Summary • Description • Constraints • Potential effects • Monitoring • Locations • Further information

Case Studies	Abbotts Hall Farm, Salcott Estuary
	Orplands, Blackwater Estuary, Essex
	Tollesbury realignment site
	North Trimley Marsh, Orwell Estuary

Summary

Managed realignment is a so-called soft engineering coastal management technique that has received considerable attention in recent years. Rather than working against nature (e.g. by fighting erosion) the approach adopts a method which allows the landward migration or creation of intertidal habitats (including saltmarsh) by the removal or breaching of an existing sea defence. Under the terms of the EU Habitats Directive (92/43/EEC) and the Rio Convention it is seen as a suitable method for the creation of compensatory habitat following the loss of intertidal through, for example, land claim, dredging, coastal squeeze or coastal defence works. Nevertheless, the potential influence of realignment sites on the wider estuarine system can be significant. Therefore, extensive baseline survey is required to model or predict these effects, and to determine whether they are acceptable, before such a scheme should be progressed.

Managed realignment is not, therefore, strictly a technique for the management of saltmarsh. However, it is an important option in the range of options available to coastal/estuarine managers for both coastal defence and the maintenance of intertidal habitats. For that reason, it is also covered here.

Description

Managed realignment broadly involves constructing a new flood defence line inland of the original, promoting the creation of saltmarsh (or a combination of mudflat and marsh) on the land between the old and new lines and, finally, removing the front sea wall either partially or wholly. In most situations, in the past, the land between the defences will have been reclaimed (from the intertidal) and thus, through shrinkage during drying out and possibly continued accretion seaward of the defence, may be at a lower elevation than the habitat in front of the sea wall. Infilling may therefore be necessary to help generate the required conditions for the new habitat. Such schemes can involve the use of dredged material, pumped onto the site in order to build up the level of the sediment to an appropriate height for marsh development. However, equally, surcharging a site with sediment may not be necessary, depending on the prevailing local conditions. This ideally results in the development of a new area of saltmarsh habitat, which acts as a protective buffer to the new sea wall and the higher ground behind.

The use of tiered defences involves creating a new sea defence line inland of the original and allowing only a degree of overtopping of the front defence. The rear defence will be at a higher level than the front line and protected by the front line and intervening land. The irregular overtopping of the front defence should allow halophytic vegetation to colonise and the creation of a new area with an enhanced ecological value.

Controlled abandonment (as distinct from managed realignment) is more suitable in locations where there is a natural rise to higher ground and no new defence line is necessary. Again, however, active management is likely to be required to create new areas of saltmarsh behind the present sea wall. In this instance, once marsh becomes established, maintenance of the coastal defence would be discontinued and, with eventual failure of the defence, full tidal inundation of the newly created saltmarsh would occur. Both forms of managed retreat produce a wider intertidal profile that is better able to respond to coastal processes and to reduce the effect of coastal squeeze.

These approaches differ from do-nothing, in that some form of active management is carried out to create either a new saltmarsh habitat or a new sea defence line, or a combination of both. In addition, all such schemes should be monitored regularly to assess the changes in the new marsh and to determine whether any additional work is required. This is more likely to result in a viable saltmarsh than simply allowing the wall to disintegrate and flood the land behind, and should therefore be more successful in terms of both sea defence and habitat creation.

Techniques for creating and regenerating saltmarsh are well documented and the reference list at the end of this section provides relevant sources of information. In particular, comprehensive guidance on managed realignment techniques is provided in the CIRIA guide ‘Coastal and estuarine managed realignment - design issues’ (Leggett et al., 2004). This report is divided into three sections, intended to provide easy access for different users. Part I explains the objectives of managed realignment; Part II discusses whether realignment is appropriate for a particular site and how it may be achieved; and Part III provides technical guidance on design and implementation. A review of managed realignment schemes that have been implemented is also provided. The review identifies examples of best practice and lessons learnt relating to scheme design and associated impacts, discussing four case studies in detail. The reader is, therefore, directed to this document for a more in depth discussion of managed realignment and the conditions required to successfully implement a scheme. The rest of this section provides a summary of the main points that need to be considered when management realignment is an option.

There are many issues that need to be addressed when selecting a suitable realignment site. The main points are summarised in the table below.

In addressing some of these considerations baseline data will need to be collected to predict/model the effects that the realignment scheme will have both in the realignment site itself and on the wider estuary. Such baseline data should include:

• Topographic surveys of the realignment site and adjacent marshes;
• Bathymetric surveys of the estuary;
• Current and suspended sediment monitoring;
• Biological monitoring.

Key points to consider in the selection of a managed realignment site
Tidal prism	The volume of water entering and leaving the estuary on each tidal cycle. This will be modified by increasing the intertidal area and could affect net accretion/erosion status of the estuary.
Morphology	Returning the estuary to a quasi-natural shape to allow processes to function ‘naturally’.
Site history	Was the site previously saltmarsh? A past history of saltmarsh growth will mean that conditions favourable to such exist in the estuary.
Soils	How modified are the original marsh soils. Ongoing research is suggesting that land-use history is important in marsh generation success.
Surface elevation	Is the site high enough for saltmarsh vegetation to survive? Following initial claim, land often sinks due to compaction and dewatering. Land which has been cut off from the sea for long periods of time may well be lower than the limit of vegetation colonisation.
Surface gradient	Marsh zonation from pioneer to mature upper communities relies on a slope which, in turn, is related to submergence time.
Sediments	All marshes are fine grained, although some degree of variation exists. Increased coarser sediment due to land use practices may mean that drainage is too great.
Creek networks	Natural creeks are complex structures. Some debate exists as to whether it is advantageous to artificially create these or allow them to develop naturally (on reclaimed sites, residual creeks are often still apparent).
Tidal hydraulics	Main ebb/flood channels and current velocities. This factor will control the required width of the breach.
Breach location	It is important to avoid breaches open to predominant wave direction, as these can lead to scour and erosion.
Sediment budget	Is there enough sediment available for marshes to grow and develop without having a detrimental impact on the rest of the estuary.

