Along the coastline the waves, currents, wind and any rivers entering the sea
all work together to transport sediment. Whenever waves, rivers and wind
lose their energy deposition occurs. That means any sediment being transported is
dropped. Along coastlines this can result in the formation of features such as
beaches, dune, spits and tombolos.
Beaches form when sediment is deposited on the shore by the swash of
constructive waves. Sediment found on the beach can include silt, sand, gravel and
boulders. The composition, color and size of sand on a given beach is the result
of the local rocks that it came from. For example, the erosion of cliffs or rivers
that transport material to the sea. This is combined with the coastal processes
that modified it over the long term. Many beaches contain a lot of silica, also
known as quartz. Quartz is the most common mineral on the Earth's
continental crust. In between the sand grains one also
finds the remains of ground-up shells which are made of calcium carbonate. The
top part of the beach is usually always dry and there is another part that is
always submerged. The inter-tidal zone of the beach is the area that is above
water at low tide and submerged at high tide. Waves can either build up beaches or
erode them. It it is normal for beaches to have a summer and a winter profile. Small
waves have a stronger swash than backwash and build beaches up in the summer. They
are called constructive waves. The deposition of material by waves can form a
sand ridge called a berm which marks the edge of the swash zone.
During winter there are frequent storms. Waves are higher and more frequent. These waves
have a stronger backwash than swash and erode the beach. These are called
destructive waves. Sand and other material is transported along the beach by a
process known as long-shore drift. How it works is that the prevailing wind
direction will cause the waves to approach the beach at an angle. As a
result the sediment that moves up the beach with swash will be moved at an
angle. Likewise the backwash will move the sediment back down the beach at an
angle. Thus sediment will travel back and forth along the beach in a zigzag like
motion. If long-shore drift occurs predominantly in one direction and then
encounters a river mouth or a change in direction of the coastline, sediment can
be deposited forming an extension of the coastline known as the spit. The
diffraction of waves bend around islands taking sediment with them. Eventually
when enough sediment has built up the spit will fully connect with an island
and form a tombolo such as this one. Sometimes long-shore drift will cause
the sand to deposit right across the river mouth forming a bar. As a
consequence many bars have lagoons or wetlands behind them. Long-shore drift
along the east coast of Australia is also responsible for the creation of the
giant sand islands of Queensland's coasts such as North and South Stradbroke
Islands, Bribie Island and Fraser Island. Sand dunes are an accumulation of
sand grains. Sand dunes form when wind carries sand up the beach and deposits in
a heap around an obstacle like a piece of driftwood or vegetation. Aeolian
processes are action of wind. Sand will not be moved until the wind
reaches a velocity fast enough to move the particles of sand. This is called the
fluid threshold velocity. Large particles require higher wind velocity to be moved
than finer particles. However, in wet climates sand particles
a more difficult for wind to pick up, as wet sand tends to clump together. Sand
moves by surface creep which is sand rolling along the ground. The suspension
of smaller particles in the air, or by saltation. The processes of saltation
involves particles being lifted a small distance in the air, only for them to
fall back to the surface under the force of gravity to strike other sand grains.
The grains that are hit by the saltating grains are forced by kinetic energy to
move themselves. Saltation accounts for most of the movement by sand. If there is
a drop in wind velocity, deposition will occur. Dunes classically have a gradual
upward slope and a steep downwind slope,
Sand dunes don't form anywhere. They require special conditions to form.
Firstly, dunes are quite an abundant supply of sediment. Beaches and deserts are
examples of environments where vast supplies of loose sediment exists.
Secondly, dunes require ample wind energy to be able to pick up sand grains and
transport them. Thirdly, beaches with large tidal ranges and a gentle sloping
gradient are more likely to have dunes as there is more time for sand to dry out
enough for wind to pick up the sand grains.
Finally, dunes require an ideal location for sand to accumulate. Dunes such as
these ones have an extensive back shore area where sand can accumulate. You don't
find dunes at places where there are cliffs preventing the inland spread of sand. The
dunes that are closest to the sea are called the fore-dune. In Australia native
plants are often the first species to colonize the fore-dune. As the roots of
these plants bind the sand together the dunes are stabilized. Close behind these
plants on the fore-dunes and on the back dunes where there's more protection for
wind and salt, small shrubs and trees such as banksias, coastal she oak and
wattles may grow. These trees have adapted to help them survive in these
low nutrient and salty environments. There are often wetlands behind frontal dunes
which are dominated by paper bark trees.