It is often assumed that in sheet flow conditions the transport rate in oscillatory flow behaves quasi-steady, i.e. showing a direct relation with the instantaneous flow velocity (e.g. Ribberink et ah, 1994). In this paper it will be shown that this assumption is not always valid. Therefore a new semi-unsteady sand transport model is developed which takes into account phase-lag effects of the sediment on the net transport rate. In order to verify this new semi-unsteady model and two existing quasi-steady models, new experiments were performed in the Large Oscillating Water Tunnel (LOWT) of WL DELFT HYDRAULICS. Together with earlier experiments these measurements form a data set of net sand transport rates of uniform sand for three different grain sizes (D50 = 0.13; 0.21 and 0.32 mm) in combined wave-current sheetflow conditions. The verification shows that phase-lag effects become important for a combination of fine sand, large flow velocities and small wave periods. Under these conditions the quasi-steady models cannot predict the behaviour of the net transport rates correctly and the predictions of the new semi-unsteady model show much better agreement.
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