Managed realignment is able to reduce both coastal flooding and erosion. It is the deliberate process of altering flood defences to allow flooding of a presently defended area. Managing this process helps to avoid uncertain outcomes and negative impacts. It also helps to maximise the potential benefits (Leggett et al., 2004). A number of terms may be used as an alternative to managed realignment. These include managed retreat, dike realignment, dike (re)opening, de-embankment and de-polderisation.
The description of this technology originates from Linham and Nicholls (2010).
Managed realignment generally involves setting back the line of actively maintained defences to a new line, inland of the original or preferably, to rising ground. Doing so should promote the creation of intertidal habitat between the old and new defences, as shown in Figure 1. In most cases, the objective of realignment is to create saltmarshes. Saltmarshes develop between mean high water springs (MHWS) and mean low water springs (MLWS), in areas shaped predominantly by tidal processes and where silts and mud are predominant (French, 1997).
The benefit of creating intertidal habitats lies in the fact that they are highly effective at attenuating wave energy. This helps to reduce offshore sediment transport and therefore erosion. Intertidal habitats also form dense root mats which increase the stability of intertidal sediments, helping to reduce erosion rates (USACE, 1989).
This section uses the creation of saltmarshes through managed realignment as an example because, to date, the managed realignment approach has only been applied in North-West Europe and North America, where saltmarshes are the dominant intertidal habitat. There appears to be no reason why creation of other wetland habitats, such as mangroves, should not be possible through realignment, although such an approach has not been undertaken to date.
Studies on saltmarshes have shown they are capable of attenuating up to 97% of incoming wave energy depending on the width of the marsh (Doody, 2008). This can have highly beneficial implications for coastal protection. For example, if defences are realigned to an inland location, the presence of intertidal habitats can greatly reduce the cost of installing and maintaining protective measures (Doody, 2008). This is illustrated in Figure 2. Alternatively, if realignment to higher ground is undertaken, defences may not be required at all.
Managed realignment may involve deliberate breaching or the complete removal of a current coastal defence. The process can be planned through abandonment or relocation of existing defences or unplanned through abandonment of defences if, for example, financial resources for maintaining defences are not available (Nicholls et al., 2007b).
In order to undertake managed realignment, a number of conditions must be present. Six of the most important conditions are given below (Gardiner et al., 2007; Rupp-Armstrong and Nicholls, forthcoming):
1) presence of coastal defences
2) availability of low-lying land
3) desire or need to improve flood or coastal defence systems
4) presence of a sustainability-oriented coastal management attitude
5) desire or need to create intertidal habitats
6) societal awareness about the benefits of managed realignment
As already mentioned, managed realignment can significantly reduce the cost of providing a given level of protection against coastal flooding and erosion. Intertidal habitats attenuate incoming wave energy, meaning that waves reaching the shore are smaller in height and less powerful. This is advantageous as it may mean hard defences are not required, or if they are necessary, that they can be of reduced height and strength. Reduced incident wave energy is also likely to result in reduced defence maintenance costs. Further cost savings can be made if realignment allows the defensive line to be shortened or completely abandoned (Nicholls et al., 2007b).
The effectiveness of saltmarshes at attenuating wave energy means that the coastal zone is less reliant on engineered hard defences for reducing coastal flood and erosion risk. By increasing the coastal zone’s natural flood and storm buffering capacity, the long-term sustainability is also improved (Leggett et al., 2004). The widespread application of managed retreat could significantly reduce the need for coastal defences in the future (Nicholls et al., 2007b). In addition, the approach is highly robust against unexpected climate change futures and generally enhances resilience to unexpected changes (Nicholls et al., 2007b).
As well as helping us respond to unexpected futures, this approach helps to mitigate carbon dioxide and methane emissions because the gases are stored within the sediment deposits. Another major benefit of managed realignment is that intertidal habitats are encouraged to return on surrendered land. This is a real benefit because coastal squeeze and human development have caused a marked decline in these habitats in many areas in recent years. Managed realignment contributes toward the reinstatement of intertidal habitats which are important to specialised birds, plants and commercially exploited fish and shellfish (Leggett et al., 2004; UK POST, 2009). A beneficial by-product of intertidal habitat creation is that these areas can then be used to promote recreation and ecotourism (Nicholls et al., 2007b). In the UK and elsewhere, intertidal habitats are popular areas for walking, sailing and bird watching.
