6. The dune system

Dune systems are sand accumulations that are situated on the dry part of the beach forming a land ecosystem containing important and characteristic flora and fauna.

Dunes are a dynamic element of the beach landscape. Their existence requires the presence of sediment (sand) and some wind blowing from sea to land. In this way, dunes grow from the sand transported by the waves from the sea bottom and settle on the beach surface in formations called beach berms. Berm sand, which dries up in periods of good weather, is transported by the wind action towards the dunes that act as reservoirs or sand banks.

Plants are very important for the dune generation; on the one hand, it reduces the wind's speed and its transportation power, whilst, on the other, plants act as a shield for the grains of sand favouring its sedimentation and giving structure to the dune. The plant species inhabiting the dune system are species with the capacity to withstand extreme environmental conditions such as high salinity levels of the ground, high temperatures in summer, the saltpetre and the continuous movements of sand. They use the remains of Posidonia oceanica seaweed to grow, that, thanks to the action of wind and surf have moved towards the inland beach.

The formation process of a dune
Classification of dune systems
The dunes of Menorca

The formation process of a dune

Cross-sectional line of the emerged part of a beach. Source: The author: CARDONA, F.

1. The wind coming off the sea moves the sand that the sea has settled in berms on the beach (a) once they are dry, forming small dunes just a few centimetres high, known as ripples (b).

2. As the wind loses intensity due to friction against ripples, these ripples come together to form the so-called embryonic dunes (c). They are not retained by vegetation and are the youngest dunes in the dune system. With regard to vegetation, this dune is found in a hostile area due to a high level of salinity and the presence of shells in the sand. These dunes are easily destroyed by storms.

3. On the upper half of these small embryonic dunes, plants appear, although their presence is rather scarce. This vegetation retains the sediment that slides down the leeward side of the dune as small natural landslides. These new dunes are known as primary dunes (e). They are older and higher than the embryonic dunes. Furthermore, they are of vital importance, since they act as a sediment reservoir that ensures the beach's continuity and also protects the vegetation of the inland dunes against wind and saltpetre.

4. Behind this foredune, the first ridge of dunes, or secondary dunes (d), appears. These are the first permanent dune accumulations. The conditions for the growth of vegetation are better in this area, as they are sheltered from the wind and sea salt and the sand is more suitable for plants. Any alteration in vegetation provokes sediment movement.

5. Dunes do not grow indefinitely and, if the sediment supply is consistent, they reach a balance where the amount of sand reaching the dune from one side equals the amount being lost from the other. This balance fosters the creation of new dunes further inland from the beach, known as tertiary dunes (f), which make up a second ridge of dunes.

In any case, this natural process is extremely slow and is rarely completed due to the action of humans.

Classification of dune systems

Regressive dune systems

The dune system slowly disappears due to the continuous erosion of primary dunes by storms and tides. They cannot recover in periods of good weather. Therefore, the primary dune vegetation is challenged by steep slopes on the sea side, with frequent slides, where blocks of vegetation are ripped from the dune and slide down to the base.

Punta Prima beach where the dune system has been radically torn off

Balanced dune system

Average wind transportation towards the dunes is maintained and balanced out by erosion from storms. Usually in periods of good weather a foredune with young vegetation is formed, which is destroyed by erosion processes. Its primary dune is usually well developed and rather high.

Cala Pilar's dune system

Dune system with a positive sediment balance

A consistent sediment supply on the dry part of the beach along with wind transportation towards it creates sand belts to parallel dune ridges as new foredunes are created on the sea side. In this case dunes are usually lower and show more vegetation as they move inland.

The dunes of Menorca

Menorca has approximately 20 dune fields and, although most dune systems are still intact, some have been lost due to coastal developments, such as in Son Xoriguer, Cala en Bosch and Punta Prima.

Menorca dune systems. Source: GRIMALT, M.; TOMÀS, M. (2009) Els sistemes dunars de les Illes Balears. Universitat de les Illes Balears - Grup de Recerca Climaris,

The conservation level of dune systems can change substantially depending on the different sedimentary processes involved (sediment supply, wind and waves), as well as on the human pressure they undergo. The intensity of the northern winds on the Menorcan northern coastline makes the dunes align perpendicularly to the coastline instead of horizontally, as usual.

Dune systems in Menorca usually present a first line of vegetation near the sea, which can therefore be affected by the surf. This is why vegetation is temporary and is made up of plants that bear high salt levels and have the ability to disseminate their seeds through the sea. They use the remains of Posidonia oceanica seagrass to grow, which, thanks to the action of wind and surf, have moved inland. The most common species are: Salsola kali, Cakile maritima, Suaeda spicata, Polygonum maritimum and Euphorbia peplus.

As the dune ridge takes shape, the pioneer vegetation fosters the emergence of more stable and permanent species adapted to sand dynamics. The most frequent are: Eryngium maritimum, Pancratium maritimum, Calystegia soldanela or Euphorbia paralias.

Calystegia soldanela

On a third vegetation strip the dune shape becomes more evident. Two kinds of grass are the characterising vegetation of this area: Elymus farctus and Ammophila arenaria. In a few certain areas on the southern part of the island we can also find Crucianella maritima. Further inland from this plant appears the the savin junipers (Juniperetum eumediterraneae, Juniperus phoenicea as well as other shrub communities not necessarily related to sandy substrata (Pistacia lentiscus, Rosmarinus officinalis, Phillyrea media, Erica multiflora, Smilax aspera, Ruscus aculeatus...). This wide variety of flora has fostered the presence of a great variety of native flora: Ononis crispa, Thymelaea velutina, Polycarpon colomense, Coronilla montserratii, Scrophularia romosissima and Scrophularietum minoricensis are rather frequent in northern dune systems.

The large variety of plants present in beach sand contributes to increasing the island's biodiversity, after millions of years. This is the best example of the proper functioning of a system made up of living organisms and the environment in which they live. Given that different species exist with different resistance levels to changes in the environment (frequently provoked by human beings), this fact guarantees the preservation of life facing external impact.

Cross-sectional line of the emerged part of a beach