Dunes - Nature's way of defending the coast

More than 100 million years ago before the Tertiary Period, Australia was connected to the Antarctic land mass that formed the super-continent known as 'Gondwanaland'.

This southern land mass was mostly above sea level until the beginning of the Tertiary Period, 65 million years ago, when a series of extreme climate cycles weathered and eroded parts of Gondwanaland.

Australia separated from Gondwanaland approximately 55 million years ago along the southern boundary of what is now the Australian continent. This splitting of Australia from Gondwana caused the sea to flood land areas which had been free of the ocean for hundreds of millions of years.

Gondwanaland super-continent

Gondwanaland super-continent

This led to a series of basins being formed and Adelaide was built on the coast of   one of the basins, Gulf St Vincent. This area offered a wide belt of coastal dunes and wide sandy beaches stretching continuously north to south for 30 kilometres in the areas stretching from Seacliff to Outer Harbour, broken only at the Patawalonga Creek.

An excellent example of Adelaide's coastal dunes can be found at Tennyson. These dunes were formed 10,000 years ago when the sea level rose to its current height, creating an extensive dune barrier system 200-300 metres wide and 10-15 high.

What are Sand Dunes?

A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when the wind blows sand into a sheltered area behind an obstacle. Dunes grow as the grains of sand accumulate. Dunes can also be formed by strong currents beneath the water moving sand particles onto the beach.

Coastal sand dunes are small ridges or hills of sand found at the top of a beach, above the usual maximum reach of the waves. They are formed by strong onshore winds (greater than 5 m/sec) blowing sand that is deposited against an obstruction such as a bushes, driftwood or rocks or at the base of cliffs and hills.

Plants seeds are blown or transported by fauna onto coastal dunes and grow in a series of stages from the pioneer plants which initially cover the bare sand e.g. spinifex, to the secondary stabilising plants which form the more permanent plant cover.
Dune fields comprise a variety of dune types including transverse and parabolic dunes. They range from relatively small shore-parallel fore-dunes that sit immediately behind the beach, tens of metres wide and a few metres thick to a series of dunes that may extend hundreds of metres to a few kilometres inland, representing long-term accumulation of large volumes of sand.

Primary Secondary Teriary Sand Dunes

Where does sand come from?

Sand is a loose granular material blanketing the beaches, riverbeds and deserts of the world.
Earth's landmasses are made up of rocks and minerals, including quartz, feldspar and mica. The most common component of sand is silicon dioxide in the form of quartz. Weathering processes — such as wind, rain and freezing/thawing cycles - break down these rocks and minerals into smaller grains.
Beaches are a common feature of a coastline and are made up of eroded material that has been transported from elsewhere and deposited by the sea.
Beach sand is made mostly of material weathered from inland rocks or sea cliff material and transported to the beach on the wind or in rivers; and/or from shells and the calcium, a mineral in seaweed and other hard fragments of marine organisms originating in the ocean.
Beach sand is composed of different materials that vary depending on location and therefore comes in an array of colors including white, black, green and even pink.

Adelaide Metropolitan Coastline
(The Adelaide Metropolitan Coastline - South Australian Coast Protection Board No. 27 April 1993)

What do Coastal Sand Dunes do?

Coastal sand dunes are nature's way of protecting the beaches from erosion. Coastal dunes provide a buffer zone between marine and land environments. They absorb and decrease wave energy and reduce storm damage to our coastline.

Dunes stop the inland flow of seawater but they erode without vegetation cover.

Dune structures support a wide variety of bird, reptile and invertebrate species.

Coastal sand dunes are extremely sensitive to disturbances. They suffer severe damage from pedestrian and vehicle traffic.

When beach sand is washed away by storms, wind or wave movement, it is normally replaced by sand stored in the dune. Human activities can severely reduce this store of dune sand.

Waves approach the beach at an angle. This sets in motion a current which moves up the coast. The sand is carried in the current. Longshore drift is defined as the movement of sand in the surf zone along the coast. It is also known as littoral sediment transportation.

Longshore drift

Waves approach the beach at an angle. This sets in motion a current which moves up the coast. The sand is carried in the current. Longshore drift is defined as the movement of sand in the surf zone along the coast. It is also known as littoral sediment transportation.

