Restoration and management of coastal wetlands

Coastal wetlands (tidal marshes, or salt-marshes) are saltwater and brackish water wetlands located in coastal areas. Coastal wetlands also include areas of marine water the depth of which at low tide does not exceed six metres (Ramsar Convention). They include shallow water ecosystems permanently or periodically inundated and intertidal habitats. 

The restoration of coastal wetlands is increasingly considered as a measure for adaptation. Wetland restoration refers to the rehabilitation of previously existing or damaged wetland or remaking of wetland which was previously re-appropriated. In some locations (see for example the Schedlt Estuary case study), coastal wetlands are used to absorb storm surge waters, and attenuating flooding. Coastal wetlands provide natural defence against coastal flooding and storm surges. They dissipate wave energy  and reduce erosion by helping to stabilise shore sediments. Their other major benefit is to preserve important habitats and enhance biodiversity. 

Wetland restoration aims at re-establishing natural functions of wetlands that have been degraded by natural processes and human activities and that are also threatened by sea level rise. 

Ways to restore coastal wetlands include: 

• Restoration of geo-morphological structures (salt marshes, mudflats), by adding sediment to raise land above the mean water level and allow wetland plants to colonise, or to contrast erosion processes which are degrading wetland areas.  
• Diverting water ways, dredging sediments and maintaining natural canals to allow water flowing in a favourable path. 

• Rewetting of coastal wetlands that had been drained in the past to gain land for human activities. This option also includes “managed realignment” and ‘depolderisation’: Those measures aim at setting back the line of hard flood defences to a new line, further inland and/or on rising ground to recreate intertidal habitats between the old and the new defence. The wetland will serve as a buffer zone where storm surges will be attenuated. Depolderisation refers to returning reclaimed or drained land (a “polder” in Dutch) to the sea. Managed realignment can involve deliberate breaching or complete removal of a coastal defence such as a dike, or the relocation of defences further inland. For example, in the Hedwige-Prosper polder project in the Scheldt Estuary (Belgium and Netherlands), outer dikes are being removed to turn reclaimed land into wetlands, while inland dikes are being strengthened. 
• Small-scale measures aimed to reduce human  pressures on wetlands and enhance habitat and landscape quality. They can include clearing trees, changes in land-use and in agricultural practices that enhance habitat and landscape quality. 

IPCC categories

Structural and physical: Ecosystem-based adaptation options

Stakeholder participation

For long-term success, it is important to involve the local communities, environmental groups, agencies, businesses, and interested individuals in the planning process, as well as in the monitoring and reporting of the progress and success of the project. Stakeholders involvement can help mitigating conflicts for land use, as restoring a coastline to a natural wetland may involve loss of properties, land use change or block access to the coast. The restoration of coastal wetlands may be part of the management plans of the Natura 2000 sites (designated specifically to protect core areas for a sub-set of species or habitat types listed in the Habitats and Birds Directives).  In order to qualify as a protected site by the Natura 2000 framework, planning requires a stakeholder participation process. Managed realignment or tree-clearing or adapting cultivation is likely to require consultation with residents and landowners living in or near the area to be flooded. Where reclaimed land is returned to wetlands through management realignment, this will affect any residents and economic activities on the site and could lead to opposition. For example, the Hedwige-Prosper polder project on the Belgium-Netherlands border saw protests by farmers and local residents. 

Success and Limiting Factors

Interventions that include the restoration of coastal wetlands and managed realignment generally can help meeting multiple goals, favouring the success of such initiatives. They recreate important intertidal habitats. , They can potentially include those playing a valuable role (nursery, spawning or feeding area) for species of commercial interest. In addition to preserving biodiversity, restored wetlands or new wetlands can be used for recreation and ecotourism. These ecosystems act as traps for nutrients and contaminants, reducing eutrophication and pollution of coastal waters. Intertidal and submerged vegetation (seagrasses) also  act as carbon sequestering habitats, with important benefits for mitigation. 

