The coastal dunes They are accumulations of sand moved by the sea wind in the coastal area, forming accumulations of different dimensions. This sand is the product of the weathering of coastal rocks or coral reefs in fragments of 0.5 to 5 mm that are washed ashore by the waves..
Once on the coast, they dry out due to the action of solar energy and the wind, and are susceptible to being carried away. When there is sufficient accumulation of fine sand and the wind has the minimum adequate speed, the sand masses are transported and accumulated.
This accumulation acquires defined patterns that are known as coastal dunes that go through a development process, and can be ephemeral or consolidate as hills fixed by vegetation..
They can develop from scattered grasslands to thickets with some arboreal elements. In turn, the latitude, climate and vegetation factors determine the presence of a more or less diverse fauna of insects, reptiles, crabs and seabirds..
Coastal dunes occur in different climatic types, from the tropics to temperate and cold zones. An example of this ecosystem is the 205 km of coastal dunes of Yucatán (Mexico), between the Gulf of Mexico and the Caribbean Sea..
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Coastal dunes are geographical forms created by accumulation of sand carried by the wind (aeolian sand), which reach up to 35 m in height. It is a dynamic system with a constant cycle of erosion and restoration that plays an important role in defending the coast against the intrusion of the sea during storms..
The coast is a transition zone between land and sea, where land, sea, atmosphere and rivers come into contact. In these areas there are three fundamental factors that act to erode coastal rocks: water, wind and solar radiation..
The mechanical action of the waves on the rocks causes hydraulic fractures, compressing the air trapped in the cracks and pores, which then expands. In this process over thousands of years, the rocks are cracking and breaking.
The fragments fall into the sea, undergoing a constant process of erosion due to the abrasive effect of the dragging of currents and waves. There is also a chemical action of the water dissolving cementitious substances in the rock.
In this way, they become more and more fragmented, creating the small grains of sand that are dragged to the beach. It is about thousands of years of a continuous process of abrasion of fragments of rocks in movement with the ebb and flow of the waves.
It is a substrate formed by mineral grains of shapes from angular to rounded, of small size. Its dimensions range from 0.062 mm in the so-called very fine sands to 2 mm in the very coarse.
This substrate is produced by weathering (action of environmental elements on coastal rocks and coral reefs). This reduces the rock to almost individual mineral fragments, including a diversity of elements, such as silica (mainly in the form of quartz) and limestone..
Feldspars, basalt, and olivine are also found in volcanic rock sands. Limestone predominates in areas of calcareous rocks, as well as in tropical or subtropical coasts with coral reefs, originating white sand beaches..
In the case of coral erosion, marine fauna contributes, such as parrotfish that scrape it to ingest the algae cover.
The coastal dune is characterized by being a somewhat asymmetrical mound of sand, as it has two uneven slopes and a peak. The windward slope (facing the wind) rises gently, while the leeward slope (on the side facing the wind) is more drastic.
In addition, the coastal dune has two lateral arms, which are like extensions of sand that remain when the central mass is dragged. Finally, between one coastal dune and the next, a depression forms that separates them, called a hollow..
Coastal dunes are classified according to different criteria, either by their position with respect to the coastline, by their mobility or by their shape..
Regarding the order in which they are formed from the coast inland, there are primary, secondary and tertiary. The primary coastal dunes form near the beach, the secondary ones are located somewhat further inland, followed by the tertiary ones, and so on..
Regarding mobility, there are mobile or transgressive dunes and immobile or stabilized dunes.
The mobile or transgressive ones move constantly, since they do not have more vegetation to stabilize them. These dunes cover all types of terrain and even invade roads and other infrastructures.
The immobile or stabilized dunes remain fixed due to the growing development of vegetation that immobilizes them. In addition, there are forms of intermediate mobility, depending on the degree of stability that the vegetation provides to the dune..
According to their shape or topography, there are embryonic, frontal, parabolic and barjan coastal dunes..
They consist of small clusters that form and cover an obstacle such as a rock or plant. They have variable shape depending on the particular obstacle.
The frontal dunes create dune cords along the coast, as barriers with little or no development of arms.
It is a dune in the shape of a half moon, with the arms (horns of the moon) pointing in the direction from which the wind blows.
They are also shaped like a half moon, but their arms spread out to where the wind blows..
Not on all the coasts of the world there are coastal dunes, since this depends on various factors, such as the conformation of the coast. Coastal dunes do not form on coasts of hard rock cliffs, coasts of very closed bays, inlets or inlets with narrow beach strips or coasts with high vegetation barriers.
For the formation of coastal dunes, the appropriate coastal relief, amount of sand formed, size of the sand grains and wind speed are required. Finally, obstacles are required to hold back the moving sand to form the core of the dune..
