Information and Advsiory Note Number 72, February 1997
1.1 This Information and Advisory Note examines the causes and consequences of coastal erosion from a conservation perspective, the relevance of coastal cells to the management of erosion and the content and purpose of Shoreline Management Plans. It is one of three Notes covering the broad area of coastal erosion and defence, the others being: No. 36 Coastal erosion and defence. I. Scottish Natural Heritage and the Coast Protection Act, which highlights recent changes in coastal defence legislation pertinent to SNH, and No. 73 III. Coastal defences and the natural heritage, which addresses the relative environmental impacts and benefits of various forms of coast protection.
2.1 Coastal erosion affects most coastlines in Scotland at rates ranging from <1mm per year on hard igneous cliffs to 4m per year or more on certain dune coastlines. In Scotland it is generally only on non-rocky coastlines, where the coastal edge is formed of mud, sand, shingle or glacial deposits, that erosion progresses sufficiently rapidly to be of concern to landowners and managers.
2.2 There is rarely any single, identifiable cause of coastal erosion. In Scotland, any of at least 15 different factors may contribute to beach erosion at a particular location (Table 1). In general, only detailed study or prolonged monitoring of a beach and other environmental parameters will allow the principal causes to be discerned. This is, however, important if an appropriate response to a perceived erosion problem is to be made. Because of the number of factors which may influence sea level and sediment supply and the unpredictability of weather patterns, it can, nonetheless, be very difficult to predict the future trends in evolution of a particular beach or coastline.
2.3 Many soft coastlines of mud, sand or shingle undergo natural cycles of erosion and deposition or ‘accretion’ linked to variations in sea level, sediment supply and wind and wave climate. These may last from a few years to many thousands of years, depending on the cause. Sandy coastlines may even exhibit apparently seasonal cycles of advance and retreat related to sand accumulation under fair-weather conditions and loss under stormy conditions.
2.4 Erosion in Scotland occurs primarily through wave attack during periods of exceptionally high tides. Other processes can, however, also loosen material and contribute to sediment loss, such as mass movement, wind action, rainfall, freeze-thaw processes, and the shifting of stream and river exits through dunes, beaches, or other coastal landforms. Wind action may affect sandy coastlines, in particular, by removing sand from areas of dune or links/machair from which stabilising vegetation has previously been lost, for instance as a consequence of wave attack or grazing/trampling pressures.
2.5 Many non-natural activities may also cause localised erosion, or remove stabilising vegetation, thereby rendering the underlying sediment more susceptible to wind scour. These include sand and gravel extraction, over-grazing, recreation pressures such as trampling and vehicle use and, ironically, inappropriate coast protection. All these issues are to be addressed in other Notes in this series, currently in preparation.
The sediment generated by erosion may accumulate in the immediate vicinity or it may be transported elsewhere. Muddy sediments and some sands may, for instance, be carried off-shore under storm conditions and sand may be blown inland by strong on-shore winds, removing such material from the beach environment. On the other hand, sand and gravel especially are often moved alongshore by wave activity (longshore- or littoral drift) to other beaches along the coast. As beaches dissipate wave energy, they reduce the severity of coastal erosion by wave attack and so perform a vital coastal defence function, in addition to their more obvious amenity and conservation roles. Erosion at one location may, therefore, be vital not just for the ‘health’ of a neighbouring beach, but also for the natural defence of that part of the coastline.
2.6 Apart from the generation of sediment for beach building, some coastal erosion is also essential for maintaining the diversity of coastal habitats (dune grass growth is, for instance, largely dependent upon the continued supply of wind blown sand) and, of course, contributes greatly to the formation of many of Scotland's most spectacular coastal landscapes and landforms, such as the Old Man of Hoy. Thus, although the general perception of coastal erosion is that it is undesirable, from a conservation perspective, erosion should be regarded as a natural process, fundamental to the natural functioning and evolution of the coastline. Except where erosion has been introduced artificially to a system (e.g. by grazing or trampling pressures) the aim of conservation management should be, therefore, not to combat it but to maintain the natural processes.
