The Western Isles (or Outer Hebrides) are situated off the North West coastline of Scotland and comprise of literally hundreds of islands, although only twelve of these are now permanently inhabited. Many of these islands are extremely important from a conservation viewpoint, on an international scale, due to their geological, botanical, ornithological and other attributes. The St Kilda group of islands, for instance, is designated as a World Heritage Site because of the large colonies of breeding sea birds. One of the most important features of many of these islands is the lack of anthropogenic influences.
These islands are largely founded on some of the oldest and most durable rocks found around the world. Because the basic geology of the islands is of a hard rock, the material available to create beaches might be thought to be sparse. However, along the western coasts the limited sand and gravel derived from glacial material has been greatly supplemented by calcareous sand, derived from biological sources (shells etc.). In suitable locations this sand has accumulated in large quantities, forming wide beaches backed by dunes and a flat, fertile coastal plain (machair). It is this type of beach type which is commonest along the western coast of the Western Isles.
However, there are a significant number of shingle beaches and ridges, sometimes partly covered by sand, which are also interesting and valuable. Much of this shingle is of glacial origin, although on Lewis, eroding till cliffs continue to supply fresh shingle to the coastline. This solid geology forms a framework for the evolution of the islands' beaches, and the sediment on the nearshore seabed and the coastline provides the basic material for those beaches. However, it is the winds, waves and tides which shape those beaches, and the associated dunes and machair.
The beaches, dunes and immediate coastal hinterland of these islands, endure some of the most vigorous wind and wave conditions in the world. In many places, the resulting morphology of the coastline is of considerable geomorphological interest, with numerous interesting beach forms having developed.
In many parts of the world the pressure for human management of beaches has arisen because of the need to protect the coastal strip against erosion or flooding. However, aspects such as the amenity and tourism value of good beaches, and the conservation of geological or biological features are now beginning to be recognised. In the Western Isles, the beaches not only fulfil a coastal defence role in many areas, because of the concentration of settlements and farming there, but they are also of considerable importance for both recreation and conservation.
A study has therefore been commissioned through the Minch Project by Scottish Natural Heritage and Western Isles Islands Council to identify the dominant causes and extent of coastal erosion on nine of the permanently inhabited islands.
It has been found that the causes of coastal erosion are different in the Western Isles compared to many other parts of the UK coastline. Factors such as sea level rise, and changes in the climate leading to more severe wave action are important. It has proved difficult to assess quantitatively whether coastal erosion is on the increase due to increased storminess in recent years. This analysis was complicated due to the lack of reliable survey information for this coastline. Although there have been a number of significant storm events in recent years (eg the "Braer" storm), it is too early to assess whether these events are part of any long term trend, or are having any significant effect on the long-term erosion of the coastline.
The impacts caused by past coastal defence schemes in the Western Isles are very minor, partly because the defences are few and far between, and partly because there is often little sediment transport along the coastline. As a consequence, adverse effects caused by interrupting longshore transport (eg by groynes), or by interfering with the supply of fresh beach material (eg by cliff stabilisation) are very rare. The major problems are related to extraction of beach material, eg for construction purposes, and damage to the fragile dune and machair areas by over-grazing or access by vehicles or large numbers of pedestrians.
In many parts of these islands grazing and trampling by cattle and sheep allows erosion of dunes to be initiated. Wind action then exacerbates these areas often leading to blowout features and large areas of deflation. In many cases future coastal management may only need to involve restricting or managing such practices in the immediate frontal dunes and from any areas where wind erosion is occurring.
Aggregate extraction from the foreshore is having a severe effect on the condition of the natural defences. The shingle storm ridges which protect the machair edge along much of the coastline, particularly in South Uist, are an integral part of the beach system. Once this largely relict shingle has been removed, the machair is exposed to wave attack with severe coastal retreat occurring. The report strongly recommends that no shingle be removed from these beaches. Mining of sand from the dunes and beaches can also have a serious effect on the stability of the coastal zone, allowing wind action to further erode the exposed area. The report recommends that any such quarrying should occur on the landward side of the dunes and should not occur within the frontal dune system.
Other human activities also have a localised detrimental effect. Pedestrian access to the beach areas down the frontal dune face and vehicle access through the frontal dunes can cause erosion of these features. Where access to the beach is required this should be concentrated where the machair or frontal dunes are lowest.
Only in a few locations it is necessary to consider "built" coastal defences, for example to protect roads, houses and cemeteries. In these circumstances, the type of defences used must be chosen bearing in mind the possible "knock-on" effects on adjacent, natural stretches of the coastline. The report recommends the use of Shoreline Management Plans to help develop a strategy for managing the coastline of the Western Isles, and a programme of regular monitoring to confirm or amend this strategy in the future. This approach should ensure that the diversity, the geomorphological processes, and the natural beauty of the Western Isles coastline, which have resulted in many of these beach areas being of considerable environmental and conservational importance, should be sustained and enhanced in the future.
