How are sand dunes formed?

There are a few conditions that are required for a sand dune to form:

• A large, flat beach

• A big supply of sand

• Time for the sand to dry (i.e. an extensive tidal range)

• Obstacles for the dune to form against (driftwood, pebbles, etc.)

• Wind blowing from the sea onto the land, pushing the sand backwards.

As the wind blows up the beach, it will transport material, including sand backwards, through saltation. This is where it is bounced across the beach’s surface as it is blown backwards. Eventually, the sand will be blown against an obstacle, and the sand will start to pile up, creating a ridge. The side facing the wind will start to reach a crest. This is because the pile of material becomes very steep and unstable and begins to collapse. When this happens, smaller particles will start to fall down the other side of the pile. Once there is a stable angle once again (anywhere between 30-34 degrees), the sand will stop sliding, and it will start to build up again. As the sand becomes an obstacle, it will cause other dunes to form in front of it. Generally, the stronger the wind is, the higher the dunes.

This explains some of the features of dunes. The sheltered side of the dune is always steeper, whereas the side that is in the wind has a gentler slope. The further from the beach, the larger that the dunes generally are.

The sections of a sand dune system

• Embryo dunes

Plants growing at the back of beaches together with seaweed, driftwood and other materials deposited by the high tide, form obstacles for the sand to form small piles. This forms another larger obstacle, which leads to more sand being deposited, growing the size of the dune. The movement of the embryo dune is dependant on the speed and power of the wind. If there is a lot of strong wind the dune may move inland, if not it will remain where it is. With enough sand, the dune may even move towards the sea. These dunes are prone to disappearing quickly, however some may remain if colonised by plants, who stabilise the sand with their roots. These dunes generally don’t grow any higher than 1m.

Conditions within embryo dunes are very extreme.

1. Highly alkaline pH values from the calcium carbonate of the shell fragments (over Ph 8).

2. Rapid drainage of water due to wind and the sand’s own incompetence at holding water.

3. High levels of salt as it is often immersed in sea water.

4. High wind speeds nearby and a lot of salt spray.

You can often find sea twitch grass, sea rocket and sea couch grass. These plants are very tolerant of salt, and therefore can grow very well in this highly salty environment. They help to stabilise the embryo dune.

• Fore dunes

If the embryo dunes persist and continue to grow, they can be colonised by plants such as lyme grass and sea holly. This develops them into fore dunes. The conditions here are slightly more favourable. The plants are drought-resistant, making their lives in the sand dunes easier (as sand doesn’t hold water) and capable for withstanding burial by the ever-moving and shifting sand. They are also able to thrive in areas with high salinity levels. However, there is less calcium carbonate, so the levels of alkalinity are lower. Marram grass replaces sea couch grass and its long root system further helps to stabilise the sand, helping to trap more sand, stabilise the dune and allow it to increase in height. Fore dunes generally grow to about 5 metres.

• Yellow dunes

As the fore dunes continue to grow in height, we start to call them yellow dunes. They have this name because of the high amounts of sand still visible within them. These yellow dunes have a greater diversity of plants, as the conditions are far more favourable. As plants die and decay, a layer of humus builds up, trapping nutrients and further helping the plants nearby to thrive. The pH is now only slightly alkaline (toughly pH 7.5) and the salinity levels are also lower. Here there is more shelter and less salt spray. Marram grass still dominates the majority of the vegetation, but other plants are also starting to take hold (creeping fescue, sand sedge, mosses, lichens, sea holly and sea spurge). The yellow dunes are now probably 5-10 metres high and almost 80% of their surface may be vegetated. Rabbits and other mammals may also venture here to add their droppings, helping to enrich the soil that is starting to develop.

• Grey dunes

The grey dunes are far more stable, and mosses and lichens fill the few remaining spaces between the plants, meaning that occasionally vegetation cover can reach 100%. Marram grass is now less common and appears now only in isolated patches as other plants are out-competing it for water and nutrients. Small shrubs such as brambles, gorse and buckthorn now start to appear. Here there is shelter from the harshest winds, humus is beginning to darken the surface layers and a true soil begins to form. Soil pH is increasingly acidic and water content is still low and plants have to search for water with their spreading root systems. Grey dunes are generally 10 metres in height and wider than the other dunes nearer the shore.

• Dune slack

The dune slacks are found between the grey dunes and the more mature dunes. They are where the water table reaches the surface causing seasonal, or in some cases permanent, waterlogging and surface water. Plants which are well adapted to these damp sheltered hollows include sedges, creeping willow, cotton grass and rushes. If decay of plants is slow, a peaty soil may develop.

• Mature dunes

  The most mature dunes are found several hundred metres from the shoreline and the sea. High humus and water content allow these plants to succeed previous vegetation. Left undisturbed, these dunes can develop a soil which can support shrubs and trees including birch, ash and hawthorn. Eventually an oak climax vegetation may develop.

Human uses of sand dune ecosystems:

Dunes, due to their proximity to the sea and beach, are often subject to development due to tourism. They can become extremely popular locations to visit, one such example is the Dune du Pilat, the largest dune in Europe, located in the south-west of France, near Bordeaux. Although they are not stable enough for construction of large houses/buildings, people still build paths through them to get to beaches. The tourism industry is wildly important to the dunes. Dunes are also often turned into nature reserves due to their unique ecosystems. They are very important for the survival of seabirds and coastal plants.