Influence of large-scale shoal and navigation channel on infragravity-wave oscillations in a harbor

This study investigates the influence of a large-scale offshore shoal and a navigation channel on infragravity (IG) wave oscillations within a harbor using Macun Port as a case study. The non-hydrostatic model SWASH was used to simulate wave fields for two channel schemes, one perpendicular and one parallel to the shoal orientation, at design high and low water levels (DHWL/DLWL). The shoal was found to significantly amplify IG waves through nonlinear energy transfer from short waves, with heights up to 6 times the incident values at DLWL. Channel alignment critically influenced this amplification. The scheme with a channel perpendicular to the shoal orientation reduced IG energy entering the harbor by deepening water (suppressing shallow-water IG wave generation) and altering radiation stress fields by wave refraction (suppressing continued energy transfer from short waves), resulting in wharf-side IG wave heights averaging 63% (DLWL) and 74% (DHWL) lower than in the scheme with a shoal-parallel channel. Harbor resonance modes at periods 10–20 times the incident peak period were more sensitive to shoal effects than longer period modes. This shows that optimizing channel alignment can significantly mitigate shoal-enhanced IG waves and associated harbor resonance, providing insights for port engineering in similar environments.