This paper presents a comprehensive numerical study into the infragravity-wave dynamics at a field site, characterized by a gently sloping barred beach. The nonhydrostatic wave-flow model SWASH was used to simulate the local wavefield for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyze the infragravity-wave dynamics at spatial scales not often covered before. Overall, the model predicted a wavefield that was representative of the natural conditions, supporting the model application to analyze the wave dynamics. The infragravity-wave field was typically dominated by leaky waves, except near the outer bar where bar-trapped edge waves were observed. Relative contributions of bar-trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity-wave growth was partly explained by nonlinear energy transfers from short waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravity-wave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the model's potential to study wave dynamics at field scales not easily covered by in situ observations.

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In this study we investigated the capabilities of the mesh-free, Lagrangian particle method (Smoothed Particle Hydrodynamics, SPH) to simulate the detailed hydrodynamic processes generated by both spilling and plunging breaking waves within the surf zone. The weaklycompressible SPH code DualSPHysics was applied to simulate wave breaking over two distinct bathymetric profiles (a plane beach and fringing reef) and compared to experimental flume measurements of waves, flows, and mean water levels. Despite the simulations spanning very different wave breaking conditions (including an extreme case with violently plunging waves on an effectively dry reef slope), the model was able to reproduce a wide range of relevant surf zone hydrodynamic processes using a fixed set of numerical parameters. This included accurate predictions of the nonlinear evolution of wave shapes (e.g., asymmetry and skewness properties), rates of wave dissipation within the surf zone, and wave setup distributions. By using this mesh-free approach, the model is able to resolve the critical crest region within the breaking waves, which provided robust predictions of the wave-induced mass fluxes within the surf zone responsible for undertow. Within this breaking crest region, the model results capture how the potential energy of the organized wave motion is initially converted to kinetic energy and then dissipated, which reproduces the distribution of wave forces responsible for wave setup generation across the surf zone. Overall, the results reveal how the mesh-free SPH approach can accurately reproduce the detailed wave breaking processes with comparable skill to state-of-the-art mesh-based Computational Fluid Dynamic models, and thus can be applied to provide valuable new physical insight into surf zone dynamics.

The physical roughness (canopies) formed by organisms within aquatic ecosystems (e.g., seagrass, kelp, and mangroves) modifies the local wave‐driven hydrodynamics within coastal and estuarine regions. In wave‐dominated environments, an understanding of the mean wave‐driven flows generated within and above canopies is important, as it governs material transport (e.g., of nutrients, sediment, and biota). However, until recently the effect of submerged canopies on wave‐current interactions and the resulting mean (wave‐averaged) flow dynamics has received relatively little attention. In this study, a combination of wave flume experiments and numerical modeling is used to investigate the wave‐induced mean flow profiles in the presence of a submerged canopy. The measured velocities and vegetation forces were used to derive bulk drag and inertia coefficients, and to validate a nonhydrostatic 2DV wave‐flow model. The numerical model results were used to conduct an in‐depth analysis of the mean horizontal momentum terms responsible for driving the mean (horizontal) flow within and above the submerged canopies. We show that the mean canopy hydrodynamics are driven by vertical gradients in wave and turbulent Reynolds stresses, balanced by the mean canopy drag forces. The wave Reynolds stress gradient is the dominant force driving the in‐canopy mean flow and is directly related to the vorticity that is generated when the wave orbital motions become rotational near the canopy interface. This study provides new insight in the mechanisms responsible for wave‐driven mean flows within submerged canopies and guidance for how these hydrodynamics can be predicted in coastal wave‐circulation models.

Wave-induced currents are an ubiquitous feature in coastal waters that can spread material over the surf zone and the inner shelf. These currents are typically under resolved in non-hydrostatic wave-flow models due to computational constraints. Specifically, the low vertical resolutions adequate to describe the wave dynamics-and required to feasibly compute at the scales of a field site-are too coarse to account for the relevant details of the three-dimensional (3D) flow field. To describe the relevant dynamics of both wave and currents, while retaining a model framework that can be applied at field scales, we propose a two grid approach to solve the governing equations. With this approach, the vertical accelerations and non-hydrostatic pressures are resolved on a relatively coarse vertical grid (which is sufficient to accurately resolve the wave dynamics), whereas the horizontal velocities and turbulent stresses are resolved on a much finer subgrid (of which the resolution is dictated by the vertical scale of the mean flows). This approach ensures that the discrete pressure Poisson equation-the solution of which dominates the computational effort-is evaluated on the coarse grid scale, thereby greatly improving efficiency, while providing a fine vertical resolution to resolve the vertical variation of the mean flow. This work presents the general methodology, and discusses the numerical implementation in the SWASH wave-flow model. Model predictions are compared with observations of three flume experiments to demonstrate that the subgrid approach captures both the nearshore evolution of the waves, and the wave-induced flows like the undertow profile and longshore current. The accuracy of the subgrid predictions is comparable to fully resolved 3D simulations-but at much reduced computational costs. The findings of this work thereby demonstrate that the subgrid approach has the potential to make 3D non-hydrostatic simulations feasible at the scale of a realistic coastal region.