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Controlled inundation of land by setting back sea defences is an increasingly used method for coastal protection and anticipation to climate change. In the United Kingdom this so-called “managed realignment” is applied widely and considered a cost-effective and sustainable response to loss of biodiversity and sea level rise. It is also applied in other countries such as the United States, Germany and Belgium.
By re-inundating land the coastline is placed backwards and new intertidal area is created. The area is enclosed by a secondary dike on the landside to ensure safety of the hinterland. The goal is to create the right circumstances for succession of saltmarsh vegetation. Once saltmarshes develop the vegetation will enhance sedimentation and the area will become higher and is able to grow with sea level rise. Saltmarshes can reduce wave energy and improve the stability of the dike.
Managed Realignment can be applied to many different situations that fall within the scope of coastal and flood management. Succes is also dependend on local boundary conditions. It is important to be clear on the aim and purpose of the proposed project at the beginning. Without this it may be difficult later to determine the success/failure of the scheme in meeting its objectives. It is also important at the outset of a project to identify any potential opportunities and/or constraints. Managed Realignment presents the opportunity for a variety of benefits, though such opportunities may also have associated constraints.
Picture: Trimley Marsh Managed Realignment Site
Definition and objective of managed realignments
Controlled inundation of land by setting back sea defence is an increasingly used method for coastal protection and anticipation to climate change (French 2006). In the United Kingdom dike setbacks, called “managed realignment”, is more and more applied and considered as a cost-effective and sustainable response to loss of biodiversity and sea level rise (Garbutt 2008). It is also applied in other countries such as the United States, Germany and Belgium. By re-inundating land the coastline is placed backwards and new intertidal area is created. The area is enclosed by a secondary dike on the landside to ensure safety of the hinterland.
Managed realignment is mostly used in low lying estuarine and coastal areas to counter-act coastal erosion and coastal squeeze as a consequence of sea level rise or other causes and improve coastal stability. Furthermore it is a method that is used to compensate for loss of intertidal areas (tidal flats and salt marshes) and as water retention area during extreme water levels in narrow river mouths.
Schematic overview of the managed realignment concept.
A clear definition of Managed Realignment is given by Rupp et al (2002):
Managed realignment 'involves setting back the line of actively maintained defences to a new line inland of the original – or preferably to rising ground – and promoting the creation of inter-tidal habitat between the old and new defences'
A conceptual picture is shown on the website of ComCoast (www.comcoast.org)
Managed realignment and wetland restoration are the same. Although minor differences exist. The word managed realignment is generally used throughout the UK and Europe for projects in which the safety against flooding is enhanced and ecology is incorporated. The word wetland restoration is generally used in the USA for projects in which the state of the ecosystem is enhanced, with high water safety as an added bonus.
A very clear explanation of a Managed Realignment is shown in this animation:
How does it work, some examples
The figures below (based on figures from CIRIA, 2004) show examples of the different approaches to Managed Realignment (from www.intertidalmanagement.co.uk).
Retreating to higher ground is when a line of defence is breached or removed as there is higher topography behind the old line of defence. This allows the landward migration of the shoreline up to the higher ground and may produce a new intertidal area.
The planform of a coast or an estuary might be modified to improve the overall functioning (hydrodynamics) of the system or provide a more sustainable position (for defences and habitats). The nature of change will depend on the situation but might include the creation of a stable bay form on the open coast, or the modification of an estuary to move it towards an equilibrium form or to accommodate increased tidal volumes as a result of sea level rise. Consideration might be given to a number of separate realignments within an estuary to improve management of the whole system or allow present land uses to continue. The separate realignment methods that can be used are presented below.
Constructing a set-back line of defence can protect property that is landward of an existing defence, or where the ground behind is either lower or not much higher topographically. This may allow for the new set-back defences to potentially be lower in height than the old defences, and also potentially reduce maintenance needs by moving the defence to a less energetic environment.
If the length of the overall sea defence is large, it may be cost effective to use Managed Realignment to reduce the overall line of defence. This may bring the expense of maintenance down in relation to the value of the protected assets. This approach may also reduce future defence cost by removing problematic sections of defence that project into deeper water.
Types of realignment
Design can greatly differ amongst managed realignment sites, depending on goals (coastal protection, nature development, retention) and specific local conditions. Pontee (2007) distinguishes three main types: banked realignment, breached realignment and regulated tidal exchange.
Breaching the line of defence (Breached realignment)
A Breach Managed Realignment allows the inundation of the land behind through a defined gap ('the breach'). This option may be a desirable option to control inundation of the area or the flows from it. Such a breach may also be armoured if it needs to be maintained for a long time. Simple dug breaches can be cost effective and provide a degree of protection (for example from waves or wash) to the landward area or allow longer residence time for sediment settling while the new intertidal area develops. The exit velocity of water will need to be considered carefully in terms of scour to seaward or impact on navigation. Pipes with tidal flaps, or other control structures, can be placed through a defence. These can be used to allow an intertidal area to develop and/or gain height (as sediment in suspension settles out) in advance of breaching the defence and are called tidal exchange systems.