3.1 Although coastal erosion is not necessarily undesirable from a conservation perspective, many areas of soft, potentially erodable coastline in Scotland are built upon or support some other form of land use or development which may, potentially, be lost or damaged by coastal erosion. Typical examples include golf courses, caravan parks, MoD establishments, roads and, more locally, housing and built leisure and recreation facilities such as promenades, slipways and footpaths. Historical and archaeological monuments are especially common in many dune and links or machair areas of the Western and Northern Isles. All these interests may lead to proposals for coast protection. Many coastal areas are also farmed or forested, though it is rare for coast protection to be cost effective in such situations.
3.2 Numerous different approaches to coast protection are utilised throughout Scotland, ranging from hard defences such as sea walls and rock armour revetments, which resist natural processes, to soft defences such as sand fencing, dune grass planting and beach renourishment, which utilise or work in sympathy with natural processes. These and their potential environmental impacts and benefits are discussed in detail in I & A Note No. 73. In many cases, and particularly with respect to hard defences, such coast protection works are more damaging to the conservation interest of a site than the erosion itself. This is because such defences obscure landforms and habitats, sterilise their interest and stabilise naturally dynamic systems. Hard defences also have a tendency to transfer the erosion due to waves to the nearest unprotected coastline downdrift of the defences, increasing land loss there (Figure 1). If inappropriately designed, groyne fields may trap so much sand and gravel that the beaches downdrift are starved of their former sediment source and so erosion is precipitated or exaggerated there too (Figure 2). Indeed, any form of defence which retards or prevents sediment loss from a previously eroding area reduces the sediment supply to the beach and, thereby, contributes to increased erosion elsewhere. This may lead not only to increased loss of valuable coastal habitat but may also result in proposals for further coastal defences at the site newly affected, particularly if that land is developed.
3.3 Erosion, therefore, not obviously attributable to anthropogenic influences does not constitute a damaging impact in itself, from a conseniation perspective, except in the rare circumstances of a unique habitat or feature, being permanently lost. Even then, or particularly if that habitat or feature is coastal in character, it may sometimes be argued that its loss is simply a consequence of natural coastal evolution and should not be interfered with.
4.1 In assessing the potential environmental impacts of a proposed coast protection scheme, the direct effects of the works, such as loss of habitat or landform, may be obvious. However, the indirect effects caused by interference of defences with natural processes of water and sediment movement may be less apparent, yet just as significant. It is important that such effects are predicted at the planning stage wherever possible. This may be done by computer modelling but as this is expensive, modelling is typically only conducted in Scotland for major schemes. Although SNH may request proponents of a planned coast protection scheme to construct such a model, to help ensure minimal damage to the natural heritage, there is no requirement to do so unless the planning authority makes it a condition of the planning application, or unless the Scottish Office or the Coast Protection Authority (generally the local authority on whose land the works are to be built) insists upon one being constructed before considering the proposal. Alternatively, where a planning department considers that the defences are likely to cause significant or SAC (including proposed SPAs and SACs), they may require the proponents of the scheme to prepare an environmental assessment of the works’ impact, under the Environmental Assessment (Scotland) Regulations 1988 (as amended), through which the EC 1985 Directive on Environmental Impact Assessment is implemented. Such an assessment would, amongst other things, be expected to consider the effects of a scheme upon natural processes. These regulations and the legislation surrounding coastal defence in Scotland are discussed in greater detail in Note No. 36.
4.2 In practice, few coastal defence schemes in Scotland have, to date, been subject to a formal EIA. Consequently, the effects of existing and proposed coast protection environmental effects to a ‘sensitive location’, such as SSSI, NSA, SPA schemes upon the stability and evolution of adjacent unprotected coasts is not known.
