School of Civil, Environmental and Mining Engineering

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Contact

Darshani Thotagamuwage

Phone: (+61 8) 6488 8121
Fax: (+61 8) 6488 7279


Start date

Nov 2009

Submission date

Aug 2014

Curriculum vitae

Darshani Thotagamuwage CV
[rtf, 210.03 kb]
Updated 30 May 2012

Darshani Thotagamuwage

Darshani Thotagamuwage profile photo

Thesis

Infra-gravity period oscillations in harbours

Summary

Infra-gravity (IG) waves, generally known as long-period waves, are generated through non-linear interaction of wind and swell waves. Periods and lengths of these waves are in the range of 25-300s and 100m-10km respectively. Propagation of IG waves towards coastal areas such as harbours and lagoons can excite oscillations in the water surface. At resonance, oscillations are generated in the IG period range with significantly high amplitudes. In such conditions, berthing operations can be interrupted due to unacceptable vessel movements causing damages to mooring lines and fenders, resulting harbour down-times and hence economic losses.

Two Rocks Marina located on the South West Australian coast facing the Indian Ocean has been considered for this research. Results of the field observations show an excitation of the natural oscillation periods in the marina during energetic wind-wave conditions. The IG energy in the region of the marina in the same time duration has spread in a wide range of periods without showing any discernible periods closer to the natural oscillation periods of the marina. Hence the study illustrates an excitation of the natural oscillation periods of a harbour in an energetic wind-wave condition, where the periods of the incident IG waves ‘bound to’ swell wave groups are not necessarily closer to the natural oscillation periods of the harbour.

A numerical model has been developed which can be used to predict the IG period oscillations in existing harbours and as well as in newly designed harbours so that the problems due to the IG period oscillations can be minimized. Setting up guidelines for optimized harbour geometry in terms of minimum harbour disturbance and cost will also be considered.

Why my research is important

Excitation of natural oscillation periods of a harbour can be done (1) by very long-period incident waves of the period same as the natural oscillation period, such as tsunamis; and (2) by wind generated short-period waves. Even though the damages are not that severe as tsunamis do, many harbours around the world are frequently impacted by long-period oscillations excited by short-period incident waves of wind origin.

The influence of incident short-period waves on long-periodic harbour oscillations in terms of narrow banded incident swell wave groups has been widely studied based on theories and laboratory experiments. However in general, incident sea spectra are nearly always broad-banded. Therefore, it is important to carry out studies under real sea conditions to understand how the IG period oscillations are forced under these conditions. This knowledge is important in designing new harbours and in modifying existing harbours in order to minimize the problems due to IG period oscillations.

Funding

  • University of Western Australia Scholarship for International Research Fees (SIRF)
  • University International Stipend (UIS)

Two Rocks Marina
 

School of Civil, Environmental and Mining Engineering

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Last updated:
Thursday, 19 September, 2013 11:39 AM

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