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 TanDecember, while the strongest ones come later in January or February (Gentilli 1971). Atypical summer weather pattern is depicted in Figure 2.8 (a-d).During the winter months, the general isobaric pattern favours offshore flow and thus landbreezes are strengthened while sea breezes often fail to occur (Gentilli 1971). The strengthand onset of sea breeze are also affected by other meteorological factors. For example, theamount of cloud cover can affect the temperature and the prevailing synoptic situation whichmay either enhance or retard its development (BOM 1993). According to Pattiaratchi et al1997), wind waves that are locally generated by the sea breeze activity are the majormechanism controlling littoral sand transport and nearshore morphology of sandy beaches.When the nearshore wave field is influenced by a sea breeze local wind waves undergodiurnal changes in height, period and incidence angles (Pattiaratchi et al 1997).Storm eventsThe low pressure systems associated with the mid-latitude depressions in winter bringwesterly cold fronts, resulting in strong sustained winds with speeds up to 14ms -1 (Rose2001). With the approach of a depression, the winds are initially from the north and shifts tothe northwest whilst increasing in strength. As the depression crosses the coast, there is arapid shift in the wind direction to become southwesterly, and the mean wind speed decreasesslightly after the change. Storm conditions may be maintained for up to 36 hours, with windsreaching up to 20 ms -1 and instantaneous gusts to 35 ms -1 and will gradually moderate(Masselink & Pattiaratchi 2001, Rose 2001). A typical winter sequence for Perth is shown inFigure 2.9 (a-d).Literature Review 21
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tan(a) Day 1: A low pressure trough extends in a generalnorth-south orientation inland of the west coast and ahigh pressure system is centred over the Indian Ocean.The typical wind profile shows a moderatesoutheasterly wind in the early morning, veering to thesouthwest and strengthening by late morning. Windwill gradually tend southeast and moderate overnight.The strongest sea breezes in Perth result from thispattern.(b) Day 2: The low pressure trough has progressedfurther inland and the high over the Indian Ocean iscloser. Wind pattern is similar to Day 1, although seabreeze may arrive later and a little weaker. The windmay turn to a gusty easterly overnight.(c) Day 3: The high pressure cell has progressed furthereast and is now located off the south coast andproducing a hot easterly flow over WA. A fresh tostrong easterly wind early in the morning will moderateduring the day as the temperature rises. A weak seabreeze may arrive late in the afternoon, but the wind islikely to return to the east and freshen during evening.On some days there will not be a sea breeze and the(d) Day 4: The formation of a low pressure trough alongthe west coast is due to the temperature differencebetween the land and sea surfaces. Another high islocated over the Indian Ocean. Winds may remain fromthe northeast until mid-afternoon when a moderate seabreeze is likely to develop.Figure 2.8: A typical summer sequence for Perth (from BOM 1993)22 Literature Review
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Directional Waves in the Nearshore Coastal Region of Perth ...

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