4.3 By studying patterns of longshore drift (i.e. the movement of beach sediment) around coastlines, it is apparent that there are points or features around or through which beach forming materials such as sand and gravel are not transported. Examples include prominent headlands, such as Fife Ness, and major river estuaries, such as the Firth of Forth, but may also include artificial constructions such as harbour breakwaters aligned perpendicular to the coastline. Sediment may move freely between these features but its movement is generally confined by them. An example is shown in Figure 3. These intervening stretches of coastline are known as coastal cells (also called littoral cells, sediment cells and process cells). Often a feature within a coastal cell, such as a headland, may block movement of beach material under most conditions but will allow occasional transport under exceptional circumstances (e.g. prolonged gales from a particular direction). Such features may be used therefore to demarcate stretches of coastline which are nearly, but not entirely, self contained, in terms of beach sediment movement. These are termed sub-cells and form divisions of the major coastal cells. Cell and sub-cell boundaries may also occur on open coastlines, away from any obvious topographic feature, but such boundaries are liable to shift along the coast under different weather conditions and so are difficult to map with any precision.
4.4 By identifying the principal cell and sub-cell boundaries, a coastline may therefore be sub-divided into a series of self-contained, more or less independent units in terms of beach sediment movement. Any development or activity which affects the supply or movement of beach sediment, such as construction of coastal defences or extraction of sand, may influence the supply of sediment elsewhere in the same sub-cell or, to a lesser extent, cell but will have no effect upon beaches in other cells.
4.5 Identification of coastal cells has, therefore, considerable significance for the management of coastal erosion since it enables areas of coastline likely to be affected by inappropriate coastal defence works elsewhere to be predicted at the planning stage and the plans modified accordingly. In practical terms this should reduce the likelihood of one local authority or landowner causing or exacerbating erosion of another's land.
4.6 Figure 4 illustrates this. In 4(a) the “natural” coastline, without defences, is shown, sub-divided into its constituent coastal cells. Should town A, in cell 1, decide to erect a series of groynes along its beach front (4(b)), this will have no effect upon the beaches of the neighbouring cell 2 but will severely deplete sediment supply to the beaches fronting towns B and C which are also in cell 1. Were such works to proceed towns B and C might thus be forced to construct their own defences where none were previously necessary. A further example is given by cell 2 which crosses the boundary of two separate councils, X and Y. Should either, for instance, permit sand extraction from the beach, this will deplete sand availability within the whole cell, not just the area of jurisdiction of one. When managing such developments and activities, therefore, there is a clear benefit in founding plans upon these natural divisions of the coastline rather than upon administrative boundaries, in order that the risk of potentially damaging effects and unnecessary expense is reduced.
4.7 Recent research, funded by SNH, SOAEFD and Historic Scotland has identified 7 principal cells around the Scottish mainland, divisible into 24 sub-cells (Figure 5). Cells and sub-cells have also been distinguished for the Western and Northern Isles. On these, and other deeply indented coastlines, such as along much of the west coast of mainland Scotland, each major bay or sea loch constitutes a distinct cell in terms of sediment movement (i.e. sediment supply and transport along the shores of each embayment are unrelated to sediment movement along shores in neighbouring bays or lochs). Because, ultimately, the purpose of identifying coastal cells and sub-cells is as a management tool, these are not all distinguished on these maps but are grouped together into “cells” and “sub-cells” defined according to other, additional, factors pertinent to shoreline management such as exposure and physical character. The “cells” in such areas may, as elsewhere, be considered as possible planning zones for the management of coastal erosion.
4.8 Preliminary maps indicating principal directions of long-shore drift have been drawn up for each cell (Figure 6). More detailed research is currently underway, the results of which should be available by early 1997 in the form of a separate report for each cell. These should assist SNH staff in predicting the location of potentially damaging effects which might arise from proposed coast protection schemes, as well as providing basic information about the natural evolution of each area's coastline.