As well as habitat benefits, the creation of new intertidal areas would also help to maintain water quality and avoid saltwater intrusion due to inappropriate land use. This is achieved by reducing the undesirable effects of eutrophication (The process whereby a water body becomes hyper-enriched by plant nutrients, therefore resulting in excessive plant growth which eventually leads to oxygen depletion which is detrimental to aquatic life) (Leggett et al., 2004). This would be of benefit in locations where drinking water supply is threatened by SLR, in highly populated locations where water availability is limited, and in areas where water bodies are required to meet a certain standard.
One of the biggest drawbacks of managed realignment is that the option requires land to be yielded to the sea. This may require the relocation of important infrastructure or buildings, potentially at significant cost. Alternatively, the land may be able to be used in other ways, such as for recreation. In both instances, valuable land on the seafront is required to be relinquished.
For this reason, the managed realignment option is often of high political and social controversy. The schemes frequently suffer from a lack of public acceptance, perhaps because of a perceived threat from the sea coming closer or because of a reluctance to lose land which forefathers fought hard to (re)claim from the sea (Rupp-Armstrong & Nicholls, forthcoming).
Managed realignment is further complicated by the frequent involvement of numerous land owners. It is important to involve those affected in the planning and decision making process in order to increase acceptability.
Managed realignment is also likely to be highly disruptive and expensive if relocation of relocation of coastal infrastructure is required (Nicholls et al., 2007b). Care should be taken to ensure that if infrastructure is abandoned rather than relocated, that nearby areas do not become isolated, thus leading to increased poverty (Nicholls et al., 2007b). As a result, managed realignment must be strategically planned to minimise problems and avoid detrimental local impacts. If a scheme is well planned, it may even be possible to improve local opportunities.
Another disadvantage of this approach which may become more significant in the future is the conflict between the need for wetland creation and the need to retain valuable agricultural and historical sites (UK POST, 2009). At present, a significant portion of realignment projects are carried out on agricultural land, largely because these sites do not require such significant relocation of infrastructure. However, inundating agricultural land may lead to reductions in local agricultural production. This is likely to become a more significant issue in future as the issue of food security becomes more pertinent and may be particularly problematic in some developing countries.
Although experience in the application of managed realignment is growing, the approach is still relatively young and uncertainties still exist. For example, it is not fully understood how long it will take to create typical intertidal habitats that deliver the full benefits of naturally occurring systems (UK POST, 2009). In addition, the approach is not necessarily conducive to all environments; wetlands and saltmarshes tend to occur in locations where wave energy is low and where high volumes of sediment are available. It is therefore important to carefully evaluate the feasibility and effects of this approach in specific locations.
As no reference on the application of managed realignment in any developing countries has been found, the authors are unable to present cost information for the developing world. Developed country costs are instead presented to give some indication of costs and how they are likely to vary.
Rupp-Armstrong and Nicholls (forthcoming) state that the average cost of managed realignment in Britain is approximately US$97,000 per hectare (at 2009 prices), where construction of a new defence was also required. However, the costs of managed realignment schemes can vary widely as a result of numerous factors outlined below:
- Cost of the land where managed realignment will be performed
- Requirement for compensation to land owners/occupiers
- The need to dismantle human-made structures present on the site to prevent marine pollution
- Requirement for and size of sea defences to protect the hinterland
- Availability and cost of human resources including expertise
- Scale and frequency of monitoring
In developed countries, where experience of managed realignment is greatest, the main cost of managed realignment is usually the cost of purchasing the land to be flooded. This may differ in developing countries where land prices are not so high and may already be owned by the state. Land costs can vary widely depending on the current landuse and as such, so too will realignment costs. As an example, agricultural land is usually less costly than land used for housing or industry, largely due to the presence of infrastructure. If land is used for housing or industry it may also be necessary to provide additional compensation for relocation.
Costs may increase further if it is necessary to dismantle human-made infrastructure present in the realignment zone. This may include structures such as buildings and roads, underground pipes for gas delivery or wires for electricity, internet or television, to name but a few.
Costs are likely to be lowest if existing defences are left to breach naturally. This saves money which would have been spent on the creation of artificial breaches. In Germany, the cost of realignment is seen as a major barrier to implementation of managed realignment, since the majority of the North Sea defences are in excellent condition (Rupp & Nicholls, 2002).
The scale of monitoring operations post-realignment will also influence costs. The more rigorous the monitoring schedule, the higher the likely costs.
Both planned and unplanned managed realignment could be achieved at the community level. Breaching or abandonment of defences is inexpensive and straightforward and is therefore unlikely to require the involvement of external organisations. However, in order to obtain the greatest benefits from managed realignment, implementation must be more carefully planned. Pre-implementation monitoring and modelling will help to determine the effect of managed realignment and will help to maximise the benefits.
