Coastal sand dunes
What are they?
Sand dunes are highly diverse coastal habitats, with a range of physical forms and associated plant communities. They develop when there is a suitable supply of sand (sediment within the size range 0.2 to 2.0 mm) and prevalent onshore winds.
The form of the dune depends on the amount of available sediment supplied by eroding coastlines and the seabed, and the ease with which it can be moved by the wind. A critical factor is the presence of a sufficiently large beach plain, on to which the sands are deposited and whose surface dries out between high t ides. A breeze of just over 4.5 m/s will start moving dry sand and the dune grows as the sand comes to rest in sheltered areas in the lee of debris on the beach.
Pioneer plant species such as sea rocket Cakile maritima and sand couch grass Elytrigia juncea colonise these areas of trapped windblown sand; however foredune and mobile dune communities are dominated by marram grass Ammophila arenaria, - the key species in dune formation in the UK.
The accumulating sand begins to form ridges, which initially lie parallel to the direction of the prevailing winds. Over time, these ridges may break up and a more irregular arrangement of hills (dunes) and hollows (dune-slacks) form. Further less specialist plants then gradually colonise, until ungrazed dune grassland becomes scrub and eventually woodland.
Sand dune systems can support an extremely diverse range of plants and animals and provide a habitat for a variety of specially adapted species, including a number of uncommon plants, fungi and invertebrates. Orchids, bees and butterflies all thrive in this habitat.
The situation in the South East
|Extent in England||11,897 ha
|Extent in the SE region|| app 800 ha
|Percentage UK resource in the SE||7%|
|Extent covered by SSSI designation|| data to be added
Rate of Change
|County||1998 extent (ha)||2008 extent (ha)|
|Isle of Wight||15||17|
|Kent||600||date to be added
The main threats to this habitat are:
Erosion and progradation. Unless artificially constrained, the seaward edges of sand dunes can be a highly mobile feature, though there is a natural trend to greater stability further inland. Very few dune systems are in overall equilibrium, and a majority of those in the UK demonstrate net erosion rather than net progradation; insufficient sand supply is frequently the underlying cause. There is no particular geographical distribution of either trend, both normally being present along any one stretch of coastline, and often within individual sites. Changes may be cyclical, both seasonally and over longer periods of time. Landward movement of mobile dunes often entails loss of fixed dune and dune heath habitat, as the latter are usually stable, or retreat may be impeded by development; in a few cases dune systems may move inland where not artificially constrained. The net loss of dune habitat in England to erosion has been estimated as not more than 2% of the resource over the next 20 years.
Falling water tables. Dune slacks support characteristic communities dependent on a seasonally high water table, including the formation of temporary or even permanent ponds. There may be considerable variation in the behaviour of the water table from year to year, resulting in a stressed ecosystem where only specialised species can survive. However, in some dune systems with important slacks, a long-term fall in the water table has led to loss of the specialist slack flora and invasion by coarse vegetation and scrub. While unusually dry summers may have contributed to this problem, the long-term causes are believed to be local extraction of water and/or drainage of adjacent land used for agriculture or housing.
Grazing. In the absence of human interference, most stable dunes, with the exception of those experiencing severe exposure, would develop into scrub and woodland. The preponderance of grassland and heath vegetation on British dunes is due to a long history of grazing by livestock. Continued grazing is normally necessary to maintain the typical fixed dune communities, but over-grazing, particularly when combined with the provision of imported feedstuffs, can have damaging effects. A more widespread problem is under-grazing, leading to invasion by coarse grasses and scrub, though rabbits are locally effective in maintaining a short turf. Parts of some stabilised dune systems have been entirely converted to agricultural use, resulting in almost total loss of the conservation interest.
