Details of the deposition and diffusion processes of soil dumped into water, such as the propagation of the water-surface wave, the behavior of the circulating flow cell and the complicated shape of the flow-sediment interface, are simulated by the Lagrangian gridless simulation of solid-liquid two-phase flow. All of the complicated characteristics of the deposition and diffusion processes of soil are reproduced well, at least qualitatively. The existence of twin peaks in the time series of the sediment concentration, which was difficult to be simulated by the ordinary Eulerian models, is clearly reproduced by the present model. Introduction The large-scale artificial islands are often reclaimed by dumping a large volume of soil from the split-hull barges with taking the efficiency of construction into account. The previous experimental and numerical approaches for the prediction of the deposition configuration of soil treated mainly a rubble stone and a coarse sediment (Oda et al., 1991; Matsumi and Kimura, 1992; Oda and Shigematsu, 1994). While the reduction of the turbidity caused by a fine sediment in dumped soil recently becomes to be significant from the viewpoint of the prevention of the coastal environment including ecosystem. Jiang et al.(1997) conducted experiment of the settling-dispersion process of fine sediment dumped into water, and compaired their experimental results with the 1 Associate Professor, Department of Civil Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan. 2 Professor, ISVA, Technical University of Denmark. 3906 COASTAL ENGINEERING 2000 3907 Fig. 1 Settling-dispersion process of sediment Eulerian numerical model. Although a general feature of the settling-dispersion process was simulated by their numerial model, some details, such as existence of twin peaks in the time series of the sediment concentration was not reproduced. The deposition and diffusion processes of soil dumped into water is a quite complicated phenomenon, hence there is no previous model capable to predict its full characteristics schematically shown in Fig. 1, such as the propagation of the water-surface wave, the behavior of the circulating flow cell and the complicated shape of the flow-sediment interface. The key of this phenomenon is how to track the solid-liquid interface and the liquid-gas interface, or a water surface. In this study, the newest gridless Lagrangian model of solid-liquid two-phase flow, which is free from numerical diffusion due to the advection terms in the momentum equation, is applied to reproduce all of the complicated characteristics mentioned above. Governing Equations of Solid-Liquid Two-Phase Flow In this study, the solid phase is constituted by a fine sediment, then the two-fluid model can be applied to describe a two-phase flow. The governing equations of solidand liquid-phases are the equations of continuity and the momentum equations as follows: