Efficient non-hydrostatic modelling of 3D wave-induced currents using a subgrid approach

• Published in: Ocean modelling (Oxford) Vol. 116; pp. 118 - 133
• Main Authors: Rijnsdorp, Dirk P., Smit, Pieter B., Zijlema, Marcel, Reniers, Ad J.H.M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.08.2017
• Subjects: Nearshore circulation; Non-hydrostatic modelling; Subgrid; SWASH; Wave breaking; Wave-induced currents; SWASH; Wave-induced currents; Non-hydrostatic modelling; Nearshore circulation; Wave breaking; Subgrid
• ISSN: 1463-5003; 1463-5011
• DOI: 10.1016/j.ocemod.2017.06.012

•We propose a subgrid approach to efficiently resolve wave-induced currents in nonhydrostatic models.•Model predictions are compared with laboratory experiments of surf zone waves.•The model captures the wave evolution and mean flow profiles throughout the surf zone.•The subgrid approach makes 3D applications of non-hydrostatic models feasible at field scales. 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.