Numerical prediction of wave boundary layer over a bed with a change in roughness

Abstract The behaviour of the turbulent oscillatory wave boundary layer over a sudden change in the bed roughness has been studied using a one-dimensional two-equation turbulence model. The 1D model is being used to describe the 2D flow by making a Lagrangian use of the 1D model, making a sudden shift in time of the bed roughness when the fluid particle passes from a region with one roughness to a region with another roughness. Attention was concentrated on two quantities: the period-averaged velocity and the bed shear stress. The results indicated that the near-bed period averaged velocities are different from zero and directed towards the bed section with the larger roughness. This near-bed streaming was found to increase with increasing difference between the roughnesses of the two bed sections and with decreasing values of amplitude-to-roughness ratio. Streaming velocities as high as 10% of the maximum orbital velocity were predicted. The bed shear stress, on the other hand, responded with a strong peak near the step change in roughness; this amplification in the bed shear stress was found to increase with increasing difference in roughness and with increasing amplitude to roughness ratio. Amplifications experienced in the bed shear stress as large as 3–3.5 were predicted.