Lowering or removing the line of defence (Banked realignment)
In some circumstances, the best option will be to remove the defence completely to allow a fully natural system to start to develop immediately. This is called banked realignment. This approach can be useful in managing the effects on estuarine processes or to provide wider intertidal areas where beaches are steepening. They can also reduce scour or navigation problems that may be caused by breaches. The reduction in defence height may be to ground level or to a slightly higher elevation to provide a sill that erodes down gradually and can encourage sedimentation to the landward side. The height of a defence may also be reduced, rather than removed entirely, to allow land use to continue for at least part of the time with a reduced standard to the flood defence level.
Regulated tidal exchange
Use of a culvert/sluice system in the coastal defence enables regulated tidal exchange to the re-inundated area. A high level and low level sluice in the dike enable inflow during high tide and discharge during low tide. The position of the sluices in the dike determines the tidal regime in the area. In a river mouth where fresh as well as salt water influence is present the sluice positioning and design can also determine water quality (salinity and nutrient) inflow (Maris, Cox et al. 2007) and determines the amount of water and sediment inflow. Regulated tidal exchange is mainly used for water retention motives, for example along the freshwater tidal zone of the Scheldt estuary (Belgium), where more retention is created by allowing reduced tide and dike overflow at several sites along the estuary. However potential of these sites concerning sedimentation and the ability of the area to grow with sea level rise also looks promising. By limiting currents and wave action, limited erosion occurs (Peeters, Claeys et al. 2009). Because water depth is not determined by the natural feedback (increase of elevation means decrease in water depth when inundating) but remains constant, continuous high sedimentation rates are possible (Vandenbruwaene, Maris et al. 2011). Vandenbruwaene (2011) calculated that an area with regulated tidal exchange could grow up to 2-2,5 times faster than a natural saltmarsh over 75 years. Drawbacks are high construction costs due to the sluices and limited exchange with the estuarine ecosystem.
Why managed realignments, what are the options?
- Do nothing
This is where no action is taken to either improve or maintain the existing defences. This involves stopping all maintenance, repair, renewal, and emergency repairs. The defences may be monitored (for example for health and safety reasons) until they eventually fail, or a new management option is selected. The areas protected by the existing defence would no longer be protected from flooding once this management option is implemented and completed. Application of this management approach may require a defined and agreed exit strategy appointed under the permissive powers of flood risk management.
- Do minimum
This involves limited intervention. Maintenance and repair works are only undertaken in an emergency for immediate health and safety issues. This will eventually lead to a reduced standard of defence over time. Under sea level rise, this may in effect be the same as lowering the defence level in relation to water level and thus provides a lower standard of defence over time.
- Advance the line
This is where the line of existing defence is advanced forward by building new defences in front of the existing defences. This approach is only likely to be relevant on advancing or emergent coastlines but in such circumstances investment in new defences is unlikely to be warranted. Where infrastructure or development of national importance is required, it may be necessary to advance the line to meet the requirements of the development.
- Hold the line
This is where the existing defence is maintained. This is achieved through repair, maintenance, and/or improvements to the existing defences. The standard of defence should be maintained with this option. This often calls for new works to be undertaken due to changing flood risks dependant on the area. Holding the line restricts natural processes, and is a leading factor in the growing problem of coastal squeeze.
- Retreat the Line - this is also known as Managed Realignment.
Managed Realignment can be applied to many different situations that fall within the scope of coastal and flood management. It is important to be clear on the aim and purpose of the proposed project at the beginning. Without this it may be difficult later to determine the success/failure of the scheme in meeting its objectives. It is also important at the outset of a project to identify any potential opportunities and/or constraints. Managed Realignment presents the opportunity for a variety of benefits, though such opportunities may also have associated constraints. For example, the realignment of the defences of a Natura 2000 site may be unacceptable for the functioning of the system or the disturbance it would cause.
Drivers for Managed Realignments
Coastal & Flood Management
Consents & Legislation
Water Framework Directive
Funding & Financial Compensation
Opposition from the community
Costs and benefits
Costs and benefits of a managed realignment project are difficult to determine and can differ from project to project. However, DEFRA (2008) mentions the cost and benefits according to experience of several projects:
The main economic benefits are reduced defence costs, due to both shorter defences and the role of inter-tidal habitats in wave energy reduction. Standard project appraisals aim to account for these benefits but currently existing scientific information on wave energy dissipation over inter-tidal surfaces is not fully utilised in predicting how much lower defences realigned inland could be for different water depths. However, inter-tidal habitats also provide other important products and services that, even though they are often not marketed, have significant economic social value.
