Book: Sand Dunes Chapter: 2 Management Planning Section: Principles of Dune Stabilisation Metadata: Details Buy this book

Unstable dune surfaces are subject to erosion and provide a source of sand which may blow and smother adjacent areas. Stabilisation may be needed to:

• Prevent the loss or deterioration of valuable natural habitat
• Maintain dune ridges which act as coastal protection for low-lying hinterland
• Protect hackdunes development of agricultural or recreational dune grasslands
• Allow maintained or increased levels of public; use (but see 'General principles'  below)
• Allow the afforestation of backdunes and stabilised dune ridges

GENERAL PRINCIPLES

• Be sure that instability is occurring on a scale and with effects which are destructive enough to warrant stabilisation.
• Where instability is due to natural factors unaffected by public or grazing pressure, it is often impractical and even undesirable to correct it unless valuable hinterland is threatened.
• Measures designed to protect pioneer strandline and foredune communities differ from those aimed at restoring stable backdune or dune grassland conditions. The choice of stabilisation strategy also depends on whether the dune system as a whole is eroding, accreting or stable, and on local wind and water regimes, access problems etc. Examples are given under 'Strategies', below and in Access management.
• Dune restoration and stabilisation does little to increase the carrying capacity of the system. For this, you need to establish plants which resist trampling better than those which occur naturally. In most cases, erosion starts again if public or grazing pressure returns to the levels which prevailed prior to stabilisation. Along access routes, carrying capacity can be increased by suitably sited and designed trackways. See the chapter on Access Management.
• Fencing is usually necessary to keep people and grazing animals off the dunes while stabilisation is in progress. This focuses pressure on the dunes just outside the fence line. Choose boundaries to the work area where adjacent erosion will not cause problems, or be prepared to expand the area as fresh erosion occurs at the boundaries.
  >On heavily used sites, rotational management may be necessary. Areas can be fenced and restored or 'rested' while adjacent areas are subjected to increased pressure. Fences can then be shifted to re-open the restored areas and enclose the previously open areas before these erode too badly.
• Allow time for dune plants to establish themselves and to spread. Maintain protection for at least this period even where biotic pressures are low. Although turf may form within a few months of sowing and fertilising, it may take several years for the vegetation to reach optimum density. Experience at Oxwich National Nature Reserve, Gower, suggests that revegetated dunes achieve their greatest diversity only after at least a decade of protection.
• Review dune restoration programmes each autumn, where seasonal public pressure is heavy, so that you can plan autumn and winter maintenance according to the situation at hand, within an overall long-term strategy.

METHODS

There are three ways of stabilising dunes: trapping sand, preventing sand blow and developing erosion-resistant landforms. These methods are rarely mutually exclusive. Usually, two or even all three must be used, concurrently or consecutively, in a complete stabilisation programme. In particular, it is seldom worth trying to create erosion-resistant landforms unless the other methods are also applied.

• Sand may be trapped by the use of solid or porous barriers or by vegetation. Solid barriers seldom work well against wind-blown sand because they often cause scour.>
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  >They may however be useful in preventing wave erosion or in trapping water-borne sediments. Porous barriers (such as fences, see Dune profiling and stabilisation) and vegetation (see Vegetation establishment) allow the wind to pass through but slow it so that it drops some of its load of sand.>

