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

Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean TanThe waves travelling into the shallower waters of the Perth metropolitan area (Figure 2.7) aremodified by a number of processes, including refraction due to changes in bathymetry anddiffraction due to exposed reefs and islands. The continental shelf along the Perth coastalregion generally has a mild slope, but the actual bathymetry may be rather complex due to thepresence of submerged limestone ridges (Masselink & Pattiaratchi 2001). The offshore ridges,shallow reef chains and the islands (Rottnest and Garden Islands) have significant shelteringeffects on the inshore wave field from the offshore wave conditions. In areas of themetropolitan coastal waters, a northwesterly wind will generate higher seas than asouthwesterly of similar strength because of the sheltering effect of the islands in thesouthwest direction (BOM 1993).2.4.2 ClimateThe ‘winter-wet south-west’ of Western Australia experiences a Mediterranean-type climatewhich is characterised by hot, dry summers and cool, wet winters (Gentilli 1972). The majorinfluence on the weather pattern along the south-west coast is the position of the ‘subtropicalridge axis’, which is a belt of semi-permanent anticyclonic high pressure systems encirclingthe southern hemisphere between latitudes of 25 and 40 degrees south (BOM 1993).According to Linacre (1977), Perth is Australia’s windiest city, being exposed to westerlygales in winter and to strong sea breezes in summer. The average annual wind speed for Perthis 4.3 ms -1 , which is approximately 30% above those of other capitals. Perth also experiencesa maximum gust of 43ms -1 , far exceeding those of most other capital cities of Australia(Linacre 1977). There are distinct variations in both the seasonal and diurnal wind patterns.The summer wind pattern is characterised by the daily sea breeze cycle, and the winterweather is dominated by the passage of low pressure systems crossing the south-westernregion of Australia every 7 to 10 days.Subtropical Ridge Axis and the Anticyclonic High Pressure SystemDuring summer, the subtropical belt of high pressure is over most of the south-west region,reaching near 40 o S and preventing most cold fronts from approaching the south coast withany strength (BOM 1993, Gentilli 1972). During winter, the belt is displaced to the north ataround 25 o S to 30 o S, allowing cold fronts and strong westerly winds to frequently penetratethe southern half of WA in some strength (BOM 1993).18 Literature Review