Field observations of bedforms and sediment transport thresholds of fine sand under combined waves and currents

Seabed video images and S4 wave-current meter data, collected during the build-up of a moderate storm on the Scotian Shelf, are analysed for bedform development and sediment transport threshold of fine sand under combined waves and currents. As the storm built up, the following sequence of bedforms was observed: (1) relict wave-dominant ripples with worm tubes and animal tracks during the preceding fairweather period; (2) irregular, sinuous, asymmetrical current-dominant and intermediate wave-current ripples under bedload transport; (3) regular, nearly straight or sinuous asymmetrical to slightly asymmetrical wave-dominant ripples under saltation/suspension; (4) upper-plane bed under sheet-flow conditions; (5) small, crest-reversing, transitory ripples at the peak of the storm; and (6) large-scale lunate megaripples which developed when the storm decayed. These data also show that only single sets of asymmetrical intermediate wave-current ripples will form when waves and currents are co-linear. The development of the crest-reversing transitory ripples indicates a high-energy transition stage under quasi-sheet-flow conditions. A direct comparison of the skin-friction combined shear velocity and the critical shear velocities for bedload, suspension and sheet-flow transport under-estimated the onset of these sediment transport modes. As the presence of ripples causes the shear stress to increase from ripple trough to ripple crest, the ripple-enhanced skin-friction shear velocity must be used to determine properly the initiation of bedload transport. At high transport stages, the boundary layer dynamics is controlled mainly by the thickness of the bedload transport layer. Thus a transport-related bedload shear velocity, predicted based upon the sum of the grain roughness and bedload roughness, has to be compared against the conventional threshold criterion to properly define the onset of suspension and sheet-flow transport modes. (C) 1999 Elsevier Science B.V. All rights reserved.