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 Tan4.2.2 Hourly TrendsThe hourly wave patterns induced by the sea breeze cycle are investigated in this section. Thesequence presented in Figure 4.2 illustrates the typical wave patterns that occur during a seabreeze event. This particular sea breeze cycle occurred on 25 – 26 January 2001 at CityBeach. Data analysis for these dates via DIWASP produced generally excellent hourly plots.The sequence begins a few hours after the onset of the wave response to the sea breeze effectoccurring on 25 January at 17:00, and ends on 26 January at 17:00 to encompass a wholeday’s cycle. However, the description presented here shall begin at 06:00 of 26 January inorder to illustrate the wave response to the arrival of the sea breeze.At 06:00 on 26 January, the prevailing southwesterly summer winds have subsided against thestrengthening offshore land breeze. The wave field was dominated by swell waves ofapproximately 0.1 Hz (i.e. wave period of 10 s) flowing in the east-northeasterly direction(60 o – 90 o ), and there were very weak sea waves flowing in the northeasterly direction(approximately 45 o ). By 08:00, the energy of the northeasterly sea waves had significantlyreduced, and the swell waves continued propagating to the east-northeast. Swell conditionsremained the same for the next six hours, as verified by the 13:00 wave pattern. At 14:00, seawaves of approximately 0.32 Hz began propagating to the northeast (approximately 40 o ) inresponse to the onset of the south-southwesterly sea breeze. By 15:00, the sea breezegeneratedwaves of 0.2 – 0.3 Hz have picked up significant energy. The east-northeast(approximately 80 o ) swell component was still present in the background, giving a maximumdifference of 40 o in the directions of wave propagation between the swell and sea componentsduring the sea breeze event. At 16:00 and 17:00, two and three hours after the onset of thewave response to the sea breeze, the propagation of the sea breeze-generated sea wavespersisted in the northeasterly direction against a background easterly swell component.Reflected waves of the same frequency as that of the sea waves were detected almost 180 o inthe opposite direction.In fact, the cessation of the sea breeze effect on the waves was less defined than for the onsetof the sea breeze effect because there was a gradual return to pre-breeze conditions during theevening. This is illustrated in the hourly wave patterns of 17:00 and 21:00 on 25 January and01:00 and 04:00 on 26 January. The pre-breeze swell wave conditions did not return until theearly hours of the next morning.Results and Discussion 35
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean TanHence, the summer wave patterns on the nearshore coastal zone of Perth waters during thesampling period at City Beach were dominated by east-northeasterly (approximately 80 o )swell waves and northeasterly (approximately 40 o ) sea waves generated by sea breezes. Amaximum difference of approximately 40 o in the directions between the propagation of swelland sea waves was achieved during the sea breeze event. The persistence of the eastnortheasterlyswell waves is due to the fact that swells are not affected by local windconditions (Section 2.1.1). Note that nearshore waves do not respond to the overnight/earlymorning offshore (easterly) winds (i.e. by propagating to the west) due to the insufficientfetch area at the sampling location for sea waves to develop appreciably in response.36 Results and Discussion
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Directional Waves in the Nearshore Coastal Region of Perth ...

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