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Understanding Coastal Processes

It's just the way it changes

... Like the shoreline and the sea

Cohen 1968

The dynamics of coastal systems

The previous chapter noted the difficulty of finding a single definition for the Australian coast. This becomes more complex when we realise that the modem coast is not a static line, even though it is often used as boundary marker. While some changes are very visible to the human eye, more subtle changes that may be just as important are occurring all the time. For this reason it is necessary to consider the time context for any coastal processes so that we can provide useful information that is relevant to coastal management.

At time scales of hundreds of millions of years, continental evolutionary processes have less influence on the coast, although many coastal forms may have resulted from older geological processes. For example, the occurrence of large granite erratic boulders along the South Australian gulf coastline does not relate to modem coastal processes but to glacial processes 250 million years ago. Since that time, Australia has split away from Antarctica and is currently drifting north at a rate of approximately 6 cm per year. A coastal classification based on the longer time scale processes of plate tectonics is discussed below.

At time scales of millions of years there have been major changes, such as the rapid climatic and associated sea-level changes of the last two million years during the Quaternary period. Within this time scale, there is also a different periodicity of hundreds of thousands of years between cold glacial cycles and intervening warmer interglacial cycles. These changes have had major impacts on the coast, and in some places the cyclical record can be seen in coastal sediments and coastal landforms (see 'Global Change and Australian Coastal Processes, page 45). More recently in geological time (i.e. thousands of years), the Australian coast has been affected by a rapidly rising sea, which has been a major influence in shaping our modem coastline. This is discussed in more detail on page 54.

At the shorter time scales of tens to hundreds of years, it is easier to detect the impact of more rapid coastal processes, particularly where they affect humans. For example, in Queensland the rapidly moving sand bars near the Burdekin River cut off the beach at the Alva Beach township within 30 years. The rapidly moving Murray River mouth in South Australia created changing administrative boundaries. Similarly, coastal sediment movement at the Tweed River mouth crosses the state boundaries of New South Wales and Queensland (Harvey 1988).

Even shorter time scales are associated with coastal processes such as daily tidal movements or complete tidal cycles (18.6 years). Long-term weather patterns such as El Nino may have coastal effects lasting for a number of years, whereas the effects of shorter-term storm events may last only a matter of days. The weather results in coastal processes of wind and waves which can vary rapidly in direction and periodicity. At these shortest of time scales, waves can be measured in seconds.

Thus it is important to think of the coast as a dynamic system which is constantly responding to changes at a variety of time scales. This chapter first explores the nature of Australia's coastal form and processes from different perspectives, at different spatial and temporal scales. It then examines a number of case studies that focus on different types of processes.

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