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Adaptation option

Dune construction and strengthening (2015)

Dune erosion results from wind action (surface erosion) and marine erosion of the toe and face of the dune. Erosion is a natural phenomenon that can be worsened by human activities. Dune construction, strengthening and rehabilitation refer to the restoration of the flood safety and sand reservoir functions of dunes. This include the following processes:

  • Dune grass planting: plant dune grasses on the face of the dune and the front to reduce wind speed across the surface and thereby trapping and holding sand. Planting vegetation helps to stabilise dunes and encourages dune recovery and may therefore be used after storm damage. Alternatively, vegetation may be planted when new embryonic dunes become high enough. In this way a buffer is created at the seaward front of existing dunes which can be eroded during storm surges. In general, the number of plant varieties that can be planted on dunes is relatively small. Selected species must be resistant to silting up, to the wind and to salinity. When the grass cover is established it can become self-sustaining. Regular monitoring and, when needed, re-planting will be necessary.
  • Dune thatching: covering the face of the dune with plant debris and branches to stabilise sand, encourage sand accretion and protect dune vegetation. Materials can be put on the ground manually or mechanically. The input of organic material favours the development of plants and grass.
  • Dune fencing: construction of fences along the seaward face of the dune to reduce wind speed on the surface and encourage foredune deposition of transported sediment. Fences are often constructed of wood, but depending on local circumstances can also use other material (including, for example, used fish neeting). Dune fencing also can increase the deposit of organic matter and the resulting growth of grasses and other plants. Fences can also act as barriers against wave impact. This technique is not suited for all types of dunes: the installation of fences will be difficult on steep slopes, and in very instable areas. Also, maintenance will be complex in touristic areas welcoming a lot of visitors.

These methods are complementary and are usually combined: grass planting usually requires fencing and thatching to succeed. Interventions on dune are more effective when they are integrated with the restoration or construction of the complete coastal transect, including retro-dune wet areas and consolidated dunes with shrub and tree vegetation.

Artificial dunes are engineered structures reproducing the form of natural dunes, often in a chain-like manner. They are built with sand brought from an external source area and shaped into dunes using bulldozers, dune nourishments or other means. This is often carried out at the same time as beach nourishment. 

Additional Details
Reference information

Adaptation Details

Category

Green

IPCC categories

Structural and physical: Ecosystem-based adaptation options

Stakeholder participation

Dune construction can provoke conflicts of interest over land use: landowner may be eager to preserve sea views while safeguarding beach use is important in touristic places. To a lesser extent, fencing and thatching can have a negative impact on the landscape and therefore can be controversial in touristic places. Another concern is that sand can be deposited in nearby residential or commercial areas.

Dune construction, strengthening and rehabilitation projects can also provide an opportunity to raise awareness of local stakeholders and visitors. If the coastal area is a Natura 2000 site, the ‘appropriate assessment’ of the infrastructure project can but not mandatorily does include a public participation process. Where the area behind the dunes is a nature area, nature managers might want to be involved when dunes are formed or strengthened to form a sand drift dike, due to possible loss of natural values.

Success and Limiting Factors

Success factors:

  • If they are well managed, dunes can offer a high degree of protection against flooding and erosion.
  • They also provide valuable habitats for animal and plant species. Dune rehabilitation or the construction of artificial dunes is beneficial to the beach ecosystem. In some cases, artificial dunes can restore the recreational value of the beach. Also, hybrid combinations of a dyke-core in a dune may provide high safety in combination with a more appealing landscape.
  • Dune thatching, fencing and grass planting are low cost solutions to reduce dune erosion. They are not however likely to succeed if erosion is very severe. These methods are also labour intensive; they have a limited lifetime and require frequent maintenance (replacing plants, put fertiliser, replacing branches blown away, repair after vandalism etc.).
  • These techniques in general do not damage the dune. Materials used for thatching can be biodegradable. However, grass planting can have a negative impact on the borrow area (locally increased erosion). Branches deposited on the dune can bring unwanted seeds or cuttings. Thatching must also be limited as transport by machines leads to deterioration; thatching is also beneficial to invasive plant species that can grow in nutrient rich grounds rather than the original planted species. Although fences are usually made of degradable timber, they also use wires and sometimes plastic that can be a long term nuisance.
  • Grass planting can be useful to dissimulate hard defences such as gabions, timber or rock structure. Sand placement and vegetation will create a more natural appearance to these structures by partially integrating them in the dune.
  • Dune construction and strengthening can be combined with beach nourishment, to improve coastal resilience and natural landscape of the coastline.

Limiting factors:

  • Construction of fences and thatching will limit access to the dune and the beach and will disturb the public use of beaches, though in many cases walkways and confined paths can bypass these areas.
  • Thatching and fences also alters the natural visual aspect of the dune, which may have a negative influence on tourists’ flows and recreational activities.
  • Land loss can be an issue for the construction of artificial dunes and can be controversial, both because dunes can be a barrier to beach access, and reduce the part of the beach suitable for recreational activities.
  • Often dune construction and strengthening and the creation of sand drift dikes repress the natural functioning of dunes. They can reduce natural values due to the restriction of the natural dynamics of both the dunes and the areas behind them. Landward dune migration, sand drift, powdering and allowing overwash or spray, can provide the necessary pioneer conditions to species. A succession of vegetation and loss of natural values is normal especially behind dune construction areas. Combining planting with allowing local wind erosion of the dunes thus generating controlled blow holes above a certain height level (for safety) and allowing inland sand drift may greatly counteract such losses and generate an exciting landscape for the visitors. This requires, however, sufficient land area.

Costs and Benefits

In general implementation costs of all these measures are low as material used is cheap. However, their limited lifetime implies on-going maintenance costs, involving in particular labour costs. According to Scottish Natural Heritage, in 2000:

  • Dune grass planting costs between £200 - £2000 (250-2500€)/100m length for each visit; the lower cost estimates are for small schemes largely carried out by volunteers; the higher costs for large schemes undertaken by contracted operators. Costs for transplanting are dependent on labour costs, sources of transplants, extent of works, the need for ongoing management and the cost of ancillary works.
  • Dune thatching costs around £200 - £2000 (250-2500€)/100m length excluding costs of transplanting and annual maintenance; Costs for thatching depends on labour, material sources, extent of works, the need for ongoing management and the cost of ancillary works.
  • Fencing costs are around £400 - £2000 (500-2500€)/100m frontage length, excluding costs of transplanting and on-going repairs. Fencing costs vary according to labour, type of material used, quality, length and spacing of posts, frequency of spurs, number of public access points, need for management and the cost of ancillary works.

Beach construction costs are of similar range as beach nourishment costs. Maintenance costs should also be foreseen, such as regular grass planting or fencing.

The benefits are however that hard defences such as dikes, which are often more expensive to build, are not necessary. Moreover, dune construction and strengthening can combine coastal defence goals with other benefits, as habitat and biodiversity preservation and eco-tourism

Certain types of natural dunes are classified under Annex I of the Habitats Directive as a natural habitat of EU interest. Dune strengthening and rehabilitation, and in some cases dune (re)construction as well, may be part of the management plan for a Natura 2000 site.

Implementation Time

1-5 years.

Life Time

5-25 years.

Reference information

Websites:
Source:
Fact sheet provided by the OURCOAST II Project

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