Sand Dunes

How are sand dunes formed?

The conditions required for sand dunes to form include:

• a large supply of sand
• a large flat beach
• time for sand to dry, so a large tidal range is needed
• an onshore wind (wind blowing from the sea to the land) for sand to be moved to the back of the beach
• an obstacle for the dune to form against e.g pebble or driftwood

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.

A beach with a large tidal range

How does wind transport sand?

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:

• suspension
• saltation
• creep

Aeolian transportation

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.

The characteristics of sand dunes

Characteristics of sand dunes

Sand dunes have:

• a gentle slope on the side the wind blows against
• a steep side on the sheltered side (30-34 degrees)
• a crest (top of the sand dune) up to 15 metres

How do sand dunes change as you move inland?

How do sand dunes change with distance from the beach?

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.

Sand dune vegetation succession

The video below shows the extent of roots and illustrates the way vegetation helps stabilise sand dunes.