Several methods may be used to prevent aeolian erosion, to increase the height and stability of existing dunes, to repair damaged dunes and to encourage sand accumulation. Where fresh sand deposits around obstructions such as grass clumps show conditions conducive to natural dune formation, plantings of native vegetation or structural barriers can be used to start and accelerate sand accumulation. Planting of native vegetation should be the primary method for erosion prevention dune, construction, improvement and repair. Structures, such as fencing or ashier hedges (see next section) can help trap sand and stabilize dunes, but they should be used as a last resort and removed when vegetation is established.
Of course one should use salt resistant plants that are able to emerge from burial by sand. At the most seaward side in the moderate climate (Europe, the Mediterranean, Australia, West coast of North America) very often Marram Grass (Ammophila arenaria) is used. As alternative, Baltic Marram Grass (Calammophila baltica) can be used.
In the Netherlands Marram Grass is the dominant sand fixing species. It is a perennial, rhizomatous grass. If covered by sand, the stems elongate, which enables the plant to emerge from the deposited sand. New roots are produced in the layer of freshly deposited sand. It has a natural ability to emerge from sand which is deposited in the vegetation and is therefore used to stabilize coastal foredunes. Two to four years after planting, the sand holding capacity can be optimal.
Marram grass may grow 1 - 1.2 m/year. The grass is most vigorous on
seaward slopes, where it is buried regularly by windblown sand. At these
sites, new healthy white roots develop in thefresh layer of sand. The
growth becomes less vigourous when the sand accumulation diminishes.
|Exclusion of nematodes improved the growth of Marram Grass seedlings significantly. Identification of the nematodes involved showed that only low numbers were present which were very variable. Further research lead to the conclusion that Marram Grass is especially damaged by the nematodes when they occur together with fungi. The nematode damages the root system of the Marram Grass with its pickle (see figure).|
|The fungus is then able to enter into the root system through the damage point with its hyphal threads. The nematodes are only able to climb to the new roots during a certain period of the year. In early spring Marram Grass is able to form new roots in the fresh sand before the arrival of the nematodes. In this way the Marram Grass survives the attack of the nematodes and the fungi.|
Marram grass can be planted, it can be sown using seed, or the rhizomes of the grass can be used. Traditionally, Marram Grass is planted manually in bundles of six tillers at a spacing of 50 cm x 70 cm. This technique is time and labour consuming and, therefore, expensive in the industrialized countries. Replanting poor-growing Marram Grass is not advised, because of the effect of the harmful organisms, as described above. In degenerating Marram Grass fields, one should either plant its natural successor, or one should allow more settlement of sand.
The natural successors of marram grass (Ammophila) in the Netherlands are red festure (Festuca rubra), sand sedge (Carex arenaria) and sea couch (Elymus athericus). When marram grass is planted in its "own" sand, the growth is as discussed above. Red festure, however, grows well in this sand. In its own sand is grows less well, and it does not grow in sand of sand sedge and sea couch. Sand sedge grows well in the sand of its predecessors (marram grass and red festure) but not in its own and not in the sand of its successor. In the adjacent figure this is shown (figure).
The complete sequence of succession of beach and dune plants in the Netherlands is shown in the following figure. In other countries, the system of succession is similar but, the species may vary according to the local climate. An inventory of the natural plants in the dune zone may reveal the local sequence of natural succession.
Then, from the plants in the sequence, those which can be planted easily have to be selected. After degeneration of one species, one should not plant the same species, but its natural successor.
For use at the U.S. Gulf coast, three species of grass have proven to be appropriate for dune vegetation projects: bitter panicum (Panicum amarum), sea oats (Uniola paniculata), and marshay cordgrass (Spartina patens). Dune and marsh plants are usually transplanted from natural stands. Transplants from the vicinity of the project are more likely to survive than imported ones. If suitable stands cannot be found on the property where the vegetation project will be undertaken, it may be possible to obtain plants from neighbouring property by agreement with property owners. The best time of the year to transplant vegetation in the U.S. Gulf area is in early spring (Feb.-March).
General tips: Take plants only from dense stands in areas that are not subject to aeolian erosion. Plants should not be taken from coppice mounds or from foredunes that are sparsely vegetated. Be careful not to trample plants. Remove individual plants in a scattered pattern at intervals of no less than half a meter. Dig them out with "sharpshooter" shovels. Pulling plants damages the small hair roots needed for re-establishment. Obtain a good root structure to ensure plant survival. On the U.S. Gulf coast, usually 2 plants per m2 are planted, while in N.W. Europe usually 3 plants per m2 are planted. During extremely dry periods the plants are watered. For some special projects along the coast of Georgia, irrigation pipes have been used.
