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Sand dunes are small ridges or hills of sand found at the top of a beach, above the usual maximum reach of the waves. They form from windblown sand that is initially deposited around obstacles on the beach such as driftwood, rocks or a fence.

As more sand particles are deposited the dunes grow in size, forming rows at right angles to the prevailing wind direction. If vegetation, such as Marram Grass and Sand Couch begin to grow on the dune its roots will help to bind the sand together and stabilise the dunes.

 

 

Embryo Dunes

The majority of sand dune systems in Ireland are derived from offshore glacial sediments which have been reworked by tides and winds. Wave action pushes this sediment onto the beach, and at low tide, deposits of sand between the high and low tide marks (the inter-tidal zone) may be exposed and start to dry. If the wind blows towards the land, these dry grains of sand will be transported up the beach and beyond the high tide mark (by saltation).

When the dried sand reaches the top of the beach it can be trapped by debris such as driftwood, dead seaweed or rocks and pebbles. The larger grains will be dropped in front of it and pushed up it while smaller grains of sand are deposited behind it.
As the dunes grow in size, causing the wind speed to lessen, they are able to catch more of the sand blown across them. This represents the first stage in the development of dune ridges and are known as Embryonic Dunes. They are mostly made up of exposed sand often only present in the summer. They can be washed away by high tides or storms releasing sand back to the beach, but will form again each year.

If the sand is not eroded again it may become colonised by small plants, or trap other windblown debris, increasing its size and thus trapping even more sand. The low-lying mounds are often vegetated by salt-tolerant species initially and over time the pioneering plants such as marram grass take root on the dune. These plants stabilise and preserve the dune in two ways. Firstly the roots bind the sand together, and secondly the above ground vegetation traps more particles of sand as they are blown over the surface. With the right conditions this could become the start of a huge sand dune.

Foredunes


Under the right conditions foredunes can develop from embryo dunes by steadily growing with sand build up on the seaward side. The landward side (lee side) becomes more stable and protected from salt spray and sand deposit and nutrients increase.
Behind the embryo dune, the fore dune often stands a few metres taller, with marram grass stabilising it a little more than the younger dunes, and more vegetation cover compared to completely bare sand. As more plants grow in the sand, this type of dune is more stable and resilient to storms and erosion, so it can stay in the same position for a number of years.

The development and morphology of foredunes into stable structures depends on a variety of factors including:

  • Sand supply and the rate of windblown sand deposit and erosion
  • Degree of vegetation cover and plant species present to help stabilise the foredune
  • Frequency and magnitude of wave and wind forces and erosion
  • The medium to long term beach state (stable, accreting or eroding)

The form of dune systems will also be dictated by a number of factors, including the shape of the coastline, shape of the beach, currents and swell of the ocean, prevailing wind, frequency of storm events, and particle size of the sand.


Foredunes with good vegetation and plant roots that reach deep into the dune are relatively stable, but there still be exposed sand on the surface. These dunes will continue to accumulate sand from the beach and to have sand blown over the ridge and inland to grow the dunes behind them. These are also known as yellow dunes because of the colour of the sand.

Backdunes


As a foredune builds up, the landward (or lee) slope becomes more stable, nutrient levels increase, and sand inundation and salt spray levels decrease. This are often referred to as backdunes or mature dunes and are furthest away from the beach. They can range from a metre or so in height on some coasts in areas of limited sediment supply, to over 30 m in height and a number of kilometres wide in some instances.

The backdunes dunes become less yellow in colour as plants die off, adding nutrient and humus to the sand dune improving the soil so more diverse plants, such as brambles, can move in.  On most sites, backdunes are relatively stable because of this increasing complexity of vegetation but in some circumstances they can be highly unstable and poorly vegetated. If left to natural processes stable backdunes will eventually be succeeded and covered with woody shrub and tree species forming coastal forest.

Backdunes can also have associated wetlands, dune lakes, rivers and streams, flood plains, dunes of all of which add to the complexity of plant and animal communities that inhabit them.


 

Coast for Kids


Coasts for Kids is a collaborative experience between children and their parents, coastal scientists, community artists, teachers, animators and coastal managers led by Irene Delgado-Fernandez, coastal geomorphologist, Edge Hill University, UK. They have produced wonderful series of short videos  aimed at kids from six years and older exploring coastal processes and coastal evolution.

Episode 2 – Our Coasts Like Moving, looks at how dunes are formed and the protection they provide to our coastline.