Summary 4: DUNE FENCING

Appropriate locations Above normal limit of wave run-up at any location with available blown sand. Unlikely to succeed where erosion is severe.
Cost Low, but requires on-going maintenance. (£400 - £2000/100m frontage length, plus cost of transplanting and on-going repairs)
Effectiveness Enhancement to natural dune recovery. Limited resistance to storm erosion. Enhanced by vegetation transplanting.
Benefits Minimal impact on natural system. Can be used to control public access and to improve other systems.
Problems Damaged fences and accumulated debris can be unsightly. Fences need regular maintenance and have a maximum life of about 5 years depending on material, frequency of storm wave damage and vandalism.

General description

Construction of semi-permeable fences along the seaward face of dunes will encourage the deposition of wind blown sand, reduce trampling and protect existing or transplanted vegetation. A variety of fencing materials can be used successfully to enhance natural recovery. Fencing can also be used in conjunction with other management schemes to encourage dune stabilisation and reduce environmental impacts.

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Dune fencing along upper beach.

Function

Sand fences cannot prevent erosion where wave attack is both frequent and damaging, but they will encourage foredune growth and resist some erosion. Fences reduce wind speed across the sand surface and encourage foredune deposition. They also act as a modest barrier to wave attack, reducing the erosion potential of waves near the limit of uprush.

Success depends on the void to solid ratio of the fence, the availability of blown sand, the frequency of wave attack at the fence and the amount of vegetation available to stabilise the accumulated sand. Success will be enhanced by a programme of dune grass transplanting (Summary 2), thatching (Summary 3) and beach recycling/regrading (Summary 5) to establish new foredunes.

Fencing and associated works can be used to enhance the appearance and effectiveness of other erosion defences. Rock, timber or gabion structures can provide a fixed line of defence, but are incongruous along a natural dune coast: partial burial by recycled or sand accreted by fencing and grasses will create a more natural dune environment.

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Fencing used to stabilise an eroded dune face

Methods

Dune fencing to control wind erosion and encourage dune stability has been undertaken over many centuries, and the methods are covered in various publications, including the British Trust for Conservation Volunteers “Sand Dunes Handbook”. Much the same methods are applicable to the management of marine erosion. Fencing materials can include chestnut palings, brushwood, wooden slats or synthetic fabrics. To be effective a void: solid ratio of 30% to 50% is required. Choice of materials will depend on required life, length of frontage commitment to maintenance and potential for vandalism.

Brushwood is normally the cheapest material, depending on labour costs, but has a life expectancy of no more than one year, assuming that it does not get removed for firewood. Synthetics vary from low cost materials such as strawberry netting up to expensive polyproplylene, nylon or composite wire/synthetic webs. Life expectancies of synthetics vary from one year to decades, and maintenance is minimal. Chestnut paling fencing is commonly used for dune management as it is widely available, easy to erect and has a life expectancy of 2-5 years. It should be noted that the standard paling fences used around construction sites have a void ratio well in excess of 50% and should not be used for dune management (see photograph below). Wooden slat fences are generally not recommended as they are prone to vandalism and are costly to erect. If used the slats should be not more than 50mm wide with gaps of the same size; wider slats will encourage local scour rather than deposition, and wider gaps will reduce efficiency in causing deposition.

Fencing can be installed forward of the toe of the dunes where it will be subject to occasional wave attack during storms. Low cost fence material, such as brushwood, can be considered expendable during very extreme events, but the posts and tensioning wires are best set up to last for several years at least as they are costly to replace. Posts should be long enough to allow burial to about 1m below the lowest expected beach level. At locations exposed to regular wave attack it may be necessary to place substantial timbers (e.g. railway sleepers) as posts to avoid annual reconstruction.

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Dune fencing with ineffective spacing of chestnut palings. Dune grasses have been planted along the eroding face.

The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both winter storms and more normal wave conditions. The toe of a freshly eroded dune face is normally at the run-up limit of the most recent severe sea.

Fencing should be set up parallel to the dune face, as no significant benefits have been found from attempting to orientate fences normal to the dominant wind. Short spurs running landward up the dune face can be beneficial to recovery in areas subject to dominant winds blowing at an acute angle to the shoreline. Spurs running seaward are less useful as they are likely to be damaged by swash zone debris or by beach users trying to walk along the shore at high tide.

Dune grass transplanting (Summary 2) should be undertaken after fencing, rather than before, to reduce trampling and to take full advantage of sand accumulated by the fencing. Successful sand fences may be buried within several seasons and a new line of fencing can be added to allow further foredune deposition. Fencing can be undertaken at any time of year, but associated transplanting is best completed in the spring.

Access routes to popular public beaches should be defined by the fencing at regular intervals (say 100m) along the dune face. Poorly planned access routes will encourage the public to damage fences in order to create their own paths.

It is possible to remove fencing for part of the year and to replace it again, though this may have little benefit and the extra work may be damaging to the dunes. Reasons for this may be to allow greater public access to the beach by removing fences in the summer, or to reduce potential damage to the fencing by winter storms. Either way, it will be necessary to place large, well bedded posts that can be left permanently in place when the fencing material is removed and stored.

Fencing costs vary according to labour, type of material used, quality, length and spacing of posts, frequency of spurs, frequency of public access points, need for management and the cost of ancillary works. Small schemes in low risk areas that are implemented by volunteers may cost less than £500/100m frontage. Contracted schemes involving fenced access routes, spurs and substantial straining piles may push costs up to £20,000/km, plus ongoing maintenance. Well constructed fences in appropriate locations should have a 5 year life.

Impacts

Sand fences limit public access to the dunes and beach, and can be visually intrusive. Fencing may accumulate blown litter or strand line debris, while damaged fences may interfere with the amenity use of the beach. Paling or slat fencing combine biodegradable timber with much longer lasting wire that may cause a long term nuisance. Synthetics are generally intended to be resistant to degradation and may remain as an unsightly part of the dune landscape for many years.

Best practice and environmental opportunities

Fencing along the dune toe allows public access to be controlled and reduces trampling of vegetation along the seaward edge of the dunes. Fencing and associated vegetation transplanting can help to stabilise the foredunes and will extend the dune habitat. They can also improve the appearance of other forms of built defences that might otherwise detract from the coastal landscape.

All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:

In addition to these general guidelines, the following are of specific importance to dune fencing: