Characteristics of low-frequency oscillations in the Hambantota Port during the southwest monsoon

The low-frequency (1/400-1/30 Hz) oscillations within the Hambantota Port, Sri Lanka, are investigated based on the in-situ measured water surface elevations in and outside the harbor. By combining analyses of spectra derived from the in-situ measurement and modal analyses solving the extended mild-slope equation, four natural frequencies of the port are identified and their modal structures are obtained. It is found that the low-frequency oscillations within the port were dominated by its natural frequencies (i.e., the eigen oscillations) instead of the frequencies of external waves. Subsequently, the correlation analysis is carried out to determine the generation of the low-frequency oscillations. Although both the offshore low-frequency and short waves contributed to the low-frequency oscillations, the low-frequency energy spectrum within the port is found to be more significantly correlated with the offshore total low-frequency wave energy than with the offshore total short-wave energy. Moreover, the empirical formula and artificial neural network models are presented to efficiently estimate the low-frequency waves inside the harbor in terms of the offshore wave parameters. Although both methods give excellent results, the artificial neural network performs slightly better.