Nonhydrostatic Numerical Modeling of Fixed and Mobile Barred Beaches: Limitations of Depth-Averaged Wave Resolving Models around Sandbars

: Along sandy coastlines, submerged, shore-parallel sandbars play an essential role in shoreline morphology by dissipating wave energy through depth-induced wave breaking. While wave breaking and sediment transport around sandbars are complex three-dimensional (3D) processes, shoreline morphology is typically simulated with depth-averaged models that feature lower computational demand than do 3D models. In this context, this study examines the implications of depth-averaging the fl ow fi eld and approximating the breaking process in nonhydrostatic models (e.g., XBeach nonhydrostatic) for the hydro-and morphodynamic processes around sandbars. The implications are drawn based on reproducing large-scale experiments of a barred beach pro fi le using the single-layer (XBNH) and the reduced two-layer (XBNH + ) modes of XBeach. While hydrodynamic processes were predicted with high accuracy on the sandbar ’ s seaward side, wave heights were overpredicted on the bar ’ s landward side. The overestimation was due to the simpli fi ed reproduction of the complex breaking process near the sandbar ’ s peak, particularly in terms of the generated turbulence in the water column. Moreover, the velocity pro fi le with a strong undertow could only be represented in a simpli fi ed way even using the two-layer mode XBNH + , thus resulting in inaccurate predictions of sediment loads around the sandbar. A parametric study is performed, and it revealed which model parameters control the simulation of the wave-breaking process. Thus, wave height predictions could be improved by tuning the energy-dissipation parameters. However, fl ow velocities and morphodynamic predictions could not be improved accordingly. Thus, this study identi fi es possible hydrodynamic model improvements, such as incorporating a roller dissipation model. Moreover, it improves understanding of key drivers and processes that should be included in nonhydrostatic depth-averaged models to simulate morphological changes around sandbars more ef fi ciently. DOI: 10.1061/(ASCE)WW.1943-5460.0000685 . This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/.