BTCV Handbooks Online

Book:	Sand Dunes
Chapter:	5 Dune Profiling and Stabilisation
Section:	Wave Barriers
Metadata:	Details	Buy this book

Sea defences are beyond the scope of this Handbook, but it is appropriate to mention a few types of wave barriers which may be useful in large-scale dune protection programmes and which are suitable for construction by groups of volunteers.

GENERAL POINTS

Uses

The main uses of coastal wave barriers of the sort described below include:

Deflecting currents and tidal scour from eroding sections of beaches and dunes
Shielding vulnerable foredunes from high tides and storm waves
Blocking the mouths of blowouts or other low-level gaps in foredunes near the sea
Providing a basis for preliminary sand buildup to encourage the formation of embryo dunes

Other considerations

Wave barriers should be of porous rather than solid construction, so that they slow the waves and cause them to lose energy and drop their load of suspended sediments. Solid barriers usually cause scour in the same way as solid wind barriers (see the diagram in Management planning) or the unequal buildup of beach material on either side of the barrier.
Ideally wave barriers or their component parts should be able to shift with the force of the sea yet retain their overall structure. It has been found that for large-scale coastal defences, interlocking polyhedrons are stronger and more effective than solid walls. Small-scale designs cannot always accommodate this factor, but gabions (see below) have considerable flexibility and are often a good choice where exposure is not too great.
It is important to prevent scour or weakness at the ends of wave barriers. If barriers project out from the dunes, their seaward ends must be far enough out that scour does not endanger the dunes and their landward ends should run well above the line of storm tides. If they run along the beach, their ends should be brought gradually in toward the dunes at either end.

DESIGNS

Fencing

Most fencing, although useful to trap wind-blown sand or control access, is too weak to withstand wave action. Even large posts (eg railway sleepers) may be undermined by the sea so that fences rapidly deteriorate and must be repositioned frequently.

The box system of brushwood fencing used on the East Lothian coast, and similar systems of brushwood or chestnut paling fencing used on the Sefton coast can be effective against wave action, as well as in trapping blown sand. These are described below.

Note the following:

The very nature of the problem means that fencing material cannot last many years, and normal wave action and the occasional storm will damage or completely remove the fencing. When erecting fencing primarily as wave barriers, which are unlikely to be permanently buried by sand, it is important to consider what maintenance is likely to be needed to keep it effective, and who will undertake this. This type of work is a constant battle, and constant maintenance with some replacement of materials is likely to be needed.
Chestnut paling is reckoned to have a life of about two years at the best when used as a wave barrier. It is not considered worth trying to lift and re-use any chestnut paling which does get buried by sand, as the lifting loosens the sand that has accumulated. It is important to repair chestnut paling immediately after any storm damage loosens it, or it will be removed by the next storm or high tide. Re-attach paling to posts, and join in new sections as required (see Fencing materials and techniques).
Brushwood does not have to be held in position by wire fences, and is better simply set in holes in the sand, provided loss to firewood-gathering and vandalism is not too great a problem. The post and wire fence serves little purpose, and is a nuisance to clear up if it becomes damaged. The brushwood should last two to three years before it breaks up.
Post and wire with brushwood or chestnut paling will usually be needed where the wave barrier also acts as an access control fence, to stop people moving back from the beach onto the foredunes.
The size of the 'box' depends on the severity of erosion and the requirement for trapping sand blown longitudinally along the beach.

The box system of 'Dutch' fencing shown below has been used to reduce tidal erosion of dunes at Hedderwick on the East Lothian coast. The box construction absorbs the impact of incoming waves, while the entrenched brushwood resists tidal scour and remains in place to trap wind-blown sand. It is important to thatch the eroded sand face (see Vegetation establishment) to increase its stability so that plants can grow. You need to set the distance between fences and their height by trial and error, but if this is done correctly the build-up can be rapid.

At Gullane, another site on the East Lothian coast, a box system with spurs has been found effective in protecting the foredune face.

As shown above, the protected stretch of dune has two 'Dutch' fences parallel to its face, about 1.8m (6') apart. The spurs are about 2m (6'-z') long and 7m (24') apart, at right angles to the main fences. These fences, besides trapping windblown sand, force the waves to break farther offshore. This has caused a sand bar to form just to seaward of the breaking waves, where the waves drop their load of sand. The bar pushes the tide line still farther seaward, giving added protection to the dunes.