• Start trapping or fixing wind-blown sand as near as possible to the source, ie at the strandline or the windward edge of a blowout, and work downwind. Otherwise, newly planted areas may be buried in drifting sand from untreated areas upwind. This sequence may vary in detail when you build up a new dune ridge (see Dune profiling and stabilisation) or where you need to rebuild a large blowout and do not want to cut off the sand supply to the back of the blowout until it is well filled with sand.
• Leave natural and relatively permanent slacks unfenced and unplanted. These wind-scour-areas are unlikely either to supply much sand or to receive significant buildup.
• Choose sand-trapping measures according to the amount of blown sand. If this is relatively small, plant or sow sand-trapping grasses (or in some cases shrubs) and encourage them as necessary with fertilisers (see Vegetation establishment) and protective fencing. If more than about 1m (3') of sand comes into the system each year, all vegetation is likely to be smothered and fences are required instead. Fencing and thatching (see Vegetation establishment) are also useful to supplement planting. Where there is insufficient blown sand to support sand-trapping grasses, the best approach is to sow meadow grasses (see Vegetation establishment) and to fertilise and bind the sand surface (see Vegetation establishment) as necessary.
• Normally the best way to stabilise an area of sand blow is to plant marram (Ammophila arenaria) or one of the other sand-trapping grasses. Planting alone may be insufficient and may cause problems of secondary erosion unless combined with contouring. This is especially true of marram dunes which tend to develop a steep windward face. Seedlings or offsets may fail to survive unless protected by fencing. It is also important to try and plant marram on a broad front, if possible covering the whole area in one season. Piecemeal work results in uneven dune growth which gives rise to new, unforeseen erosion patterns.
• Sand-trapping plants, especially the grasses of open foredune communities, do little to fix the sand surface. Fixation is, in general, only possible on secondary dune ridges or in relatively stable systems where there is enough shelter for other plants to seed in or to be introduced artificially. You can encourage new or existing stands of vegetation by fertilising, mulching and binding. But this may not be justifiable in nature conservation areas since it increases soil productivity and may allow uncharacteristic species to invade. Wherever possible, use seed mixtures containing only native species which are adaptable to the site.
• Steep sand faces, particularly on the windward sides of dunes, are prime erosion sites. You can make these more erosion resistant by thatching and planting or by contouring them to a gradually sloping, aerodynamically stable profile (see Dune profling and stabilisation).
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  Combine contouring with other measures, since it does nothing in itself to trap sand or fix the surface. Usually planting and fencing are both required in addition to contouring.

• Surface roughness increases wind turbulence and erosion. Make dune profiles as smooth as possible. Blowouts, gullies, notches and holes cause the wind to funnel and concentrate its scouring action. Fill these in by contouring, planting, fencing and thatching.
• When stabilising a slope, start at the top and proceed downhill. Finish treating the section from top to bottom as soon as possible - within a few weeks at most. Otherwise the bottom edge will erode and threaten the work which has been done above.

STRATEGIES

Eroding systems

Eroding systems may be in a state of overall dynamic equilibrium (see A look at sand dunes) or they may be suffering from the lessening of sand supplies which in earlier times balanced the losses caused by natural erosion. Most often, though, they receive enough sand to make up for natural erosion but not enough for the extra erosion caused by increased biotic pressure. Strategies for eroding systems apply also to blowouts within otherwise accreting or stable systems.

At Ainsdale NNR, Merseyside, trampling along a popular footpath had created a blowout through the foredunes, with the sand being blown through onto an area of slack beyond. The dunes are eroding quite severely at this point, so it was felt that action had to be taken. The gully through the foredunes was fenced with a series of parallel sand trap fences, of either chestnut paling or 'Enkamat' (see Dune profiling and stablisation), leaving a fenced footpath through to the beach. This followed a dog-leg route, to prevent creating a wind tunnel. As the fences fill with sand, more will be built on top, until the former gully has reached a sufficient height and stability for replanting with marram.

At the same time, the most severely eroding foredune face was 'thatched' with a synthetic material (see Vegetation establishment), through which marram was planted. .A major fencing programme was carried out to fence all along the front of the foredunes, so forcing people to use the designated route, which joins to a boardwalk on the landward edge of the foredune.

A similar but more serious situation at Hayle, Cornwall was tackled by first using a bulldozer to contour the eroded area, followed by access control and sand trap fencing and then replanting of the entire area with marram. This is described in Dune profiling and stabilisation.

Accreting systems

In accreting or prograding dune systems, the supply of fresh sand is greater than that of the sand lost, at least under natural conditions. Normally this sort of system needs no stabilisation unless public pressure is such that embryo dunes are literally trampled flat and the vegetation of existing foredunes is largely killed off. Even then there is little danger to coastal defences and the major effect, aside from loss of dune habitat, is increased sand-blow inland.