Apply mulch either before or after planting to minimize wind erosion, to moderate soil temperature and to help retain moisture. Hay is the most economical. Use at least 1500 kg hay per acre. Pack the hay into the soil to prevent it from blowing away. In areas where high winds are common, burlap or screens anchored with stakes are recommended instead of hay. All three materials will eventually break down under normal weathering. A transplant survival rate of 50-80,an be expected. If the survival rate is less than 10%, areas should be replanted after investigation on the origin of the low survival rate.
In some cases a good method is placing small bundles of reed between the
grasses. These bundles are usually planted in a grid of 35 cm x 35 cm, and they
are placed in planting holes 20 cm deep in the sand, and about 30 cm remains
above the sand surface.
This is a very good, but labour-intensive method. Therefore in the Netherlands alternatives are investigated. In countries where the labour-costs are not as excessive as in the Netherlands, this method can be recommended. Because of the natural decay process, the reed will disappear after a number of years.
Instead of planting, one can also use the rhizomes (placing them with a harrow) or using seed. These methods proved to be more cost-effective in the Netherlands, than planting bundles of tillers, because of our very high wages. In case of seed or rhizomes, one has to protect the surface with a layer of straw. For the protection with straw one can use a quantity of approx. 5000 to 6000 kg per ha. Mixing with the upper layer of the sand can be done by hand. On rather horizontal areas it also can be done with equipment (using a harrow). When the straw is mixed with rhizomes, the amount of straw can be reduced to approx. 3000 kg per ha.
In large-scale planting experiments one has found that similar results can be achieved, using one of the following techniques:
To prevent blowing away one of the methods is placing a wind screen. This can be an artificial, vertical screen, of an hedge of branches from some brushwood. In western europe very often osier hedges are used (wood from willows). In Denmark one often uses fir trees, and in the Netherlands and in the United States discarded Christmas trees were also used. In principle on can place three types of screens:
In the figure the three variations are
indicated, each of them leads to a different type of sedimentation behind the
screen. On the beaches very often one screen is placed parallel to the dune
foot, catching sand blown from the beach into the dunes. In this way the dune
foot is improved. When a screen is placed on top of the dune, the dune will
Screens perpendicular to the dune foot will trap the sand which is blown parallel to the dunes. One has to realise that these perpendicular screens act like groynes. This means that they indeed may catch sand, but as a conse- quence, the also cause a lee-side erosion.
In some cases screens have also been constructed using wooden piles and plastic sheets or special geotextiles with wide mesh. In the plastic sheets some holes were made to create some wind velocity directly behind the screen, which decreases the overall turbulence. Less turbulence gives more sedimentation. Plastic fencing has the advantage of being strong, non-degradable, and reusable. Because plastic is non-flammable, it will not be taken for campfire fuel. Plastic fencing, however, is much more expensive as wooden sand fencing. Moreover, it proved to be impossible to remove the plastic sheets later on from the dunes. Because they do not decay, they have an considerable environmental impact and are no longer used in the Netherlands.
A height of 1.5 m, measured from the ground surface after installation, is recommended for dune building structures. In areas where sand conditions are poor for dune building, a height of 0.75 m is appropriate. The fencing can be supported with wooden, metal or concrete posts at 3 m intervals. Wooden posts should be hardwood, redwood or treated pine. Bamboo may be used, too. The minimum practical length for posts is 2 m, a length of 2.5 m is optimum. Wooden posts should be no less than 10 cm diameter. Secure the fencing material by fastening it to each post with four ties of galvanised wire and weave the material between the posts, so that every other post has fencing on the seaward side. Trees, brush and seaweed can be held in place with smooth wire strung between supports posts. Another method is to anchor the vegetation to stakes driven into the ground. If the base of a sand fence is placed at ground level, dunes will be built over the structure. If the base is elevated 10-15 cm above the ground, dunes will be built on the downwind side of the structure, and the fencing can be retrieved for reuse as the dunes are formed. In this case place the fences 1.5 to 3 m seaward of the area to be restored.
In dunes blow-outs can very often be found. At such places the sand is blown away, and the hole becomes deeper and wider. Such a hole also slowly walks in an upwind direction, because the sand is deposited at the downwind side of the hole. Because sometimes such holes may endanger the stability of the sea defence, the manager wants to control them. One of the ways to do so is building a fence with reeds in the blow-out. Such a screen consists of wooden piles covered with reeds. A practical value is that the screen is 40 cm in the sand, and 60 cm above the sand. Because a blow-out is very dynamic, such a screen has to be replaced (higher or lower) very often (sometimes several times in a month).
In a wide blow-out or a breach in the foredunes, multiple tiers of fences should be used to increase sand entrapment and raise the ground elevation. Place the first fence at the landward end of the dune-building site. When sand has built up along this fence, erect a second one about 7.5 m seaward of it. After sand accumulates here, place a third fence between the first two. Offset the breaks in the tiers to facilitate sand entrapments.
The fences should not extend seaward of the dune-foot on either side at the breach. The area should be vegetated for natural stabilisation.Exit