Numerical Study of Sediment Suspension Over Bedforms in Combined Flows

Sediment suspension in the coastal environment is greatly enhanced by the presence of bedforms under the combined influence of both waves and currents. In order to assess the relative importance of the effects of bedforms, waves, and currents on suspended sediment concentrations, we simulate sediment suspension over sand ripples in a wave-current flow using a three-dimensional Navier-Stokes solver. Hydrodynamics of the wave-current flow is solved in a boundary-fitted curvilinear coordinate system incorporated with Large- Eddy Simulation (LES) to model the turbulence, and transport of suspended sediment is modeled using the Eulerian approach. Using this solver, a laboratory-scale numerical study is conducted in a 1 m x 1 m x 0.25 m (L x W x H) domain with 288 x 288 x 64 grid points. The flow is forced with the sum of oscillating and constant pressure gradients to produce combined wave-current flows in which the mean flow is either aligned with or orthogonal to the wave direction. We adjust the physical parameters based on the experimental set- up of Lacy et al. (2007)* such that hydrodynamics and suspended-sand concentration (SSC) profiles can be obtained under the influence of different wave-current conditions and bedforms. By comparing instantaneous and mean velocity fields from different experimental settings, turbulent features and vortex structures associated with different flow parameters and bedforms are investigated. From the resulting SSC profiles, we assess the relative effects on sediment suspension due to waves, currents and bedforms. The results are validated via comparison to the experimental data of Lacy et al. (2007)*. * J. R. Lacy, D. M. Rubin, H. Ideda, K. Modudai, and D. M. Hanes, "Bed forms created by simulated waves and currents in a large flume", Journal of Geophyiscal Research, 112, C10018, 2007