Natural and Human impacts on Dunes

Prior to European colonization, the Adelaide area (Kaurna Meyunna country) hosted a three-dune system (fore, mid and hind) with two swales (dips or hollows). The dunes had a natural cycle of sand replenishment and supported extensive tidal mangrove, swamp paperbark and samphire wetlands east of the hind dunes. The area supported a rich biodiversity and a resilient ecosystem.

Since 1836, the Adelaide region, including the coastal dunes, offered settlers an abundant source of building materials, grazing land, timber for fuel, and residential land.

However, since European settlement the dunes on Adelaide's coast have suffered disrespect and degradation from cattle and pig grazing, tree felling for building and fuel, sand-mining for building and glass making, WW2 25lb guns test fired into the dunes, housing developments, rubbish dumping, illegal encroachment, deliberately-lit and accidental fires and other destructive actions by humans and feral animals.

Much of the foreshore development occurred in the 1940's, and major storms in the 1940's and 50's highlighted that the metropolitan coastline was far from stable. These storms created widespread damage to properties along the foreshore, and increased public awareness about the need to protect both public and private property due to the effects of the natural coastal processes.

Storm Damage Glenelg 1964
Storm Damage - Glenelg, 1964

The Tennyson Dunes (named after Lord Tennyson, Governor of SA 1899-1902) are 22 hectares of coastal dunes with natural grass and shrub land. They are the last true remnant of the Adelaide metropolitan coastal plains and a coastal conservation reserve. Located 12 kilometres north-west of the city, Tennyson Dunes display a large diversity of native flora and support the presence of rare and regionally significant plants, birds, reptiles and invertebrates.

The plants in the Tennyson Dunes are an insight into what can survive and thrive in our changing climate and the best example of Adelaide's natural coastal biodiversity. For instance: Coastal Spinifex - a grass that is important in stabilising dunes; Coastal Daisy Bush - a good screening plant; Coastal Wattle - excellent shelter and attracts birds and insects; Native Pigface - succulent ground cover and its fruits were a food source for the Kaurna people; Dryland Tea-Tree - a hardy plant providing good shelter, attracting birds and insects.

Native plants are naturally adapted to harsh conditions and attract native fauna.

Tennyson Dunes are extremely significant for reptile life including the regionally threatened Painted Dragon. Although rarely visible (as they try to avoid humans) Brown Snakes can sometimes be seen. They are a vital part of dune ecology because they keep mice and rat populations under control.

Tennyson also provides breeding grounds and habitat for many bird species including birds of prey such as the Nankeen Kestrel and the Black-shouldered Kite.

The Dune Group has recently restored populations of the Coastal Bitterbush which is the only food plant for the endangered Bitterbush Butterfly.

Tennyson Dunes
Tennyson Dunes
Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage

The dunes are being heavily tested by competing factors such as invasive non-native plants, pedestrian traffic, feral and domestic animals and a proposed bike track. The Tennyson Dunes Group are a dedicated volunteer conservation group working to provide a safe sanctuary for flora and fauna and to improve the dunes as a valuable community asset by enhancing the  natural values of the dunes as well as protecting their heritage and educational significance.


(adapted from website: http://www.tennyson.org.au/)

Climate Change and Dunes

Over the last 4,000 years the sediment supply from offshore sources has declined to the point where far less sediment is being transported ashore to Adelaide's beaches. Ongoing sea level rise and subsiding land in Adelaide is also contributing to beach and dune erosion.

In 1993, the South Australian Government's 'Coast Protection Board' recommended that allowance be made for 0.3 metre rise in average sea level from the year 1991 to 2030, and a further 0.7 metre rise from 2030 to 2100.

(The Adelaide Metropolitan Coastline - South Australian Coast Protection Board No. 27 April 1993)

Another way in which sand dunes are eroded is by the wind remobilizing sand and blowing it out of the dune, a process known as deflation. The most common deflation feature is the blowout, a bowl-shaped depression with a flat floor that lies below the elevation of most of the adjacent dunes. Blowouts are flat-floored because the sand is blown away until the sand surface reaches the top of the water table.

As storm and wind events increase in frequency and ferocity due to climate change, the frequency of blowouts in dune systems also increases.

Combined with natural and climate change events, loss of vegetation through the tracks made by pedestrians and vehicles through dunes also increases the chances of blowouts and dune erosion.

