Infragravity

Infragravity refers to a class of long-period gravity waves observed in the coastal zone, with play between the ocean and shoreline dynamics. These waves have periods longer than typical wind-generated waves, usually ranging from about 20 to 200 seconds (roughly 0.005 to 0.05 Hz), and they propagate primarily in the nearshore and surf zones.

The origin of infragravity energy is diverse. It can arise from nonlinear interactions among shorter wind waves, resulting in bound long-wave components. It can also be generated or amplified by shoaling and resonance with coastal topography, the presence of beach slopes, and edge waves that travel along the shoreline. Infragravity waves may exist as freely propagating modes or as bound components locked to the phases of shorter waves. Their behavior is influenced by coastal geometry, bathymetry, and wave forcing, often leading to low-frequency oscillations in water level that interact with shoreline run-up and inundation.

Infragravity plays a significant role in coastal hydrodynamics and sediment transport. The low-frequency oscillations can drive substantial shoreline motion, contribute to extreme run-up events, and influence coastal erosion and landscape evolution. They are typically detected with bottom pressure sensors, surface buoys, or coastal radar, which can separate infragravity signals from higher-frequency wind waves. Researchers study infragravity to improve understanding of nearshore energy partitioning, improve coastal hazard assessments, and inform engineering designs for seafront structures and beach nourishment projects.