Camber, East Sussex, is an example of a system where the quantities of sand liberated by passing feet can be considerable. Dune stabilisation has been required mainly to reduce damage to poorly sited roads and housing just back of the dunes. Pizzey (1975) describes the piecemeal methods which were tried without success from 1945 until 1967 as well as the coordinated approach which eventually brought the problem under control. This began with contouring the dunes, which had become oversteepened at their inland side where they had shifted back towards the road.

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Because the problem was one of too much sand, excess material could safely be removed from the inland edge of the dunes and was taken away for use in building the Dungeness Power Station. The dune ridge was then reshaped to the more stable profile shown above.

Plants have trouble seeding into loose sand, so at Camber it was essential to stabilise the graded surface by hydraulically seeding and mulching the bare areas. This was accompanied by a bitumen spray to bind the straw mulch and sand in place and to improve germination of the grasses. The straw was disk harrowed to partly bury it and improve its stabilising effect. Fences were then erected around the entire work site, including the sides of such paths as were needed to channel people to and from the beach. The fences at the seaward side were positioned along the strandline to help form new embryo dunes and prevent smothering of the grassy slopes behind. Further steps are to plant marram grass and native shrub species to develop a more natural vegetation. Marram on its own would be inadequate because the need is to fix the sand surface rather than simply trap more sand.

Stable systems

In stable dune systems, little sand goes into or is lost from the system. Many east-coast dunes, such as those in East Lothian and Lincolnshire, have a very slow accretion rate and can be considered more or less stable.

Gibraltar Point, Lincolnshire, shows the interesting situation of a system which was until recently gradually accreting but which has had its sand supply virtually cut off by foreshore salt marsh development. A new dune ridge may eventually arise to seaward of the marsh, causing it to become brackish or fresh, but in any case the present dune system is essentially static.

Wilcoek and Carter (1975 and undated) emphasize that with the stable system at Portrush, County Antrim, Northern Ireland, 'traditional' restoration methods of planting and fencing could not succeed alone because the sand supply was inadequate and the eroded slopes too steep. Nor was it necessarily desirable to initiate new dune formation had this been possible. Contouring provided the key. Regraded, smoothly sloping dune faces formed a much better rooting medium for marrarri, at least when supplied with additional nutrients and mulch. Marram may gradually decline on this site but it only needs to last long enough here to allow other plants to colonise and fix the surface. More difficult than the contouring and planting, but equally essential, was the erection of relatively vandal-proof fences to keep people out of the restored area.

Dune grasslands, especially in their extreme form of machair plains, illustrate flat, virtually unbroken sand systems which can be considered stable in the short run even though they may go through cyclical erosion and deposition over the course of decades or centuries (see Vegetation management). Machair is easily disturbed by overstocking of grazing animals or poorly timed ploughing, or by caravan and car tracks which cut through the turf and create wind funnels, especially where the vehicles park and turn. Unlike dunes, machairs often end at a rather steep cliff at the beach edge. This area is particularly sensitive to erosion where people clamber down, creating gullies which then eat farther into the plain behind.

Seaton (1968) describes various attempts at restoring a large-scale blowout on machair at Bornish, South Uist, caused primarily by agricultural mismanagement. The most successful measures involved beginning restoration at the point of scour and increasing the fertility and stability of the soil through heavy applications of organic mulches. Mulching with dung also introduced many quick-growing weeds which helped fix the sand surface. Other measures, essential to prevent a recurrence of the blowout, included restricting the numbers of sheep and controlling the rabbit population.

At Achmelvich, Sutherland, and on the Isle of Mull, machair-edge restoration became necessary where public pressure increased without adequate restrictions on parking or the provision of suitable pathways to the beach. Stabilisation has focussed on the cliff edge itself. This area has been fenced with post and wire at the back, against people and grazing animals, and with post and rail plus brushwood along the seaward edge to encourage sand buildup and protect against wave erosion. After fencing, the face is graded and returfed (see Vegetation establishment), seeded and sprayed with fertilisers and binders to encourage a variety of grasses. Large gullies are treated as continuations of the machair edge and are 'toughened' in the same way, while smaller gullies are filled with sand and brushwood prior to seeding.

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