GCSE Geography | Landforms of Deposition: Sand Dunes (Coastal Landscapes 8)

Deposition has led to the formation of distinctive coastal features - we call these depositional landforms.

Sand dunes are large piles of sand that form at the back of sandy beaches.

What conditions do sand dunes need to form?

Sand dunes need the following...

• large wide flat beach
• plenty of sand
• big tidal range, so there is enough time for the sand to dry before the tide comes in again
• onshore winds so sediment can be moved to the back of the beach
• obstacles on the beach that dunes can use to aid formation, such as tree roots or drift wood

How do sand dunes form?

Sand is moved by the wind in three ways:

• Saltation - this is where grains of sand bounce along as the wind picks them up and drops them again (95% of movement)
• Suspension - this is where the wind picks up and carries grains of sand (4% of movement)
• Creep - this is where grains of sand collide and push other grains along (1% of movement)

Sand dunes form where obstacles lay on the beach - this causes the heaviest grains of sand to settle on the obstacle, meaning that a small ridge will start to form, and the lighter grains of sand will start to settle on the other side of the obstacle. Over time a crest will form on the side facing the wind because the pile of sand has become really steep. This makes it unstable and causes the top of the dune to collapse under its own weight. After the collapse the light grains of sand will slip down the leeward side (this is the side facing away from the wind), and will keep slipping until the slope has reached an angle of about 30 degrees. At this gradient the slope is now stable. However, the wind will blow more material to build up the windward side, and the crest will collapse again, causing more sand to slip down the leeward side. This process will keep repeating and will cause the sand dune to move inland. Over time the sand dune itself may become an obstacle, causing more dunes to form.

The size of sand dunes is dependent on wind strength. The stronger the wind, the higher the sand dunes.

Sand dune succession

Sand dune succession is about how sand dunes and their vegetation change as you move inland (see diagram below). Several lines of sand dunes may run parallel and they will have distinctive vegetation.

• Sand dunes become taller as you move inland - embryo dunes closer to the sea are only a few metres high, compared to mature dunes furthest inland, which may be up to 15 metres high
• Dunes get bigger because the sand is bound together by the roots of vegetation, such as marram grass, which has very long tough roots. Binding the sand together enables the sand dunes to grow high. Marram grass is also quick growing which aids this further
• Dunes change colour as they move inland - the ones closest to the sea are sandy in colour due the lack of vegetation, whereas the ones further inland have been colonised by vegetation, so have less sand content, and are more grey than yellow in colour
• Between each line of dunes you will fine a slack - this is a trough that has been formed by removing sediment from the leeward base of one line of dunes and the windward side of the next line. The removal of sand can be so significant that the slacks are as deep as the water table, which leads to salty ponds forming
• Sometimes a blowout may occur - this is a big depression caused by strong winds removing all the sand following the loss of the protective vegetation layer