Norwegian Institute for Air Research
Netherlands Institute for Ecology
Tyndall Centre for Climate Change Research
Institute for Environmental Studies, Free University Amsterdam
University of Plymouth
Centre for Social and Economic Research on the Global Environment
Land-Ocean Interactions in the Coastal Zone
 


Climate Change and Coastal Management

Case Study: Climate Change and Coastal Management in Practice – A cost-benefit assessment in the Humber, UK
 
4. Managed realignment - Introduction
 
The term ‘managed realignment’, also referred to as ‘managed retreat’ or ‘coastal setback’ (Reed et al., 1999), involves deliberately breaching engineered defences to allow the coastline to recede to a new line of defence further inland (Box 5).

Managed Realignment

Managed realignment schemes have been implemented at a number of sites along the east coast of England, for example, at Tollesbury in Essex and Brancaster in North Norfolk. French (1999) suggests that managed realignment can provide “adequate coastal protection for the remaining hinterland...for significant time periods”. French undertook an investigation of the Medway estuary in Kent, to determine the effectiveness of realigned sites in protecting the hinterland. Two sites where defences had been naturally breached circa 1900, allowing reclaimed land to flood, were studied to examine the rates of erosion experienced in retreat environments. These were compared to un-reclaimed, open marshes in the same system. He found that over the same period of time, erosion was greater in the open marshes. As an analogue for contemporary managed retreat sites, the breached sites have provided significant coastal protection for the hinterland for over 70 years.

Box 5. Managed Realignment (Source: English Nature)
 
Managed realignment schemes generally aim to realign defences in a manner that will not only reduce the length of defence required, but will also increase the overall area of intertidal habitat. This is partly to create intertidal habitat as a means to comply with the Habitats Directive and also because it has recently been recognised that the intertidal zone may act as a natural sea defence (O’Riordan et al., 2000). Intertidal habitat within an estuary reduces tidal amplitude by creating frictional drag on the water surface (Burd, 1995, cited in Reed et al., 1999). Locally, the surface of the intertidal land, particularly if it is vegetated, dissipates wave energy thus reducing wave run-up and overtopping of sea defences (Brampton, 1992, cited in Reed et al., 1999). This may provide benefits in terms of decreasing the effects of severe flood events and unexpected inundation. In addition, intertidal areas provide a store for excess water, reducing the amount of water available for flooding (Wetlands Ecosystems Research Group, 1998). Furthermore, the sediment deposit itself provides mechanical support for the toe of the sea wall (Reed et al., 1999). Hence, an intertidal area in front of a sea wall can decrease the wall’s height and routine maintenance required (O’Riordan et al., 2000). However, the presence of intertidal habitats in front of sea defences not only provides increased protection from flooding, reduced maintenance costs, reduced replacement costs and a means to combat coastal squeeze, but also secondary benefits, such as recreational value.

In the identification of sites suitable for managed realignment, five key issues have been identifies which should be considered. These issues are outlined in Box 6 . Based on these criteria, GIS can determine areas that are suitable for re-conversion to intertidal habitat (Coombes, 2003).

The conversion of an area to intertidal is a three-phase process. The site is flooded up to a point determined by the land surface elevation and tidal height; this area undergoes subsequent conversion of flora to salt tolerant species, and a creek network is established (French, 1999).

 

Criterion 1 – The Area below the High Spring Tide Level
The high spring tide level is the highest point at the coastline that is reached by the sea during a spring tide (Pugh, 1997). The area below the high spring tide level illustrates the maximum area of intertidal habitat that could be created, before other factors are considered.

Criterion 2 – The Present Land Use of the Area
Land uses can be divided into two main categories: developed and undeveloped. Undeveloped land is considered as being more suitable for conversion to intertidal habitat than developed land, both in terms of the physical ease of converting the land (Reed et al., 1999) and its economic value. Within the undeveloped land use category, some land uses are considered more suitable for conversion than others. These included: Meadow/Verge/Semi-natural Grass, Rough/Meadow Grass, Ruderal Weed, Inland Bare Ground, Mown/Grazed Turf and Tilled Land *. It may not be appropriate to carry out realignment where protected flora or fauna would be put under threat. Therefore Sites of Special Scientific Interest (SSSI), Special Areas of Conservation (SAC) and other similarly protected areas may not be considered suitable for realignment.

Criterion 3 – The infrastructure of the area
The transport network, including roads, railway lines and canals, must be taken into account. It should be ensured that a strip of land, a minimum of 10m in width, exists between any transport line and realigned defence. Transport lines that lead directly to the estuary, where disturbance would be minimised, were considered suitable for inclusion in realignment areas (Pilcher et al., 2002).

Criterion 4 - The Historical Context of the Area
It is assumed that land that has formerly existed as intertidal habitat within the 20th century, and that has remained undeveloped, is more suitable for return to intertidal habitat than that which existed as intertidal only before the last century.

Criterion 5 – The Spatial Context of the Areas
Once the criteria above have provided the basis for identifying areas physically suitable for realignment, the size, shape, land elevation and proximity to existing habitats can be considered.

SIZE: Pilcher et al. (2002) suggest that it is not cost-effective to realign areas under 5ha in area. Larger areas of intertidal habitat are likely to provide greater benefits. For example, a larger intertidal area provides more area to store excess water (Coombes, 2003).

SHAPE: The optimum shape for realignment areas can be considered as a trade-off between creating a wide intertidal area to maximise benefits, while ensuring that the length of realigned defences to protect the surrounding land is no greater than those which already exist (Pilcher et al., 2002).

ELEVATION: It is preferable to realign defences in such a way that elevation may be used as a natural defence to absorb wave energy. This reduces the maintenance costs of the realigned defences (O’Riordan et al., 2000) and in cases of retreat to above the high spring tide level, a minimal or no defence may be required. Retreating to areas of high elevation also helps to maximise habitat creation benefits, as due to the progressive reduction in wave energy over the rising elevation, a range of associated habitats may develop.

PROXIMITY TO EXISTING INTERTIDAL HABITATS: It is preferable to create intertidal habitats where they will fit in with the overall vegetation succession to facilitate the movement of species between habitats.

* These categories are based on the Institute of Terrestrial Ecology (now Centre for Ecology and Hydrology) 25 land cover classification system.

Box 6. Identification of sites suitable for managed realignment
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