The primary objective of wetland restoration can be three-fold. These projects can serve to reduce coastal flooding and erosion and can also provide new habitats and environmental benefits.
The term ‘wetland’ refers to a diverse range of shallow water and intertidal habitats, which occur in various locations around the world. Wetland restoration relates to the rehabilitation of previously existing wetland functions from a more impaired to a less impaired or unimpaired state of overall function.
Although similar to managed realignment, wetland restoration can be distinguished by the goal to maintain the present position of the coastline as opposed to realigning landward, as occurs under managed realignment.
The description of this technology originates from Linham and Nicholls (2010).
The most commonly restored wetland ecosystems for coastal protection are saltmarshes and mangroves. Seagrasses may also be employed as a coastal defence, to dampen waves but on their own they are seldom considered an adequate shore protection alternative (USACE, 1989).
Wetland habitats are important because they perform essential functions in terms of coastal flood and erosion management. They induce wave and tidal energy dissipation (Brampton, 1992) and act as a sediment trap for materials, thus helping to build land seawards. The dense root mats of wetland plants also help to stabilise shore sediments, thus reducing erosion (USACE, 1989). Wetland restoration re-establishes these advantageous functions for the benefits of coastal flood and erosion protection.
Restoration is required because many of the world’s wetlands have become increasingly degraded through both natural and human activities.
Techniques have been developed to reintroduce coastal wetlands to areas where they previously existed and to areas where they did not, but conditions will allow. The diversity of wetland types means there are numerous methods for restoring wetlands. The method adopted will depend on the habitat which is being restored.
Saltmarshes are widely re-established through managed realignment schemes. However, this involves retreating the present line of defence. Saltmarshes can also be re-established whilst maintaining the present coastline position through vegetative transplants from healthy marshes. Transplant types often include sprigs, stems with leaves or pot-grown seedlings; seeding is not likely to be effective on sites subject to erosion (USACE, 1989). Re-establishment of saltmarshes may require the site’s elevation to be raised using appropriate fill material.
For mangrove restoration, it is necessary to collect plant propagules (a structure, such as a cutting, seed or spore that propagates a plant) from a sustainable source, prepare the restoration site for planting and directly plant propagules at regular intervals at an appropriate time of year (de Lacerda, 2002). In re-establishing mangroves, it may also be desirable to establish nurseries to stockpile seedlings for future planting (de Lacerda, 2002). Mangrove re-establishment can also be achieved by planting dune grasses. These grasses provide a stable, protective substrate for mangroves to establish their root systems in. However, as the mangroves grow, they will eventually overshadow the dune grasses, causing them to die. Thereafter, the mangrove becomes the dominant species (USACE, 1989).
In terms of climate change adaptation in the coastal zone, the main benefit of wetland restoration is the reduction of incoming wave and tidal energy by enhancing energy dissipation in the intertidal zone. This is achieved by increasing the roughness of the surface over which incoming waves and tides travel (Nicholls et al., 2007b). This reduces the erosive power of waves and helps to reduce coastal flood risk by diminishing the height of storm surges.
A reduction in installation and maintenance costs of sea defences may occur when such structures are located behind large areas of saltmarsh. A similar effect exists for mangroves which absorb the energy and slow the water flow of storm surges (Barbier, 2008). Evidence from the 12 Indian Ocean countries affected by the 2004 tsunami disaster suggested that coastal areas with dense and healthy mangrove forests suffered fewer losses and less damage to property than those areas in which mangroves had been degraded or converted to other land use (Kathiresan & Rajendran, 2005). Observations indicate that a mature mangrove stand will reduce the costs dike maintenance by 25-30% assuming a stand width at least comparable to the characteristic wavelength of incident waves (Tri et al., 1998).
In contrast to hard defences, wetlands are capable of undergoing ‘autonomous’ adaptation to SLR, through increased accumulation of sediments to allow the elevation of the wetland to keep pace with changes in sea level (Nicholls & Klein, 2005). Provided wetlands are not subjected to coastal squeeze, and the rate of SLR is not too rapid to keep pace, wetlands are capable of adapting to SLR without further investments.
Coastal wetlands also provide a number of important ecosystem services including water quality and climate regulation, they are valuable accumulation sites for sediment, contaminants, carbon and nutrients and they also provide vital breeding and nursery ground for a variety of birds, fish, shellfish and mammals. They are also a sustainable source of timber, fuel and fibre (White et al., 2010).
The restoration and recreation of wetlands can also reduce or even reverse wetland loss as a result of coastal development. This is important in terms of maintaining the global area of wetlands and in sustaining wetlands in the face of climate change. Wetland creation may also fulfil legal obligations for the compensation of habitats lost through development.
The disadvantages of wetland restoration are minimal. The restoration of natural ecosystem services, including flood and erosion protection benefits, largely outweighs any disadvantages.
One possible disadvantage is the space requirement in locations which are often of high development potential. This must be carefully weighed against the range of benefits accrued.
Wetland restoration is also likely to require a degree of expertise, especially in locations where wetland re-colonisation has to be encouraged by transplanting wetland plants. Some wetland habitats will no doubt be more difficult to recreate than others and could require greater expertise.
Tri et al. (1998) studied the costs and benefits of mangrove restoration in Vietnam. The project involved the expansion of an existing mangrove forest on the seaward side of a dike system. The study estimates planting, capital and recurrent costs at approximately US$41 per hectare of mangrove planted, at 2009 price levels. This estimate includes planting costs and the cost of thinning from year six onwards (Tri et al., 1998).
