Bottom Roughness for Wave and Current Boundary Layer Flows over a Rippled Bed

摘要

Virtually all theoretical models for turbulent wave and current bottom boundarylayer flows characterize the effects of the bottom in terms of a single roughness length scale, kn. To investigate the validity of this basic assumption, an experimental study of combined wave and current boundary layer flows over an artificially rippled bed was completed. The rippled bed was simulated by 1.5-cm-high triangular bars placed in intervals of 10 cm along the bottom of a wave flume. Additional experiments were completed with a wide bedform spacing of 20 cm. Estimates were obtained for the bottom roughness experienced by pure waves, pure currents, and waves and currents in combined wave-current flows. Pure current roughnesses were determined by analyzing time-averaged near-bottom velocity profiles and the wave roughness estimates were determined by analyzing wave attenuation measurements. The wave roughness determinations included careful analysis of the phase difference between the bottom shear stress and near-bottom horizontal orbital velocity.