Wave plus current over a ripple-covered bed

This paper concerns the combined wave and current boundary layer flow over a ripple-covered bed, The study comprises experiments as well as a numerical modelling study: the experimental part comprises laser Doppler anemometry (LDA) velocity and turbulence measurements, and a flow-visualization study in the laboratory with ripples, 22 cm in length, and 3.5 cm in height. One wave-alone, three current-alone, and three combined waves and current tests were conducted. The wave-velocity-to-current-velocity ratio ranges from 1 to 2.4. The orbiral-amplitude-ro-ripple-length ratio (at the bed) is 0.41. The effect of superimposing waves on a current is to displace the velocity profile to higher elevations. The velocity profiles exhibit two "logarithmic layers", one associated with the actual roughness of the bed (the actual ripple roughness), and the other with the apparent roughness induced by the waves. The apparent roughness is, for the tested cases, found an order of magnitude larger than the actual bed roughness. The turbulence near the bottom increases markedly during the time when the lee-wake vortices are washed over the ripples. The numerical part of the study gives a derailed numerical description of the now around fixed ripples by use of a k - w model to calculate the roughness, and friction of a rippled bed. (C) 1999 Elsevier Science B.V. All rights reserved.