Great sandy land
Summer means getting sand between your toes. But have you ever thought about the sand dunes that shape the Australian landscape from the coast to the outback?
Although they may appear just a big pile of sand, dunes come in many forms, and may be found on the coasts, in deserts, in alpine regions and even underwater.
Terrestrial dunes are shaped by the predominant wind direction. They may be linear — long thin dunes that stretch across the landscape or parallel the coast; crescent-shaped lunettes that form when the wind blows from one direction; or U-shaped parabolic dunes that can have 'arms' up to 12 kilometres long.
Underwater dunes, known as sandwaves, are caused by tides, changing sediment deposition and erosion patterns. Up to nine metres high, and more than 200 metres long, they have been found in deep water on the North West Shelf, in Bass Strait and in Torres Strait.
Across Australia, 30 per cent of dune systems are active or migrating, while 70 per cent are vegetated, stable systems, says coastal geomorphologist Dr Andrew Short from the University of Sydney. A really active system can advance inland up to ten metres a year, with the most active systems tending to be found in the most arid areas.
When: Dune systems are active when wind speeds top 18-20 km/h. The strongest winds blow in the cooler months along Australia's northern and southern coastlines.
Where: Coastal dunes are found all around Australia's sandy coast, continental dunes are found in the Great Sandy, Gibson, Great Victoria, Simpson, Tanami and Strzelecki Deserts and mallee country. Alpine dunes are found predominantly in Tasmania adjacent to Lakes Petrach, Augusta, St Clair and the now-drowned Pedder.
If you live near the beach you'll be familiar with coastal sand dunes, which form when sand blows inland from the beach and accumulates. As well as wind direction, their shape and size depends on the shape of the beach: if a beach has a shallow slope, more sand is deposited onshore, resulting in bigger dunes. On a steeply sloping beach, however, more sand washes back out to sea, making smaller dunes.
With such an extensive coastline Australia is home to many active dune systems, including those on Fraser Island and a massive, but little known, system on Cape York. Other more recognisable systems occur along the Myall Lakes and Stockton Bight dunes north of Newcastle, while the most extensive systems are found on the Eyre Peninsula and at Steep Point, the westernmost tip of the Australian mainland.
"Australia's coastal dunes cover 23,500 square kilometres, and back 85 per cent of [beaches] on the sandy coast," says Short. While they can reach 260 metres in height, on average, coastal dunes are 10 metres tall, and extend 1.8 kilometres inland. Short estimates these dunes contain 280 cubic kilometres of marine sand that has been blown inland from the beaches — equivalent to 22,000 cubic metres of dune sand for every metre of beach.
One of the landscape's most dynamic features, coastal dunes are the first casualty of changing sea levels, which Short says have risen and fallen around 20 times in the past two million years.
"Most of Australia's obvious present day coastal dune systems are Holocene (less than 12,000 years old), but when we dig deeper into the dunes we find that many of these have blanketed older systems.
"These tend to be stacked on top of one another, while other older dune systems are found much further inland. Known as inner barrier systems, these are usually fairly stable and well vegetated, and examples can be found in The Coorong in South Australia."
For example, a barrier of sand dunes was formed two million years ago by the ebb and flow of an ancient inland sea known as the Murravian Gulf. Buried beneath what is now western Victoria and South Australia, these sand deposits record important changes in climate over this time.
Blowing in the wind
Although most large dune fields form in dry conditions, dunes don't actually require a dry climate to form, only a dry surface and sufficient wind speed, which can occur during afternoon sea breezes. A wind speed greater than 18 - 20km/h is needed to move sand on a sloping surface (or 30km/h to move sand on a flat surface), but once it gets above that threshold, the volume of sand shifted increases exponentially with wind velocity.
The strongest winds are northern Australia's trade winds and southern Australia's westerlies, both of which blow in the cooler months, says Short. But because it is wetter in the north, the dune systems are better vegetated making them more stable.
"In southern Australia, it's not necessarily windier but the aridity means the dunes are less vegetated making the dry sand more potentially volatile."
