2.1 Long-term evolution of dunes

2.1.1 Sources of sand for dunes

Most dunes in Scotland are formed from sand produced and delivered to the coast by glaciers, or rivers draining from them. This means that the major source of sand started to decline about 10,000 years ago at the end of the last Ice Age, and has now filely disappeared. Present day supplies of sand are modest in comparison. The generally hard rocks forming cliffs and the nearshore seabed around much of Scotland yield little fresh sand for the beaches. Some rivers in Scotland still supply sand to the coastline, although much of this settles out in estuaries rather than travelling directly to the open coast.

Plate 3 White calcareous sand beach, Lingay Strand, N Uist.

Plate 3 White calcareous sand beach, Lingay Strand, N Uist.

In many areas, the only new sand for beaches and dunes is derived from broken shells (Plate 3). This serves a double purpose, firstly replacing sand lost from the beaches and dunes and secondly providing minerals that are taken up by the dune vegetation, encouraging growth and therefore sand stabilisation and retention.

Plate 4 Little fresh sand is being added to Scotland's beaches today. Sand or shingle removed from beaches is not replaced naturally and can lead to beach lowering and subsequent dune

Plate 4 Little fresh sand is being added to Scotland’s beaches today. Sand or shingle removed from beaches is not replaced naturally and can lead to beach lowering and subsequent dune erosion.

2.1.2 Losses of sand from dunes

Sand is continually being lost from the open coast. Waves transport beach material into more sheltered areas (e.g. estuaries) or carry it offshore, to settle out on the seabed. In addition wave action slowly abrades even the hardest pebbles and sand grains, with the very fine particles being carried away by winds or currents either to seaward or landward. The rate of sand loss from beaches in Scotland generally exceeds the supply of new sand and results in long term landward retreat.

A further cause of long-term shoreline retreat is the rise in mean sea level relative to the land. In recent centuries, Scotland has not greatly suffered from the increase in global sea levels, which has been averaging about 1mm to 1.5mm/year. This is because much of mainland Scotland is itself increasing in elevation as the landmass recovers isostatically from the superimposed weight of the ice laid over it during the last Ice Age. In the western Highlands of Scotland, this process is still occurring faster than the increase in sea level, leading to a nett local lowering of sea level relative to the coastline. Elsewhere in Scotland the rate of global sea level rise now matches or exceeds that of the landmass, with a resultant nett increase in local sea levels. As sea level rises relative to a beach, there is an inevitable tendency for the shoreline to move inland.

In the future, the consequences of atmospheric pollution, and hence global warming, may include an acceleration of the increase in mean sea levels around the world. As a consequence, large parts of the coast of Scotland may begin to experience, for the first time in thousands of years, a nett increase in sea levels. In other areas, the existing rate of sea level rise may substantially increase.

2.1.3 Consequences for dune evolution

Most dunes in Scotland can be expected to retreat in the long-term, as a result of both a general loss of the existing sand and in response to beach retreat under the influence of sea level rise.

There are, however, exceptions to the general trend of long-term erosion. There are some sandy beach/ dune areas around the coast of Scotland that have been gaining sediment and advancing seawards. Usually, the sand that is accumulating in these areas has been removed from other parts of the coastline (often from dune erosion), and carried along the beaches by longshore drift. This long-term re-distribution process typically occurs where sand has been deposited in large dune systems thousands of years ago, when sea levels and wave conditions would have been very different from those of today. Erosion of these ancient dune systems may continue indefinitely, with the sand they lose being transported and deposited elsewhere along the coastline, often leading to the creation of new dunes.

In the vicinity of river mouths, tidal inlets or estuaries, other redistribution mechanisms are at work. The combination of wave action and tidal currents will transfer sand from one side of an estuary to another. In some cases, they also remove sediments from inside the estuary and deposit it outside on the open coastline to produce new dunes. Elsewhere, the opposite process occurs: beaches and dunes outside the estuary mouth erode, and the sand they lose travels into the estuary, slowly filling it.

Overall, the long-term pattern of evolution of sandy beaches and dunes in Scotland is of loss of sand and a general landward retreat. Where beaches and dunes are gaining sand, then this is almost always at the expense of beach and dune erosion elsewhere along the coast. Even these gains might prove transient as the processes leading to growth may well change, resulting in rapid erosion of the accumulated sand.