Experiments on bottom roughness experienced by currents perpendicular to waves

Abstract

Near-orthogonal combined wave-current flows are more representative of hydrodynamic conditions in near-shore coastal waters than co-directional wave-current flows. In this paper we present results from laboratory experiments on currents perpendicular to waves propagating over a movable bottom. It is shown that the experimental set-up is capable of producing a uniform current of the same depth-averaged velocity both for a current alone and in the presence of orthogonal waves. Log-profile analysis of current velocity profiles obtained for currents alone over a wave-rippled bed as well as combined orthogonal wave-current flows is used to obtain values of the movable bed roughness experienced by the current. For current alone over a wave-rippled bed the bottom roughness is found to be of the order of the ripple height. In the presence of waves, the roughness experienced by the current is greater than in the absence of waves by a factor of the order twenty (20). The latter result is explained as a result of wave-current interaction. In both cases the bottom roughness is roughly the same as one would expect for currents that are perpendicular, rather than parallel, to the predominant direction of the ripple crests. Although a contributing factor to this result is suggested, a solid explanation of this surprising result awaits further experimentation. © 2009 World Scientific Publishing Co. Pte. Ltd.