Compared to the 'Holding the Line' alternative, the situations where Managed Realignment is likely to have the higher net benefits include:
- areas with low value agricultural land;
- sites where the topography allows shorter defences inland or no additional defences where retreat is to higher ground; and
- sites where the topography is such that only minor or no engineering works are necessary to ensure natural succession to the desired type of ecosystem.
Experience shows that the costs of engineering works are likely to be minor compared to land opportunity costs. In some cases, Managed Realignment leads both to the loss of freshwater or brackish habitats and to the creation of salt marshes or mudflats. It is difficult to generalise as to which type of habitat has the higher value, though in some cases one type of habitat may clearly be providing more valuable goods or services.
There is still considerable uncertainty regarding benefits and costs of Managed Realignment. Results from case studies show that costs can be higher than expected, as it is difficult to predict the success of habitat recreation, what further works might be necessary to improve or accelerate habitat succession, and what the cost of maintenance will be. There can also be costly delays in the process of Managed Realignment due to planning complexities that were not foreseen. The benefits of managed versus unmanaged realignment are not always clear. There is no consensus amongst ecologists about whether managed retreat sites lead to higher quality habitats than unmanaged ones. Furthermore, the potential costs of unmanaged realignment are likely to depend on risk communication and accompanying safety measures.
It is worth noting that with climate change and sea level rise, holding the line options are likely to become increasingly costly. Furthermore, Managed Realignment schemes are likely to become increasingly preferable on economic grounds, both along the coast and rivers, as it becomes possible to evaluate sea-defence cost savings more accurately based on scientific information.
For more specific information on costs of managed realignment also see Building with Nature Building Blocks of Managed Realignment
Design and Construction
The Managed Realignment electronic platform (www.intertidalmanagement.co.uk) provides a schematic picture of the total planning and design process of a Managed Realignment (figure 2.1). All steps are mentioned in this picture. The detailed descriptions can be read from the website.
Figure 2.1: Schematic picture of the planning and design process for a Managed Realignment (www.intertidalmanagement.co.uk)
A complete overview of design issues for coastal and estuarine managed realignments is given in CIRIA (2004). This guidance is intended to disseminate knowledge on design issues for managed realignment projects by providing information on the design and management processes. It provides coastal managers and their consultants with comprehensive information on the design process and help with the practical application of this management option. As this is a extensive guideline, no further reference will be made. It is recommended to use this guidance during the whole process of managed realignment design. Some of the design steps are mentioned in more detail below.
An important part in the planning and design phase is the data collection. Relevant baseline information is crucial for the design and implementation of a successful Managed Realignment scheme. A desk study to determine what information already exists is traditionally carried out prior to collection of any new data. The desk study should also assess the usefulness of existing data to the design or assessment of the restoration, identify gaps and effectively focus new data collection.
Areas for investigation include geomorphology and hydrodynamics, engineering characteristics, environmental characteristics and assessment, economic viability, and consents, permissions and licences (http://www.intertidalmanagement.co.uk/).
Baseline information about coastal processes, morphology and ecology is necessary (already within the initial phase) to support potential coastal ecosystem restoration activities (Royal Haskoning, 2010). An understanding of the site and the coastal system within which it fits will:
- help to inform the initial identification of the site and its potential suitability for restoration;
- assist in the design of the restoration;
- and support the assessment of impacts of the restoration on the local and wider environments.
Tabel 2.1 summarizes the key baseline data
Areas of investigation
Regional morphological environment
Satellite images, aerial photographs, land maps and marine charts can be used to gather information on large areas providing a broad-scale impression of the morphological characteristics of the coast
The elevation of the restoration site relative to the tidal frame is the primary factor which determines the fate of the new wetland over the long term. If the land receives less than 300 tidal inundations a year then saltmarsh will be created. If the landreceives more than 300 tidal inundations a year then the land will turn into mud flats. Three main methods are generally adopted to collect topographic data; ground survey, photogrammetry and Laser Induced Direction and Range (LIDaR).
Water levels and tidal range
The elevation of tidal water levels relative to the topography of the site is important for wetland development. If the tidal inundation is too high or too low it can restrict habitat development. Tidal asymmetry will influence the net import/export of sediment in the estuary.
Collection of wave data enables assessments to be made of the degree of exposure of the potential restoration site to wave action, and hence the potential for inhibiting deposition and for re-suspension of deposited sediments. Waves influences erosion and deposition. Wave energy must be below the level that erodes saltmarsh.
Sediment composition and distribution at and adjacent to the site can be evaluated through a campaign of surface and sub-surface sampling followed by laboratory analysis.
Suspended sediment concentrations
Monitoring suspended sediment concentrations is important in the analysis of morphological change in wetland environments. There must be enough sediment to develop saltmarshes without harming the coastline or estuary.
