Parameterization of intrawave ripple-averaged sediment pickup above steep ripples

Near-bed sediment pickup is critical for predictions of intrawave suspension and in turn net sediment transport in coastal models. In the present study, numerical results from a two-dimensional Reynolds-averaged Navier-Stokes model are used to assess the functional relationship of intrawave ripple-averaged sediment pickup above steep ripples. The numerical model provides intrawave time histories of ripple-averaged near-bed velocities and turbulence, which are qualitatively interrogated to determine pickup functional relationships. Several specific sediment pickup formulations are implemented within the numerical model: Expressions relating pickup to near-bed velocity or near-bed turbulent kinetic energy via the bed shear stress; and expressions relating pickup to near-bed shear production of turbulent kinetic energy. These are then tested via model-data comparisons of near-bed suspended sediment concentration. The results show that the traditional functions relating sediment pickup to near-bed velocity cannot lead to reasonable intrawave suspension predictions above vortex ripples under a ripple-averaged framework. Instead, relating sediment pickup to near-bed turbulence quantities, such as turbulent kinetic energy or shear production of turbulent kinetic energy, significantly improves the numerical predictions for these conditions.