[1] We present two depth- and phase-resolving models, based on single- and two-phase approaches for suspended sediment transport under water waves. Both models are the extension of a wave hydrodynamic model Cornell Breaking Wave and Structure (COBRAS). In the two-phase approach, dilute two-phase mass and momentum equations are calculated along with a fluid turbulence closure based on balance equations for the fluid turbulence kinetic energy k f and its dissipation rate ∈ f . In the single-phase approach the fluid flow is described by the Reynolds-Averaged Navier-Stokes equations, while the sediment concentration is calculated by an advection-diffusion equation for the conservation of sediment mass. The fluid turbulence is calculated by k f -∈ f equations that incorporate the essential influence of sediment, which can also be consistently deduced from the two-phase theory. By adopting a commonly used sediment flux boundary condition near the bed the proposed models are tested against laboratory measurements of suspended sediment under nonbreaking skewed water waves and shoaling broken waves. Although the models predict wave-averaged sediment concentrations reasonably well, the corresponding time histories of instantaneous sediment concentration are less accurate. We demonstrate that this is due to the uncertainties in the near-bed sediment boundary conditions. In addition, we show that under breaking waves the near-bed sediment pickup cannot be solely parameterized by the bottom friction, suggesting that other effects may also influence the near-bed sediment boundary conditions.

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Toward modeling turbulent suspension of sand in the nearshore

This study is undertaken to find out the most reliable breaker height formulas that predict well for a wide range of hydraulic conditions. The applicability of 24 existing formulas, for computing b...

Verification and modification of breaker height formulas

A new breaker height formula is developed based on a re-analysis of available laboratory data. The analysis shows that the breaking wave steepness is mainly governed by its deepwater wave steepness...

A proposal of new breaker height formula

To better understand the effect of wave-breaking-induced turbulence on the bed, we report a 3-D large-eddy simulation (LES) study of a breaking solitary wave in spilling condition. Using a turbulence-resolving approach, we study the generation and the fate of wave-breaking-induced turbulent coherent structures, commonly known as obliquely descending eddies (ODEs). Specifically, we focus on how these eddies may impinge onto bed. The numerical model is implemented using an open-source CFD library of solvers, called OpenFOAM, where the incompressible 3-D filtered Navier-Stokes equations for the water and the air phases are solved with a finite volume scheme. The evolution of the water-air interfaces is approximated with a volume of fluid method. Using the dynamic Smagorinsky closure, the numerical model has been validated with wave flume experiments of solitary wave breaking over a 1/50 sloping beach. Simulation results show that during the initial overturning of the breaking wave, 2-D horizontal rollers are generated, accelerated, and further evolve into a couple of 3-D hairpin vortices. Some of these vortices are sufficiently intense to impinge onto the bed. These hairpin vortices possess counter-rotating and downburst features, which are key characteristics of ODEs observed by earlier laboratory studies using Particle Image Velocimetry. Model results also suggest that those ODEs that impinge onto bed can induce strong near-bed turbulence and bottom stress. The intensity and locations of these near-bed turbulent events could not be parameterized by near-surface (or depth integrated) turbulence unless in very shallow depth.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JC009854

A numerical investigation of wave-breaking-induced turbulent coherent structure under a solitary wave

A wave-resolving model for nearshore suspended sediment transport

This paper describes the mechanism of sand suspension due to wave breaking on the basis of detailed laboratory measurements of suspended sand concentration near the breaking point with a light-absorption-type probe. The distribution and the total amount of suspended sand were evaluated using the data obtained, which were carefully corrected for the contamination due to air bubbles entrained through wave breaking. A predictive formula for the amount of sand suspended by a breaker was proposed for a wide range of conditions.

Kiichi Homma

Papers

Laboratory Study on Sand Suspension Due to Breaking Waves