Recreation. Recreation is a major land use on sand dunes. Many dune systems are used extensively by holidaymakers, mostly on foot but also for parking cars and in some cases for driving four-wheel-drive vehicles or motorcycles. Moderate pressure by pedestrians may cause little damage, and may even help to counteract the effects of abandonment of grazing. However, excessive pedestrian use, as on routes between car parks and beaches, and vehicular use in particular, have caused unacceptable erosion on many dune sites. Many dune systems also support one or more golf courses. Here much of the original vegetation may be retained in the rough, but the communities of the fairways, and particularly the greens and tees, are often severely modified by mowing, fertilising and re-seeding. Fragmentation of dune systems by golf courses makes grazing management much more difficult.
Sea defence and stabilisation. Many dune systems are affected by sea defence works or artificial stabilisation measures such as sand fencing and marram planting. These practices are particularly prevalent on the more developed coastlines where drifting sand may be perceived as a threat to urban or holiday developments. While carefully applied dune management measures can help to counteract severe erosion which may threaten the existence of a dune, engineered defence systems usually reduce the biodiversity inherent in the natural dynamism of dune systems, and may cause sediment starvation down-drift. UK dunes as a whole suffer from over-stabilisation and poor representation of the mobile phases.
Beach management. The seaward accretion of dune systems takes place through the accumulation of wind-blown sand caught by plants or debris along the driftline; the initial accumulations are colonised by pioneer plant species and form embryo dunes. On some heavily used beaches this process is inhibited by pressure of pedestrian or vehicular traffic, or by beach cleaning using mechanical methods, where the organic nuclei for sand deposition may be removed. These factors may remove the minor obstacles which would catch the sand initially, or destroy the embryo dunes at an early stage in their formation. In either case a dune system in a location where the physical conditions exist for accretion may actually be static or eroding.
Forestry. Afforestation of dunes is not as prevalent in Britain as it is in parts of continental Europe, but in a few locations it has had a major effect on large areas of dune landscape. Some sites hold large conifer plantations that have the effect of suppressing the dune vegetation communities and lowering the water table. However, both routine fellings and permanent removal of conifers have shown that vegetation close to the original can be restored in a relatively short time.
Military use. During the Second World War the majority of dune systems were used for the construction of defensive installations, for military training or both. The resultant widespread erosion had a severe effect on dune vegetation that has since been reversed by protective measures and natural recovery. A significant number of major dune systems, particularly in Scotland, are still used for military training, but fortunately most retain good dune habitat. Military use can be beneficial in restricting other activities or developments.
Ownership. A substantial proportion of the UK coast is in the ownership of Government departments/agencies or voluntary conservation bodies, though the degree of influence over management is variable due to legal complexities.
Other human influences. Sand dunes have also been affected in the past by housing developments, industrial development, waste tips on or adjacent to them, fly tipping and sand extraction. Indirect effects on dunes include atmospheric nutrient deposition, and coastal squeeze due to rising sea levels and increased storminess. The potential for dredging and marine aggregate extraction, through the disruption of coastal processes, to have cumulative and long-term effects on sand dunes is an area for further investigation.
Vision for Coastal Sand Dunes
The South East Biodiversity Forum’s vision for this habitat is that there should be:
No further loss of existing habitat
Good management, including visitor management, on all extant sites
No damage to site integrity from activities arising outside the sites, eg. inadequately managed public access
Re-creation of sand dunes on appropriate sites to restore some past losses, including the linking up of fragmented sites
Greater public appreciation of sand dunes and their specialist wildlife, including greater awareness of the impacts of human pressures, such as dog-walking, mountain-biking, dumping of waste
Creation of alternative green space around important sand dunes under pressure for increasing new housing
How we can deliver this vision
- MoD Integrated Land Management Plans (www.defence-estates.mod.uk/conservation/2_biodiversity.php)
- Provision of land management advice by statutory (Natural England) and non-statutory agencies (NGOs)
- Agreements under Higher Level Stewardship
- Project funding (SITA Trust, WREN etc)
- Site management plans
- Minerals after-use (http://www.afterminerals.com/)
- Land purchase/management agreements by NGOs
- River restoration linked to Water Framework Directive Programme of measures
- Catchment Sensitive Farming tackling diffuse pollution issues