RANS-based simulation of wave-induced sheet-flow transport of graded sediments

A one-dimensional vertical (1DV) turbulence-closure flow model, coupled with sediment transport capabilities,is extended to incorporate graded sediment mixtures. The hydrodynamic model solves the horizontalcomponent of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations coupled with k–ωturbulence closure.

The sediment transport description includes both bed and suspended load descriptions. Socalledhigh-concentration effects (turbulence damping and hindered settling velocities) are likewise included.The sediment transport model treats the bed and suspended load individually for each grain fraction, includingeffects associated with increased exposure of larger particles within a mixture.

The suspended sedimenttransport model also makes use of modified reference concentration approach, wherein reference concentrationscomputed individually for each fraction are translated to a common level, conveniently enabling use of asingle computational grid for the simulation of suspended sediments.

Parametric study shows that these twoeffects combine to help alleviate an otherwise systematic tendency towards over- (under-) predicted transportrates for fine (coarse) sand fractions. The sediment transport model is validated against sheet-flow experimentaloscillatory tunnel measurements beneath velocity-skewed wave signals, and demonstrates similar accuracy(transport rates generally within a factor of two) for both graded and uniform sands.

The model is likewisevalidated against an extensive data set involving sheet-flow transport beneath acceleration-skewed wave signals(limited to uniform sands). It is then utilized to study potential effects of gradation on the net transport beneathsuch flows. The simulations suggest that gradation effects can both increase, as well as decrease, the totaltransport rate, depending largely on the behavior of the fine sand fraction.

The model is implemented within theMatlab environment, and is freely available upon request to the corresponding author.