Wave-Breaking Modulation by Infragravity Waves

An extreme typhoon, named Lan, passed through Sagami Bay, Japan, and left severe damage along Seisho Coast at the head of the bay in 2017. The typhoon excited large short waves (>0.04 Hz), and consequently infragravity waves (0.005 Hz–0.04 Hz) were also excited in nearshore area. Herein, we present coastal wave records under the historically extreme conditions caused by the huge typhoon. Extreme incident waves exhibiting a height of greater than 8 m at a water depth of 14 m were rapidly attenuated toward the shore through wave breaking, and the infragravity waves, which were 2 m in height, were observed to retain their energy level in shallow water. The infragravity waves showed the negative correlation with the short wave group envelop in the shoaling zone, suggesting that the infragravity waves originally developed as bound waves. However, the correlation turned to positive in the surf zone, which suggests that the infragravity waves gave some effects on the nearshroe evolution of the short waves. Therefore, we conducted further analysis on the correlation shift from the shoaling zone to the surf zone. An individual wave analysis clearly demonstrated that the instantaneous water-level rise/drawdown caused by the infragravity waves decelerated/accelerated the breaking of the short waves under extreme conditions. This mechanism transmitted the large short-wave energy on crests of the infragravity waves to the shore, eventually increasing the height of the nearshore waves. This study provides in situ evidence that the infragravity waves significantly affect the nearshore wave statistics under extreme conditions.