To understand the interaction between wave‐breaking induced turbulent coherent structures and suspended sediment transport, we report a Large‐Eddy Simulation (LES) study of wave‐breaking processes over a near‐prototype scale barred beach. The numerical model is implemented using the open‐source CFD toolbox, OpenFOAM^®, in which the incompressible three‐dimensional filtered Navier‐Stokes equations for the water and air phases are solved with a finite volume scheme. A volume of fluid (VOF) method is used to capture the evolution of the water‐air interface. The numerical model is validated with measured free surface elevation, turbulence‐averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60%∼70% of coherent bottom stress events are associated with surface‐generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave‐breaking turbulence approaching the bed.

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.

Turbulence characteristics in the swash zone are investigated using a 3-D large eddy simulation model. The numerical model is implemented based on OpenFOAM which solves the filtered Navier-Stokes equations for two immiscible fluids with a standard Smagorinsky subgrid-scale closure. The numerical model is validated with laboratory data for swash flow driven by a dam-break apparatus. The model results demonstrate that the main characteristics of turbulence in the swash zone are different from those in the surf zone, which are mainly induced by surface wave breaking. During uprush phase, bore-generated turbulence has 2-D turbulence characteristics because of limited water depth. Near-bed-generated turbulence is mainly observed during backwash. Turbulence production and turbulent dissipation rate estimated from the model results indicate an imbalance, possibly due to advection at swash front and large roughness used. Touching down of turbulent coherent structure (TCS) is observed during uprush, which drives intense bed shear stress. During the backwash, interaction between TCS and bed is less clear. However, finger-like patterns in the spatial extent of bed shear stress and vertical components of vorticity are predicted during the backwash. The location of the strongest finger patterns in the vertical direction is collocated with that of maximum turbulence production. These finger patterns are likely caused by boundary layer instabilities injected vertically from the bed.

The nonhydrostatic surface and terrain‐following coastal model NHWAVE is utilized to simulate a continually forced stratified shear flow in a straight channel, which is a generic problem to test the existing nonhydrostatic coastal models' capability in resolving shear instabilities in the field scale. The resolved shear instabilities in the shear layer has a Reynolds number of about 1.4 × 10⁶, which is comparable to field observed value. Using the standard Smagorinsky closure with a grid size close to the Ozmidov length scale, simulation results show that the resolved energy cascade exceeds 1 order of magnitude and the evolution and turbulent mixing characteristics are predicted well. Two different approaches are used to estimate the turbulent dissipation rate, namely using the resolved turbulent energy spectrum and the parameterized subgrid turbulent dissipation rate, and the predicted results provide the upper and lower bounds that encompass the measured values. Model results show significantly higher turbulence in braids of shear instabilities, which is similar to field observations while both the subgrid turbulent dissipation rate and resolved vorticity field can be used as surrogates for measured high acoustic backscatter signals. Simulation results also reveal that the surface velocity divergence/convergence is an effective identifier for the front of the density current and the shear instabilities. To guide future numerical studies in more realistic domains, an evaluation on the effects of different grid resolutions and subgrid viscosity on the resolved flow field and subgrid dissipation rate are discussed.

To better understand the effect of wave-breaking-induced turbulence on nearshore sand transport, we carry out a 3D Large Eddy Simulation study of breaking solitary wave in spilling condition. Using a turbulence-resolving approach, we investigate the formation and evolution of wave-breaking-induced turbulent coherent structures, commonly known as obliquely descending eddies (ODEs), and how they may interact with the bed and enhance the suspended sediment transport. The numerical implementation is based on an open-source CFD library of solvers, called OpenFOAM®, where the incompressible 3D 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 are approximated with a Volume of Fluid (VOF) method. With the dynamic Smagorinsky closure, the numerical model results show good agreement with measured wave flume data of solitary wave breaking over a 1/50 sloping beach. Simulation results show that 3D hairpin vortices are generated under breaking wave, and they possess counter-rotating and downburst features, which are the key characteristics of obliquely descending eddies (ODEs) observed by earlier laboratory studies with Particle Image Velocimetry. A suspended sediment transport formulation (Liu and Garcia 2008) has been incorporated into the present hydrodynamic solver as part of the OpenFOAM® framework. Model results suggest that those ODEs that impinge onto the bed can cause significant bottom sediment suspension, and the location of the sediment plume is highly associated with the impinging points of ODEs but with notable time-lag.