An ecological assessment is important to document existing on-site and surrounding biological communities, identify environmental concerns and help define biological goals and objectives
Geotechnic / soils
The geotechnical properties of the substrate in the restoration site would provide an indication of the sediment stability, shear strength and susceptibility to erosion. Soil information: The less modified the original marsh soils, the higher the success rate. Saltmarshes require fine-grained soils but not sandy soils as the lack of nitrogen inhibits plant growth. Mudflats require sand or soft mud.
The presence of potentially contaminated materials across the restoration site should be investigated in order to identify any constraints that may affect the design process.
Water quality measurements provide data on the health of the watershed, wetlands and lagoons in order for appropriate actions to be taken to protect and improve desired conditions. Because fish and other aquatic organisms cannot survive without oxygen, dissolved oxygen is one of the most important parameters. Other key parameters include nitrates (may stimulate excessive plant and algae growth), pH and organic carbon content.
Landowners need to be willing to give up the land. The land can be bought or compensation given.
Once the dike is breached and the managed realigned area is subject to tidal influence, it will start to develop. Development of salt marshes is crucial for two reasons. Firstly, once salt marshes develop, large scale erosion is unlikely to occur. Salt marsh vegetation will enhance sedimentation and acreation of the area that will reduce waves and improve safety. The second reason is that managed realignment is often used as nature development and compensation measure. This is aimed at creation of saltmarshes because they are an valuable habitat that can support biodiversity and ecosystem services. The practice of managed realignment should therefore by aimed at development of salt marsh vegetation and apply to several boundary conditions in order to be succesful. These boundary conditions are elaborated in Building with Nature Building Blocks of Managed Realignment
From Royal Haskoning (2010)
In designing a tidal wetland restoration scheme, the intent is to restore physical processes that create and sustain the particular form or structure that supports the desired wetland ecological functions. This approach should not attempt to 'engineer' a predetermined replicate of a tidal wetland, but should instead provide a setting for the natural evolution of wetland functions and interplay of natural ecological processes.
In order to take advantage of the physical processes that would allow the tidal wetland to evolve, the site is typically graded before the re-introduction of tidal action. This grading is the site template and, if appropriately designed, can steer the progress of the wetland towards maturity (Figure 2.2). The site template should aim to create conditions that allow the wetland landscape to evolve through hydrodynamic and sedimentary processes, without the need for further management intervention.
Figure 2.2: Example of site template from Hamilton Airfield, San Francisco Bay (Royal Haskoning, 2010)
Some more design considerations are listed in ComCoast (ComCoast Brochure):
- New realigned defences - These should be designed as per the usual procedure for sea defences, using overtopping analysis and including an allowance for consequent erosion, deposition and sea level rise.
- Wave breaks - They can be used at sites vulnerable to wave action to decrease the erosion of sediment recently deposited in the intertidal area, or to decrease the erosion of the old defence.
- Creek networks - They can be left to develop naturally or can be artificially created. The latter has the advantages of increasing the distance of flow of the water and sediment so increasing sedimentation and vegetation, and aiding the drainage of water from the site. They can be formed to match old creeks identified from aerial photographs, and created with vertical sides and bends.
- Vegetation - Short vegetation or a ploughed field should be left to encourage quick establishment of new vegetation.
- Breaches - They should be placed where there are existing channels and should encourage even distribution of water over the site. They must not be open to the predominant wave direction to avoid scour and erosion. They need to be wide enough to allow the tide to exit on the ebb tide without causing negative consequences from increased flows in part of the tidal cycle. This is controlled by the velocities of the current. They may need to be very wide to achieve this. Having one breach will cause more sedimentation, but increased scour on both sides of the breach. The bed level of the breach should reflect the depth of seaward creeks, accounting for predicted changes once breached. The breach ends of earth embankments will need armouring if widening of the breaches is not desired. Breaches should be monitored and if there are problems with erosion or lack of inundation, the breach size should be increased.
- Drainage - Sluices must not be created in a location where they might silt up.
- Counterwalls - They should be constructed to protect surrounding development or sensitive areas.
- Impacts - Design must spread the impacts of the managed realignment to decrease the rate of change e.g. adding sediment to an inter-tidal site if it is predicted to erode quickly.
- Cost - The expense of removing or maintaining the old defence must be considered as it could prove a hazard if it is left to erode naturally.
- Navigation reasons - If modelling of the breaches is miscalculated, managed realignment could produce increased scour of the channel resulting in changes to navigation.
Project Delivery / Construction
Royal Haskoning (2010) provides some recommendations about the construction phase of the Managed Realignment. The recommendations consider the way of creating desired elevations and establishment of vegetation. Further recommendation on the construction of managed realignments are listed in CIRIA (2004).
