Sand dunes are ridges or hills of sand found at the top of a beach, above the usual maximum reach of the waves.
The conditions required for sand dunes to form include:
The image below shows Harlech Beach, North Wales. Its large tidal range supports the development of sand dunes. The tidal range is the difference between the high tide and the following low tide.
Aeolian Transport is the first process of coastal dune formation and involves the movement and weathering of sand particles behind and along the shoreline. Aeolian transportation is when the wind transports sediment. Wind transports sand in 3 ways. These are:
1% of the movement of sand is caused by suspension. This is when sand is picked up and carried within the wind. 95% of sand movement results from saltation. This is when grains of sand bounce along the beach as they are picked up and dropped by the wind. Finally, 4% of transportation is by creep. This is when sand grains collide and push each along other grains.
The video below shows a combination of these processes of transportation.
As the wind blows up the beach it will transport material. Larger pieces of sediment will rest against an obstacle forming a ridge while smaller particles will settle on the other side of it. On the side facing the wind, the material begins to reach a crest. This is because the pile of material becomes steep and unstable and begins to collapse. When this happens smaller particles fall down the other side. Once there is a stable angle (30-34 degrees) the sand stops slipping. This cycle repeats. As the sand becomes an obstacle itself more dunes may form in front of it. The stronger the wind the higher the dunes.
Sand dunes have:
Moving inland sand dunes become taller. Embryo dunes (youngest sand dunes) are only a few metres high whereas mature dunes are up to 15m high. This is because marram grass and other vegetation colonise the sand dune and hold it together with long roots, stopping the migration of the sand dune. Sand dunes closer to the beach are more yellow in colour whereas further away they are grey due to humus and bacteria from plants and animals being added. Each sand dune is separated by a trough (dip). This is known as a slack. They are formed by the removal of sediment from the sheltered lee side of the dune and the windward side of the next dune. Slacks can be eroded so much that they reach the water table resulting in the formation of salty dunes. The video below illustrates how vegetation in a sand dune ecosystem changes as you move inland (known as vegetation succession).
The image below shows vegetation succession on sand dunes at Harlech, North Wales.
The video below shows the extent of roots and illustrates the way vegetation helps stabilise sand dunes.
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