Online coupling of spectral and non-hydrostatic models for wave simulation from offshore to nearshore

A wide range of well established numerical models are now routinely used in coastal engineering studies. Phase-averaged spectral models are fundamentally based on linear theory, with non linear processes represented via ad hoc parametric submodels, and are thus best suited for simulating wave propagation from offshore to nearshore. Better representation of nonlinear waves in the nearshore is sought for with use of time-domain models, such as non-linear shallow water (NLSW) equations, Boussinesq-type (BT) and nonhydrostatic (NH) models. Although the NLSW equations can be used to simulate effectively broken waves and wave runup on a dry bed, they cannot correctly represent the onset of breaking. On the other hand, after a high effort has been addressed in the last two decades to improve the dispersion and nonlinear properties of both BT (Madsen & Fuhrman, 2010) and NH multi-layered models (Zijlema & Stelling, 2008), in principle they can now accurately represent wave propagation from offshore. However, such models are still excessively time-consuming to be considered for use in large-scale practical engineering problems. Therefore, the much cheaper phase-averaged approach is currently used to compute wave propagation from offshore, providing offshore boundary conditions to a phase-resolving model running in the nearshore. The above procedure is currently carried out manually, with use of distinct codes for the spectral and the phase-resolving models. Higher efficiency is expected to be achieved by online coupling the two models, resulting in one code and one executable, for seamlessly simulating wave evolution from generation to runup and land inundation. Developing an online coupling between the phase-averaged SWAN (Simulating WAves Nearshore, Booij, et al. 1999) and the time-domain SWASH (Simulating WAves till Shore, Zijlema et al. 2011) models, both open source, is the main aim of this work. Herein, results of the coupled-model are compared with laboratory data of unidirectional random wave runup carried out on a gentle, smooth and impermeable slopes (Mase, 1989). Furthermore, comparisons are presented with McCabe et al. (2010, 2011) findings obtained with a coupled SWAN – NLSW model.