Numerical analysis of harbor oscillation under effect of fluctuating tidal level and varying harbor layout

9/5/2014Long waves may excite harbors to slosh intensively, and may severely damage the ship mooring facilities and threatening lives, especially when waves periods approach to the fundamental periods of the harbor and the ship mooring system. Earlier studies of harbor oscillation were largely focused on the resonance based on constant tidal level and the resulting harbor layout. However, the tsunami events that occurred on Feb. 27th, 2010 and Mar.11th, 2011 generated unusually more spikes in wave spectrum in the Crescent City harbor, and in San Diego harbor. Earlier studies noticed this phenomenon, the present study conducts specific frequency domain and time domain analysis to investigate the reasons that lead to these spectral spikes in connection with resonant behavior of the harbor basin. ❧ The present study proceeds in 5 phases, which are model verification, study of harbor oscillation based on constant mean tidal level, study of harbor oscillation based on five separate tidal levels , study of harbor oscillation with continuously fluctuating tidal levels, and simulation of historical tsunami events with realistic incident wave trains. Each phase is explicated in details to determine the reasons that lead to the unusual resonant modes in historical tsunamis. ❧ One Boussinesq-type model is used and validated by laboratory experiments. This model is used to predict response of harbors under impact of long incident waves in time domain. Three major harbors in California and one harbor in American Samoa are studied with white noise analysis on constant mean tidal level. The predicted responses at tidal gauge station are transformed into spectra, which matches fairly with the primary resonant mode simulated by mild slope equation in frequency domain simulation. The present study compares the time domain analysis with the frequency domain analysis to complement the understanding of harbor response. ❧ The fundamental period is observed to be 22 minutes in Crescent City harbor, 60 minutes in Los Angeles/Long Beach harbor, and 273 minutes in San Diego harbor, based on constant mean tide level. In some historical cases, however, the resonant periods are not the same as earlier predicted. For example, Crescent City harbor shows multiple spectral spikes in tsunami event on Feb. 27th, 2010 from Chile source and in tsunami event on Mar. 11th, 2011 from Japan source. The fluctuating tidal level as well as the corresponding change of harbor layout are two important reasons, whereas the energy content of incoming waves also influence harbor oscillation. The present study takes state of art process to consider the change of harbor layout due to the fluctuated tidal level, and proves it as one of the reasons for some of the multiple spectral spikes. This phenomenon is ascertained by tidal gauge records on normal days without tsunamis events. ❧ The data from the tide gauge records are used in present study to validate numerical model and to prove the hypothesis. The reasons of the multiple resonant modes in Crescent City harbor, during the tsunami events of 2010 and 2011, are proved to be the fluctuating tidal level with corresponding change of harbor layouts, and the fundamental period for the larger region including continental shelf (72.5 min). The resonant mode of Los Angeles harbor keeps fairly stable, and the topography of larger area outside the harbor also influence the incoming waves. San Diego demonstrates different primary resonant modes at different tide level, but the fundamental period for the larger sea area is observed to be 58.5 min. ❧ The present Boussinesq-type model is crosschecked with earlier study by mild slope equation on constant tide level, and the agreement of both results indicate the nonlinear effect in harbor oscillation does not appear to be significant. The movement of tidal level will alter the fundamental period in harbors, through the changing bathymetry and the associated layout of harbors. In Crescent City harbor, the tide level impacts response intensity, and smaller water depth will cause response of larger amplification. In Los Angeles/Long Beach port and San Diego harbor, the response amplitude remains stable on different tide levels. White noise analysis with tide fluctuation demonstrates multiple spectral spikes between 20 min and 25 min in Crescent City harbor, whereas the spikes of lower frequencies are due to the fundamental period of larger sea area outside the harbor as well as the energy content of the incoming waves. It has been noticed that the multiple spectral spikes occur on normal days also, which means this phenomenon generally exists in harbor oscillation. The spectra shows larger energy magnitude on tsunami events than on normal days. The simulation with MOST generated incident waves fairly matches with the records by tide gauge station. Therefore, the combined effects of fluctuated tide level, change of harbor layout, fundamental period of larger sea area outside harbor, and the energy content of incoming waves all lead to the multiple spectral spikes on normal days and on tsunami events