Naturally occurring sand dunes are wind-formed sand deposits representing a store of sediment in the zone just landward of normal high tides (French, 2001). Artificial dunes are engineered structures created to mimic the functioning of natural dunes.
Dune rehabilitation refers to the restoration of natural or artificial dunes from a more impaired, to a less impaired or unimpaired state of overall function, in order to gain the greatest coastal protection benefits.
Artificial dune construction and dune rehabilitation are technologies aimed at reducing both coastal erosion and flooding in adjacent coastal lowlands.
The description of this technology originates from Linham and Nicholls (2010).
Dunes naturally occur along most undeveloped, sandy coastlines. A typical example is shown in Figure 1. Where they are present, their coastal defence role is two-fold:
- They represent a barrier between the sea and land, in a similar way to a seawall
- Dunes are ‘dynamic’, i.e. the dune/beach system interacts a great deal and is constantly undergoing small adjustments in response to changes in wind and wave climate or sea level. As such, dunes are able to supply sediment to the beach when it is needed in times of erosion, or store it when it is not (French, 2001).
Clearly natural sand dunes are an effective defence against coastal flooding and erosion. However, a problem arises in that wide, sandy beaches – the environment where most sand dunes occur – are highly appealing for development. As such, natural sand dunes are in decline. Coupled with an increased chance of dune erosion caused by SLR and more energetic wave climates, sand dunes are at risk.
The importance of dunes in coastal protection has now been recognised however, and the construction of artificial dunes and rehabilitation of existing ones are potential technologies for adapting to climate change in the coastal zone.
At its simplest, artificial dune construction involves the placement of sediment from dredged sources on the beach. This is followed by reshaping of these deposits into dunes using bulldozers or other means. As a result, dune construction is most frequently carried out at the same time as beach nourishment, because sand is readily available.
There are a number of methods of dune rehabilitation. One such method is to build fences on the seaward side of an existing dune to trap sand and help stabilise any bare sand surfaces (USACE, 2003). This method can also be used to promote dune growth after a structure has been created using bulldozers (Nordstrom & Arens, 1998). Natural materials such as branches or reed stakes are commonly used for fence construction, because they break down once they have accomplished their sand-trapping objective (Nordstrom & Arens, 1998).
Alternatively, vegetation planting may be used to stabilise natural or artificial dunes. This promotes the accumulation of sand from wind-blown sources around their stems – over time, this causes dune growth. Planting can be achieved by transplanting vegetative units from nursery stocks or nearby intact dunes (USACE, 2003). It can be undertaken at the community level using widely available tools. Over time, dune vegetation root networks also help to stabilise the dune.
Artificial dune creation and dune restoration can be carried out on existing beaches, beaches built through nourishment, existing dunes, undeveloped land, undeveloped portions of developed areas and areas that are currently fully developed but may be purchased so that dunes can be restored (Nordstrom et al., 2000).
Advantages of the technology
Sandy beaches are very important for dissipating wave energy. However, sandy beaches are in a constant state of flux, because they continuously react to constantly changing wave climates and sea levels. As such, the volume of sand held upon a beach is constantly fluctuating. During periods of low beach volume, the shoreline is susceptible to erosion and it is at these times, that sand dunes can be particularly valuable as a store of sediment which can be accessed in order to satisfy erosional forces. This compensates for the sand removed from a beach and helps to maintain wide, sandy beaches which will continue to dissipate incoming wave energy. This process is illustrated in Figure 2 The volume of erosion can be calculated using the Vellinga (1983) equation which requires knowledge of wave height, extreme water level and sediment fall velocity.
With careful management, dunes are able to offer a high degree of protection against coastal flooding and erosion. Because dunes provide both a physical and tangible defence, they may even serve to encourage sustainable development within the coastal zone.
Dunes are naturally occurring features, and provided the construction/initiation of artificial dunes is completed in a sympathetic manner, they do not necessarily spoil the local landscape. Many sandy beaches would have had naturally occurring sand dune complexes prior to coastline development; as such, the initiation of artificial dunes may even restore a degree of natural character to the site.
Sand dunes also provide a valuable coastal habitat for many highly specialised plants and animals. As such, sand dunes may be considered important both ecologically and recreationally.
Disadvantages of the technology
Despite being a natural feature of many sandy coastlines, dunes also represent a barrier to beach access. In many cases, dunes have been removed as a result of development and communities have grown used to direct access to beaches and views straight onto the sea. Reconstruction of dunes may receive local opposition if it affects these factors.
Land loss is another issue; dunes have a reasonable sized footprint. This space requirement increases further if dunes are to be given sufficient room to adapt to SLR, thus avoiding coastal squeeze. It could be controversial to use land with development potential for dune creation and rehabilitation if the full benefits are not made clear. Alternatively, sand dune construction may take place on an area of beach important for tourism and recreational purposes, therefore restricting its use by the public.
Financial requirements and costs
Since the most basic sand dune construction projects consist simply of the deposit of dredged material onshore, followed by shaping using bulldozers, simple dune construction costs are not expected to be significantly different from beach nourishment costs in terms of cost per cubic metre of sediment used. Additional costs may however, be introduced through the requirement for dune grass planting and fencing.
