Long period oscillations and tidal level in the Port of Ferrol

ABSTRACTA new container terminal will soon be inaugurated in the Port of Ferrol (Spain). Sea level observations show the occurrence of seiche events in the basin. The objective of this work is to investigate the long wave oscillations and their dependence on the tidal level. Two analysis techniques, fast Fourier transform (FFT) and short time Fourier transform (STFT), are applied. Time-averaged spectra corresponding to different tidal levels are obtained with the FFT, whereas seiche events are identified on spectrograms computed with the STFT. The time-averaged power density spectra features eleven well-marked peaks, with moderate to high amplification. A clear influence of the tide on the spectral peaks is found, with most peaks presenting higher frequencies and greater power densities at high tide. The analysis of the individual seiche events shows that the behavior of long wave energy in the basin varies across the spectrum; on these grounds, three frequency bands are proposed: LF (low frequency), VLF (very low frequency), and ULF (ultra low frequency). The LF band exhibits a high correlation with the offshore swell energy, both outside and inside the harbor. At the other end of the long wave spectrum, the ULF band is only weakly correlated with the swell; it responds to a different forcing, possibly related to atmospheric disturbances. Finally, the intermediate VLF band presents a mixed character, with influences both from the swell and the other driving agent. The contributions of the paper are as follows. First, the long wave behavior at the Port of Ferrol, a major port in Spain, is characterized for the first time. Second, two contrasting behaviors affecting different frequency ranges are identified—one is proven to be swell-driven, whereas the other is proven, on the contrary, to not be swell-related. And, finally, the tidal oscillation is found to be relevant to the long wave behavior within the port, for it affects both the frequencies and power densities of the spectral peaks—but not to the generation of long waves outside the harbor.

▸ We study long waves in a port and their relation to tidal level. ▸ We use FFT and short time Fourier transform (STFT) to analyze sea level data. ▸ Tidal level does not affect the generation of infragravity waves outside the harbor. ▸ Tidal level does affect the resonant characteristics of the harbor basin. ▸ We find a dual band that responds to swell-driven and swell-independent excitations.