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Dune Building
Sand dune systems can provide an important natural coastal flood defence and
are also of great importance from nature conservation, recreation and tourism
perspectives. However in order to create or enhance a dune system there MUST be a suitable sand source in the first place. Very little attempt has been made to devise techniques for the artificial provision of a source area, although the dumping of sand on a beach has been attempted.
Central to all dune building is the process of sand transport known as saltation, in which sand is bounced across the ground by the wind.
Very small changes in wind can have a dramatic affect on transportation. |
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Therefore a slight increase in wind speed will cause erosion of the beach or dune, as more sand will be transported. Conversely a slight decrease in wind speed will lead to the deposition of any sand being carried around by the saltation process and is the principle on which dune building is based.
Most techniques for stimulating dune building interrupts the wind flow in some way.Most commonly fences are used with impede wind flow causing a slight decrease in speed.
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This forces some of the sand load to be deposited but allows the remainder to travel further into the dune allowing an even spread of deposition. Fences must therefore be porous so that they impede rather than halt the wind flow.
The effect of a fence on sand deposition is felt for approximately eight times its height downwind and fence spacing should reflect this relationship. |
Planting dune vegetation is a more permanent and perhaps the most cost effective method of stimulating dune growth as fences soon become covered in sand and lose their effectiveness whilst vegetation, particularly Marram grass grows through the sand deposits. The combination of both planting and fencing is particulary effective.
Dune systems generally comprise several distinct features:
| FOREDUNES |
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These are the youngest actively building dunes – often the most attractive for
recreation, but also the most susceptible to erosion.
A few well-adapted plants grow here
such as Marram Ammophilia arenaria and Lyme-grass Leymus arenarius.
Many invertebrate
species utilise the warm varied habitat provided by these dunes, especially spiders, butterflies,
hoverflies and moths.
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| FIXED DUNES |
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Dune grassland occurs as a more stable dune develops. Sand trapping plants
give way to sand “fixing” plants such as mosses and lichens which
raise the humus content of the soil, creating a carpet through which sand is unable to escape.
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| LANDWARD DUNE EDGE |
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Where dunes are fairly stable ranker grasses and scrub species might
seed in, shading out some of the more interesting low-growing vegetation. Some invaders,
such as Sea buckthorn Hippophae rhamnoides leave an enriched soil which changes the nature
of the vegetation. Where grazing predominates, grassland may persist – otherwise scrub or
woodland is typical.
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| DUNE SLACKS |
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These are low-lying wetland areas between dune ridges. Dragonflies and
damselflies, as well as molluscs are found. Succession is largely determined by whether the
soil remains waterlogged and becomes increasingly acidic where rushes and bog moss can
dominate. If the soil dries out seasonally shrubs like Creeping willow Salix repens may occur
which provide a useful source of nectar and a foodplant for invertebrates.
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While beach and dune vegetation have adapted to a harsh, demanding environment, they are quite fragile to human disturbances.
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Activities that damage or destroy beach and dune vegetation can initiate dune erosion. Large gaps in the dunes, called ‘blowouts’, can occur as a result. |
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Over the past few years the importance and increasing vulnerability of sand dune flora and fauna has lead to measures to protect them against erosion. The use of wooden walkways, exclusion zones and grazing exclusion fences are all necessary forms of dune management carried out in order to prevent erosion and is beneficial to the long term stability of the area.
A sand dune restoration project undertaken by the National Trust,at South Milton Sands, south Devon won a national environmental award in 2009.
| Prior to the commencement of the scheme the wooden sea defences protecting the National Trust car park at South Milton Sands were reaching the end of their life. They were continually under attack from the sea a more sustainable approach to protect the beach was required. |
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Four options were proposed:
| Do nothing |
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The existing defences will fail, creating a potential hazard to the public and serious erosion. |
| Hold the line with rock armour |
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This would be technically possible, but boulders the size of a people carrier would be needed, and there would be a very significant landscape impact and loss of usable beach. |
| Hold the line by re-establishing the sand dune system |
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This would involve re-establishing the dune system, relocating part of the car park further back from the beach, and allowing the dune system to gradually flex in response to sea-level change. |
| Retreat the line |
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This will involve physically moving the dune inland with the loss of the car park and an impact on the nature reserve and track. |
Option 3 was the most popular choice for the long term future of the beach and attempted to return the beach to the position that it was over 40 years ago when the dunes were removed o make way for a car park and timber sea defences.
Over 100 tonnes of waste material was removed. Sand was moved from the beach onto the old site of the car park and sculpted and graded according to exact measurements determined by the Engineering Consultants.
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Work also included improving pedestrian access to the beach via two wooden boardwalks and new parking spaces.
The National Trust and over 100 local residents |
and schools planted 15,500 Marram grasses across the dunes.As the Marram grasses establish they will trap more and more sand creating important habitats for insects and birds as well as making them a stronger defence against the sea.
The restored dunes cover a stretch of 200 metres at the beach and are part of the trust's Shifting Shores policy of managed retreat from the encroaching sea.
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