BED-LOAD TRANSPORT FOR STEADY FLOWS AND UNSTEADY OSCILLATORY FLOWS

The validity of a bed-load transport formula, based on the bed-shear c oncept of Meyer-Peter and Mueller, was investigated for steady unidire ctional flows, oscillatory flows and oscillatory flows with superimpos ed net currents. The aim of the study was to develop a general bed-loa d transport concept for a wide range of flow and sediment conditions, as occurring in the marine coastal environment. The results of more th an 150 laboratory experiments, including more than 75 recent bed-load transport measurements in oscillating water tunnels, were used for the study. For oscillatory flows, the time-dependent bed-load transport w as treated in a 'quasi-steady' way and an equivalent Shields parameter theta'(eq) was defined in order to enable a comparison between 'stead y fow' and 'unsteady oscillatory flow' measurements. A good correlatio n was found between the non-dimensional transport parameter Phi(b) and the non-dimensional excess bed-shear theta'(eq) - theta(c) for differ ent sets of oscillatory bed-load transport measurements. However, espe cially in the lower Shields regime, the oscillatory flow data show a c lear deviation from an extensive set of steady flow bed-load data. A g ood agreement between the (oscillatory flow and the steady flow) data sets could then be obtained by reducing the lower limit of the roughne ss height k(s) for oscillatory flows from the (steady flow) value 3D(9 0) to D-50. Based on the obtained concept a generalized bed-load formu la was derived for both flow types, including oscillatory flows and su perimposed currents under an arbitrary angle. A verification, carried out with a set of (co-Linear) oscillatory + superimposed current measu rements, showed a good agreement between calculated and measured bed-l oad transport rates. Finally, the validity and limitations of the obta ined bed-load model are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.