An experimental investigation of wave-induced sediment responses in a natural silty seabed: New insights into seabed stratification

The widely recognized global phenomenon of wave-seabed interactions and the resulting secondary stratification processes are complex and poorly understood. To better understand the mechanisms by which the differentiation between fine-grained and coarse-grained materials occurs and how new sedimentary sequences form, laboratory flume experiments were performed under varying wave conditions using natural silty sediment samples collected from the Yellow River Delta (China). Detailed observations led to the description of variations in the grain-size composition and texture of sediment under waves of progressively increasing height and the recognition of the typical, secondarily formed seabed stratification that forms during processes of increasing pore pressure and subsequent liquefaction. The development of a stratified seabed, which is characterized by a superficial fluid mud layer, a subsurface that alternates between coarse and fine sediment banding, and an internal liquefied layer from top to bottom, is closely related to wave-driven pore-water flow through the sediment. Localized fluidization and the onset of piping leads to progressive changes in the grain composition with initial particle dislocation, the development of macro-voids and micro-conduits and, finally, pipe formation. Internal liquefaction occurs in the presence of a critical value of excess pore pressure, during which most fine particles can be scoured by the pore-water flow along the sliding surface while the coarse sediment grains settle, leading to a more compact, homogeneous seabed. These elements are combined to produce the first model of wave-induced stratification initiation and evolution in an originally uniform silty seabed. In addition, the implications of this model for future studies are explored.