Managed Retreat
Restoration of reclaimed marshes in the United Kingdom is both a scientific and a political issue(5) and there are a number of conflicting interests that are having a bearing on any plans for marsh restoration; European legislation has designated large areas of reclaimed marshes as Special Areas of Conservation due to their unique flora and fauna assemblages and so cannot legally be restored to their former tidal processes; public opinion is largely against the idea of returning land to the sea, a position that can be summed up in the words of a former Government Environment Minister who publicly stated that “not one square foot of England will be given back to our old and implacable enemy the sea!”; there is also the issue of not all the scientific community agreeing that managed retreat is even necessary due to the belief that the prevailing hypothesis about the cause of salt marsh erosion is wrong. (6)
The main concepts behind managed retreat are a combination of flood defence requirements and habitat creation to replace lost intertidal habitats and is particularly important in areas which are suffering from coastal squeeze which occurs where rising relative sea levels raise the low water mark whilst the high water mark is artificially held in place by the presence of hard sea defences. (1) It is this hypothesis that some scientists are not convinced about. According to some scientists an alternative hypothesis is that an increase in the numbers of the polychaete worm Nereis diversicolor and a decrease in the abundance of intertidal sea grasses are to blame. (6) N.diversicolor is a burrowing animal and in large numbers could possibly cause sediment instability which would be exacerbated by the loss of sea grass, and their sediment stabilising roots. Sea grass beds have been in decline around our coast since the 1930’s when they were devastated by disease.
As the coastal squeeze theory and the need for some form of salt marsh restoration programme is accepted by the major conservation and flood defence agencies (The Environment Agency, English Nature, Royal Society for the Protection of Birds, The Wildlife Trusts) it is likely that this will form the basis of future coastal planning decisions.
It is widely accepted that relative sea-level fluctuations during the Holocene period have combined with medieval to present saline floodplain embanking to reduce the inter-tidal salt marsh area in Essex from 40000 ha to 4400 ha. The present loss by erosion is estimated at 2% per year for the country as a whole and the reduction in these areas is not only detrimental to related habitats but has caused an increasing requirement for flood-defence financial investment. (3)
Managed retreat involves setting back the line of actively-maintained defence to a new line inland of the original, or preferably to rising ground, and promoting the creation of intertidal habitat on the land between the old and new defences. (1) The creation of saltings acts as sea defence in itself, absorbing wave energy and creating a nature conservation resource. (2) As some of the wave energy has been dissipated the new line of sea defences can be considerably smaller and cheaper
In suitable locations managed retreat is an economical option and a number of trials are in operation in Essex, at Orplands, Tollesbury Fleet, Northey Island and Abbots Hall Farm. (2)
When managed retreat takes place, the creation of intertidal habitat occurs on soils that have undergone physical and chemical changes, some of which are irreversible. (4) When the sea is allowed to re-cover the formally reclaimed land rapid sedimentation creates conditions in which salt marsh plants germinate and become established above the layer of altered soil. At the experimental site at Tollesbury the sea was temporally allowed to cover the areas to be converted on two occasions to test the immediate effect on soil conditions and to kill off the terrestrial plants established there.
In the initial studies conducted at the sites mentioned above there are indications that there may be some worthwhile benefits from a managed retreat approach to estuarine management including; lower tidal levels and therefore a decreased risk of flooding; reduced pressure on the existing sea defences; an increase in the area of inter-tidal habitat; an increase in the natural absorption of nutrient effluent such as that caused by run-off from arable land or discharge from sewage treatment works and an increase in the space for a natural adjustment to change. (8)
These and other benefits, however, are not automatic and do not arrive purely by knocking down sea walls and letting the sea in. Careful analysis is necessary to ensure that the locations chosen and methods employed will achieve the desired positive results detailed above. There is also need for further research and also a willingness on the part of the scientists and politicians involved in the research and decision making to discuss the plans openly with the public and landowners.
References:
(1) Burd F, 1995, Managed retreat: a practical guide, p7, English Nature.
(2) MDC, 1996, Blackwater Estuary Management Plan, Maldon District Council.
(3) Dixon A M, Leggett D J, Weight R C, 1998, Habitat Creation Opportunities for Landward Coastal Re-alignment: Essex Case Studies, Water and Environment Journal 12 (2) , 107–112.
(4) Hazelden J, Boorman L A, 2001, Soils and ‘managed retreat’ in South East England, Soil Use and Management (2001) 17, 150-154.
(5) Pethick J, 2002, Estuarine and Tidal Wetland Restoration in the United Kingdom: Policy Versus Practice, Restoration Ecology Vol. 10 No. 3, pp. 431-437.
(6) Hughes R G, Paramor O A L, 2004, On the loss of saltmarshes in sout-east England and methods for their restoration, Journal of Applied Ecology 2004, Vol 41, pp 440-448.
(7) Burd F, 1993, Erosion and vegetation change to the saltmarshes of Essex and North Kent (1973 to 1983), NCC, Peterborough, Research Survey 42.
(8) Townend I, Pethick J, 2002, Estuarine flooding and managed retreat, Philosophical Transactions of The Royal Society, London, A 360, No 1796/ 15 July 2002, pp 1477-1495.