The benefits of distinguishing coastal cells can be realised most fully by preparing, for each cell or sub-cell, a Shoreline Management Plan (SMP). A SMP is “a document which sets out a strategy for coastal defence of a specified length of coast, taking account of natural coastal processes and human and other environmental influences and needs” (MAFF, 1995).
In England and Wales MAFF, which has Departmental responsibility for coastal defence, has issued guidance recently encouraging coastal defence authorities (essentially the local authorities with coastlines) to prepare a SMP for each sub-cell in their jurisdiction. Where sub-cells traverse council boundaries, the authorities concerned must agree a joint SMP to ensure a consistent approach across the sub-cell. Should other major landowners, or organisations with coastal defence responsibilities such as road, rail or harbour authorities exist within the sub-cell concerned, then these too should also be involved in funding and/or preparing the SMP. Grant aid for future coastal defence work will only be available from MAFF where a SMP, approved by them, is in place.
5.1 The aim of a SMP is to “provide the basis for sustainable coastal defence policies within a sediment cell and to set objectives for the future management of the shoreline”. (MAFF,1995) Specific objectives of a SMP are summarised in Table 2.
5.2 Four key issues need to be addressed in the preparation of a SMP - these are:
5.3 From a natural heritage perspective such plans have considerable importance since any coastal defence options proposed within the plan must:
5.4 Consequently, although SMP5 have no statutory basis, where such a plan is in place, the risk of having poorly designed or ad hoc defences which are damaging to the natural environment should be significantly reduced. They may also inform Local Plans and help guide coastal development away from shorelines prone to erosion.
5.5 In Scotland, the policy of the Scottish Office is to reduce the risk to life and the natural and built environment from flooding and erosion or encroachment by the sea. This is achieved by:
The Scottish Office does not require preparation of SMPs in Scotland at present, although their potential benefits are to be highlighted in the forthcoming NPPG 12 Coastal Planning.
5.6 Because of their potential benefits in economic as well as environmental terms, Highland Council has funded preparation of a SMP for an area of the Inner Moray Firth, supported by SNH. In addition, that part of the Scottish coastline east of St Abbs in Berwickshire is encompassed within the SMP being prepared for the Northumberland coast, as the northernmost sub-cell on the east coast of England extends between St Abbs Head and the Tyne in Northumberland.
5.7 SNH is currently assessing the applicability of Shoreline Management Planning to developed areas of the Scottish coastline subject to erosion, such as along parts of the east coast. Should the results prove positive, it is hoped to co-ordinate the preparation of a SMP for a cell or sub-cell in one such area, possibly as a pilot project to demonstrate the relevance of Shoreline Management Planning elsewhere in Scotland.
Bird, E.C.F., 1993. Submerging Coasts. The Effects of a Rising Sea Level on Coastal Environments. John Wiley and Sons, Chichester.
H R Wallingford, 1993. Coastal Management: Mapping of Littoral Cells. Report to MAFF. H R Wallingford Report SR328.
H RWallingford, 1995. Coastal Cells in Scotland. Report to SNH, Scottish Office Agriculture Environment and Fisheries Department and Historic Scotland. H R Wallingford Report EX 3176.
H RWallingford, 1996. Shoreline Management Plan. Inverness Firth and part of Moray Firth (Burghead to Sutors). Report to Highland Regional Council. H R Wallingford Report EX 3230.
MAFF, 1993. Coastal Defence and the Environment. Ministry of Agriculture, Fisheries and Food. Report 1993 PB 1191.
MAFF, 1995. Shoreline Management Plans. A Guide For Coastal Defence Authorities. Ministry of Agriculture, Fisheries and Food Report 1995 PB 2197.
An overview report of the Coastal Cells Of Scotland should be available for each SNH Area Office with coastal responsibilities by spring 1997. A Regional report covering each of the 11 cells identified should be available by autumn 1997.