Tides and currents
The gravitational and centripetal forces of the Earth, Moon and Sun ('gravitational' tides) cause tidal movements in the oceans which are modified by interaction with the shape of the ocean basins and continental shelves ('equilibrium' tides). These processes are explained in standard works, such as Open University (1989).
Standard records and description of tides in Australian coastal waters are found in the Australian National Tide Tables (1993), and Radok (1976) produced an atlas of tides.
Microtidal conditions (spring tide range less than 2 m) are found around the south-east, south and south-west coasts of Australia, from Brisbane to north of Perth, so the principal ports of the country (other than Port Hedland, 3.6 m) have spring tidal ranges near to 1 metre. In contrast, northern Australia is macrotidal (spring tide range more than 4 m). Darwin, for example, has a spring tide range of 5.5 m. Some northern gulfs and estuaries have ranges twice this value. In general, tidal amplitude is greatest where the continental shelf is wide and low where the shelf is narrow. Along the narrow continental shelf of the New South Wales coast, strong and persistent onshore winds may raise coastal water levels more than the variation caused by neap tides. In inlets and gulfs, funnelling tidal movement may also affect amplitude; thus spring tide range at Port Lincoln in South Australia is 2.0 m, but at Port Augusta (at the head of the tapering Spencer Gulf) it is 3.9 m.
Tidal fluctuations of water level affect the configuration and stability of both sandy and rocky shorelines, and have an important bearing on the distribution of coastal biota. Plants and animals of the intertidal zone form a narrow shoreline ribbon along the microtidal coasts of southern Australia, in contrast to the wide mangrove belt of the Northern Territory, where tidal range often exceeds 6 metres.
At macrotidal coasts, tidal movement generates powerful currents which may transport sediment, flush pollutants, or disperse eggs and seeds. Tidal current streams are an important mechanism contributing to the flushing of inshore waters and estuaries. Tides thereby play a fundamental role in the health of these inshore and estuarine systems. Tidal currents may reach considerable velocities where water movement is constrained.
A knowledge of tidally driven water-level fluctuations is important for many reasons, including commercial and recreational fishing, navigation, design and construction of coastal and offshore engineering structures, and an understanding of the development of coastal landforms. At many ports and entrances knowledge of water depth is critical, and even a few hours warning of deviation from forecast tide heights may allow great efficiencies in the management of shipping movements. Tides are most significant in coastal management at the extremes of variability, or when some unpredicted change occurs; for instance, an extreme high tide leading to flooding, or an unforeseen very low tides leading to exposure of oyster racks.
A number of factors may lead to water level being different to the forecast tide; these factors are known as 'residuals'. They include: variation in water temperature; variation in salinity; change in atmospheric pressure; run-off from rivers; and winds. Thus, for example, sea level may seasonally vary due to regular atmospheric pressure changes (the 'inverse barometer' effect). Interannual variation in sea level occurs along Australia's east coast during El Nino-Southern Oscillation (ENSO) events: changing seasonal winds and pressures lead to a lowering of tide levels during the El Nino years.
Stonn surges, which are caused by low atmospheric pressure, strong onshore winds (wind set-up) and large waves (wave set-up), may locally raise sea level
Figure 2.5 Tide gauge at Spring Bay, Tasmania, part of the 'Seaframe' set of precision gauges collecting data for the National Tidal Facility
Photography: Bill Mitchell
Figure 2.6 Major ocean currents affecting Australia
Source: after SEAC 1996
above tide height, leading to coastal erosion and flooding. At Port Adelaide on 4 July 1981, the recorded water level was 1.5 m above the forecast tide, due to low atmospheric pressure and strong westerly winds. The largest recorded surge in Australian waters was over 7 m, observed in 1892 at Bathurst Bay, Queensland. Storm surges are especially damaging at the coast when they coincide with high tide. For example, Cyclone Tracy inflicted disastrous wind damage on the city of Darwin on Christmas Day 1974, but little flooding or erosion occurred because the cyclone passed over the shore at the time of low tide.
Very accurate measurement of tides is undertaken by the Australian Baseline Sea Level Monitoring Project under the national Greenhouse Climate Change Core Research Program. This project (and the South Pacific Sea Level and Climate Monitoring Project) are managed and supported from the National Tidal Facility, Flinders University. Driven by the search for a sea level change that would be a signal of Greenhouse climate change, this major monitoring project has a wide range of applications for coastal managers. Very accurate monitoring of changes in absolute sea level also involves monitoring the vertical motion of the Earth's crust at the tide gauge location.
Figure 2.6 shows the major ocean currents affecting Australia. These currents operate seaward of the continental slope, only occasionally operating strongly in the shallow waters of the continental shelf. Nevertheless, the East Australian Current and the Leuwin Current are responsible for the warm temperate conditions of the continent's eastern and western coastal waters, in contrast to the cool waters of the southern coast. The temperature gradient at the SE and SW waters of the continent appears to mark a boundary which has contributed to the uniqueness of the species of Australia's southern coastal waters (Poore 1995).
The East Australian Current, the westerly limb of the great South Pacific gyre, is a powerful and persistent force moving polewards along the east coast of the continent, and spinning off eddies as it meanders its way south. In contrast, the Leeuwin Current, driven by thermal expansion effects in the warm waters of the North West Shelf, is more irregular in its pulses down the Western Australian coast. The Antarctic Circumpolar Current, driven by the strong winds of the 'Roaring Forties' (and fifties), runs well south of the continent.