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. 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. Lewisian Gneiss dominates the geology of the landforms, although there are also granites in Harris and sandstones and conglomerates around the shores of Broad Bay. 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.
The coastal processes in the islands are dominated by the passage of atmospheric depressions approaching the islands from the Atlantic, with their associated strong winds. Gale force winds occur during about 50 days/year, and even on the relatively sheltered eastern shoreline, winds of Beaufort Scale 6 or more occur for some 21% of the year, and reach gale force for some 3.1% of the time. Such severe winds can occur from any direction, but at Stornoway are most common from the South to West .
These exceptional wind conditions can create coastal erosion problems directly, by enlarging weak patches on dune faces and machair, and creating "blow outs" and more general erosion (deflation). In addition, they have indirect effects both on water levels and by creating waves.
On most shorelines around the world, it is waves, generated by wind action, which dominate both the movements of beach sediments and damage to coastal structures. Locally generated waves, often called "wind-sea", are usually the most commonly occurring and most damaging waves around the coast of the UK. The character of wind-sea depends not only on the wind speed, but also on the length of time for which the wind blows, and the size of the area over which the wave are generated. The size of the generation area is normally characterised by a single length, in the direction of the wind, and known as the "fetch length" or simply "fetch".
On oceanic coastlines, it is common to find significant wave activity arriving at a beach despite calm or offshore winds for many days previously. Swell waves have been generated by wind action further out to sea, and are characterised by long, smooth crests, and very little variation in direction or in period . Generally swell waves are smaller than wind-sea, although their longer period means that they can run further up a beach (or seawall), and transport more beach material, than wind-sea of the same height.
The western seaboard of the Western Isles experiences both swell waves from the Atlantic Ocean, and wind-sea which is likely to be very large on occasions because of the vigorous wind climate. Swell waves will also occur on the north-east facing coast of Lewis, eg Broad Bay.
Evaluating wave conditions can be tackled in two ways, namely by direct observations and measurements or by numerical modelling. Data obtained by direct observation are available for the Western Isles, using measurements taken by non-directional waverider buoys. However, the limited information available on wave direction makes it difficult to use these data for use in coastal management, and can be used only to identify a suitable "general" offshore wave climate. The main value of the existing wave measurements off the Western Isles is probably as calibration data for numerical modelling, which is capable of accurate predictions of "wind-sea".
Wave energy can be substantially reduced by frictional effects between the water and the seabed. Measurements taken in South Uist demonstrate that wave energy is being dissipated by friction some distance offshore, and in relatively deep water. Swell waves, which are largely generated many hundreds of miles away from the Western Isles in the Atlantic, have long wave periods, and hence long wave-lengths. As a consequence, the waves begin to "feel" the seabed in much deeper water than in areas where only wind-sea occurs. The rocky and rugged nature and the comparatively gentle gradient of the seabed west of the islands provides a considerable amount of shelter to the sandy coastline. Even by the time waves have reached a depth of 25m, they have been substantially dissipated, and at depths between 25 and 15m this process is even more marked. The consequence of this is that the sandy coastlines of the western part of the Isles typically seem to be low energy environments, despite the extremely high wave energy levels offshore. Further north, however, the western coastline of Harris and Lewis is less well protected, as water depths increase more rapidly as one travels offshore.
Tidal currents are strong off the major headlands, for example reaching 4-5 knots close inshore around the Butt of Lewis, and can also be strong in narrow channels at the head of tidal inlets. More typically, however, they only reach 1-2 knots off other headlands, and the speeds are much less in the intervening bays, or further offshore. Such currents are probably of little consequence in transporting sediment and the shaping of the beaches. Tidal levels, however, can have an important influence on the effectiveness of friction and (offshore) wave breaking at reducing wave energy at the beach. It can be expected that wave activity on the beaches will be at a minimum at low tide. Conversely, at times of exceptional high tides, the shelter provided by the offshore seabed will be at its smallest, and wave action on the coast will be at its harshest. It is therefore likely that the greatest damage to beaches and coastal structures will occur during Spring tides, or more particularly when storm surges produce an extreme high tide. Since such conditions are likely to occur when deep atmospheric depressions (anti-cyclones) move toward the Western Isles from the Atlantic, they are almost bound to also coincide with strong wave activity. It is this alliance between extreme tide levels and large wave heights which will cause the erosive episodes on this coastline.
At the longest timescale, the gradual change of sea level relative to the land is often an important factor in the development of the coastline. In contrast to the north-west coast of mainland Scotland, where there is still a substantial isostatic recovery taking place (i.e. the landmass is still rebounding after the removal of the ice sheets during the last glaciation), the relative change in level of land and sea in the Outer Hebrides is much less obvious. It appears from the available evidence that there is a slow nett increase in sea-levels here of about 1.5mm/year. As a consequence of widely predicted global warming, this will increase to about 4-5mm/year over the next 50 years or so which is likely to produce an equivalent increase in the rate of rise of tidal levels relative to the land of the Western Isles. This in turn will increase coastal erosion and landward recession of beaches, particularly in areas where the amount of beach material is limited.
The botanical interest of the machair is sufficient importance that many such areas have been designated as Sites of Special Scientific Interest or Environmentally Sensitive Areas. The land use of the machair is also important in forming and sustaining the environmental characteristics of these areas. Small scale cultivation has occurred since Viking times and such cultivation is important to the stability and fertility of this land.
In addition there is 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. The winds, waves and tides shape these beaches, and the associated dunes and machair. Important causes of coastal erosion are likely to be 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 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. Recognising the value of healthy beaches, and the pressures they face, is an important first step in managing the coastal zone. Since much of the infrastructure and habitation of the islands is concentrated close to the coast, it can be appreciated that good management of the coastal zone is particularly important.
