For a sand
dune to form there
must be a reliable
supply of sand, a
means of transporting it (wind), and somewhere
for it to be deposited more quickly than it is eroded.
•The dry grains of
sand can then be transported by the wind through a process of saltation. If the wind
blows towards the land, sand will be transported up the beach and beyond the
high tide mark. When the dry
particles of sand reach the top of the beach they might be trapped by driftwood
and rocks.
If the sands
is not eroded again, plants might start growing on it increasing the size and
hence trapping more sand.
Diapositiva 3
Strong onshore
winds
Large
expanses/area of dry sand ( spits, cuspate forelands, bays)
Reliable
supply of sand
Large tidal
range
Vegetation to
act as obstacles to limit/trap sand movement and deposition can occur –
vegetation cause wind velocity to drop.
Pie de foto: : http://www.s-cool.co.uk/a-level/geography/coastal-processes/revise-it/coastal-deposition
Sand Dune Characteristics
Diapositiva 5
Embryo – this is the
first part of the dune to develop and are formed in sheltered areas. Once the
pioneer plant species such as Lyme grass, sand couch grass and sea rocket
stabilise, the sand is trapped and accumulated over time - the dune grows.
Conditions in this dune are dry thus plants adapt by growing long roots and/or
thorny leaves to reduce evapotranspiration. The early colonising plants will
eventually die off and decompose. They
add nutrients to the soil which makes the dunes more hospitable for new and
larger plants to grow therefore giving them the capability trap more sand –
larger dunes develop.
Vegetation
makes the dunes stable.
Characteristics of a sand dune
Diapositiva 6
Fore dune – They tend to
be very yellow as they have limited vegetation so no real humus layers develop.
Marram grass is the main coloniser and stabiliser.
Yellow dune – The colour
darkens as organic material add nutrients and humus to the soil. Sea couch and marram grass stabilise the
dune.
Grey dunes – Developing
humus layers changes the colour of the dune form yellow to grey. Water retention is also increased
(soil becomes damper and richer) due to more humus therefore more species of plants
thrive e.g. Creeping willow, red fescue and dewberry. In time, the dune stops growing because the
sheltered means that there is a lack of wind to blow the sand.
Diapositiva 7
Mature dunes –
The dunes can
sustain more plants such as thistle, evening primrose, bracken, bramble and
heather because the humus layers grow more. Less than 10% of sand is exposed on
these dunes.
Dune slacks - are low lying
depressions between the dunes. Plants in
the dune slacks include wild strawberries, buttercup and violets with some flag iris and willow in the
slacks at the rear of the main dune system. In the slacks, vegetation covers almost
everything.
Diapositiva 8
It
is key to remember that during the development of mature dunes, the embryo
dunes are continually being formed at the base of the dune system consequently,
the dune system is advancing seaward.
