SEDIMENT TRANSPORT IN OSCILLATORY BOUNDARY-LAYERS IN CASES OF RIPPLEDBEDS AND SHEET FLOW

An experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action, Wave-induced oscillat ory flow conditions near the seabed were simulated at full scale (1: 1 ) in a new large oscillating water tunnel. Two sets of experiments (se ries A and B) were carried out. During series A, bed forms and wave-cy cle averaged suspended sediment concentrations were measured under sin usoidal waves. Series B focused on measurement of the wave-cycle avera ged sediment transport rates under regular and random asymmetric oscil latory flows (upper shoreface conditions) and was aimed at the verific ation of quasi-steady formulas for the description of cross-shore sedi ment transport. Bedform dimensions appeared to decrease considerably u nder the influence of wave asymmetry and wave randomness. Only the mea sured ripple dimensions under asymmetric waves (series B) showed good agreement with Nielsen's (1979) relations. For most of the series B ex periments the bed was plane (sheet flow), the net sediment transport w as directed ''onshore'' and the measured transport rates showed a stro ng correlation with the velocity moment [U3]. An empirical quasi-stead y transport model is proposed which is based on the new tunnel data an d other existing data sets. The limitations of the quasi-steady model approach became clear in the rippled-bed regime and through the presen ce of a consistent influence of the wave period in plane-bed condition s. In rippled-bed conditions the suspended concentration profiles foll owed a negative exponential distribution. For most of the experiments the measured concentration decay length showed a linear relation with the ripple height. In plane-bed/sheet flow conditions the measured sus pended concentration profiles followed a negative power function. The power or concentration decay parameter was constant (alpha congruent-t o 2. 1) for a wide range of velocity conditions. It is suggested that the mobile bed (sheet flow layer) has a strong damping effect on the m ixing of suspended sediments (turbulence damping).