plane. The gradients of radiation stress components are added to the shallow water equations to take into account the affect of waves on current. The combined wave-current models are applied to the calculation of bed shear stress instead of the simple addition of wave and current. The instantaneous sediment transport of suspended and bed load are computed, and the sediment transport rates are time-averaged over one wave period. The Lagrangian drift velocity and the bed shear stress due to streaming are considered when calculating the suspended sediment transport rate. A conservation equation of sediment mass expressed by time-averaged sediment transport rates is solved for simulating the topographical evolution. Equations expressed by the time series of wave surface elevation for computing the radiation stress components are proposed. The critical shields parameter is modified when the sediment grain moving on a slope beach and the modified shields parameter is applied to the numerical model. The computation results using the models in this thesis are compared with experimental data and other numerical results. The influence of KC number, sediment grain size, and scale affect on process of scour are examinated numerically in this thesis.The local scour around a large-scale vertical cylinder due to wave and current is studied experimentally and numerically. The scour processes of wave and current acting on cylinders with various diameter are tested. The numerical model for simulating the scour process around large-scale structures due to wave and current is established, and the numerical results are compared to with experimental data. Because the current velocity is smaller than the wave particle velocity in real coastal engineering, the affect of current on waves is neglected in the numerical model, but the affect of radiation stress of waves on current was considered.