In this study, we conducted hydraulic model experiments on mixed soil containing clay and investigate the phenomenon of interparticle clay escaping from sand-clay mixed sediment with different wave conditions and clay content. Furthermore, we examine the effect of the clay escape phenomenon on the topographic change characteristics of mixed sediment. On mixed sediment having a high clay fraction, suspended clay behavior depended on bottom shear stress. On the other hands, in lower clay fraction condition, suspended interparticle clay behavior depended on wave steepness. It was also found that the corrugation gradient and the erosion rate of the clay escaping from the sand particles have a linear relationship. Through the above investigation, it was suggested that difficult to evaluate the washout of clay from sand particles only by evaluating critical shear stress.


In coastal projects, it is very important to evaluate the tendency of erosion and to predict the topography change. Many of the past studies have focused on either non-cohesive sediment or cohesive sediment (Van Rijn, 1993; Whitehouse et al., 2000). However, treating natural sediment mixed with clay and silt as a single particle is not suitable for accurate prediction. The reasons are the mechanism of topographic change on mixed sediments contains of non-cohesive coarse particle and very fine cohesive particles under wave action is established by a complicated interaction of aggregation, sedimentation, deposition, erosion and consolidation (e.g. Grasso et al., 2015).

It is possible to investigate the erosion process of sediment on the bed by evaluating the critical condition of movement of the bottom sediment process. It is important to evaluate the initial threshold of sediment movement which some particles protruding from the surface start to move. It is known that the initial critical condition depends on the Shields number expressed of the ratio of the bottom shear force by the wave and the resistance force of the weight of the sediment (Shields, 1936). When considering the movement of sand in sediment composed of non-cohesive single particles on the rough surface, it is considered that the movement of particle diameter is dominated by repose angle (White, 1940). When a movement model that takes repose angle into account is applied to adhesive sand mixed with adhesion, the movement of sand is suppressed by filling fine voids with fine particles (Wiberg and Smith, 1987). However, the fine grain fills the voids between the sand particles when the fine grain content increases, and a matrix is formed by fine grains when the sand grains come into the condition of non-contact with each other and the resistance force rapidly increases. In this condition, the characteristics of fine grains are dominant. In the case where the fine particle content has adhesiveness, the boundary at which transition from the non-cohesive dominated condition to the cohesive dominated condition is when the fine particle content exceeds about 30%, and under the high mud containing condition, the migration limit depends on the mud content (Panagiotopoulos et al., 1997). Actually, it is suggested that topographical variations can be suppressed by increasing clay (Amos et al., 1996). Ashida et al. (1977) theoretically calculates the estimation formula of the critical shear stress of the mixed sediment, assuming the parameters that determine the critical condition of mixed sediment depending on the clay intrinsic cohesive force, water content, and clay content. However, in these past studies, the water content and the clay fraction are not taken into time variation under fluid action. In the actual mixed bed material, the clay fraction and the water content in surface layer of the mixed sediment change over time as the clay escapes from between the sand particles. Therefore, by considering the change in the ratio of sand and clay and the water content, it is thought that the method of evaluating the critical condition of sand movement can be improved. Especially, it is suggested that variations in clay content can cause variations in the number of species (DeFlaun and Mayer, 1983; Cammen, 1991) and the time variation of clay content in bottom layers greatly affect subsequent erosion resistance (Te Slaa et al., 2013). Therefore, evaluating the ratio of clay and sand for each layer under wave action seems to help evaluate the erosion tendency by controlling the clay content, it is considered that the optimum management of coastal sediment structure can be realized from ecological and engineering aspects. Studies on the formation of layer structure in such mixed soil are directed to seabed having a particle size composition for liquefaction, and temporal change in particle size, composition inside the ground is observed (Jia et al., 2014). However, in these studies, we consider the movement of clay from the inside of the ground in the higher clay content condition, and few studies focused on the movement of clay between sand particles on the surface layer. When sand particles on the surface layer are in a state of difficulty to move under low energy conditions and lower clay content condition, non-equilibrium movement of only clay from sand particles is observed. This phenomenon is called "washout" by us. There is not enough to study about this clay washout effect of mixed sediment and the details of the movement mechanism of interparticle clay are not well known in the lower clay content condition under low energy conditions.

This content is only available via PDF.
You can access this article if you purchase or spend a download.