Parameterization of intrawave ripple‐averaged sediment pickup above steep ripples

L O Amoudry,P L.‐F Liu,P D Thorne,A J Souza

doi:10.1002/2015jc011185

Abstract

AbstractNear‐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.

Highlights

Numerical coastal models cannot resolve all spatiotemporal scales
Following Vittori [2003], we present in Figure 6 the intrawave time histories for the different near-bed terms appearing in the balance equation of turbulent kinetic energy (TKE) (equation (7)): time derivative @tkb, shear production Pb, dissipation rate eb, advective transport Ab, and diffusive transport Db
It is clear that using pickup functions of the type of Eu would always lead to maximum pickup at maximum near-bed flow, and near-bed sediment concentration incorrectly peaking near maximum flow with little to no asymmetry between the two half-cycle peaks

Summary

Introduction

Numerical coastal models cannot resolve all spatiotemporal scales This well-known issue leads to the necessary parameterization of processes that occur at the unresolved small scales. For sediment transport, this is especially pertinent for near-bed processes, and how the unresolved small scales are represented is fundamental for the predictive ability of numerical models. The intrawave correlation between velocity and concentration can generate or significantly modify net sediment transport. This has been observed for fine sand for which so-called phase-lag effects can be significant [e.g., Dohmen-Janssen et al, 2002; O’Donoghue and Wright, 2004]

Results

Discussion

Conclusion