Generation Of Net Sediment Transport By Velocity Skewness In Oscillatory Sheet Flow

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The velocity of oscillatory flows resulting from nearshore short waves usually become skewed with peaked narrow flow crest and flat wide flow troughs in wave propagation and shoaling. The interaction between sediment and water and the collision between sediment particles are obviously strong in the sheet flow layer with large shear stress and high sediment concentration. An extra current is usually imposed on the velocity-skewed oscillatory sheet flow and produces much complex boundary layer flow and sediment transport process that is considered essential for morphological studies.

A qualitative approach and a two-phase numerical model are applied to study the generation of net sediment transport in sheet flow resulting from velocity-skewed oscillatory flow combined with the current. The qualitative approach is an integration of exponential approximations of pseudo-laminar boundary layer velocity and sediment concentration which considers mass conservation.

The wave boundary layer thickness is shown as important in sediment transport process by the qualitative approach, and the exponent for instantaneous sediment transport rate (q/qm) against the exponential function of velocity (U/Um) can generally be unified to decrease with the increment of phase-residual. The instantaneous variables are reasonably obtained by the two-phase model, and the differences (i.e., erosion depth, boundary layer thickness, sediment flux, and transport rate) between positive and negative flow stages caused by velocity skewness are shown, with considerable importance in the generation of net boundary layer flow and sediment transport.

The effects of phase-lag and boundary layer flow for sheet flow transport in pure velocity-skewed oscillatory flow combined with current were studied. The wave boundary layer developments of positive and negative flow stages are different and the wave boundary layer thickness near the flow crest is larger than that near the flow trough due to velocity skewness in a purely velocity-skewed flow. The asymmetric wave boundary layer leads to a negative net wave boundary layer flow component due to velocity skewness. An extra positive current enhances the turbulent wave boundary layer’s asymmetry to enlarge the negative net wave boundary layer flow component, and a negative current reduces the turbulent wave boundary layer asymmetry to reduce the net wave boundary layer flow component.

In a large phase-residual case, the periodic erosion depth and concentration distribution are almost constant, leading to q/qm≈U/Um, and the total net boundary layer flow almost decides the net sediment transport. In a small phase-residual case, the net sediment transport is dominated by the total net boundary layer flow in the sheet flow layer bottom and by the wave-related transport above the initial bed, where periodic concentration and velocity variations are obvious. In this case, the instantaneous sediment transport can be approximated by q/qm≈(U/Um)3, which is close to existing instantaneous-type formulas without phase-residual.

In conclusion, the phase-lag effect is shown to be important but insufficient, while the asymmetry between positive and negative flow stages in wave boundary layer development also contributes a lot to the generation of net sediment transport in pure velocity-skewed sheet flow combined with a current.

These findings are described in the article entitled Generation of net sediment transport by velocity skewness in oscillatory sheet flow, recently published in the journal Advances in Water Resources. This work was conducted by Xin Chen, Fujun Wang, and Xuelin Tang from China Agricultural University, Yong Li from China Geological Survey, and Genfa Chen from China Institute of Water Resources and Hydropower Research.

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