Modulation of Short Infragravity Waves by Tide

Kovalev P. D., Kovalev D. P.

Institute of Marine Geology and Geophysics FEB RAS performs regular registration of sea waves in the coastal zone of Sakhalin Island and Kuril Islands for the purpose of investigating infragravity (IG) waves and dangerous marine phenomena using autonomous bottom wave loggers (sea level fluctuations). The one-second discreteness is used in devices and allows recording not only IG waves, but else swell and wind waves without distortion. As a result of longterm observations of sea level, it was possible to detect the phenomenon of IG waves modulation by the tides on the shelf of Sakhalin Island. Analysis from field data and numerical models of marine surface gravity waves showed that in the coastal zone the energy was transferred from the low-frequency waves to higher-frequency motions in the result of nonlinear wave-wave interaction. This explains the tidal modulation of the energy of the short infragravity waves, that was observed in the records of the bottom pressure on the southern shelf of the Sakhalin Island. The obtained results confirm the possibility of using the Longuet-Higgins and Stewart models for determining the coefficients for increasing the amplitude and the period of the IG waves both in the surf zone and outside it. Similar changes in tidal beaches are common throughout the world and thus, tidal modulation of IG energy in the surf zone can be an effect of coastal processes and regional seismic activity in many coastal areas. The study of the processes considered here is important, since IG waves contribute to the formation of the shoreline.

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