Cross-shore hydrodynamics and morphodynamics modeling of an erosive event in the inner surf zone

Abstract

The phase-averaged and depth-integrated coastal morphodynamic model, XBeach-Surfbeat, was investigated for its capability of predicting the cross-shore hydrodynamics and morphodynamics in the inner surf zone by simulating the storm-induced berm erosion, sediment transport, and subsequent sand bar formation. By utilizing a comprehensive hydrodynamic and morphodynamic dataset measured in a large wave flume and high-fidelity 3D large-eddy simulation (LES) data, a rigorous model validation was conducted to assess its capability in predicting inner-surf zone hydrodynamics and to explore how the improved hydrodynamic performance impacts the predicted morphodynamics. Using the default model parameters of the model, the undertow was overestimated with the peak magnitude being 30%–35% larger in the inner surf zone. Combining Monte Carlo simulation, the optimum hydrodynamic calibration for the simulated undertow was achieved when the roller energy dissipation parameter ($\beta )$ was maintained below 0.1, and the threshold water depth ($h_{\min }$) exceeded 0.25 m. The calibrated undertow improved the morphodynamic predictions by reducing the excessive berm erosion (Event I) and sand bar growth in the inner surf zone (Event II). Further improved morphodynamic predictions were achieved by calibrating sediment transport parameters, including the onshore sediment transport coefficient (${\gamma }_{ua}$) and the bore interval coefficient ($T_{bfac}$) associated with turbulence-bed interaction. A consistent set of optimized model coefficients for the model is shown to be effective in simulating the entire erosive event (combined Events I and II). This study reveals that further improvement of the model's capability may require incorporating new parameterizations and physics, such as wave-breaking-induced turbulence and wave nonlinearity associated with sediment transport in the inner surf and swash zones.