Although they don't travel far, dunes cover large areas of arid and semi arid central Australia. These so-called continental dunes cover about 40 per cent of Australia's surface and are found in a vast anticlockwise whorl around Central Australia. Some of the dunes began to form in the dry, cold ice age two and a half million years ago at the start of the Pleistocene, with additional dunes forming during successive ice ages.
Very dry conditions also extended across Bass Strait to eastern Tasmania, and as a result several sets of linear dunes on the mainland and on Flinders Island date from that period, says Emeritus Professor Eric Colhoun from the University of Newcastle.
Eastern Tasmania is also home to many sets of ancient lunette dunes, which formed leeward of lagoons when evaporation was reduced and groundwater levels higher during the maximum of the last glaciation. The lunettes probably started forming more than 20,000 years ago and the most recent began forming 8000 years ago during the early to middle post-glacial period when conditions were wetter.
While a few dunes have been found on the top of the Blue Mountains and in the Snowy Mountains, most of Australia's alpine dunes are found in Tasmania, formed on the leeward shores of lakes by sand blown from narrow beaches themselves formed of sediments deposited by glaciers and rivers. Dunes in the modern alpine and sub-alpine environments are located at the southern end of Lake Petrarch in the formerly glaciated Cuvier Valley, at the south-east corner of Lake St Clair, Tasmania's biggest glacial lake, and adjacent to Lake Augusta beyond the last glaciations ice limits.
Extensive arcuate-shaped sand dunes also formed leeward of the renowned Lake Pedder, but these were drowned when the lake was dammed as part of the Gordon River Hydro Electric scheme.
Teeming with life
Dune systems have tended to be regarded as less biodiverse than other ecosystems, and only limited research has been done on the plants and animals that call them home. But throughout Australia, they have always been important to Indigenous people: abundant shell middens and other artifacts attest to their importance, so much so that the entire Stockton Bight dunefield was handed back to the Worimi Aboriginal people in 2007 in recognition of the culturally significant sites and diverse ecosystems found in the area.
"There is a fairly good understanding of the mechanics of sand dune systems now — how they are built, stabilised, destroyed and rebuilt," says Associate Professor William Gladstone from the University of Newcastle, "and there is growing interest in the biology and ecology of dunes and the role dune ecosystems play in the coastal environment."
Coastal dunes are a home to uniquely adapted species, including a range of dry and salt-tolerant plants and grasses that stabilise the shifting sands as well as crustaceans like ghost crabs and other invertebrates. Scavengers and birds of prey search for food thrown up by the sea, or in the protected swales that lie on the dune's leeward side, where small rodents and reptiles eke out an existence. Some birds also nest in protected spots while insectivorous bats are known to hunt in desert dunes and probably do the same in coastal dune systems.
Recent research near Roxby Downs in South Australia has shown that not only do small animals like bilbies and bettongs forage in the shelter of dunes, they also actively contribute to their stabilisation. When they dig for food buried in the soil, they create a patchwork of small depressions that capture seeds, leaf litter and water. Areas disturbed by their digging contain up to twice as much nitrogen and carbon nutrients as the surrounding soil, and more than three times the number of seedlings following desert rains.
"It is a very dynamic environment," says Gladstone, adding that an advancing dune can completely swallow a forest, leaving mature trees to reappear years later when the dune has moved on as bleached skeletons.
But even though dune ecosystems are used to surviving in a dynamic environment, "they will be under greater pressure from greater wave heights, stormier conditions and rising sea levels caused by climate change", he says.
Short agrees. "Rising sea levels will lead to the erosion of the foredunes, which are presently stable. This will cause that sand to be released, and the reactivation of the dune system." He anticipates there will be blowouts, where holes are knocked in the foredunes that back the beach systems causing landward movement of the sand.
"But on the bright side, dunes are relatively easy to manage, as long as they are kept vegetated, which is fairly easy to do in urban areas." Keeping to boardwalks and marked tracks as you make your way to the beach will also help keep the dunes in one place.
Published 10 February 2010