Experimental investigation of evolution of infragravity waves over a large-scale shoal

Abstract

This study investigates the amplification of infragravity waves induced by short-wave groups over an underwater shoal, as well as their irreversible growth following the processes of shoaling and deshoaling across front and rear slopes of the shoal under various wave conditions. Through laboratory experiments, we observed that, in scenarios without short-wave breaking, the amplification of infragravity waves increased with the peak frequency and spectral width of incident short waves, while decreasing with the water depth atop the shoal. These dependencies were further conceptualized through the introduction of a normalized shoal length, which represents the spatial accumulation of local energy transfer from short to infragravity waves. The normalized shoal length was proved to be equivalent to the normalized bed slope by Battjes et al. (2004), expanding its physical interpretation to include the travel distance of the wave groups. Additionally, the near-resonance solution for infragravity waves by Liao et al. (2021) was extended to incorporate wave attenuation effects, especially under conditions of short-wave breaking within the surf zone where infragravity waves rapidly decay. The modified solution aligns closely with experimental observations regarding infragravity-wave height and phase coupling with short-wave groups. The detailed wave measurements from shoaling to deshoaling zones are useful for enhancing understanding of infragravity wave dynamics over complex seabed features.