Infragravity Waves Across a Steep-Sloping Rough Rocky Seabed on the Inner Shelf

Abstract

Observations of infragravity (IG) waves were conducted from June to October 2023 along the rocky shores off China Rock, CA, on the inner shelf at depths of 8–13 m. The study examines inner-shelf IG wave dynamics over rough, steep rocky shores and compares them with those over smoother, gentler sandy shores and coral reefs. IG energy correlated with sea-swell (SS) energy but exhibited a flat spectral distribution across the IG frequency band, showing minimal temporal variations, except during storms. Cross-spectral phase analysis revealed a constant progressive IG wave structure. Theoretical spectral-phase computations based on linear wave dispersion agree well with observations, indicating that IG waves are predominantly free, a finding further supported by a newly derived theoretical relationship showing that the squared correlation coefficient estimates the bound-wave energy fraction. During storms, the IG waves behave increasingly as progressive bound and free waves. These findings contrast with sandy shores, where standing waves typically produce frequency-dependent nodal structures. A cross-shore energy-flux balance indicated that nonlinear energy transfer dominated the IG energy-flux gradient for local generation. Excess energy is balanced by bottom dissipation over the rough seabed, with friction factors larger than, though comparable to, those of coral reef flats. Relative IG-to-SS energy was lower than in other environments, likely due to the absence of reflected and refractively-trapped waves along the extended stretch of rocky shores. Findings suggest that IG waves in this region are locally generated, propagate shoreward as a progressive wave, and are dissipated by the rough, rocky seabed.