Under stormy waves on a sandy beach, a flat movable bed may appear when ripples have been washed out. Studies of the sheet flow have recently received much attention because a large amount of sand is transported under such conditions. However, most of the existing studies on the sediment transport are dealing with the phenomena under small tractive forces because this is easily reproduced in a small wave tank. Until now, very little understanding has been obtained on the sheet flow phenomena. The first systematic study on the sheet flow is likely the work of Horikawa et al.(1982). They measured the sediment transport velocity and sediment concentration in an oscillatory flume. The reported velocity and concentration profiles were different from existing results obtained under the condition of small amount of sediment movement. Yamashita et al.(1985) and Ahilan- Sleath(1987) performed similar experiments with light artificial particles as the sediment. These experiments have increased the understanding of sheet flows. However, difficulties in the measuring densely mixed sediment-fluid flow prevent investigating the detailed structures of the flow. In addition, the limited size of the experimental flume may cause several problems in generating idealistic conditions. Since a numerical model has an advantage of being free from the restrictions imposed on the experiment, several models on the sheet flow have recently been proposed. Ahilan-Sleath modelled sediment-fluid mixture flow as a mono-phase flow with varying density in time and depth. Since this model ignores the sediment-fluid interaction, the accuracy is likely to decrease as the sediment concentration becomes higher.
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