Toward a more realistic modelling of coastal hydrodynamics: impact of the boundary conditions in the three-dimensional phase-resolving model Symphonie 3.8.9

Abstract. This study focuses on improving wave boundary conditions for phase-resolving coastal simulations using the three-dimensional non-hydrostatic SYMPHONIE model deployed on the rocky beach of Ars-en-Ré (France), located on the exposed northwestern coast of Ré Island. With regard to waves leaving the numerical domain, a new radiative condition scheme is proposed. With regard to waves entering the domain, two approaches for forcing offshore boundary conditions are compared: using direct measurements and downscaling from larger-scale, lower-resolution spectral wave models. A particular frequency de-coherence procedure is applied to reconstruct the phase-resolving boundary signal from the directional wave spectrum.

The model’s performance is evaluated against a comprehensive hydrodynamic and bathymetric field campaign conducted during the winter of 2020–2021. Comparisons with in situ observations demonstrate that both configurations successfully reproduce the variability of significant wave heights and frequency spectra across the nearshore zone, as well as the main directional wave transformation from offshore to nearshore. While both offshore-forcing strategies provide realistic results, uncertainties associated with the incident spectral forcing and its phase-resolving reconstruction, the numerical representation of bathymetry and dissipation processes remain a major source of model variability.