Creating desired elevations
If the potential restoration site is subsided below the elevation required for the desired wetland habitats, then filling may be required. This is particularly so if the desired habitat is saltmarsh, but the site is not at elevations conducive to vegetation colonization. Two different strategies can be adopted to raise the elevation of a restoration site:
1) Take advantage of natural sedimentation
In this technique the required elevations are achieved by taking advantage of the natural deposition of suspended sediments brought into the restored site on flood tides. The rate of accretion will depend on the suspended sediment concentrations carried into the site, the amount that is deposited from suspension and the amount of sediment that is eroded and carried out of the site on ebb tides. The expected rate of sedimentation at a restored site can be predicted from measurement of nearby suspended sediment concentrations, observed rates of sedimentation at similar restoration sites and/or local established saltmarsh areas.
If the desired wetland elevations cannot be achieved by natural sedimentation and artificial infill is too costly or impractical, then techniques can be adopted to accelerate sedimentation rates. This means maximizing the amount of sediment that is deposited on the flood tide and/or minimizing the amount that is eroded and leaves on the ebb tide. Options are:
- Wave breaks
- Sedimentation fields
- Re-introducing river sediment inputs
2) Fill with imported material
If the rate of natural sedimentation is predicted to be too low to reach the target
elevations, then the alternative strategy (if practical and affordable) is to fill the site with imported material. The technique could use sediment derived from various places including borrow pits on the site, nearby ponds or newly created tidal channels. Larger volume fill could be derived from the navigational dredging of ports and harbours (or other remote areas), providing a beneficial reuse for this material. The most suitable method to directly place material on the restoration site is by hydraulic pumping
Establisment of vegetation
1) Natural Colonisation, Seeding or Planting
In situations where saltmarsh habitat is the restoration objective, natural vegetation colonisation is generally preferred over seeding or planting. This is because natural colonisation will reflect the existing species and allow the vegetation community to change over time from initial colonisation to site maturity. This will provide a range of plant species that can adapt to future change and are suited to the niche environment offered by the restoration site. However, there may be some situations where natural colonization will not take place and seeding or planting is necessary. In these cases, the plant source should be from an existing saltmarsh close to the restoration site so adaptation to local conditions can take place.
2) Soil Treatment
Vegetation colonization to restore saltmarsh requires a suitable substrate in the rooting zone in terms of its soil chemistry, particle size and bulk properties. Wetland plants are adapted to take advantage of and thrive in naturally deposited sediments. Filled sites may have unsuitable substrates, perhaps due to high acidity, low nutrients or excessive compaction. A potential strategy for dealing with this problem (in addition to fill removal)is to modify the soil substrate, by artificial addition of sediment. In general, wetland plants prefer to grow in sediment finer than sand (which is not compacted or polluted).
When tidal action is restored to a subsided tidal wetland, physical processes are set in motion that dictate the rate and manner in which the site will evolve. As long as the site is sheltered from significant wind-wave action and is at the appropriate elevations, it will evolve in response to coastal sedimentation processes, from intertidal mudflat, to initial mudflat colonization by salt tolerant marsh plants, to ultimately a fully mature vegetated marsh plain (Figure 2.3). Subtidal (lagoon) habitats could also form across lower parts of the site, if the site is low relative to the tidal frame.
Figure 2.3: Cross-section across mudflat and saltmarsh of increasing maturity (Royal Haskoning, 2010)
Operation and Maintenance
Adaptive management and monitoring
A managed realignment project does not end when the construction is finished. After construction the management and maintenance phase starts. Different strategies can be applied. One of the main post-construction strategies of a managed realignment is Adaptive management and monitoring.
Adaptive management and monitoring will be necessary after realisation, to address uncertainties and achieve project success. Although the design features should direct the restoration to meet its ecological objectives, a number of uncertainties and data gaps exist that can only be resolved by implementing management actions and learning from the results to improve future management actions. This process of learning by doing is called adaptive management. Adaptive management is iterative, evaluating actions through carefully designed monitoring and subsequently proposing adjustments. The adjustments are, in turn, tested with appropriate, and perhaps redesigned monitoring.
Moderate levels of uncertainty are associated with creating tidal wetlands and they warrant adaptive management with 'implementation' monitoring. Implementation monitoring is designed to evaluate the effectiveness of adaptive management actions and to steer the restoration towards its ecological objectives. This type of monitoring provides data on whether a given management action to improve the quality of tidal wetland habitat has been successful using a reasonable, previously tested action (presumably the best available technology developed by other projects). The data generated can be compared to a baseline condition or reference area to determine if the management action provided the predicted change in resource quality. Data from the implementation monitoring would be used to confirm that restoration actions are producing the desired trajectory to meet the success criteria for the developing habitat.
The critical determinants for adaptive management of tidal wetlands are:
- Rate of accretion indicates trajectory toward vegetated marsh
- Tidal wetland habitat establishment
- Impacts on intertidal habitats
- Endangered species use
- Bird use
- Fish use
- Water quality.
The critical determinants for adaptive management of seasonal wetlands are:
- Seasonal wetland habitat establishment
- Bird use
- Water quality.
