A Tow-phase Flow Model For Simulating Sheet Flow Sediment Transport Under The Wave-current Conditions

In the surf zone, sheet flow sand transport regime occurs under extreme wave conditions such as storms, when the bed shear stress is so large (Shields parameter> 0.8-1.0) that the sea bed becomes plane. In sheet flow regime, a large amount of sands is predominantly transported in a near bed thin layer with high concentrations, leading to a significant change in the near-shore bathymetry. As an important sediment transport regime in the near-shore morphodynamic progress, an accurate prediction of sediment movement under sheet flow condition is needed. Accordingly, many experimental and numerical studies of sheet flow sediment movement have been carried out.After considering the various two-phase models, we proposed an enhanced two-phase flow model to simulate the sheet flow sediment movement under two different sheet flow conditions, i.e., the sinusoidal oscillation and the combined wave/current flows. This model applies the simple mixing-length turbulence closure with a newly-introduced time-varying eddy viscosity and sediment diffusivity based on Trowbridge and Madsen (1984). Model’s validation was conducted by comparing to a series of classic sheet flow experiment measurements, considering the spatiotemporal variation of sediment concentration, the time-averaged concentration distribution, and the net sand transport rate estimation under the combined wave/current flows. Intercomparison among various existing two-phase flow models (Dong and Zhang,2002, Liu and Sato,2005b, and Amoudry,2014) shows that there is no clear improvement on the sheet flow sediment concentration prediction after using the sophisticated high order turbulence closure in the two-phase flow simulation. Comparing with the present model, applying the two-equation k-ε or k-ω turbulence closure (Amoudry,2014) in the two-phase flow model does not produce better temporal concentration estimations for the sediment-laden sheet flow conditions. Nevertheless, comparing to the experimental measurement, the present two-phase flow model with the simple mixing-length turbulence closure shows similar, if not better, simulation accuracy as that of the k-ε or k-ω closure based two-phase flow model from both the qualitative and quantitative viewpoints. In addition, significant improvement on the net sand transport rate estimation using the present two-phase flow model was confirmed after comparing to all 25 sheet flow measurement data in Dohmen-Janssen (1999).