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 Tan4.3 STORM EVENTSThe data recorded at Cables ASR during August 1999 (i.e. winter) were used to demonstratethe wave response to storm events in the Perth coastal area.4.3.1 Overall patternThe time series of wind and wave conditions over the sampling period for the Cables ASRdeployment are shown in Figure 4.3.The wind pattern (Figure 4.3a) clearly indicate the passage of storms associated with midlatitudedepressions (Section 2.4.3). One storm event occurred on days 229 – 231 (i.e. 17 – 20Aug) and two closely-spaced storms occurred on days 236 – 237 (i.e. 24 – 25 Aug) inclusive.Offshore variable winds initially flow from the north, shifting to the northwest and increasingin speed as the depression approaches the coast. Winds then abruptly shift to southwesterly,indicating that the depression crossed the coast, and the winds decrease slightly in strength.The present wind conditions persist for a few hours before gradually moderating. The meanand maximum wind speeds for the winter sampling period at the Cables ASR deploymentwere 12.05 knots and 34.05 knots respectively. The maximum wind speed was achievedduring the storm event on days 229 – 231.The response of H s to storm events is straightforward and apparent, as shown in Figure 4.3b.During the storm event on days 229 – 231, H s increased more than 1.5 m. At the height of thestorm on day 230, the peak H s of 2.75 m was achieved in response to the highest wind energyfor the sampling period. The mean H s for the winter sampling period was 1.17 m.The wave frequency spectra (Figure 4.3c) clearly show the wave response to storm winds.With the onset of northwesterly winds, short period sea waves became superimposed on thebackground swell. The generation of sea waves began in response to storm winds at 0.3 Hz,decreasing down to around 0.15 Hz for the whole duration of the storm event. The constantbackground swell waves of less than 0.1 Hz (i.e. wave period of more than 10 s) were almostalways present. The highest energy waves were generated in response to the strongest windspeeds, especially for the storm event on days 229 – 231.Results and Discussion 41
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean TanThe directional wave spectra (Figure 4.3d) indicate that the dominant wave direction for theduration of the sampling period was in the southeast quadrant (i.e. 90 o – 180 o ), correspondingto the prevailing northerly winds in winter. An interesting point to note is that during the latterpart of the storm events, the predominant direction of wave propagation remained to thegeneral southeasterly direction although the wind was blowing from the southwest. This isfurther discussed in Section 4.3.2.42 Results and Discussion
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