Also Friess et al (2008) address the importance of monitoring during the post construction phase of a managed realignment:
To achieve maximum benefit from realignment schemes, the continued monitoring of site conditions before, during, and after implementation is critical. Monitoring results from several managed realignment schemes in the UK suggest that thresholds exist within the evolution of these ecosystems that alter their resilience and vulnerability to future hydrodynamic, sedimentological, and climatic changes. Such thresholds still need to be adequately quantified. To achieve this, continued monitoring is necessary and adds to the cost of scheme implementation.
See also the knowledge page on Adaptive Monitoring Strategies for more information.
Design / technical issues
- The planning process is complex and often causes long delays, both in terms of technical details and obtaining consents. Many of the issues seem to be due to the relatively novel nature of Managed Realignment. Experience from these early cases could be documented to provide useful information for future cases and accelerate the process (DEFRA, 2008).
- There is a need to better understand risks and uncertainty associated with Managed Realignment, particularly when compared to "traditional" Hold the Line schemes. Uncertainties include the lack of ability to predict physical processes, anticipation of longer periods required to obtain consents and licences and estimating long-term maintenance costs or other similar factors. Quantifying these would assist decision-makers, who are usually risk-averse, in bringing forward more schemes (DEFRA, 2008).
- Technical issues about how best to help natural succession of habitats, model channels and the development of creeks, as well as obstacles such as the presence of heritage resources and how best to protect them, can be costly and timeconsuming to resolve. It appears that much could be gained from a better monitoring of intertidal environments (both natural sites and those resulting from human intervention) and from EU-wide collaboration / exchange of technical information (DEFRA, 2008). See below for succes factors.
- Monitoring periods of five to ten years, commonly required as permit conditions, may provide valuable information on whether the site is evolving as anticipated. However, this period is generally not long enough to inform improvements in planning and design of future projects (Williams, 2004).
- The design of many early restoration projects was focused on the achievement of vegetated marsh functions as rapidly as possible and, in doing so, discounted the value of interim habitats and the value of a mosaic of evolving habitats (Williams, 2004).
- Early restoration projects were not planned and designed following a rigorous methodology that clearly established the linkage between design decisions and predictions of how the site would evolve to meet restoration objectives. This has sometimes made it difficult to assess performance in a way that would help us improve design decisions (Williams, 2004).
- Some early restoration projects were based on goals for developing suitable habitat for a particular target species. Overall project objectives should be clear at the outset with due consideration given to the needs of other target species and the marsh as a part of the whole ecosystem (Williams, 2004).
Succes factors for wetland restoration
Wolters et al (2005) evaluates the success of many de-embankments in north-west Europe, especially in the UK, Netherlands and Germany. In this (scientific) review, success has been measured as a saturation index, where the presence of target plant species in a restoration site is expressed as a percentage of a regional target species pool. Also factors that affect restoration success are determined as well as recommendations for future restoration schemes.
Factors that affect restoration are:
- Surface elevation
In salt-marsh systems, elevation in relation to tidal inundation is generally accepted as the major abiotic factor governing the establishment and survival of halophytes at different zones within the range from mean high water neap (MHWN) to mean high water spring (MHWS) tide levels. Other studies reveal that the rate at which vegetation develops in de-embanked sites is determined by the initial elevation or that sites lower than 1.5 m below high water spring tides will fail to colonise with salt-marsh vegetation. However, initially low elevation in itself may not be a problem if sedimentation rates are high enough. For the sustainability of re-created and established salt marshes it is required that rates of surface elevation change are at least equal to local rates of relative sea-level rise.
Williams (2004) further suggests that:
- Restoration projects (and unplanned restorations) that took advantage of natural sedimentary processes to form an accretionary marsh that evolved over time have performed as well or better than highly engineered projects that attempted to replicate the form of a mature marsh.
- Many early restoration projects had unrealistic expectations of the rate at which a fully vegetated marsh would form. One should expect restoring wetland sites to take at least several decades to evolve towards a mature state in balance with sea level rise and sedimentation.
- Size of restoration sites
It has been established that, for a variety of organisms and habitats, a linear relationship exists between the number of species and the size of the area (plotted on a log scale). Therefore, the size of restoration sites may be an important determinant of success in restoration sites. The best results are found for sites larger than 100 ha. It should be noted however, that the width of a site (i.e., the line perpendicular to the coastline) is likely to be more important than the length(i.e., the line parallel to the coastline), due to zonational processes leading to higher species diversity.
- Soil Salinity
Soil salinity is an important factor affecting the composition of salt-marsh vegetation. High salinities for example may prevent germination and seedling establishment, whereas low salinities allow glycophytes to outcompete halophytes.
2) Accessibility of the site
- A prerequisite for the successful restoration of salt marsh communities is the availability of a target species source and the ability of the species to reach the target area. The best results may be expected when the target species are still present in the community species pool of the target area, which consists of the established vegetation and the soil seed bank.