Experience with fences in the severe winter storms of 1977-78 suggests that, even when the sand in the boxes is repeatedly washed away, the fences should remain intact for renewed sand collection provided the brushwood is entrenched as deeply as possible with some side branches buried to act as anchors. Where even this is inadequate and the boxes are demolished by waves, it is likely that the battle is a losing one and that damage will occur no matter what measures are attempted to safeguard the dunes.

The following pattern of post and chestnut paling fencing is used at Ainsdale, Merseyside. The fence serves several purposes. The 'boxes' trap sand, whichever way the wind blows, and build up a low dune, as shown. The dune helps provide a buffer against wave action. If the dune is eroded by storms or high tides, the fencing should remain as a barrier, although damage is inevitable in exceptional conditions. The fence also discourages access from the beach into the foredunes, except when it is fully buried by sand. The posts are of tanalised timber, 2m (6'6") x 100mm (4"), knocked in 1m (3') by a 'Unimog' mechanical post driver. The chestnut paling is three strand, 1.07m (3.5') high with pale spacing of 75mm (3"), supplied in 9.2m (10yd) lengths.

The other designs shown, used at Formby, Merseyside, serve similar functions, but are built of cheaper materials and have more widely-spaced spurs. Only a single longitudinal fence is built, and the spurs are unenclosed on the seaward side. This does not build up as much sand as the box design above, and is also less robust. It is however very much cheaper to erect. The length and spacing of the spurs varies according to the dune profile required, the spurs being longer to make a wider dune, and closer spaced to trap more sand. The spurs can be made of chestnut paling, brushwood and wire or brushwood only. A post spacing of 1.8m (6') or 3.05m (10') on the longitudinal fence is useful, being exact divisions of 30 foot, which is the standard length for rolls of chestnut paling. This avoids having to join lengths of paling between posts. The closer spaced posts make a more robust fence.

Stone banking

At White Sands, East Lothian, an eroded beach-grassland edge which drops 1.2-1.5m (4-5') to a stony beach has been repaired by the simple expedient of throwing stones from the beach against the bank. The beach is sheltered so only a minimal amount of protection is needed against waves. While isolated boulders might promote scour, many smaller stones can be built up into a protective facing which evens out the gullies caused by people jumping down. No effort need be made to achieve the correct slope because as people descend the stony bank it gradually develops a more stable angle of rest. After this happens, the surface, which by this time has trapped some sand, may be planted up with sea lyme or other grasses.

Gabions

Gabions are most useful in somewhat sheltered conditions. If exposed to the full brunt of the waves, the motion of the rocks within the gabions may wear through the wire of the baskets as has happened at Northam Burrows, Devon. This problem can be minimised by packing the baskets by hand and adding more stones as necessary after initial settling has occurred.>

Large prefabricated gabions are made by River and Sea Gabions (London) Ltd, 2 Swallow Place, London W1R 8SQ and by The BRC Engineering Company Ltd, Stafford ST17 4NN. River and Sea Gabions Ltd recommend that their 'Maccaferri' gabions should be assembled and filled as shown below.

Smaller home-made gabions, using chain-link fencing rolled into stone-filled 'sausages' which are 'stitched' with straining wire, have been used at Braunton Burrows, Devon, to help block a tidally eroded gap in the dunes, and at Lindisfarne, Northumberland, to protect a tern nesting island from tidal scour. The design is shown in cross-section below. Further details and construction methods are given in 'Waterways and Wetlands' (BTCV, 1976).

Although this design is only suitable for blocking small waves, it can easily be added to in succeeding years if it becomes covered with trapped sand and the materials are cheap, readily available and easily handled by volunteers.

Timber 'soldiers'

This technique has been used by the National Trust at Formby on the Sefton coast, Merseyside, to break the force of the waves where they were undermining a small pine wood on the seaward edge of the dunes. The 'soldiers' are set 1-1.2m (3-4') into the sand, in a double row, staggered as shown. In this case unpeeled local pine was used, 2.4-2.7m (8-9') long by 75-125mm (3-5") diameter, but any available large timbers would be suitable.

Timber groynes

Timber groynes are beyond the capabilities of most volunteer work groups, and require careful design and digging-in to be effective. Aaron (1954) discusses methods and materials. Gaskin (1970) gives a detailed plan for a system of straight and zig-zag permeable groynes used in conjunction with wave screens at Culbin Forest, Moray. It is important that any such system ends gradually with successively shorter groynes, otherwise scour may occur around the ends.