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.

Shoreface nourishments with the artificial sandbar are effective strategies to mitigate coastal erosions mainly by wave breaking and reflection. Thus, a better understanding of the contributions of breaking and reflection in wave energy loss is important for the prediction and description of the performance of the shoreface nourishment. Experiments are conducted in a wave flume, and data analysis is complemented with numerical simulations performed with a phase-resolving model. Both incident and reflected wave heights are well reproduced by the numerical model. The ratio of reflection to reflection-plus-breaking energy loss ranges from 4% to 11%, implying the dominant role of breaking-induced dissipation over reflection in present cases. The wave transmission coefficient decreases with the decrease in the bar crest depth and onshore implementation of the artificial sandbar. The trapezoidal artificial sandbar promotes an intense wave breaking over the berm width and is more effective in eliminating waves than the triangular artificial sandbar. Due to the erodible nature of the artificial sandbar, future works will consider the use of a hydro-morphological coupled approach to study the time-varying characteristics of wave breaking and reflection during morphological evolution.