Contouring

General points

Uses

The uses of contouring include:

1. Lessening the windward slopes of dunes or blowouts which are too steep to permit an effective use of fences or a good growth of dune grasses
2. Filling in blowouts and decapitating remnant dune peaks to achieve a regular dune crest and profile which reduces problems of wind funnelling and scour (see third diagram here).
3. Remove excess drifting sand from rapidly accreting dunes

Other considerations

It is almost always easiest and cheapest to contour by mechanical means, except on the smallest scale, where the work may be suitable for volunteers. Volunteers can carry out site protection once the earth moving is finished. In the last few years, bulldozers have increasingly been used as a means of contouring badly eroded dunes in Britain. As the expertise of dune management has developed, the confidence to take these rather drastic measures has been gained. Points to note are:

1. Contouring by bulldozer is immediate. In a few days many tons of sand can be shifted, which might take years, if ever, for fencing to accumulate. It is difficult to effectively position sand-trap fencing in eroded dunes, as-the complex pattern of humps and hollows creates wind tunnels and eddies that make the effect of the wind hard to predict. The wind may also re-erode fences that have been buried, so that in spite of taking measures, the system is locked in an eroding pattern that cannot be broken without first shifting sand mechanically.
2. Eroded dunes can he contoured to a fairly safe profile, so that even before marram planting or other stabilisation work commences, there should be little danger of major wind-blow. The smooth profile means that there is nothing to create turbulence. If the contoured dune is not high enough after bulldozing, and more sand is required, it is relatively easy to then build effective sand-trap fences as the smooth profile ensures a fairly predictable movement of sand by the wind.
3. Use of a bulldozer can look rather drastic, and an unusual way of ‘conserving’ the dunes. Work is normally done in early spring, before many visitors are around, but care should be taken to inform local residents so that there is no misunderstanding about the aims of the work.
4. Plan machine use carefully, to avoid secondary erosion. Bulldozers should not be used where they will damage existing vegetation. If possible, provide access from the beach rather than through the dunes.
5. Save any existing remnants of vegetation, so that it can be re-planted. Dig up all the marram before the bulldozer starts, and keep the plants moist in fertiliser sacks. In some cases it may be possible for the bulldozer driver to carefully sweep whole tumps of vegetation from one place to another.
6. Theoretically, windward dune faces should be regraded to a slope of as little as 3°-6° (1:20-1:10), provided the site is wide enough and there is adequate sand. In practice, space and sand are normally limited, and it is necessary to grade to a maximum of 1:2 for hard planting or 1:5 for mechanical sowing of marram (Adriani and Terwindt, 1974). Do not build dunes in low areas where the spoil must be taken from the beach storm platform, since this lowers the buffer zone between the dunes and storm tides.

Examples

Two of the areas in Britain where bulldozing has been used on quite a major scale are on the north coast of Cornwall, at Hayle, Perranporth and Constantine Bay, and at Formby on the Sefton coast, Merseyside. Many of the details which follow, both of contouring and fencing, are based on the work done in these areas.

To shift sand, a caterpillar tracked vehicle is needed. Suitable machines are the Caterpillar D6 which has a 1.8m (6′) blade, or the D8, with a 2.4m (8′) x 4.2m (14′) blade. Hire prices in Cornwall in 1986 were £22 and £30 an hour respectively. The larger machine is normally preferred, as it can get the job done more quickly. The D8 can move 5-8 tons of sand per sweep, and at Hayle was used to shift 80,000 tons of sand over an area of about 7 acres. On the Sefton coast a D6 is normally used, and is reckoned to deal with about an acre a day. Each area has been fortunate in having the services of the same local driver for several years, thus building up expertise in sand dune contouring. On the Cornish sites the general aim is to re-grade to a 1 in 3 gradient. This has proved to be stable, and so far there has been no problem with wind blow in the period before planting is finished. A netting fence is erected along the landward side as soon as possible, by way of insurance. Planting is begun in late spring when the weather begins to improve.

In Cornwall, the operator works by starting at the top and back of the eroded dunes, pushing the remaining peaks into the troughs. This creates a fairly level broad-topped ridge called a ‘bench’. The bench is then pushed down in oblique sweeps as shown. The tracks remain visible for a short time, but soon become obscured by wind action.

Contouring is only the first stage, creating the basic dune shape, on which the new ‘landscape’ is formed. This involves delineating pathways by barriers or access-control fencing, and planting marram to stabilise the surface, and continue the process of dune building by trapping more sand. Sand-trap fences are usually also constructed, and may double as access-control fences to discourage people going into the newly-planted areas.

The resulting dune profile is very even, and will look ‘un-natural’ compared to any eroded dunes nearby. This even profile will become more complex over the years, with the effects of the wind, vegetation, public pressure and further management.

An interesting example of bulldozing curing a problem that sand-trap fences had failed to solve is at Trearddur Bay, Anglesey. Here the sea wall had caused the wind to scour away the sand to leeward, leaving a drop of several feet behind the wall. The sand was blown across the car-park, and was in fact causing problems as it partly buried the public toilets. Sand trap fences had failed because they were in the sheltered lee of the wall. A bulldozer was used to move the sand from the leeward side of the car-park up against the sea wall.

The work was done over a period from January to April, and there was enough seed and plant material within the sand to create a good cover of vegetation by the summer. The area was fenced off with chestnut paling, leaving paths from the car-park up to the sea wall and beach.