On the sea floor, ripples (sand waves) are formed by the oscillatory flow due to small height amplitude waves. However, under the action of the larger waves the sand ripples disappear and the sand moves over flat bed (Sheet flow). However the reason why the sand ripple disappears has still been unknown while a lot of experimental study has been performed. In the present paper it is assumed that the upward seepage flow is generated by an oscillatory flow and this upward flow fluidizes the sand bed under the crest of ripple. To clarify these phenomena, laboratory experiments and numerical calculations are conducted.
1) The upward seepage flow toward the crest of ripples is experimentally confirmed.
2) Numerical simulation simulates the upward seepage flow and predicts the fluidization of sand particle under the crest of ripple.