Waves deliver energy to coasts, where they power littoral currents, determining sediment mobility and hence the stability of many coastal landforms. The level and range of wave energy, the 'wave climate', may limit the use of a coast, determining for example the location of favourite surfing beaches, boat ramps, or safe family recreation. A detailed knowledge of wave climate is needed in the design of coastal structures, since 'flexible' structures such as boulder walls may progressively deteriorate in response to high wave energy, while some rigid structures may suddenly and catastrophically fail if a critical design energy is exceeded.
There are few Australian coasts that do not at some time experience great peaks of wave energy during storms. Wave climate also is a powerful limitation on the survival of life forms in the coastal zone: plants and animals have evolved quite different adaptations to high-energy coasts, where habitat and niche opportunities contrast sharply with low-energy zones.
The offshore wave climate of Australia is dominated by a persistent moderate-energy wave regime with waves of 2 to 4 metres in height. It is characterised by south to south-westerly swells along the south and west coasts, and low-energy waves from the west and north-west along the northern coast. The east coast of Australia is characterised by a strong south-easterly swell, interrupted only briefly by a locally generated choppy north-easterly following the passage of an anticyclone. The inshore wave energy along the northern coasts of the continent is generally low, since it is dissipated across a
Figure 2.7 Wave climate
Source: after Harris 1995, and Davies 1972
broad continental shelf. In late summer and autumn, tropical cyclones may generate high-energy waves during intense but short-lived storms.
The swells of the west, south and east coasts are generated in the storm belt of the Southern Ocean, between 50° and 60°S. As Davies (1972, p. 30) stated:
The consistent occurrence of a large number of gale force winds ... their relative persistence in location and the long sea distances over which they blow make the southern storm belt much the most important and clearly definable wave generating area in the world. Although very few of these southern gales impinge on the coast, or produce storm waves impinging on the coast, they produce a very high proportion indeed of world ocean swell... there is direct evidence that the effect of this swell is felt even along coasts in the northern hemisphere.
Davies categorised temperate Australia's coastal wave climate (average wave conditions) into West Coast and East Coast Swell Environments (see figure 2.7); in contrast, tropical Australia has, on average, more gentle wave conditions.
Despite the strong to moderate offshore wave energy of south-western Australia, the inshore wave energy is considerably less due to dissipation via refraction and diffraction processes around reefs and headlands. This effect is particularly apparent on the west coast, where an extensive reef chain parallels the coast and may attenuate wave energy by up to 50%. Hence, the shoreline of south-western Australia experiences modally low wave energies. Isolated reefs and offshore islands offer some degree of local protection of the beaches along the southern coast. However, wave dissipation along the southern coast is considerably less than on the west coast and beaches there are subjected to a moderate to heavy south-westerly swell. Refraction around headlands is an important form of wave energy dissipation along the southern and eastern coasts of the continent. The shelter provided by gulfs and estuaries from the strong swells of the open coast continues to be significant to trade, traffic and recreation at the shore, as in Moreton Bay, Sydney Harbour, the Derwent Estuary, Port Phillip Bay and the Gulf St Vincent.
Inshore, the wave climate responds rapidly to the afternoon onset of the sea breeze. Wind waves associated with the sea breeze have a significantly shorter period and length than the swell, and are less affected by the inshore bathymetry.
Figure 2.8 Types of coasts around the continent
Source: after Davies 1977
As the sea breeze continues to blow the sea waves increase in period and height. Towards late afternoon or early evening, along the West Coast, the sea-breeze generated waves may exceed the energy of the prevailing swell.