What Are Coastal Dunes?

What Are Coastal Dunes?

Case Study: Coastal Dunes

What are coastal dunes?

Coastal dunes are large accumulations of sand located immediately behind the active beach zone.They are formed when sand is deposited onto the shore by wave action, dries out and is blown to the back of the beach. This is referred to as accretion.

Prevailing windsshape the sand into a system of dunes.

Where the tidal range is small, wave actions deliver sand.

Sometimesdesert dunes, pushed by wind merge with coastal dunes formed by landward movement of beach sands.

Spatial distribution and dimensions of coastal dune ecosystems

  • Coastal dunes form where sand is available. They are found on all the world’s continental land masses, except for Antarctica.
  • Sand (grains of weathered rock) can be very coarse, coarse, medium and very fine. Windblown sand is usually finer than beach sand. Beaches generally obtain sand from rivers flowing to the sea and from the weathering of cliffs.
  • The source of coastal sediments is important in determining whether sand is available for construction of coastal sand dunes.
  • Another important factor is wind; it determines the shape and extent of the dunes.Strong onshore winds develop dunes on the coast. Therefore the most extensive dunes are found in the worlds tropical zones.
  • The occurrence of dune systems is, dependent on local topography, direction and power of prevailing winds and other factors.

Interactions within the Biophysical Environment

The four components of the biophysical environment interact and this determines the shape, extent and nature of coastal dune ecosystems.

The Role of the Atmosphere:

The main components of weather are wind, temperature and precipitation. The climate also plays a key role.

-Wind

  • Aeolian transportmoves the sand.
  • The amount of sand transported depends on size of sand particles, velocity of wind, topography and nature of vegetation.
  • Smaller grains are easily moved, but can be too small to be dislodged. Larger grains are too heavy to be lifted.Generally, stronger winds result in more volume of sand being lifted.
  • Where sand is covered by vegetation, Aeolian forces are minimized. The vegetation disturbs the flow of win, reducing velocity. The roots of plants also help bind the grains of sand together.

-Temperature

  • Temperature plays a key role in determining the rate at which beach sand dries, extent and type of vegetation.
  • The temperature also affects the functioning of ocean currents, which have the potential to move sediment or cause beaches to be eroded.

-Precipitation

  • Levels of precipitation determine the nature and extent of vegetation which grow on the dunes.
  • Areas that receive regular rainfall are more likely to have more vegetation.

Hydrological Processes:

  • Are those that involve water.
  • River systems transport sediment to the coast, where it is distributed by longshore drift and ocean currents. Waves erode cliffs and headlands, providing additional sediment. Storm waves cause damage to beaches, but constructive waves rebuild them.
  • Wavestransport the sand that is needed for dune construction and they create change in dune systems by attacking them during storms
  • The importance of longshore drift lies in its role as a transporter of sediment. Sedimentwhich is produced or depositedin one part of the coastal system, is transportedto other locations within the system by the process of longshore drift. This allows dunes and other sediment-based landforms to developin areas some distance from the source of coastal sediments.
  • Rainfall is another important factor, while rainfall infiltrates sand dunes with relative ease; heavy downpours can produce runoff, which erodes the sandy surface, exposing wind.
  • Another important change caused by rainfall is the development of dune lakes. These lakes form in low-lying areas of the dune system.
  • In times of heavy rainfall, surface runoff accumulates in a swale (a trough or shallow depression between the crests of coastal dunes). Eventually the sand becomes saturated and the water tablerises. This only goes if the weather is warmer. By this time the vegetation has drowned the sand is left exposed.

Composition of sands

  • Different places have sand which varies in chemical and mineral composition because different sands derive from different types of rock.
  • Lithification is when water combines with calcium found in some sands to form hard calcium carbonate or limestone.

Dune Formation

  • Dune systems are both complex and dynamic.
  • Main sources of material for sand dune developments come from offshore sandbars and sediment delivered from the rivers.
  • Sediment is transported along the coast via long-shore drift until trapped by a projecting landform. Then sand is deposited on beaches by wave action.Prevailing winds then transport dry sediments landwards, then it is trapped in and protected by vegetation.

Types of Dunes:

  • Handout in book.

Dune Profile Picture:

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Dune Profile

  • The windward face of a sand dune is characterized by a gentle slope, while the opposite face has a steep slope.
  • Sand grains accumulate high on the back slope of the dune and slip down. This side is called the slip face.

