Managed retreat
Encyclopedia
In the context of coastal erosion
, managed retreat (also managed realignment) allows an area that was not previously exposed to flooding by the sea to become flooded by removing coastal protection. This process is usually in low lying estuarine areas and almost always involves flooding of land that has at some point in the past been claimed from the sea.
Managed retreat is often a response to sea level rise exacerbated by local subsidence of the land surface due to isostatic rebound in the north.
Although land claim has been an important factor for salt marsh loss in the UK in the past (Allen 1992) the majority of current salt marsh loss in the UK is believed to be due to erosion (Morris et al. 2004). This erosion may involve coastal squeeze, where protective sea walls prevent the landward migration of salt marsh in response to sea level rise when sediment supply is limited (Hulme 2005; Morris et al. 2004). Salt marshes are protected under the EU Habitats Directive as well as providing habitat for a number of species protected by the Birds Directive
(see Natura 2000
). Following this guidance, the UK’s biodiversity action plan aims to prevent net losses to the area of salt marsh
present in 1992. It is therefore a legal requirement that all losses in marsh area must be compensated by replacement habitat with equivalent biological characteristics (Crooks et al. 2001). This equates to the need to restore approximately 1.4 km² of salt marsh habitat per year in the UK.
Sediment flow is also restored to its natural state, beaches can be naturally replenished due to erosion of the coast, providing protection and the balance of the coastline returns.
There are no agreed protocols on the monitoring of MR sites (Atkinson et al. 2001) and, consequently, very few of the sites are being monitored consistently and effectively (Wolters et al. 2005c). Due to the low levels of monitoring there is little evidence on which to base future managed realignment projects. This has led to the results of Managed Realignment schemes being extremely unpredictable.
in Essex
flooded in 1991, followed by larger sites at Tollesbury
and Orplands (1995), Freiston Shore
(2001) and Abbott's Hall Farm, at Great Wigborough in the Blackwater Estuary, it is one of the largest managed retreat schemes in Europe. It covers nearly 280 hectares of land on the north side of the estuary (2002) and a number of others.
Atkinson, P.W., Crooks, S., Grant, A. and Rehfisch, M. M., 2001. The success of creation and restoration schemes in producing intertidal habitat suitable for waterbirds. English Nature Research Report 425.
Available online in three sections: http://www.english-nature.org.uk/pubs/publication/PDF/ENRR425_1.pdf
Coastal erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage...
, managed retreat (also managed realignment) allows an area that was not previously exposed to flooding by the sea to become flooded by removing coastal protection. This process is usually in low lying estuarine areas and almost always involves flooding of land that has at some point in the past been claimed from the sea.
Managed retreat is often a response to sea level rise exacerbated by local subsidence of the land surface due to isostatic rebound in the north.
Coastal defence
In the UK the main reason for implementation of Managed Realignment is generally to improve coastal stability, essentially replacing artificial ‘hard’ coastal defences with natural ‘soft’ coastal landforms (Pethick 2002). This process can be used to protect areas of land further inland rather than that near the coast by relying on natural defences to absorb or dampen the force of waves.Habitat loss
In addition to being used as a means of coastal defence, Managed Realignment has also been used in a number of cases to mitigate for loss of intertidal habitat.Although land claim has been an important factor for salt marsh loss in the UK in the past (Allen 1992) the majority of current salt marsh loss in the UK is believed to be due to erosion (Morris et al. 2004). This erosion may involve coastal squeeze, where protective sea walls prevent the landward migration of salt marsh in response to sea level rise when sediment supply is limited (Hulme 2005; Morris et al. 2004). Salt marshes are protected under the EU Habitats Directive as well as providing habitat for a number of species protected by the Birds Directive
Birds Directive
The Birds Directive is a European Union directive adopted in 2009. It replaces Council Directive 79/409/EEC of 2 April 1979 on the conservation of wild birds which was modified several times and had become very unclear...
(see Natura 2000
Natura 2000
Natura 2000 is an ecological network of protected areas in the territory of the European Union.-Origins:In May 1992, the governments of the European Communities adopted legislation designed to protect the most seriously threatened habitats and species across Europe. This legislation is called the...
