Understanding the state of the muddy seabed - a numerical study utilizing multiphase flow approach

Understanding various modes of sediment transport under combined wave-current flows is critical to better predictions of coastal morphodynamics and hydrodynamic dissipation. For sediment transport in water, intergranular interactions and particle-fluid interactions are highly complex and represent a classic particle-laden flow problem with relatively low Stokes number and Bagnold number as comparing its gas-particle flow counterpart. For sediment transport induced by surface waves, the highly unsteady nature of the oscillatory flow, which is not fully turbulent, warrants a turbulence-resolving approach. In this study, recent findings on a diverse range of muddy seabed states revealed by 3D, turbulence-resolving simulations are reported. Assuming a small Stokes number, the Equilibrium approximation to the Eulerian two-phase flow equations is adopted. The resulting simplified equations are solved with a high-accuracy pseudo-spectral scheme in an idealized oscillatory bottom boundary layer (OBBL). For a typical energetic muddy shelf, the Stokes Reynolds number is no more than 1000 and all of the scales of flow turbulence and their interaction with sediments are resolved. With increasing sediment availability or settling velocity, the seabed state evolves from well-mixed sediment distribution, to the formation of lutocline and a complete laminarization of the OBBL. These findings have critical implications to offshore delivery of fine sediment and wave dissipation. New findings regarding the effect of rheological stress on the seabed state and new extensions to the model to improve the efficiency and flexibility of the numerical scheme for a wider range of sediment transport processes will also be discussed.