Infra-gravity Waves and Cross-shore Transport -- A Conceptual Study

TL;DR

This study investigates how infra-gravity waves, originating from ocean wave groups, significantly influence cross-shore currents and water movement near the shore using linear wave theory and numerical modeling.

Contribution

It demonstrates that infra-gravity waves cause substantial cross-shore motion, providing a theoretical and numerical analysis of their role in nearshore wave dynamics.

Findings

Infra-gravity waves lead to strong cross-shore currents.

Particle trajectories show large horizontal excursions due to infra-gravity waves.

Infra-gravity waves contribute to back-and-forth water motion beneath gravity waves.

Abstract

Infra-gravity waves are generally known as small-amplitude waves of periods between 25 seconds and 5 minutes. They originate from the presence of wave groups in the open ocean waves and can move freely after being released near the surf zone where they can be further fueled with energy from the spatially varying break point of swell waves . As these waves approach the shore, the relative importance of the infra-gravity wave signal increases, and its impact on the shorter waves gets stronger. In addition, infra-gravity waves drive strong cross-shore currents, which lead to significant back-and-forth motion of the underlying sea water. This strong cross-shore motion has been made visible by recent field studies, where significant cross-shore movement was detected and found to be correlated with the infra-gravity wave signal. In the present work, the connection between infra-gravity waves is explored further using linear wave theory and an established numerical nearshore wave model (BOSZ). It is shown that in all cases, the presence of infra-gravity waves leads to strong cross-shore motion. This behavior can be understood by considering the infra-gravity waves as separate free waves, and then following the fluid particle trajectories excited by these waves. As it is shown, these trajectories have a very large horizontal extent which -- if not separated from the main gravity wave field -- appears as a large, but often not directly visible back-and-forth motion, underlying the more readily observable gravity ocean waves.