It is generally assumed that the distance between the target area and a target species source will largely determine the chance of a species arriving in the target area. Thus, better results may be expected when an established salt marsh (i.e., local species pool) is directly adjacent to a de-embanked site.
- Construction and maintenance of drainage structures
At the start of the restoration, artificial creeks may be required to improve drainage, increase colonisation rates and may assist in supplying sediment to the salt marsh surface. In some restoration sites therefore, meandering creeks are dug deliberately. Another factor affecting creek development is the composition of the soil subsurface. Apart from the role of creeks, drainage is also affected by the size of the opening in the embankment. For example it is reported that enhanced salt-marsh vegetation development when culverts were enlarged by ca. 1m in diameter. Lowering of the elevation at which the culverts were placed did not increase success.
- Grazing or mowing regimes
Species diversity may be maintained over time by the implementation of a grazing or mowing regime. The dominance of tall species results in the suppression or disappearance of species of shorter statue and loss of diversity, so the level of species diversity must be maintained over time. Apart from the effect on the vegetation, grazing may also affect soil salinity and surface elevation change. So, the implementation of a grazing or mowing regime creates create heterogeneity in the soil and vegetation and prevent dominance of a single species. A prerequisite for this type of management is that the sites are high enough for the establishment of vegetation communities suitable for grazing or mowing regimes.
Communication / public perception
Appropriate consultation and public participation are important in developing any scheme (DEFRA, 2008). Involving stakeholders is not easy. It is time consuming, intensive in management time and can lead to outcomes that are not in the best interests of strategic flood management. Managed Realignment is a complex issue, which needs to be explained, and expectations need to be managed. Constructive ways to inform and involve the local communities both at an early stage of the scheme, and for monitoring purposes need to be explored. This can help to reduce delays, for example resulting from a Public Inquiry. The difficulty is in finding how best to "sell" the scheme to local communities, as this will vary across the sites.
CIRIA (2004) provides some recommendations on reporting and the use of the media:
- As the managed realignment scheme develops, periodic reports should be issued in an easily understood and accessible format. The frequency of reporting should be in proportion to the size, complexity and sensitivity of the project and the extent to which the scheme has developed. For large or sensitive projects, quite frequent reports may be required at inception, perhaps annually for the first five years and subsequently at significant milestones. The timing of reporting might also be aligned to monitoring requirements and the provision of updated or new information about the project.
- The reporting should include the problems faced and how they were overcome. It is advisable to be open and transparent about what has occurred, including detrimental outcomes or where the reality of what occurred differed from what was expected, both during and after the scheme implementation. This approach is more likely to reduce problems with the local community in future years, especially if part of the scheme develops differently from the project expectations and changes have to be made. Also, understanding the issues will help professionals to develop further tools and techniques to support managed realignment and to share best practice solutions to problems. This will help reduce uncertainty and risk in future schemes and would provide a positive contribution to future R&D work.
- Particularly for large or sensitive sites (or several smaller linked sites) public perception issues should be addressed through local or national media.
- Local television and newspapers are important additional media that can deliver messages regionally, while for large schemes with high risks it may be helpful to include national television and newspapers.
- It is important to ensure that messages come from an identified individual (a spokesperson) for consistency and familiarity.
Finance and regulations
Some lessons learned on finance and regulations are:
- Financial compensation to landowners appears to have been a key factor in the success in some earlier projects. The desirability of increased provision of financial compensation to individual stakeholders such as landowners who are adversely affected by Managed Realignment is a strong theme identified (DEFRA, 2008).
- Apart from land acquisition, the largest restoration cost is usually earth moving. Many design decisions have significant grading cost implications yet are based on very limited data and analysis (Williams, 2004).
- Environmental benefits and costs should be included explicitly in economic appraisals of schemes and be taken into account by the scheme prioritisation system, while taking care that there is no overlap between the economic and environmental criteria (DEFRA, 2008).
- The Habitats Regulations and planning process are likely to cause significant delays, especially where European nature conservation sites are involved. Experience suggests that Managed Realignment schemes will almost always take longer than hold the line (or non-intervention) schemes, which can be an important issue in the context of urgent flood works. A realistic time scale needs to be allowed for at the outset (DEFRA, 2008).
- Reducing the political sensitivity of Managed Realignment would contribute to enabling more balanced consideration with other coastal defence options and therefore better integration into the strategic planning process. Mechanisms for this could include public education about benefits of Managed Realignment in situations where it is appropriate and providing financial compensation to landowners, so as to reduce the perception that such benefits are achieved at the expense of private loss (DEFRA, 2008).
Do they work?
New salt marshes created as part of managed coastal realignment are failing to meet European conservation regulations, according to a recent study by Mossman et al. (2012) from the University of East Anglia (UEA), http://www.guardian.co.uk/environment/2012/sep/20/man-made-saltmarsh-plants.