Constraints

The Defra/Environment Agency (2002) review of managed realignment as a flood and coastal defence management option identified a number of potential constraints affecting opportunities for realignment. The relative importance of these factors will vary between schemes but, in general, consist of the following:

• Consents and legislation.
• Environmental issues.
• Funding and financial compensation.
• Opposition from the community.

Each of theses constraints are discussed in more detail in the CIRIA managed realignment guide (Leggett et al., 2004).

Potential effects

• Risk of increased erosion in other areas of the estuary due to an increase in the tidal prism, which causes faster tidal currents.
• Short term loss of grazing in the interval between the die-off of terrestrial plants (due to saltwater irrigation), on what was rough grazing pasture, and colonisation by saltmarsh.
• Loss of ecologically valuable terrestrial habitat.
• Creation of saltmarsh and mudflat habitat that counteracts that lost to other mechanisms (such as coastal squeeze).
• Providing the opportunity for the coast line to respond naturally to changes in estuary processes.
• Increased coastal habitat for nature conservation purposes.

Monitoring

Monitoring will have an important role, at both the implementation and post-project stages, in the assessment of the impacts of the project and to determine if the design is operating as intended. Normally, implementation monitoring will involve observing and recording any particular features, such as archaeological finds, that become exposed during construction or any impacts on, for example, shellfisheries. Post-project monitoring, and an associated action plan, may be set as a condition of a consent or license for the works (and will often be covered in an Environmental Impact Assessment).

The results of post project monitoring might lead to a re-design (or some other form of intervention or even compensation) where an unacceptable outcome is shown to occur. Intervention should only occur, however, where the degree of change is unacceptable (compared to pre-defined criteria) and/or where an unacceptable change has been shown to exist for a sufficiently long period of time; it is important to recognise that sites will evolve over time and so the need (or otherwise) to react to initial change should be carefully evaluated. Monitoring of projects can also feed into other projects by providing an understanding of scheme design and performance.

The principal monitoring techniques which apply to managed realignment include:

• Topographical survey.
• Monitoring intertidal accretion rates.
• Monitoring intertidal erodability.
• Flow monitoring.
• Monitoring scour and counter wall erosion.
• Ecological monitoring.

For a more detailed discussion on monitoring for realignment sites the reader is referred to Defra (2002) and CIRIA (Leggett et al., 2004).

Locations

The first managed realignment scheme in the UK was implemented in 1991 at Northey Island , Essex and was followed by schemes at Orplands and Tollesbury, also in Essex. Several schemes have been carried out since, largely in the south east region, and most recently at Abbots Hall Farm (Essex) and Frieston (Lincolnshire). The reader is referred to the CIRIA guide (Leggett et al., 2004) for a listing of managed realignment sites established in the UK to date.

Further information

The application of managed realignment for coastal defence and habitat creation has resulted in a number of papers and publications detailing best practice and lessons learnt. For example, Garbutt et al. (2003) and CEH (2002) note that colonisation of saltmarsh species will only occur in areas where the flooding frequency is considerably lower than that which characterises the environments in which the species concerned would normally grow. At both Northey Island and the Tollesbury realignment sites, pioneer vegetation has colonised at levels at which middle and high marsh species might be expected to grow. It is also becoming clear that sediment condition is critical to species colonisation, where the formation of compact substrates can preclude colonisation. Crooks et al. (2002) and Hazelden & Boorman (2001) discuss the relationship between sediment condition and drainage and suggest that realignment schemes should take into account pre-existing drainage systems and the possible benefits of engineering these systems prior to tidal inundation to facilitate saltmarsh establishment and growth. The RSPB have also produced a useful report detailing lessons learnt from a realignment scheme in the Cromarty Firth in Scotland (Chisholm et al., 2004) and provide recommendations for project management.

The reader is referred to a very useful report published by English Nature discussing the success of various restoration schemes in producing intertidal habitat for birds (Atkinson et al., 2001). The full document can be downloaded in three parts at: Part 1 , Part 2 & Part 3

Atkinson, P.W., Crooks, S., Grant, A. & Rehfisch, M.M. (2001). The success of creation and restoration schemes in producing intertidal habitat suitable for waterbirds. English Nature Research Reports. Report No 425.

Crooks, S., Scuhtten, J., Sheern, G., Pye, K. & Davy, A. (2002). Drainage and elevation as factors in the restoration of saltmarshes. Restoration Ecology Vol. 10 No. 3, pp. 591-602

Defra and Environment Agency (2002). Implementing managed retreat as a flood and coastal defence strategic option. R&D Project number FD 2008, Halcrow, 2002.

Garbutt, A., Gray, A., Reading , C. & Brown, S. (2003). Saltmarsh and mudflat development after managed realignment. Proceedings of the 38 th DEFRA Flood and Coastal Management Conference, 2003.

Hazelden, J. & Boorman, L.A. (2001). Soils and managed retreat in South East England. Soil Use and Management. Vol 17. pp 150-154.

Leggett, D .J., Cooper, N. and Harvey, R. (2004). Coastal and estuarine managed realignment – design issues. CIRIA. Report No 681.