Mobility Parameter and Sand Grain Size Effect on Sediment Transport Over Vortex Ripples in the Orbital Regime

Abstract

AbstractThe objective of this numerical study is the assessment of the effect of the mobility parameter ψ and the ratio ao/Dg (ao is the orbital amplitude and Dg is the sand grain diameter) on sediment transport induced by oscillatory flow over orbital vortex ripples. A large eddy simulation model was employed for the coupled simulation of the three‐dimensional turbulent oscillatory flow and the corresponding bed and suspended sediment transport. The immersed boundary method was implemented for the imposition of fluid flow and suspended sediment transport boundary conditions on the bed surface. Results are presented for oscillatory flow over fixed ripples with dimensions based on three values of ψ and for three cases of ao/Dg that span the range of typical values for vortex ripples in the orbital regime. It was found that the net bed load (nondimensional), qb, was always in the onshore direction and its magnitude decreased with increasing ao/Dg, while the effect of ψ on qb was stronger for ψ < 50 than for ψ > 50. On the other hand, the net suspended load (nondimensional), qs, was always in the offshore direction, and its magnitude increased with increasing ψ and increasing ao/Dg. The relative contribution of bed load versus suspended load on the total sediment load was found to depend on both ψ and ao/Dg, and the distinction between bed and suspended load dominance was quantified with a single parameter based on the ratio of the free‐stream velocity to the sand grain settling velocity.