). Following this guidance, the UK’s biodiversity action plan aims to prevent net losses to the area of salt marsh
Salt marsh
A salt marsh is an environment in the upper coastal intertidal zone between land and salt water or brackish water, it is dominated by dense stands of halophytic plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh...
present in 1992. It is therefore a legal requirement that all losses in marsh area must be compensated by replacement habitat with equivalent biological characteristics (Crooks et al. 2001). This equates to the need to restore approximately 1.4 km² of salt marsh habitat per year in the UK.
Advantages
There are no direct costs apart from that of removing any defences already in place and maintenance costs are very low.Sediment flow is also restored to its natural state, beaches can be naturally replenished due to erosion of the coast, providing protection and the balance of the coastline returns.
Disadvantages
A certain amount of land will inevitably be lost in this process while beaches are being built up resulting in settlements, farmland and other property being destroyed. Because of this, managed retreat is often not a socially acceptable plan and may invoke the need for compensation to land-owners. Intertidal sites are often a rich archaeological resource and the loss of heritage is a factor to weighed in managed retreat projects.There are no agreed protocols on the monitoring of MR sites (Atkinson et al. 2001) and, consequently, very few of the sites are being monitored consistently and effectively (Wolters et al. 2005c). Due to the low levels of monitoring there is little evidence on which to base future managed realignment projects. This has led to the results of Managed Realignment schemes being extremely unpredictable.
Examples
In the UK, the first managed retreat site was an area of 8,000 square metres at Northey IslandNorthey Island
Northey Island is an island in the estuary of the River Blackwater, Essex. It is linked to the south bank of the river by a causeway, covered for two hours either side of high tide. The island is approximately 2km to the east of Maldon, Essex and 2km to the west of Osea Island.The whole island and...
in Essex
Essex
Essex is a ceremonial and non-metropolitan county in the East region of England, and one of the home counties. It is located to the northeast of Greater London. It borders with Cambridgeshire and Suffolk to the north, Hertfordshire to the west, Kent to the South and London to the south west...
flooded in 1991, followed by larger sites at Tollesbury
Tollesbury
Tollesbury is a village in England, located on the Essex coast at the mouth of the River Blackwater. It is situated nine miles east of the historic port of Maldon and twelve miles south of Colchester....
and Orplands (1995), Freiston Shore
Freiston Shore
Freiston Shore is a village in the civil parish of Freiston, approximately east of Boston, Lincolnshire. In Victorian times the village enjoyed a boom, with visitors looking to enjoy the healthy unindustrialised atmosphere of the area.-History:...
(2001) and Abbott's Hall Farm, at Great Wigborough in the Blackwater Estuary, it is one of the largest managed retreat schemes in Europe. It covers nearly 280 hectares of land on the north side of the estuary (2002) and a number of others.
Current progress
At present approximately 6 km² of salt marsh have been restored by MR in the UK (Mossman et al. In prep). One of the major reasons cited for the slow pace of current salt marsh restoration in the UK (Morris et al. 2004) is the uncertainty associated with the practice (Foresight).External links
The following references review the UK and US experience respectively:Atkinson, P.W., Crooks, S., Grant, A. and Rehfisch, M. M., 2001. The success of creation and restoration schemes in producing intertidal habitat suitable for waterbirds. English Nature Research Report 425.
Available online in three sections: http://www.english-nature.org.uk/pubs/publication/PDF/ENRR425_1.pdf
- http://www.english-nature.org.uk/pubs/publication/PDF/ENRR425_2.pdf
- http://www.english-nature.org.uk/pubs/publication/PDF/ENRR425_3.pdf
- Board on Environmental Studies and Toxicology, 2001. Compensating for Wetland Losses Under the Clean Water Act
- Pictures of the Tollesbury and Orplands managed retreat sites
- Information on Abbott's Hall from Essex Wildlife Trust
- The UK Environment Agency’s Managed Realignment Electronic Platform
- The Online Managed Realignment Guide (OMReG): A website designed to act as a 'collecting point' for information about coastal Managed Realignment and Regulated Tidal Exchange projects in the UK and Northern Europe