Because the term ‘wetland’ refers to a diverse range of habitats, it is difficult to give accurate cost estimates. Different types of wetland will require different restorative measures with varying costs and labour requirements. A number of factors which are likely to contribute toward variations in costs are given below (Tri et al. 1998):
- Type of wetland to be restored, expertise availability, and consequent chances of success
- Degree of wetland degradation and consequent restoration requirements
- Intended degree of restoration (for example, it may not be possible to restore all the ecosystem functions of a wetland if it is located in a highly industrialised/urbanised environment and the planned restoration measures may be less ambitious)
- Land costs if land purchase is required to convert to wetlands
- Labour costs
- Transportation distance between seedling source and planting site
- Seedling mortality rate between collection and planting
- Cost of raising specific species in nurseries before transplantation because they cannot be directly planted on mud flats due to strong wind and wave forces
- Scale of post-implementation monitoring operations
Clearly, estimating the costs of wetland restoration is complex and depends on a large number of factors. The cost of individual projects should be calculated on a case-by-case basis.
At a local level, proactive measures can be implemented to ensure wetland habitats are maintained and used in a sustainable manner. This will preserve habitats into the future and reduce or even avoid the cost of restoration and planting schemes. By preventing wetland loss or degradation, it is also possible to avoid the many potential problems encountered in the course of wetland restoration efforts (NRC, 1992).
It is important that the multiple agencies involved in shoreline management avoid providing conflicting guidance. In the Pacific islands, many communities were advised to clear mangroves on medical advice in the 1930s and 1940s because these areas were seen as a breeding ground for malaria-transmitting mosquitoes. Today however, the ecosystem services provided by mangroves, including their coastal protection function, is valued. As such, many communities have been encouraged to replant mangroves to prevent shoreline erosion (Mimura & Nunn, 1998).
Past wetland restoration projects have been conducted on an experimental basis through ‘learning by doing’ with limited technological experience (e.g. Saenger & Siddiqi, 1993). Using this approach, it is foreseeable that communities could implement wetland restoration on a local scale, although with improved understanding, failures could be minimised and costs reduced.
At a larger scale, it is useful for governments to adopt proactive coastal management plans to protect, enhance, restore and create marine habitats. Without such a framework, action to restore wetlands is likely to be fragmented and uncoordinated (NRC, 1994). This is compounded by the involvement of multiple federal agencies with overlapping responsibilities and different policies (NRC, 1994).
One of the most significant barriers to the use of wetlands as a measure to combat coastal flooding and erosion is a lack of public awareness of the flood and erosion protection benefits offered by these ecosystems. Unless the public is educated on the benefits that wetlands provide, the link between coastal flood and erosion protection and wetland restoration is likely to be unclear. This will hinder the uptake of these projects as communities press for more tangible, hard defence options, for which the protective benefits are more widely understood.
Another barrier to successful implementation is an incomplete understanding of the ability of a degraded wetland to recover, and of the success rates of wetland creation. We still do not fully understand the needs of wetland plants and animals. As such, uncertainty also surrounds the effectiveness of wetland restoration activities and whether the full range of ecosystem functions will be restored during wetland repair. Monitoring of completed schemes will enhance our understanding of wetland restoration.
The adoption of wetland restoration and (re)creation as a response to coastal flooding and erosion requires a sustainability-focussed and anticipatory coastal management plan. The establishment of wetlands which provide full coastal flood and erosion protection takes time, and the approach does not offer immediate benefits. As such, wetland recreation may not be practicable where coastal management is reactive and focussed on hard defences. A desire to improve wetland habitats also needs to exist before the strategy can go ahead. This may involve raising public awareness of the benefits of wetland restoration and (re)creation.
Wetlands only exist under specific conditions and it is not always clear if habitat restoration will be achievable or successful, especially when coastal managers have limited predictive capabilities for shoreline change (NRC, 1994). Although studies have shown that it is possible to create wetlands in areas where they did not previously exist (Platong, 1998), sites with the potential for wetland restoration or creation should be identified on a case-by-case basis.
Identifying individuals and organisations qualified to undertake wetland restoration and recreation work can also prove a barrier to implementation. The qualifications and know-how of the implementing organisations directly influence the effective application of scientific knowledge and engineering capabilities and ultimately, project performance (NRC, 1994). To address problems associated with limitations in knowledge and capabilities, it is advisable to seek direct involvement or guidance by experienced and qualified organisations.
One of the biggest opportunities that exists to aid implementation of wetland restoration programmes is a growing concern regarding wetland loss and the associated loss of ecosystem functions such as habitat provision, food production and water quality improvement. The implementation of wetland restoration projects not only compensates for wetlands lost through development and natural processes but also provides the additional benefits of coastal flood and erosion protection. This option also helps reduce wetlands losses as a result of climate change.
Wetland creation can bring about various economic, social, and environmental benefits to local communities. For example, it has the capacity to improve the productivity of coastal waters for fishing. Given the importance of the fishing sector in many coastal communities in developing countries, this is likely to be highly beneficial. Such an effect may increase incomes of local communities and contribute toward local sustainable development. Other goods and services provided by wetlands, such as the provision of wood and fibres could also prove highly beneficial to local communities, especially in developing countries. Wetland recreation can also create opportunities for eco-tourism and increase recreational opportunities. Creation of wetlands, especially in or in close proximity to urban areas can even serve to increase awareness of the important functions performed by these habitats.
Because wetland restoration meets multiple management objectives – such as habitat protection, public access to environmental and recreational resources and hazard mitigation – and is less expensive and more aesthetically pleasing than some engineering solutions, the approach is likely to find broader public support in the future (Moser, 2000).
There is also the opportunity to implement wetland restoration or creation together with hard defences such as dikes or seawalls. In such a case, the presence of wetlands on the seaward side of the defence leads to lower maintenance costs over the lifetime of the structure (Tri et al., 1998).
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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