The main difficulty in implementing managed realignment involves changing land use and requires high coordination at different levels of governance. It can result in the relocation of buildings and activities, possibly at high costs (including expropriation).  This could also mean the loss of land used for recreation and agriculture. Possible soft mud-like soils can be created, which could become a drowning hazard if too nearby human-visited areas. These projects require real-time monitoring to manage critical phases of water redistribution, sediment and ecosystem dynamics. Lack of proper monitoring and not adequate intervention planning, together with the ever-shifting nature of these ecosystems, makes it difficult to get predictable long-term results. Costs may also be a limiting factor, since larger projects may require significant investment. 

Costs and Benefits

Purchasing the land to be flooded is usually the main cost in case of managed realignment. The relocation of infrastructures or activities might be required as well and could be very costly depending on the local situation. The total costs for an integrated project may include logistics, planning and execution of re-routing water, raising or lowering coastal bed, depositing new substrate, as well as planting and creating new habitat. For restoration of existing wetland, indirect costs are generally lower, as purchasing land is not necessary. However, cost can increase if required sediments are not easily available.  These ecosystems are ever-changing and need to be managed to preventing erosion and maintain the restored areas. Therefore, monitoring these projects and maintaining the functionality of wetlands  requires significant and long-term budget plans. Conversely, improper maintenance approaches have the potential to cause more damage than benefits. New field-scale studies are expected to provide new information about the effectiveness of different maintenance operations on wetlands ’functionality. 

Despite costs, wetland restoration and managed realignment have many advantages compared to other techniques in terms of adaptation to climate change and preservation of coastal ecosystems. In general, coastal wetlands can enhance energy dissipation in the intertidal zone, by reducing incoming wave and tidal energy. This supports protection against storm surges and erosion. Healthy wetlands can also help coping with a certain rate of sea level rise. A process called accretion, where plants trap sediment, increases the elevation of the wetland's surface. Wetlands reduce the need for hard coastal defences. Even in combination, these approaches can reduce the need to heighten and broaden dikes, leading to a positive impact on the aesthetic value of landscape. 

Sediments in the wetlands store or slow water runoffs, and filter pollutants in a similar way as riparian buffers. They protect and create important habitats, harbour and protect biodiversity. Fish populations benefit by wetlands providing spawning or feeding habitats while the micronutrients and micro-fauna in wetland substrate are perfect feeding grounds for birds. This also provide aesthetic and cultural value.  

Finally, wetland contribute to mitigation of climate change through absorption and retention of CO2 in wetland sediment and vegetation. This way wetland rrestoration efforts help reducing the human-induced carbon footprint.  

Although costs of wetland restoration can be very high, the water damages from severe water events due to climate change would be significantly higher. The value of restoration can save a lot of money with regards to water supply, air quality, climate and water flow regulation, erosion prevention, nutrient cycle, water purification, moderation of extreme events like floods or storms, habitat and cultural services. 

Legal Aspects

Coastal wetland habitats, such as different types of salt marshes, are considered habitats of EU interest under Annex 1 of the EU Habitats Directive, and some of these are priority habitats. The EU Birds Directive recognises the need for the protection of wetlands as a vital habitat for water birds. Projects for restoration of coastal wetlands should be undertaken respecting goals and requirements of both directives. Restoration of coastal wetlands may also be part of the management plan for sites protected under the EU Natura 2000 network, or may create a new Natura 2000 site.  If a project has a significant impact on a site belonging to the Natura 2000 network, it should undergo an ‘appropriate assessment of its implications for the site’ to determine whether it will adversely affect the integrity of the site. Restoration actions can also be required under Natura 2000 as compensation for other interventions. The restoration of coastal wetlands may be supported by habitat compensation requirements under the EU Habitats Directive. For example, in the Scheldt Estuary (Belgium), habitats destroyed by port expansion were compensated via the restoration of wetlands that provide storm surge protection.

Implementation Time

Implementation time will vary greatly based on the extent of the site and the specific conditions and magnitude of restoration. Implementation time can involve both works as well as also related communication and legal actions, for example expropriating land. This can often take at least 5 years or more. Maintenance and monitoring actions should continue in the long-term. 

Life Time

The lifetime of restoration interventions of coastal wetlands will depend on local conditions, in particular erosion and sedimentation processes and on the strategy implemented. Regular maintenance may be required to keep wetland conditions, considering that they can be are naturally unstable and ever shifting ecosystems.  

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