Bound infragravity wave observations at the Nowshahr beaches, southern Caspian Sea: Fid-Bau Portal

Bound infragravity wave observations at the Nowshahr beaches, southern Caspian Sea

Mahmoudof, Seyed Masoud / Siadatmousavi, Seyed Mostafa

Highlights • The Ursell numbers of waves with high bound infragravity fraction (bii), are in range of 30–40. • The short wave groupiness envelope (G.F.) has a negative correlation with high bii values for G.F. >0.5. • The condition of having high values of bii are determined based on dimensionless wave parameters. • The dependency of bound IGWs on wind waves is considerable for nonbreaking waves. • The bii values of breaking waves have a strong exponential relation with low values of the peakedness factor.

Abstract This paper presents the results of bound infragravity observation at the Nowshahr beach located in the southern part of the Caspian Sea. The water level data were measured at 6 stations along a transect perpendicular to the coastline. The bispectral method was utilized to show that the extreme values for the bound infragravity energy fraction (bii) were occurring when dimensionless values of wave height to depth ratio, relative depth, and wave steepness were in the range of 0.18‒0.22, 0.4‒0.5 and 0.015‒0.017, respectively. These detailed results could be equivalently summarized with the condition of having the Ursell number of peak waves within the range of 30‒40. The corresponding short wave groupiness factor (G.F.) values for waves with high values of bii were between 0.5 and 0.55. Also, it was revealed that the wave groupiness envelope had a positive correlation with infragravity wave energy level, but its correlation with the bound infragravity wave was negative when G.F. > 0.55. On the other hand, the low values of bii against the spectrum peakedness parameter for breaking waves followed an exponential relationship. The correlation of bound and free infragravity waves with wind waves was also studied. For non-breaking condition, the correlations of bound infragravity waves with swell and sea waves were considerable. The correlation of free waves with swells was also significant. In contrast, for breaking waves, a negligible correlation was observed between bound infragravity waves and wind waves.