A local phase-shift approach for predicting wave-group-forced infragravity waves under directional seas

Accurate prediction of Wave-Group-Forced (WGF) InfraGravity (IG) waves depends on resolving the corresponding phase shift, typically achieved through a coupled phase - amplitude equation. However, this approach requires a grid resolution that resolves the structure of the wave groups making it computationally expensive at regional scales. To address this limitation, an existing local expression for the phase shift of normally incident WGF-IG waves has been extended to account for directional seas. The extended formulation is verified against predictions from the coupled phase - amplitude model using bi-chromatic wave forcing over a uniformly sloping beach for a wide range of sea-swell conditions. Results show that the local approach performs well in the off-resonant region for obliquely incident waves. When applied outside this regime, however, its accuracy decreases, with performance varying depending on sea-swell and bathymetric conditions. The coupled and local phase shift approaches are also validated with observations obtained during the Coast3D field experiment. The total, incoming and outgoing IG waves are predicted with comparable skill and root mean square error for both methods. The good match using the local expression is attributed to the fact that the conditions during Coast3D correspond to directionally broad sea-swell spectra with relative short peak periods propagating over moderately sloping bathymetry for which the verification showed significant skill. Additional validation with field observations at other locations are necessary to firmly determine the limitations of the use of a local phase shift.