Coastal dunes occur on flat to slightly undulating shores facing open plains or valleys. On beaches with abundant sand, predominantly that around 0.2 mm in size, in such a way that it can be pushed by the wind.
On the other hand, marine winds of not less than 4.5 m / sec should prevail, since a lower speed would be insufficient..
When the required factors are present, as the sand dries the wind that blows from the sea, carries it inland. This causes a slow mass displacement, which if it did not find any obstacle, would continue to disperse the sand towards the interior..
When there are obstacles on the ground, such as rocks, plants, small elevations, the wind speed decreases and the sand accumulates. Therefore, when the sand is in motion with these obstacles, it begins to accumulate, forming a coastal dune, particularly between the branches of the plants..
Coastal dune movement occurs when the wind blows sand from the gully to the top down the gentle windward slope. Once up, the wind loses speed and deposits the sand on the opposite slope (leeward), from where it rushes forward..
The coastal dunes are a very permeable substrate (with little water retention), unstable and with a high salt content. These plants are tolerant to salinity (halophytes), resistant to water deficit and strong rooting and in many cases creeping.
The plant species present are associated with the development phase of the plant succession in the coastal dune. For example, in consolidated or stabilized dunes, erect plants such as palms and small trees can grow..
Various species that contribute to the formation and fixation of the dunes, such as the fanny pack or batatilla (Ipomoea pes-caprae) and the barrón (Ammophila arenaria) which are halophytes with highly developed root systems and extensive stolons.
In the first case it is a creeping plant of the convolvulaceae family typical of tropical and subtropical climates and the second is a grass of temperate and cold climates.
Other plants that grow in the tropics include the beach bean (Canavalia rosea), the croton (Croton punctatus), the saladillo (Sporobolus virginicus), the beach purslane (Sesuvium portulacastrum) Y Atriplex spp.
While in temperate zones the poppy or marine poppy (Glaucium flavum) and the sea horn (Lotus creticus).
Once the coastal dune has stabilized, denser and taller vegetation can develop, such as shrubs or bushes. There are even arborescent species or small trees, in tropical areas there are palms of genera such as Coccothrinax.
Similarly, small trees such as the beach grape (Coccoloba uvifera) and various species of Capparis. While in temperate zones there are shrubs such as cambronera (Lycium intrincatum) and jujube (Ziziphus lotus).
Although coastal dunes are not very biodiverse ecosystems, they are home to a diverse fauna of insects, crustaceans, reptiles, birds and even mammals. Insects abound, especially butterflies, coleopterans (cocci), flies, bees, wasps and ants.
On the other hand, crustaceans are common inhabitants of coastal dunes, specifically various species of crabs. Among them the blue crab (Guanhumi cardisoma), which migrates to the beach to lay its eggs.
Another characteristic group are reptiles, with various species of small lizards. For example, the pochitoque (Kinosternon leucostomum), although there are also larger lizards such as the green iguana (Iguana iguana).
There are also species of turtles such as the painted turtle (Trachemys spp.). There is also a great variety of seabirds that rest, breed or hunt among the vegetation of the dunes..
Mammals such as rabbits, raccoons and others can enter the consolidated dunes, with a greater presence of vegetation. For example, in the coastal dunes of Mexico inhabits the northern cacomixtle (Bassariscus astutus).
By definition, coastal dunes develop in an oceanic climate, where the nearby mass of water exerts a determining influence by tempering the climate. Coastal dunes are formed in coastal areas of all continents, both in tropical and temperate and cold areas.
Depending on the latitude, rainfall can be very scarce or abundant, and there are also large variations in temperatures.
In Mexico there is an important coastal dyne system along the entire coast of the Gulf of Mexico to the coast of the Caribbean Sea. Four coastal dune zones are recognized with differences even in climate and vegetation.
In Tamaulipas and northern Veracruz, a dry climate prevails with low to medium rainfall and low temperatures, with dunes of calcareous sand and quartz. Here are dune cords and coastal lagoons.
On the other hand, in the center of Veracruz there is a subhumid and warm climate, with quartz sands forming complex dune systems. The vegetation here is more complex, mainly due to the influence of adjacent tropical forests.
Then, in the south of Veracruz and Tabasco there is a more humid and warmer climate, with rainfall of up to 3,500 mm per year. Quartz sands also dominate this region and the dune vegetation is less complex..
Finally, in the Yucatan peninsula, there is a hot and dry climate, with calcareous sand dunes. In this region there is a varied scrub vegetation in the consolidated dunes, with various species of herbs, shrubs and even palms and trees..
In the coastal dunes of Mexico, species such as the kidney belt (Ipomoea pes-caprae), the beach purslane (Sesuvium portulacastrum) and the sea lily (Hymenocallis littoralis). As well as small trees like the uvero de mar (Coccoloba uvifera) and palms like nakax (Coccothrinax readii) and chiít (Thrinax radiata).
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