Directional Waves in the Nearshore Coastal Region of Perth ...

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Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tanwave field was greatly influenced by the energy of swell waves which are not affected bylocal wind conditions such as the sea breeze events. The effects of the swell and seacomponents of H s for the data collected at City Beach are discussed in Section 4.4.1. Therewere no distinct daily peaks of H s observed on days 28, 30 – 31 and 35 – 37 (inclusive) due toweak or no sea breezes occurring on those days. Furthermore, H s appear less variable duringthe periods of little to no sea breezes. The mean H s for the summer sampling period was 0.77m. A maximum H s of 1.33 m was achieved during a sea breeze event within the samplingperiod.The wave frequency spectra (Figure 4.1c) clearly indicate that during the sea breezes, shortperiod sea waves became superimposed on the background swell. The generation of seawaves begin in response to the start of sea breezes at around 0.3 Hz, decreasing down toaround 0.15 Hz when the sea breezes subsided (i.e. wave period increased from 3 s to 6 s).Swell waves of approximately 0.1 Hz (i.e. wave period of 10 s) were almost always present.The frequency spectra verify the lack of sea breeze generated sea waves on days 28 – 32 and35 – 37, when relatively high energy swell waves dominated instead.The directional wave spectra (Figure 4.1d) show that the dominant wave direction for theduration of the sampling period was in the northeast quadrant (i.e. 0 o to 90 o ), corresponding tothe prevailing southwesterly winds during the summer months.Results and Discussion 33
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean TanFigure 4.1: Wind and wave conditions measured in the nearshore region of City Beach from 21January to 09 February 2001:(A) wind speed and direction measured at Swanbourne – note that wind speed is given in kilometresper hour (kph); (B) significant wave height; (C) wave frequency in terms of wave spectral energy; (D)wave direction relative to the north in terms of wave spectral energy. The solid line in (A) representsthe wind speed and the arrows are hourly wind vectors with their length proportional to the wind speed(up = southerly; right = westerly; down = northerly; left = easterly). The shading in (C) and (D)reflects increasing levels of spectral energy as defined in the adjacent colour bars (in units of m 2 /Hz)34 Results and Discussion
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Directional Waves in the Nearshore Coastal Region of Perth ...

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