Fencing

General points

Uses

Fencing may be used for a number of purposes directly or indirectly concerned with dune stabilisation, including:

1. Trapping sand at the eroding windward faces of dunes to help maintain their positions and stop blowing sand smothering planted vegetation
2. Creating new embryo dunes in front of existing foredune ridges. In eroding systems the new dunes protect the existing foredunes, and in accreting systems the new dunes reduce sand supplies to the foredunes. In eroding systems the gain may only be seasonal, with sand accumulating in the summer, and eroding in the winter and early spring. In this case fencing is required as a permanent fixture.
3. Reducing the scouring effect of wind in blowouts and at the same time trapping sand to help fill them in
4. Indicating dune restoration areas, alone or with other fencing and signposting, in order to reduce access
5. Closing off and helping fill in access routes and channelling people onto boardwalks or other non-erodable surfaces
6. Providing sheltered bases or ‘nests’ for people in areas less easily eroded than the dunes themselves, eg in front of or between dune ridges

Fences which are designed or sited primarily for the last three reasons are discussed further in the next chapter. Even fences which are designed primarily to trap sand affect public access and can be sited in part to restrict use of the dunes.

Other considerations

1. Fencing is the best way to trap sand where so much sand accumulates that marram grass and other plants cannot survive on their own. Such areas are unusual, however, and it may be possible even here to alter the pattern of deposition so that more of the site becomes suitable for planting.
2. Wherever possible, coordinate fencing with planting so that when the fence becomes buried or is removed for reuse the vegetation remains to continue trapping sand. Fences should remain in place until the plants have a chance to become established and can cope with the amount of sand blow.
3. Often, fencing and planting can be carried out at the same time. Where fencing is likely to cause an immediate rapid buildup of sand, wait until this has occurred before planting in the accretion zone. In the meantime, use a mulch to help stabilise the sand surface.
4. Planting is most successful downwind of fences, but you can also plant a strip just to windward after the fences are at least half buried. This helps broaden out the resulting dune profile.
5. Fencing is relatively expensive and laborious. You can reduce costs by using local materials, such as brushwood, or by retrieving fences for re-use before they become buried. Retrieval requires close monitoring of the sand buildup, is usually very time consuming and difficult and almost always disrupts the sand surface. For these reasons it is best, if possible, to use cheap, disposable and biodegradable designs and let them become buried so that some material remains in the sand to help bind it.
6. Fences seldom keep their alignment and may be badly damaged by storms, but they will continue to function as long as they remain exposed and at a steep angle to the wind.
7. The object of sand-trap fencing (with the exception of ‘piggy-bank’ fencing) is to become quickly and permanently buried so that no trace remains. The job should not be considered completed until the resulting sand mound is planted and stabilised. Fencing that is not working should be removed.
8. The success of sand-trapping fences depends on such factors as the supply of blown sand, wind speeds and fence placement and design (see below). In suitable locations, fences on coastal sites in Britain may accumulate 0.9-1.8m (3-6′) of sand in a year.

Design and placement

Porosity

Optimum porosity (the proportion of holes to solid material) for a sand-trapping fence, at right angles to the wind, is between 30% and 50%. At lower porosities there may be a reverse flow of sand at low wind speeds and scouring at high wind speeds. At higher porosities the wind is not slowed enough to drop the maximum load of sand. The optimum porosity for shelter fences is about 35%-40% so this is the range to aim at when erecting fences which serve both purposes (Phillips, 1975). The porosity of the material used for fencing affects both the amount of sand trapped and the shape of the ‘dune’.

Sand build-up and scour

1. The effect of a porous fence on wind speeds determines the way in which sand is trapped. If the fence is at right angles to the wind, the greatest reduction in wind speed occurs in the area from five to ten times the height of the fence (5h-10h) downwind. Here the wind is reduced to about 25% of its free speed. There is also a smaller, but still effective, slowing of wind on the windward side of the barrier. Along a narrow strip just to both sides of the fence the wind is at full speed, causing a notch effect at the fence (Searle, 1972). Because winds are frequently at angles other than 90 degrees to the fence, this effect is minimised and most sand tends to accumulate about 1.2m (4′) behind the fence in the general pattern shown here.
2. Around the ends of the fence, turbulence produces wind speeds of about 110% of free speed. This can cause scour unless the fence ends are positioned in areas which are already as stable as possible or brushwood is used as shown below, to finish the fence.
   
3. The velocity of wind required to transport sand is given here. It is the bottom 600-900mm (2-3′) of fence which traps most sand since even at high wind speeds the sand remains quite near the surface. While a strong wind moves much more sand than a weak one, the ratio of sand trapped by the fence to sand in motion is greater at low than at high wind speeds. Even so, more sand actually accumulates when wind speeds are 25 mph-30mph (40km/h-48km/h) than at lower speeds, especially where two rows of fences are used in parallel series (Phillips, 1975, p12).
4. The angle of slope of the sand which accumulates around the fence, and the depth of any notch at the fence line, are determined by the wind speed rather than by fence design or positioning. Provided the wind speed stays constant for a long time, the profile reaches equilibrium with sand on the lee side building up about as high as the fence and with the lee slope being very shallow.

