Laboratory validation of the extended SWASH model for modeling wave dynamics under depth-uniform ambient currents

Abstract

In complex coastal systems such as estuaries and tidal inlets, surface gravity waves and currents often coexist. Phase-resolving wave models are limited in modeling the effects of ambient currents (e.g., tidal and wind-driven) on wave dynamics due to the high computational cost of resolving the flow propagation. Recently, the non-hydrostatic model SWASH has been extended to embed depth-uniform ambient currents provided by external sources (e.g., observations or circulation models) into the control equations in the form of additional terms (Rijnsdorp et al., 2024), circumventing the computational burden of directly simulating the flow field.

This study evaluates the performance of the extended SWASH model in predicting wave responses to spatially varying depth-uniform currents (following, opposing, and strong opposing currents) under diverse wave conditions using laboratory experiments. Key findings reveal that under weak current conditions, the model accurately predicts current-induced changes in amplitude and wavelength even with coarse vertical resolutions (e.g., 2 layers). Finer vertical resolution (e.g., 20 layers) is needed to capture the nonlinear shallowing, wave breaking, and blocking induced by strong opposing currents. In particular, the model successfully predicts the frequency downshift of waves as they approach the theoretical blocking point and reproduces the modulation of monochromatic and bichromatic wave patterns by strong opposing currents, albeit with an overestimation of the wave height. The results of this study demonstrate the extended SWASH model can be a practical tool for simulating coastal wave dynamics under depth-uniform ambient currents that vary slowly relative to the wave-time scale.