To avoid unwanted consequences of managed realignment, detailed planning and pre-implementation modelling studies will be required. These studies will furnish decision makers with information on how the scheme is likely to function and whether the full range of benefits will be realised. Managed realignment schemes completed to date have used modelling to determine if alterations to the site before defence breaching, such as creek excavation or elevation raising, can encourage formation of beneficial features. Additionally, pre-implementation modelling will provide information on environmental changes caused by the scheme, such as changes to estuarine ebb/flood dominance. A higher degree of certainty regarding the behaviour of managed realignment sites can be gained through modelling but this activity is likely to require the involvement of external organisations.
It is essential that coastal managers involve stakeholders including local communities in the realignment planning process. Leggett et al. (2004) claim that effective stakeholder and local community engagement is essential to successful implementation of managed realignment schemes. They also claim participation can help to:
- Understand legitimate concerns and interests
- Explain and convince the local community of a scheme’s merits
- Manage expectations
- Develop stakeholder ownership
Barriers to implementation have only been investigated in developed countries; to date, managed realignment has only been applied in North-West Europe and North America. However, barriers which are relevant to developed countries will also have relevance in the developing world.
Rupp-Armstrong and Nicholls (forthcoming) investigated the main barriers to implementation of managed realignment in England, Scotland, the Netherlands and Germany. Their findings are summarised below:
A lack of public acceptance is the main barrier. It is thought that opposition is caused by the perception that loss of land is a retrograde step. Concerns over loss of land with high perceived property value and development potential may also contribute to a lack of acceptance (Leggett et al., 2004). Public acceptance may also be reduced by peoples’ understanding of how the technology mitigates coastal flooding and erosion. In order to overcome this barrier, it is important to communicate the true advantages and disadvantages of the approach and fully engage stakeholders in the process of managed realignment.
The second most important barrier in the studied countries relates to farming communities. These groups are frequently affected by managed realignment which is mainly implemented on agricultural land. The main barrier in this case is a lack of adequate compensation for the loss of land. If sufficient compensation were available, many farmers would be more willing to sell their land (Rupp-Armstrong & Nicholls, forthcoming).
The potentially high cost of managed realignment also poses a barrier. An analysis of existing British schemes has shown an average cost of approximately US$97000 per hectare, for schemes involving the construction of a substantial new defence line. In addition, the relocation of infrastructure located in the managed realignment zone is potentially costly.
In other studies, legal and financial difficulties have been identified as a barrier to implementation. As previously stated, it is frequently the case that the process of managed realignment must deal with numerous coastal land owners who will be affected by the scheme. As a result there can be difficulties concerning the responsibilities and liabilities of certain land owners or authorities.
Availability of land is another significant barrier to implementation. The relocation of any infrastructure present in the realigned area requires land elsewhere. In densely populated coastal areas this may be very difficult.
As stated at the beginning of this chapter, managed realignment is not necessarily an option that can be applied in any location. Rupp-Armstrong and Nicholls (forthcoming) identified a number of criteria which are required in order to implement managed realignment. Firstly, the presence of low-lying land sheltered by existing coastal defences is an essential requirement. Without low-lying land, intertidal habitats will not be created and the full benefits of managed realignment will therefore not be realised. This must be coupled with the presence of a sustainability-oriented coastal management attitude and a societal willingness to entertain the notion of managed realignment. Without these conditions, managed realignment is either prevented from going ahead, or is likely to encounter further, significant barriers.
An extra barrier to implementation may be related to the existence of important or protected habitats behind existing coastal defences. Managed realignment can bring about detrimental impacts on such areas through tidal inundation. In the UK, coastal grazing marsh frequently occurs behind coastal defences. This environment is important for many plants, animals and endangered aquatic invertebrates. As such, coastal grazing marshes are of national and international importance for nature conservation. Managed realignment can lead to the destruction of such important habitats, causing negative impacts on the local environment.
Managed realignment can be part of a ‘strategic’ shoreline management plan. These plans typically consider tens of kilometres of coastline in a holistic sense, and address a variety of needs within the targeted area. This approach is often seen as a desirable way to maximise benefits and overcome potential constraints (Leggett et al., 2004).
Managed realignment can also help recreate intertidal habitats lost through human development and SLR. In this way, provision for coastal defence may be made but not at the expense of important intertidal habitats. In some cases, legal obligations to offset previous and predicted losses of these habitats may exist – the managed realignment response could play a role in meeting these requirements.
Opportunities for the implementation of managed realignment may also occur as a result of more site-specific factors (Leggett et al., 2004). These may include, the opportunity to reduce defence maintenance costs, opportunity to create a new nature reserve and the availability of funding for realignment.
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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