TY - JOUR

T1 - Nonhydrostatic Numerical Modeling of Fixed and Mobile Barred Beaches

T2 - Limitations of Depth-Averaged Wave Resolving Models around Sandbars

AU - Elsayed, Saber M.

AU - Gijsman, Rik

AU - Schlurmann, Torsten

AU - Goseberg, Nils

N1 - Funding Information: The financial support of the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG) for the first author (Fund No. EL 1017-1/1) for the ReFresh project is gratefully acknowledged. The remaining authors acknowledge the support of the Ministry of Education and Research of Germany (BMBF) through the STENCIL project (Contract No. 03F0761). The authors wish to thank the SINBAD project researchers for providing the processed measurements, particularly Joep van der Zanden. Special thanks for their thorough and highly constructive critique go to Editor and the two anonymous reviewers.

PY - 2021/11/9

Y1 - 2021/11/9

N2 - 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 flow field 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 profile 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 simplified 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 profile with a strong undertow could only be represented in a simplified 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, flow velocities and morphodynamic predictions could not be improved accordingly. Thus, this study identifies 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 efficiently.

AB - 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 flow field 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 profile 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 simplified 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 profile with a strong undertow could only be represented in a simplified 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, flow velocities and morphodynamic predictions could not be improved accordingly. Thus, this study identifies 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 efficiently.

KW - Depth-averaged Nonhydrostatic models

KW - Reduced two water layers

KW - Sandbar evolution

KW - Undertow

KW - Wave breaking

KW - XBeach

UR - http://www.scopus.com/inward/record.url?scp=85118952654&partnerID=8YFLogxK

U2 - 10.1061/(ASCE)WW.1943-5460.0000685

DO - 10.1061/(ASCE)WW.1943-5460.0000685

M3 - Article

AN - SCOPUS:85118952654

VL - 148

JO - Journal of Waterway, Port, Coastal and Ocean Engineering

JF - Journal of Waterway, Port, Coastal and Ocean Engineering

SN - 0733-950X

IS - 1

M1 - 04021045

ER -