Blowouts Beach Buggy Damage to Dunes
Beach buggy trails through the dunes. Unrestricted vehicular (and pedestrian) traffic is damaging to dune vegetation.
http://coastalcare.org/educate/sand-dunes/

Negative Economic Impacts of Dune Loss

The negative impacts of the loss of coastal sand dunes include:

The huge costs of dredging sand from other areas of the coastline and inland sources or sand pumping activities to replenish and nourish Adelaide beaches where there is insufficient natural supply of sand or degradation of dunes by human activities or feral and domestic animals.

The cost of rock-wall and groyne construction where there is beach erosion is substantial due to human activities such as residential and commercial construction on foreshores and dunes.

A greater threat to coastal properties leading to increased property and business damage, repairs and insurance costs through wave action and storm energy without the natural protection of dunes systems.

Loss of income from recreational and tourism activities associated with eroded beaches and poor quality foreshores and estuaries.

The ripple effect of the loss of dune vegetation causing the destruction of coastal habitat and loss of diversity of dune flora and fauna species that can lead to the eventual loss of dunes through lack of stabilizing vegetation. Bird-watching and other passive nature-tourism activities are contemporary economic drivers heavily affected by dune loss.

Semaphore Park 1999
Semaphore Park 1999
Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage

What next for Adelaide's Dunes?

The coastline is dynamic, always shifting in response to the wind and waves. However, human impacts have altered the dynamics of the coast to such an extent that natural processes can no longer sustain the beaches.

In 1972 the Coast Protection Act was passed by State parliament. It established a Coast Protection Board to provide management expertise not only for the metropolitan beaches but for the entire 4,000 kilometres of the South Australian coastline. However, the coastal erosion and encroachment of development within the Adelaide region has meant that the metropolitan beaches continue to require considerable research and management attention.

Today, there is no naturally continuing replenishment source of sand. When this is combined with a net northerly littoral drift and an increasing mean sea level, the long term effect is the need to artificially maintain the beaches, or eventually lose them.

Adelaide's coastline is now a highly managed one. The sand that forms the beaches is a scarce and moving asset. The future management of the beaches needs to be responsive to changing conditions to ensure that future generations are not disadvantaged by decisions today.

The Government's Living Coast Strategy (2004), the beach management strategy for 2005-2025, continues to manage erosion risks to metropolitan coastal assets by replenishing beaches and using structures in critical locations to slow the northerly drift of sand. In addition, the strategy contains three important initiatives.

The first is to recycle sand more effectively using pipeline transfer systems.

The second is to add sand from external sources to the beach system to counter the ongoing loss of dune volume and beach width caused by sea level rise and other factors.

The third is to integrate sand bypassing at harbours with beach management.

These initiatives will contribute to achieving the actions listed in South Australia's Strategic Plan (2004), including maintaining the lifestyle quality of South Australians, seeking creative solutions to environmental issues and increasing investment in strategic areas of infrastructure.

Geotextile Groyne Somerton Park 2003
Geotextile groyne at Somerton Park, 2003
Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage

The main components of the strategy for managing Adelaide's beaches from 2005 to 2025 include:

Continue beach replenishment -

Continue the existing program of beach replenishment, placing 160,000 cubic metres of sand each year at strategic locations on southern and central beaches to maintain the sandy foreshore, build up dune buffers, and protect coastal infrastructure.

Recycle sand more effectively using sand slurry pumping and pipelines -

Existing sand supplies will be recycled more effectively using sand slurry pumping and pipelines, which will minimise the need for trucks to cart sand along beaches and suburban roads.

Add coarse sand from external sources -

Coarser, more stable sand will be added to the system from external sources such as Mount Compass to tackle the ongoing loss of dune volume and beach width caused by sea level rise and other factors.

Build coastal structures in critical locations -

Structures such as groynes and offshore breakwaters may be used in a few critical locations to slow the northerly drift of sand.

Integrate sand bypassing at harbours with beach management -

Integrating sand bypassing requirements at harbours with the beach replenishment program will result in more effective recycling of sand and reduced harbour management costs.

(Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage, Coastline No. 35 - 2005)

(The Adelaide Metropolitan Coastline - South Australian Coast Protection Board No. 27 April 1993)

Somerton-Park-Groyne-2005
One of the new groynes installed at Somerton Park in 2005
Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage

Dune Conclusions

Dune restoration involves reducing the disturbance of dunes as a method of controlling erosion problems. Land-use planning needs to ensure buildings and infrastructure are positioned behind the dune system.