Positioning

1. In general you should orientate fences at right angles to the dominant winds (winds of greatest force) for maximum sand buildup, provided these winds occur fairly often.
   The best sand-carrying winds are strong and dry. In Britain this means winds from the north or northeast provided the site is suitably exposed. Strong winds from some other quarters may transport surprisingly little sand if they occur with heavy rains and high wave levels.
   The easiest way to determine fence alignment is to observe how the sand moves on a really windy day and to position the fences about 1.2m (4′) in front of where vou want the most sand to build up. Since fences are often moved or replaced every year or so, it is usually easy enough to adjust the layout in accordance with the observed pattern of sand buildup.
2. Damp depressions on the beach cut the amount of loose sand which can be blown by the wind. Position fences some way back from creeks, layers of shells and other moisture-retaining areas.
3. Ideally, you should site fences along or in the lee of areas of fine sand, not coarse sand or ‘beach gravel’ which is seldom set in motion bv the wind. Unfortunately, such areas are often within reach of high storm tides so some compromise may be necessary.
4. Position fences no lower than 1m (3′) vertical distance above the mean high water level. Even at this level wave damage is possible and it is usually best to keep fences 2m (6′) above MHW.
   When in doubt about water levels, or where there is rather little room between existing dunes and the sea, place the seaward fences at the strandline or where dune-building grasses are starting to colonise the beach. Where other indicators are absent, set the fences no more than 7-9m (24-30′) in front of existing dunes. You can extend the fences seaward later if necessary, but it is generally best to leave room for embryo dunes to develop to help protect the main dune ridge.
   Where it is essential to protect a foredune ridge which is being eroded by storm tides, try one of the systems of fencing described on page 00.
5. Where you want to develop a dune ridge, it is best to erect fences in straight parallel rows about four times the fence height (4h) apart. With fences of normal height (1m) this results in a 4m spacing. On open flat areas, use a spacing of six times the height, and on leeward slopes use a wider spacing of eight to ten times the height (CCS, 1982). Transverse fences may be needed, as detailed below. The greater the number of rows up to a limit of four, the greater their sand trapping efficiency and the less the amount of scour and size of the notches at the fencelines.
6. In normal conditions, straight fences work better than either spur or zig-zag patterns except at very low wind speeds (Phillips, 1975, p10). However, spurs at right angles to the main fences improve performance where the backshore is narrow and sand supplies are low or where occasional strong winds blow parallel to the main fences and scour between them.
   The Countryside Commission for Scotland (CCS, 1982) recommend that spurs or transverse fences should be spaced half as much again as the main fences, as shown below.
   
   Hewett (1973, p57) recommends using a spur or zig-zag design wherever dunes have formed along the line of the prevailing wind, rather than at right angles to it. When in doubt, start with simple straight fences and observe their performance before modifying the design.
7. Dune ridges can be built up most easily by a programme of sequential fencing carried out over a period of three to five years or so. Of the various ways to do this (eg Phillips, 1975, figure 10), the method recommended by Adriani and Terwindt (1974, p52) is simple and effective. They suggest using low fences (about 1m, 3′ high) at first to achieve width, followed by higher fences (about 2m, 6′) to develop height. The diagram shows how to do this.
   
8. Fences on gradual slopes trap more sand than fences on the flat. But on steep slopes, fences shelter only a small area in relation to their height so that sand buildup is highly localised and erosion is likely to continue between rows. To prevent this, contour very steep dunes before fencing them. Where this is not possible, you can place fences close together in series to reduce scour, but where sand supplies are relatively meagre only the first fenceline is likely to trap much sand (Wilcock and Carter, undated, p3). In this situation, fences – especially those near the dunecrest – are likely to be undermined by scouring on the windward side.

Blowouts, corridors and bowls

1. When fencing blowouts and other gaps in dune ridges, first concentrate on trapping sand at the downwind end. If you first fence the mouth, or upwind end, you will cut off the sand supply to the rest of the area. Extend the fences as far up the slopes as possible, since a great deal of sand moves along the steep sides of blowouts.
2. When filling a corridor, space the fences as shown (CCS, 1982). There is no need to build transverse fences as winds are funnelled through the gap. Make sure that the fence ends are extended into the sides of the dunes, so that scour does not occur.
   
3. In eroding bowls, wind patterns are complex, and a box system of fencing is the most satisfactory, so that sand is trapped whichever way the winds blow.
   

Contoured areas

Where large areas of dune have been bulldozed, sand trap fencing may be needed in addition to marram planting, to continue the process of dune building. In North Cornwall, the basic plan is a grid of fences, creating boxes about 25m square. It has been found that wind direction and the amount of sand blow is difficult to predict, especially on sites where the local topography of cliffs, headlands and valleys complicates the wind pattern. These fences are economical to build in terms of straining posts, as each one can be used in more than one direction. The grid makes an obvious and formidable obstacle which discourages people going into the area, and divides the area up to ease the rather monotonous job of marram planting. Sand is first trapped in the corners of each box, gradually building up until the whole level has been raised. More boxes are then built as necessary. In this case the fences are of strained wire and ‘strawberry netting’, but other types of material are also suitable. No attempt is made to retrieve fence material, as this is too time consuming and disturbs the accumulated sand. However, once sand has stopped building up, any fence wire or netting showing at the surface is removed and disposed of to keep the site tidy.

Access routes are usually needed through restored areas. The plan below shows the layout of fencing used at Hayle, near St Ives in Cornwall. This is a heavily used access point from a large caravan site to the beach, which started as a footpath, and eroded to a 100ft deep gully. A seven acre area was contoured by bulldozer, and then fencing was carried out.

Where an area is at an acceptable height and profile after bulldozing, and wind blow is unlikely to be a problem, sand trap fencing is not necessary. However, access control fencing will be needed to keep the public out of the newly planted areas.