WATER-LEVEL OSCILLATIONS IN ESPERANCE HARBOR

Measurements of water-surface elevation in a harbor were analyzed to determine the forcing mechanisms causing the excessive motion of moored ships. The spectral density of water-surface elevation measured at four locations is presented by a time-frequency distribution. A numerical model based on a modified mild slope equation approximation of wave refraction, diffraction, and reflection in and around the harbor was employed to simulate the transformation of waves entering the harbor, The performance of the model was validated against the field data by comparing predicted and observed wave heights at field measurement locations within the harbor; deviations of predicted ratios from those observed are discussed. Ship motions recorded at berth during the field study were analyzed in context of the concurrent wave spectra in order to isolate the dominant forcing mechanisms causing the moored ship response. The groupiness of incident swell waves was analyzed to investigate the origin of long-period (>40 sec) water-level oscillations measured in the harbor; and harbor resonance is discussed in relation to the amplification of these long-period oscillations.