Harbours are essentially protected against swell waves (<15s) by the construction of breakwaters. However, long period waves (25-300 s) known as infragravity waves impinge on many harbours causing periodic horizontal and vertical water motions (oscillations). Oscillations can interrupt berthing operations and affect harbour procedures. The study investigates oscillation problem and potential generation mechanisms of oscillations in small harbours (length scale <1 km with an average depth of 3-10 m) fronted by rapidly changing offshore topography and in a swell dominated wave climate. Two Rocks Marina in Western Australia was selected as the harbour in the above conditions. Measured water levels and numerical simulations using a Boussinesq wave model were undertaken for the investigation.
The field observation results of the marina revealed four dominant oscillations in the infragravity band. Periods of the oscillations were in closer range to natural oscillation periods (NOPs) of the marina, suggesting that the oscillations were generated through excitation of the marina’s NOPs. Continuous occurrence of these oscillations suggested that they were forced by an external energy source continuously fed into the marina. The oscillations correlated with offshore swell waves hence, bound infragravity waves associated with swell wave groups were identified as an external energy source for the oscillations. In storm events, energy of these oscillations was significantly amplified. Excessive wave heights and high occurrence of the oscillations during storm events caused undesirable conditions within the marina.
The numerical model results revealed significant level of free infragravity wave generation over submerged offshore reef systems, independent of the external incident wave forcing. As a result, infragravity wave energy in the marina region was increased which in turn continuously excited marina oscillations. The free infragravity waves generated over the reef systems were identified as another potential external energy source for the oscillations. Marina oscillations were excited even though the energy spectrum outside the marina varied smoothly without showing dominant frequency peaks, which can be matched with the marina oscillation periods. However, a broad energy plateau was seen in between 90-325 s in the energy spectrum outside the marina.
The initial value of the aspect ratio which defined by the background infragravity wave period range outside the marina was used to estimate preliminary dimensions for new harbour. Dimensions for alternative layouts were decided based on the initial aspect ratio. The results showed that the significant wave heights of the infragravity band in the harbour decreased as the aspect ratio increased for a constant harbour depth and entrance width. Furthermore, the wave heights in the infragravity band increased with the aperture ratio, for the same harbour depth and the entrance width. This study confirms that ‘free wave simulation’ is an efficient tool for identifying NOPs of a harbour under free oscillation conditions.
The findings of the study are important in understanding the infragravity period oscillations in small harbours as Two Rocks Marina, and in estimating preliminary dimensions for a new harbour in order to minimize the oscillation problem.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - Dec 2014|