Sand dunes

Follow one grain of sand

Five stages, from loose grain to a dune that stays put. Tap any stage to see what it adds.

1. 1Two ingredients: sand and windEvery dune starts with just two things: a supply of sand, and wind energy to transport it. Our grain sits on the beach until the wind carries it off.+
2. 2The wind drops itOur grain is carried until the wind slows down or the surface gets much rougher. There the sand is dropped. That is deposition, which is why dunes count as a depositional landform.+
3. 3An embryo dune beginsGrain after grain lands in the same place, and a first heap of sand builds on the backshore (the back of the beach). This young dune is called an embryo dune.+
4. 4Pioneer plants take holdThe first plants to grow here are pioneer species. They normally need to cope with salt and to withstand a lot of sand landing on them.+
5. 5A foredune that holdsHealthy marram, or similar plants, trap blowing sand and bind it together with their roots. The result is a well-vegetated frontal dune, or foredune.+

Predict, then check

Commit to an answer before you reveal it.

A stretch of backshore has no plants growing on it at all, but there is a thick line of litter along the drift line. Wind is blowing sand across it. What happens?

The deciding factor

Grow or shrink? Ask the sediment budget

Choose the beach's sediment budget, then follow the route to see what happens to its frontal dunes.

Beach sediment budget → How severe

On A beach with frontal dunes. 2 branches to choose from.

The shape of the frontal dunes on any beach is strongly influenced by the sand exchanged between beach and dunes, and above all by the balance of their sediment budgets.

What you need to know

• Sand dunes are a coastal landform resulting from deposition, alongside beaches, spits and bars.
• The two basic requirements for dunes are a supply of sand and wind energy to transport it.
• Blowing sand is likely to be deposited wherever wind speed falls or surface roughness increases significantly.
• Embryo dunes develop on the backshore, and can start without vegetation (for example at a large amount of drift-line litter).
• Pioneer plants normally need some tolerance of salt and the ability to withstand high rates of sand deposition.
• Vegetation is not essential for dunes to form, but it strongly affects dune shape and how fast dunes move.
• The beach sediment budget strongly influences whether frontal dunes grow or erode.

The big picture

Sand dunes are a coastal landform made by deposition. Every dune needs two things: a supply of sand and wind energy to move it. Sand is dropped where the wind slows or the surface roughens, and an embryo dune can begin on the backshore, even with no plants. Vegetation is not essential, but salt-tolerant pioneer plants help embryo dunes become permanent, and marram traps and binds sand. A beach's sediment budget strongly influences whether its frontal dunes grow or erode.

⚠ Watch out

Saying that dunes are built by plants, or that a dune cannot form without vegetation. The two requirements are a supply of sand and wind to move it. Vegetation is not essential, but once it arrives it traps and binds sand and strongly shapes the dune.

Memory hook

Sand to start it, wind to cart it, a slow-down to drop it, plants to hold it. Only the first two are essential.

Check yourself

Cover the page. Name the two things every sand dune needs, then say what changes once pioneer plants and marram arrive.