Propagation of Free Infragravity Waves Generated at Distant Beaches

Infragravity (IG) waves are important drivers of extreme run-up, morphological changes, and seiches. While locally forced IG waves have been extensively investigated, recent studies have revealed the significance of free IG waves generated on distant beaches. This study focuses on free IG waves generated at a coast facing southeast in Japan during the passage of a typhoon. The relationship between incident short waves and free IG waves as well as their contributions to nearshore IG waves, in particular to seiches in a small port, are analyzed using unique measurement data and numerical experiments. During the typhoon passage, more than 75% of the observed IG wave energy originates from free IG waves at an observatory located at a depth of 23 m, and their peak direction is alongshore. Six-year measurement data demonstrate that peak directions of free IG waves strongly depend on the incident wave angles of short waves and that swells from the south generate alongshore propagating free IG waves. A numerical model can reproduce the alongshore propagating free IG waves accurately when using a large computational domain. Moreover, numerical experiments performed using the model demonstrate that the alongshore propagating free IG waves are IG waves reflected from distant beaches. The free IG waves from distant beaches are small outside the port, but seiches in the port are amplified by more than 10%. The relationship between seiches and incoming free IG waves is further discussed based on the numerical experiments.