Factors which are likely to influence the unit costs of dune construction are:
- Whether dredged material is required for dune construction/restoration or whether fences or vegetation can be used to promote sand accumulation
- Availability and proximity of appropriate construction material from onshore or offshore sites
- Dredger type, size and availability
- Requirement to fence newly constructed dunes to prevent erosion
- Requirement for planting new dunes with vegetation
- Frequency with which the dune needs to be artificially replenished or whether the structure naturally accumulates sand
- Project size and resulting economies of scale
Institutional and organisational requirements
While dune construction using dredged sand may require specialised knowledge and equipment, rehabilitation and maintenance of naturally occurring and artificially created dunes is accomplishable at a community level.
The application of fences to stabilise bare sand and encourage dune growth is possible using local, naturally occurring materials such as branches and reed sticks (Nordstrom & Arens, 1998). The measure therefore requires very little external provision of materials or guidance. Fencing can also prevent dune erosion caused by human access.
As already mentioned, vegetation planting is frequently accomplished at the community level with subsequent maintenance also left to communities (Nordstrom & Arens, 1998). The success of this approach has been found to vary considerably with local commitment (Nordstrom & Arens, 1998). Local awareness raising campaigns could help local communities better understand the coastal protection role of dunes, which may promote local efforts to continue to preserve dunes.
Once sufficient material for the creation of dunes is available, dune creation either through naturally occurring processes or through artificial placement, movement and reshaping of the material is another task achievable with limited technology requirements. The use of a bulldozer or other earth moving equipment is sufficient to undertake ad-hoc operations to reshape or repair dunes. Sediment may even be bulldozed from dune crests and placed in lower areas if the dune crest height exceeds design specifications (Nordstrom & Arens, 1998).
Barriers to implementation
Previous experience of artificial dune creation or rehabilitation projects has shown that one major barrier is the difficulty in convincing the public and municipal officials of the need for dune construction or heightening (Nordstrom et al., 2000).
Conflicts of interest may also arise, especially if dune construction takes place in an area primarily used for residential or tourism purposes, where local landowners may be concerned about maintaining sea views. In these cases it may be possible to keep new dunes relatively low and linear, although this could affect the level of protection offered. If the full coastal protection benefits of dunes are communicated, opposition may be kept to a minimum.
In the USA, coastal managers have sometimes constructed sub-optimal dunes to minimise public opposition and to familiarise local communities with the presence of dunes. By gaining acceptance in this way, it may be possible in future to gain approval for dunes of larger dimensions, offering better levels of protection (Nordstrom et al., 2000).
Opposition may also be caused by the land-take requirements of dunes. Greater width on the ocean side could reduce beach space and on the landward side would bring dunes closer to human settlements such as housing.
Sand dunes are a dynamic form of coastal defence which respond to coastal processes such as the wave and wind climates. For example, in the summer months, dunes may grow as they accumulate sediments, while during winter storms, the sediment stored in the dunes may be accessed by the beach to satisfy erosion. Many communities are only familiar with static defences which do not react to the local conditions. The drastically different way in which dunes react to storm events may cause communities to object to their use, especially in communities where coastal stabilisation has been the long-term goal (Nordstrom et al., 2000).
Opportunities for implementation
Dune restoration can be much more than mitigation or reparation, in that it can lead to increased understanding and appreciation of a threatened ecosystem (Nordstrom et al., 2000). Restoration programs can be linked to environmental education initiatives aimed at re-establishing an appreciation for naturally functioning coastal landscapes. This may increase the likelihood of implementing similar programs elsewhere (Nordstrom et al., 2000).
Due to factors such as urbanisation, development, trampling and conversion, sand dunes are becoming increasingly damaged and in decline (French, 2001). With an improved understanding of the role of sand dunes in coastal defence and with greater awareness of the ecological importance of sand dunes for coastal species, dune construction and rehabilitation is likely to become more popular. This will bring advantages for coastal defence and nature.
Dune protection meets multiple management objectives, such as habitat protection, public access to environmental and recreational resources and hazard mitigation. Because of these benefits and the fact that they are less expensive and more aesthetically pleasing than some engineering solutions, dunes are likely to find broader public support in future (Moser, 2000).
- French, P.W. (2001) Coastal defences: Processes, Problems and Solutions. London: Routledge.
- Linham, M. and Nicholls, R.J. (2010) Technologies for Climate Change Adaptation: Coastal erosion and flooding. TNA Guidebook Series. UNEP/GEF.
- Nordstrom, K.F. and Arens, S.M. (1998) The role of human actions in evolution and management of foredunes in The Netherlands and New Jersey, USA. Journal of Coastal Conservation, 4, 169-180.
- Nordstrom, K.F., Jackson, N.L., Bruno, M.S. and de Butts, H.A. (2002) Municipal initiatives for managing dunes in coastal residential areas: a case study of Avalon, New Jersey, USA. Geomorphology, 47 (2-4), 137-152.
- Nordstrom, K.F., Lampe, R. and Vandemark, L.M. (2000) Re-establishing naturally functioning dunes on developed coasts. Environmental Management, 25 (1), 37-51.
- USACE (United States Army Corps of Engineers) (2003) Coastal Engineering Manual – Part V. Washington DC: USACE.
- Vellinga, P. (1983) Predictive Computational Model for Beach and Dune Erosion during Storm Surges. Delft Hydraulics Laboratory, Publication No. 294.
- Matthew M. Linham, School of Civil Engineering and the Environment, University of Southampton, UK
- Robert J. Nicholls, School of Civil Engineering and the Environment and Tyndall Centre for Climate Change Research, University of Southampton, UK