Sand fencing is built primarily to trap sand and encourage dune stabilisation, but may also be used for access control. The principles of sand fencing are outlined below, followed by some recommended designs. For full details see Sand Dunes – Fencing.
Principles of sand fencing
- The optimum porosity of a fence for trapping sand is 40%. This means that 40% of the fence should be open, and 60% impermeable. Individual components of the fence should not be more than about 75mm (3″) wide, in order to avoid local eddying and scour around the component.
- Given suitable sources of dry sand and correct wind conditions, a mound will accumulate to leeward of the fence, and extending horizontally 12-16 times the fence height.
- It is therefore important to choose the height to suit the intended extent of the trapped sand. The strength of the fence must increase with increasing height in order to withstand the force of the wind in the period while sand is accumulating. However, in spite of using heavier posts and other components, it is more cost effective to build one very high barrier where necessary, rather than to build successive lines of low fences.
- As the fence will be buried, hopefully within a few months, there is no point in using preserved or durable timber. However, the fence needs to be robust enough to withstand the wind, and not to look too much of an eyesore in the period it is still exposed. Degradable materials are preferable, in order that no evidence of the fence is left if changing conditions eventually cause erosion of the dune. Plain wires can be removed if re-exposed, but wire or plastic mesh is more difficult to tidy up. Barbed wire should never be used in sand fencing.
Brushwood fencing
At its simplest, this type of fence is made only of brushwood. This has the advantage of being degradable, and well suited to volunteer labour because it is usually a ‘free’ material, needing only the labour to gather and transport it. The lower branches (brashings) cut from commercial woodlands are suitable, spruce and fir being the best species. Pine and birch can also be used, but larch is not sufficiently bushy. Discarded Christmas trees make good sand fencing material. Erect the brashings as shown below, locking them against one another to make the fence strong enough to withstand the wind.
A more robust type of brushwood fence uses posts and strained wire. Straining posts should be set about 900mm in the sand, with foot and breastplate. Wide-topped holes can usually be dug fairly quickly in dry sand, using a shuv-holer. Sand does not compact with tamping, so the hole is merely refilled. Struts are not usually necessary as only low tension is needed on the wires, and fence lines are normally fairly short. Other posts can be driven in using a post driver or mell.
Various methods of wire and brushwood fencing can be used, two of which are shown below. It requires specialist knowledge and experience of local conditions to place sand fences effectively, and normally a few trial fences should be built before embarking on any large programme of sand fencing.
Timber slat fencing
In areas where brushwood is not available, or where a less labour intensive method of fencing is needed, timber slat fencing can be used. An advantage is that the panels can be made in a workshop, and quickly erected on site.
Attach the panels to the posts with wire as shown. To span deep gaps in dune ridges, several tiers of panels can be used on larger posts, with wires and ground anchors to hold the panels firm against the wind.
Chestnut paling
This is expensive, but is also effective as a barrier against people, and is quick to erect. For maximum efficiency in sand-trapping, use the size with 50mm gaps between the pales. Chestnut paling can be simply strained by hand, and stapled direct to posts, but this is liable to sag and lose tension. A more wind-resistant fence is made by attaching it to separate strained wires, 4.00mm diameter, using netting rings or short lengths of wire.
Another method is to only attach the paling at the top of the posts, so that when it is half-buried it can be pulled up and re-attached to continue the dune building process. Extra supporting posts will be needed.