Under the EU Habitats Directive, new salt marsh must be created each time natural salt marsh is lost to coastal development or to coastal erosion caused by sea-level rise. The new marshes must display “equivalent biological characteristics” to their natural counterparts - but the new findings, published in the Journal of Applied Ecology, reveal that artificially created salt marshes suffer significantly reduced biodiversity.
The UEA researchers analysed the vegetation of 18 marshes created through deliberate coastal realignment since 1991 and 17 marshes created accidentally by storm surges since 1881. They compared the salt-tolerant plants at these sites with those at 34 natural salt marshes and found that some key species were very poorly represented. These included sea lavender, thrift, sea arrowgrass and sea plantain.
“Salt marshes such as those in North Norfolk, Essex and around much of the coast of England are loved by naturalists and tourists alike for their natural beauty and rare/rich ecology,” said lead author Dr Hannah Mossman of UEA’s School of Environmental Sciences. “These unique tidal areas also provide vital habitat for invertebrates, a staging post for migrant birds, and the only environment in which a number of salt-tolerant plants can survive.”
Prior to this in-depth five-year study it had been assumed that this process caused no loss of species richness and that manmade marshes compensated well for the loss of natural ones. The researchers found, on the contrary, that plant life on artificially created salt marsh tended to be dominated by early-colonizing plants such as marsh samphire because of a lack of oxygen in the sediment. Shrubs such as sea purslane can also quickly become unduly dominant. The sites tended to be flat and featureless with scrappy vegetation. Even the older accidentally-created marshes were deficient in the characteristic salt marsh perennials such as sea lavender, thrift, sea arrowgrass and sea plantain.
“Our findings demonstrate very clearly that marshes created by managed realignment are not biologically equivalent to natural ones and therefore fail to satisfy the biodiversity requirements of the EU Habitats Directive,” said co-author Prof Alastair Grant, of UEA’s School of Environmental Sciences.
Prof Anthony Davy, of UEA’s School of Biological Sciences, added: “These created marshes are certainly better than losing our precious natural salt marsh altogether, but they are not a good enough replacement. We are currently developing ways of enhancing their biodiversity, such as improving local drainage conditions and planting the deficient species.”
This page shows some pictures of example cases of Managed Realignments. Detailed information can be found on this website:
This website shows an extensive overview of Managed Realignments in North-western Europe.
CIRIA C628 – Coastal and Estuarine managed realignment – design issues, London 2004.
ComCoast. A wider approach in coastal management (www.comcoast.org)
ComCoast Brochure: Innovative flood management solutions and spatial development
DEFRA / Environmental Agency. (2008). Managed Realignment Review – Project Report.
French, P.W. (2006). "Managed realignment - The developing story of a comparatively new approach to soft engineering." Estuarine, Coastal and Shelf Science 67: 409-423.
Friess, D., Moller, I., Spencer, T. (2008). Managed realignment and the re-establishment of saltmarsh habitat, Freiston Shore, Lincolnshire, United Kingdom. Prepared for the report 'The Role of Environmental Management and Eco-Engineering in Disaster Risk Reduction and Climate Change Adaptation', www.proactnetwork.org
Garbutt, A. (2008). Restoration of intertidal habitats by the managed realignment of coastal defences, UK. ‘Dunes and Estuaries 2005’ – International Conference on Nature Restoration Practices in European Coastal Habitats, Koksijde, Belgium.
Mossman, H.L., A.J. Davy & A. Grant (2012). Does managed coastal realignment create saltmarshes with ‘equivalent biological characteristics’ to natural reference sites? Journal of Applied Ecology DOI: 10.1111/j.1365-2664.2012.02198.x.
Philip Williams & Associates, Ltd., and P. M. Faber (2004). Design Guidelines for Tidal Wetland Restoration in San Francisco Bay. The Bay Institute and California State Coastal Conservancy, Oakland, CA. 83 pp.
Pontee, N. I. (2007). "Realignment in Low-lying coastal areas." Proceedings of the Institution of Civil Engineers 160(4): 155-166.
Royal Haskoning, (2010). Methods and Tools for Coastal Ecosystem Restoration, report ref. 9V5892/R00001/303996/Pbor.
Royal Haskoning, (2009). Trimley Managed Realignment Site: Annual Monitoring Report 2009, ref. 9S5953/R12009/PBor. December 2009.
Rupp, S. and Nicholls, R.J. (2002). Managed realignment of coastal flood defences: a comparison between England and Germany, Proceedings of 'Dealing with flood risk', Middlesex University, 10 pp.
Williams, P. and Faber, P. (2001). Salt Marsh Restoration Experience in San Francisco Bay. Journal of Coastal Research, SI 27, 203-211.
Wolters, M., Garbutt, A. and Bakker, J.P. (2005). Salt Marsh restoration: evaluating the success of de-embankments in north-west Europe, Biological Conservation, 123 (2005) 249-268.
Humber Estuary realignment
Huntington Beach Wetland Restoration