Evolution of infragravity waves on a macro-tidal low-tide-terrace beach

This study investigates infragravity (IG) wave dynamics on a low-energy macrotidal low-tide-terrace beach through field observations and numerical simulations using the one-dimensional nonhydrostatic SWASH model. Field measurements were conducted in the intertidal zone to capture wave transformation across varying tidal stages. The collected data were used to validate model performance and to carry out detailed analyses, including incoming/outgoing signal separation, cross-correlation, and bispectral analysis. Results show that, in the IG band, tides chiefly regulate wave height and the partitioning of energy between bound and free modes: at high water the bound long-wave component dominates the IG energy budget, whereas at low water the contribution from free long waves exceeds 80 %. At low tide, a transition in IG wave generation mechanism is observed from bound wave dominance offshore to breakpoint-forced IG waves in the surf zone, with wave group structure weakening progressively after breaking. At high tide, IG wave amplification occurs under non-breaking conditions, attributed to nonlinear energy transfer from short waves during shoaling, supported by a time lag between wave groups and IG waves. A notable slow-down in outgoing IG wave celerity to its half is observed from the surf zone, indicating substantial energy dissipation and dispersion.