In this study, a new three-dimensional model was developed to provide a better understanding of the mechanism for wave-induced seabed response around the mono-pile. Based on the poro-elastic theory, the fully dynamic formulations were adopted in the present model to simulate the pore pressure, soil stresses and the displacements of both soil and mono-pile. Good agreement between the numerical simulation and experimental results was obtained. Based on the parametric study, the numerical results concluded that (1) the wave diffraction and reflection have significant effects on the pore pressure and soil displacements around the mono-pile; (2) the largest discrepancy of pore pressure due to the variation in seabed parameters appears in front of mono-pile, while the smallest discrepancy is at the position of angle 3π/4 with respect to the incident wave direction and (3) the increase in the mono-pile horizontal displacement corresponds to the increase in the wave height and the decrease in the seabed Young's modulus.

The erosion threshold of sand-mud mixtures is investigated by analyzing the momentum balance of a sand particle or a mud parcel in the mixture bed surface, and a formula for the critical shear stress of sand-mud mixtures is developed, which also applies for pure sand and mud. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture. The developed formula reproduces well the variation of the critical shear stress of sand-mud mixtures over mud content and can predict the critical shear stress of both sand-mud mixtures and pure mud in the process of consolidation. The developed formula promises to be convenient for application by relating the critical shear stress to mud content and the dry bulk density of sediment.

Implementing shoreface nourishment is an effective method to protect sandy beaches. A better understanding of the equilibrium mechanism of shoreface nourishments is necessary for coastal engineering designs and constructions. Two experiments on the beach profile equilibrium of the shoreface nourishment are carried out under mild wave conditions on the reflective and intermediate beach. It is observed that the shoreface nourishment increases local wave height and strengthens wave nonlinearity by its shallow water depth. The most intense wave breaking dissipation has been found on the crest of the shoreface nourishment, and the distribution of wave energy dissipation rate is more uniform on the quasi-equilibrium profile than that on the initial profile. A process-based numerical model is used to reproduce bed profile evolution successfully. On that basis, it is found that onshore bedload transport is the primary cause for the onshore migration of the shoreface nourishment. The magnitude of bedload transport decreases during the evolution of the shoreface nourishment towards equilibrium. The most intense sediment transport rate occurs over the shoreface nourishment or in front of the shoreline, depending on the ’lee effect’ of the nourishment. Furthermore, the effects of incident wave height, wave period, and sea-level rise on the equilibrium profile of the shoreface nourishment under mild wave conditions are analyzed.

The seabed is usually non-homogeneous in the real marine environment, and its response to the dynamic wave loading is of great concern to coastal engineers. Previous studies on the simulation of a non-homogeneous seabed response have mostly adopted a vertically layered seabed, in which homogeneous soil properties are assumed in the governing equations for one specified layer. This neglects the distribution gradient terms of soil property, thus leading to an inaccurate evaluation of the dynamic response of a non-homogeneous seabed. In this study, a numerical model for a wave-induced 3D non-homogeneous seabed response is developed, and the effects of the soil property distribution gradient on the wave-induced response of a non-homogeneous seabed are numerically investigated. The numerical model is validated, and the results of the present simulation agree well with those of previous studies. The validated model is applied to simulate an ideal two-dimensional (2D) vertical non-homogeneous seabed. The model is further applied to model the practical wave-induced dynamic response of a three-dimensional (3D) non-homogeneous seabed around a mono-pile. The difference in pore pressure and soil effective stresses due to the soil distribution gradient is investigated. The effects of the soil distribution gradient on liquefaction are also examined. Results of this numerical study indicate that (1) pore pressure decreases while soil effective stresses increase (the maximum difference of the effective stresses can reach 68.9 $\%p_0$ ) with a non-homogeneous seabed if the distribution gradient terms of soil properties are neglected; (2) the effect of the soil property distribution gradient terms on the pore pressure becomes more significant at the upper seabed, while this effect on the soil effective stresses is enhanced at the lower seabed; (3) the effect of the soil distribution gradient on the seabed response is greatly affected by the wave reflection and diffraction around the pile foundation; and (4) the soil distribution gradient terms can be neglected in the evaluation of seabed liquefaction depth in engineering practice.