Long-term Dune Stabilization

  • Invasion of dunes by scrub vegetation stabilize the dune system. Then the growth and decay of the scrubs create a layer of humus (dead plant matter). Over a long period of time, the soil eventually replaces the sand and the dune is colonized by a new species of plant.
  • Trees and larger shrubs move in and continue to produce humus, leading to the development of a more mature soil profile. The more complex plant species transform the dune a almost-permanent feature of topography.

Dune Vegetation

  • Dune vegetation occurs in zones, these zones relate to the degree of exposure to harsh coastal conditions.
  • The oldest dunes support the most complex plant communities, while the recently formed dunes only support the pioneering species.
  • Dune vegetation is divided into these three groups:

Primary Species

  • Closest to the sea is the pioneer zone.
  • Only some plants can colonize areas exposed to salt spray, sand blast, strong winds and inundation by the sea
  • Protected from salt and sand by covering on their stands and leaves and they grow close to the ground. Therefore offering little resistance to the wind
  • While these pioneers plants provide some resistance for the dune during storm. Their main role is to quickly stabilize the sand.
  • Pioneering plants have strong roots and spread rapidly, which create a system of roots which bind the sand together. One part of the plant is buried beneath the sand, allowing another part to start to grow.
  • The pioneering plant systems stabilize sand,reduce impact of wind erosion, forward the dune-forming process and initiates the development of soil profile.

Secondary Species

  • The foredune vegetation is usually composed of semi-permanent heath-like shrubs and small trees that stabilize the foredune sand mass.
  • The actual mix of species found in this zone depends on climate.
  • The composition of this zone reveals the conditions of the beach.
  • Extensive areas of herb land suggestsand is accumulating. Where is it absent, erosion is likely to be occurring.

Tertiary Species

  • In the zones beyond coastal heath, trees dominate.
  • Here, vegetation is protected from wind and salt spray, and soils have larger amounts of organic matter.
  • The type of vegetation depends on local topography and climate.
  • Banksia and she-oaks give way to more complex plant communities.
  • These zones are not fixed, as plants grow taller, exposure to sun and soil conditions change. The soil becomes richer and holds more water, this enables a change of the type of vegetation. This process is called succession.
  • This is why it’s called climax communities, because they are communitiesthat are in equilibrium with their environment.

Human impacts on dune systems

  • Coastal environments are one of the most stressed environments on earth
  • The stress is most pronounced in the active zones of the coastal environment
  • Areas are being changed constantly by natural processes, including wind and wave erosion
  • Because their proximity to the beach , sand dune ecosystem are particular vulnerable
  • Coastal dunes are used for variety purposes including mining, water extraction, waste – water disposal, housing, agriculture and recreation.

Reduction and alteration of sediment flows

  • The flow of sediment along the coast is complex and depends on a range of factors, including wind direction, near

Coastal development

  • Dune systems can be partially flattened or they can even be completely removed to make space for coastal development.Coastal development disrupts sediment flow. This removes the protective barrier between land and sea.
  • One of the most dramatic forms of development is reclamation,which involves creating a wall behind the dune and filling it, resulting in the sand dune being forced outwards.

Recreational uses

  • Research shows coastal areas exposed to visitors are subject to significant degradation. The main reason is because of loss in plant life.
  • People trample on plants, which make holes for weeds to grow which are more resilient but are not beneficial to the dune.
  • The foredune vegetationare most easily damaged and are the most important because they protect the least stable part of the dune system.
  • Tires of vehicles destroy vegetation and force the dunes soil to become more compact. Compacted soil has less oxygen reducing plant growth and prevents plant recovery.
  • Dunes are also flattened for recreational facilities and parking areas.

Sand mining and extraction

  • Sand is filled with valuable minerals used in many industrial processes. Often the sand is taken from coastal sand dunes.
  • Sand mining is the most controversial landuse because it involves wiping out a whole system of vegetation.
  • After mining, rehabilitation begins along the dredge path of the concentrator plant. Tailings of dunes are reshaped and revegetated to resemble past plant communities.

Introduced species and weeds

  • Dunes are dominated by highly specialized species because they can tolerate poor soils, strong winds and salt spray. But they can fall to invasion to weeds.
  • Infestations are mainly caused by human activities such as deliberate introductions to stabilize dunes. Weeds can also be introduced via wheels of vehicles or even shoes.