Land conservation groups recommend the careful retention of vegetation in sensitive areas and re-vegetation activities in cleared areas. This needs to be combined with the restriction of vehicular and human traffic. For this reason damaged and sensitive dunes might need to be fenced and access tracks for vehicles and people provided. This includes drift fencing, dune re-vegetation and access controls, together with the installation of educational signs and viewing areas at some locations

On many beaches, it is legal with the correct permits to bulldoze beaches to form "dunes" or at least to form piles of sand up against buildings for storm protection.

Bulldozed "dunes" have a different shape to natural dunes, and often look like large piles of sand that have been dumped on the beach. These piles of sand contain a lot of shell material which is a rare component of natural dune sand.

However, there are problems with bulldozing sand and artificial dunes. After a few days on the upper beach, wind will blow away the finer sand on the surface, causing a layer of shells or "shell lag" to form on the surface of a bulldozed dune. Because wind often can't blow shells into dunes, the shell lag is a positive indicator that the bulldozer has paid a visit.

Natural dunes, especially those with roots entangled throughout the sand, provide a solid (if temporary) barricade against a minor storm. When attacked by waves, a scarp or small bluff quickly forms. Subsequent waves are at least partly reflected from the scarp, rolling back down the beach and smashing into the next wave coming ashore. Bit by bit, however, the dune scarp moves landward under wave attack.

Helping to reinforce and strengthen natural dunes (in addition to the beneficial effect of plant roots) are electronic forces, or Vanderwaal forces, between the uniform sized sand grains and the water between the grains. In bulldozed "dunes", which are made up of beach sand with a wide size range of quartz grains and shell fragments, neither plant roots nor Vanderwaal forces are at work to stabilize the deposit.

Consequently the artificial dunes erode with much greater ease. The bulldozed sand, minus the animals that once lived there, returns to the beach (usually during the next storm) and the dunes are flattened and washed inland.

Bulldozing sand is not a good thing for beaches. Taking sand from any part of the beach is a form of beach erosion and it kills the organisms in the beach-the crabs, clams etc. and all the microscopic organisms that live between the sand grains.

For days after bulldozing, seagulls have an unexpected bonanza; swooping and grabbing the stranded and struggling critters of the beach that are now high and dry in the bulldozed dune. The odor of rotting organisms during the post-bulldozing time also can provide an unpleasant atmosphere for beach strollers. The process affects the whole food chain including the shorebirds and the near shore fish.

http://coastalcare.org/educate/sand-dunes/

The Tennyson Dunes Group supports a Conservation Reserve Discovery Trail that protects the fragile dune ecosystem, while allowing children and adults to learn about it. A current State Government trail plan is cause for concern over its proposed alignment, width, construction materials, boardwalks and fencing to accommodate cycling on the trail path.

The Tennyson Dune Group recommends the following Coastcare Code:

If you want to help dunes recover to their natural state here are some simple things you can do:

Adelaide Coastline 1992
The Adelaide coastline, 1992 (South Australian Tourism Commission)
" The Beach Ecosystem is made up of living and non-living parts. Plants and animals and sand and water influence each other, often amidst breathtaking scenery. Greater than the sum of its parts, beaches sustain major portions of global biodiversity. With over half the world's population living within 50 km of the coast, human influence on that biodiversity is inevitable, making the study of beaches even more important.
- http://coastalcare.org/educate/sand-dunes/

The environmentalist Rachel Carson wrote,

" In every curving beach, in every grain of sand, there is a story of the Earth.
- https://preserveamerica.noaa.gov/week06/carson_pioneer.html

Researched, compiled and composed by Dr Steve Gration, January 2018

References

The Adelaide Metropolitan Coastline - South Australian Coast Protection Board No. 27 April 1993

Adelaide's Living Beaches - A Strategy for 2005-2025. Department for Environment and Heritage, Coastline No. 35 - 2005
https://www.ehp.qld.gov.au/coastal/ecology/beaches-dunes/coastal_dunes.html

http://www.tennyson.org.au/


http://www.tennyson.org.au/assets/gardens.pdf

http://coastalcare.org/educate/sand-dunes/

City of Charles Sturt

https://preserveamerica.noaa.gov/week06/carson_pioneer.html