Closed Basin Modes of a Dual Basin Harbour
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Pure and Applied Geophysics
Closed Basin Modes of a Dual Basin Harbour JON HINWOOD1 and JOHN LUICK2 Abstract—Closed basin seiches have been studied in Port Kembla, Australia, a compound harbour comprising two basins linked by a narrow channel. Despite the irregular geometry, there are well defined closed basin resonances which may be excited by long waves incident on the harbour entrance. The March 2011 Japanese tsunami excited open-basin modes, but did not significantly excite closed-basin modes. This is attributed to the very low incident wave energy at those frequencies. While the direct forcing of a closed basin mode has been extensively studied, the indirect forcing via an independent open-basin mode found here has not been extensively studied. It was found that single basin modes are more readily excited than the higher dual basin harmonics and the role of the irregular geometry in inhibiting some modes is discussed. The non-linear generation of a higher frequency mode unrelated to the forcing mode is demonstrated. Key words: Seiche, resonance, harbour, model, Port Kembla, tsunami.
1. Introduction Long period waves have been studied in Port Kembla Harbour, on the southeast coast of Australia. Port Kembla is a compound harbour with the nearly circular Outer Harbour linked by a narrow channel (The Cut) to the more-recently constructed Inner Harbour with a forked plan form (Fig. 1). Port Kembla Harbour has a history of occurrence of long period waves, commencing with the original Outer Harbour which suffered extreme problems of ship ranging alongside certain wharves (DANCE, 1961). The Inner Harbour was designed to avoid these problems but waves of periods from 1 to 26 min have been recorded
1 Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC 3800, Australia. E-mail: [email protected] 2 South Australian Research and Development Institute, Adelaide, SA 5024, Australia.
there and the resultant unpredictable current speeds in The Cut are an inconvenience to navigation. This paper examines how the closed basin seiches are generated and what effect the dual basin geometry has on their periods and amplitudes. The oscillations were simulated using a three-dimensional hydrodynamic model, supported by spectral and other analyses, a simple analytical model and limited field data. Open basin modes, forced by long period waves in the ocean, have been identified and discussed in LUICK and HINWOOD (2008). Previous studies (e.g., MILES and MUNK, 1961; MILES and LEE, 1975; MEI and PETRONI, 1973) have demonstrated the importance of the balance between energy inflow through the entrance forcing a mode and energy leakage out through the entrance. Harbours of complex form have been modelled (e.g., MILES and LEE, 1975; LEE and RAICHLEN, 1972; GAILLARD, 1982) with most authors considering the same balance of energies. MILES (1974) provided a good review at the end of a very active period of research into the topic by one of the leading mathematical researchers who was aware of the older litera
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