Cp Tsai

WAVE-INDUCED LIQUEFACTION POTENTIAL IN A POROUS SEABED IN FRONT OF A BREAKWATER

Abstract A three-dimensional short-crested wave system produced by waves reflecting obliquely from a breakwater is more conducive to soil instability in the porous seabed than the action of a two-dimensional progressive wave. The excess pore pressure induced within the sedimentary seabed can result in liquefaction and trench scouring; hence causing structural failure. General solutions are presented for soil response in a porous seabed induced by a partial short-crested wave formed by angled incident waves and partially reflected waves. The potential of liquefaction under the action of such a wave system is evaluated for the regions in the seabed fronting a breakwater. The effects of several pertinent wave and soil parameters on the liquefaction potential are then examined in detail—these including reflection coefficient, relative water depth, degree of soil saturation and soil permeability.

Numerical Fourier solutions of standing waves in finite water depth

Abstract Numerical Fourier solutions for time-dependent two-dimensional standing gravity waves of finite amplitude in water of uniform depth are presented in this paper. While using a truncated double Fourier series for the velocity potential which satisfies the Laplace equation, an implicit function, rather than a series approximation for the surface elevation, is preserved in the nonlinear free surface boundary conditions. An algorithm involving Newtons iteration method is developed to calculate the unknown Fourier coefficients. The properties of standing waves in water of finite depth, including variations of angular frequency, surface profiles and wave forces, and even the maximum wave steepness are then calculated. The accuracy of the truncated series is validated by the convergence of the solutions for the angular frequency. The null residual pressure at the free surface then implies high accuracy of the Fourier solutions. The present results agree well with the experimental data available.

COMPUTATIONS OF THE ALMOST HIGHEST SHORT-CRESTED WAVES IN DEEP WATER

Abstract The highest short-crested waves have been studied analytically and numerically by several workers, but without a conclusive view. An efficient numerical scheme is proposed in this paper which retains the water-surface elevations in an implicit form in the governing equations, rather than using a series approximation, thus improving the accuracy of the numerical results. Convergence of the numerical scheme is verified. The almost highest short-crested waves in deep water are then evaluated, which are defined for the condition with the largest wave energies. It is found that the critical angle for wave frequency reversal also demarcates the wave characteristics near breaking, for either kinematic or dynamic prominence. The known results available for the limiting two-dimensional cases of standing and progressive waves are compared favourably.

Effect of wave non-linearity on the standing-wave-induced seabed response

A standing wave in front of a seawall may reach a height more than twice of its incident component. When excess pore pressure occurs, it may even induce seabed instability, hence endangering the structure. This issue was studied previously using only linear wave theory. In this paper, standing-wave theory to a second-order approximation is applied, in order to demonstrate the differences between these two solutions. The spatial arid temporal variations in the instantaneous pore pressure are first calculated, in addition to their vertical distributions. The effects of wave height, water depth and the degree of soil saturation on pore pressure distributions are then discussed, followed by the net pore pressure averaged over one wave cycle. The results suggest the existence of a residual pore pressure in the seabed and its net pore pressure can be used to estimate the wave-induced liquefaction potential in a soil column. It also indicates that, in deep water, the second-order solution predicts that a negative pore pressure at an antinode which may be greater than a positive pressure. Overall, the second-order solution is found to agree better with the experimental results of the pore pressures available, compared to the linear solution.

Ocean waves propagating over a porous seabed of finite thickness

In the past few decades, considerable efforts have been devoted to the phenomenon of wave-seabed interaction. However, conventional investigations for determining wave characteristics have been focused on the wave nonlinearity. On the other hand, most previous works have been only concerned with the seabed response under the wave pressure, which was obtained from the assumption of a rigid seabed. In this paper, the inertia forces and employing a complex wave number are considered in the whole problem. Based on Biots poro-elastic theory, the problem of wave-seabed interaction is first treated analytically for a homogeneous bed of finite thickness and a new wave dispersion relationship is also obtained, in which the soil characteristics are included. The numerical results indicate that the effects of soil parameters significantly affect the wave characteristics (such as the damping of water wave, wave length and wave pressure). Furthermore, the effects of inertia forces on the wave-induced seabed response cannot always be ignored under certain combination of wave and soil conditions.

Ocean waves propagating over a coulomb-damped poroelastic seabed of finite thickness: An analytical solution

Abstract In this paper, the Coulomb-damping friction is considered in the problem of the wave–seabed interaction in a porous seabed of finite thickness. A closed-form analytical solution will be presented here. In the model, we couple the water waves and porous seabed with a complex wave number. Thus, the effects of soil properties on the wave characteristics can be examined. The numerical results indicate that the coulomb-damping friction significantly affects the wave-induced pore pressure in a finer seabed (such as fine sand and clay). Also, the wave characteristics have been significantly affected by the wave-induced seabed response.

Wave Transformation and Soil Response Due to Submerged Permeable Breakwater

This study reports the experimental results of the wave transformation and the wave-induced soil response when the waves pass through the submerged permeable breakwater. The model of the submerged breakwater was built on a horizontal sandy bottom. The experimental results of the spectrum of the wave transformation and the wave-induced pore-pressure are first analyzed in this paper. It is found that the wave spectrum is similar to the condition of the impermeable bottom that the higher harmonic mode appears when the waves pass over the submerged structure. However, the higher harmonic mode is not found in the spectrum of the wave-induced pore pressure, showing that the nonlinearity of the pore pressure is damped by the porous bed. The influences of the geometry of the submerged breakwater to the transformation of the wave height and the pore-pressure are also investigated. Based on the experimental results, the regression formulas for the coefficients of the wave reflection, the wave transmission and the wave energy dissipation are obtained in the paper.© 2006 ASME