Rabbits – One of the most destructive species ever introduced. They spread quickly and cause significant damage by eating grasses, vegetation, reducing ground cover thus exposing dunes to wind erosion.

Impact of global warming and sea level rise

  • Affects sea level in two ways:
  • Increases sea level by melting polar ice caps
  • Thermal expansion of world’s oceans, hotter water particles means expansion thus causing sea level rise.
  • Scientists believe rate of coastal erosion will increase and the sea will soon inundate low-lying coastal terrain.
  • Sea level rise will increase amount of washovers, destroying more parallel dunes,
  • Beaches will disappear and dunes will be pushed inland.

Traditional and contemporary management practices

Traditional management of coastal dune ecosystem

  • There is evidence that suggests Aboriginal land management practices in dune ecosystems involved use of fire.
  • Early explorers reported the sight of migrating dune systems along the eastern coast.
  • It seems that the spread of fire management into the coastal dunes destroyed vegetation, exposing sand and allowing migrating dunes to develop.

Contemporary management of coastal dune ecosystems

Successful dune management involves maintaining the natural functions of the ecosystem while allowing people to use the coastal environment in a sustainable way.

Dune protection

It is better to manage existing dunes to protect them from further damage. Some methods of protection are Landuse Controls, Fencing, Fenced accessways, Board and chain paths, Dune reconstruction, Sand fencing, Mats and netting, Brushwood and finally Revegetation.

Landuse Controls–Stricter laws on landuse helps reduce destruction of dune systems.Local authorities have the power to limit development and allow dune systems to function more naturally.

Fencing–Use of fencing protects dunes from human trampling, which is most important when immature vegetation is vulnerable.

Fenced Accessways–Specifically designed accessways enable people to access the beach and at the same time protect vegetation.

Board and Chain Paths–These flexible materials can conform to the shape of the dune. The timber slats reduce effects of wind erosion on exposed sand.

Dune restoration and stabilisation

Migration of sand occurs when vegetation on stabilized dunes is removed or damaged.Dune stabilisation aims to secured exposed sand and stop it from blowing away, these are the ways it can be done:

Dune reconstruction–Dune reconstruction involves using earthmoving equipment to manipulate dunes into ideal slopes which allow vegetation to grow easily.

  • Chemical stabilisers are often used to bind the sand on the reconstructed dune.

Sand Fencing–Panels of woven material which trap airborne sand by slowing wind velocity.

  • They are quickly buried by sand.

Mats and Netting–Used to protect sand surface, best used to protect transplanted immature vegetation.

Brushwood–A temporary stabiliser placed over bare sand which involves placing dried branches on the surface of the sand.

Revegetation–Most cases, vegetation is least expensive, most durable, most appealing and self-maintaining technique available.

  • Plants trap are good a trapping and holding wind-borne sand, reducing wind velocity and interrupting movement of sand by surface creep.
  • Degraded dunes are stabilised by planting pioneer grasses, then secondary and finally tertiary species.
  • Dune vegetation species must be able to adapt to the special coastal conditions. That is why selection of species is important.
  • Cover crops are used on unstable dunes, their role is to hold the dune together while other species which take longer to establish grow.
  • Revegetation is a labour-intensive activity and must be combined with other management strategies to work, such as fencing.

Fire management

Dune vegetation has low combustibility and regenerates quickly.Coastal dunes also have dune lakes and swamps which act as natural fire barriers. But the activities of people have reduced the effectiveness of these natural factors by introducing high-risk species of plants and destroying dune lakes.

Best first management is prevention. Barbeque facilities, fencing off camping spots and providing bins help prevent fires in dune systems from breaking out.

Dealing with weeds

Management strategies against weed infestations must have these three elements

  1. Practices that prevent infestations in unaffected areas
  2. Practices that contain existing weed infestations
  3. Programs which eradicate weeds and replace them.

Questions pg. 85

3.Outline the concept of dune reconstruction

Dune reconstruction involves forming a dune which is aerodynamic using earthmoving equipment.Dune reconstruction also involves using inorganic stabilisers tobind the dune.

4. Outline the primary objective of Revegetation

7.Explain the impacts of using stabilizers, such as bitumenemulsion, on coastal dunes