Suppression of Wind Waves in the Presence of Swell: A Physical Modeling Study

Laboratory measurements of the interaction between high-frequency (HF) wind-waves and low frequency (LF) paddle waves are presented. The measurements were made in a wind-wave flume with wind and paddle waves propagating in the same direction. The primary objective is to examine the physical mechanisms proposed to explain the suppression of wind-waves due to presence of swell (here paddle waves). For this purpose, the precise time scale of the temporal transition from wind-only to wind-plus-paddle conditions is examined. The results reveal that the majority of wind-wave suppression occurs too quickly to have been caused by reduced wind-input, instead indicating that enhanced HF wave dissipation is the primary suppression mechanism. The spatial variation in HF wave energy along the LF wave phase indicates that HF suppression mainly occurs on the LF wave crest, and high on the windward face, which are the locations that experience the highest wind velocities. This observation also suggests that the reduced wind-input is not the primary suppression mechanism. Quantification of HF wave suppression versus a broad range of wind velocities, paddle wave conditions and fetch did not reveal any critical dependence of suppression on LF wave breaking or wind separation at LF wave crests. It is concluded that suppression occurs primarily due to enhanced dissipation of HF waves near LF wave crests. This mechanism is proposed to be coupled to the wind velocity, where an increased wind velocity near LF wave crests exceeds the level of forcing that HF waves of low C/u* can withstand without breaking.