How to Use
Once the dike is breached and the managed realigned area is subject to tidal influence, the area will start to develop. Development of salt marshes is crucial for two reasons. First, once salt marshes develop, large scale erosion is unlikely to occur. Salt marsh vegetation will enhance sedimentation and creation 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 salt marshes because they are a valuable habitat that can support biodiversity and ecosystem services. The practice of managed realignment should therefore by aimed at the development of salt marsh vegetation. Several boundary conditions need to be met in order to be successful.
In the table below important boundary conditions as elevation, sediment load etc. for development of a managed realigned area are described. These boundary conditions can serve as guidance for area selection and design.
| Criteria | Threshold | Habitat | Comments | References |
|---|---|---|---|---|
| Site selection | Location | Salt marsh | Formerly reclaimed land where salt marshes were present before embankment and where there is limited erosion (relatively sheltered) are most suited for managed realignment. | (French, 2006) |
| Site selection | Land use | Salt marsh | Areas which are relatively undisturbed in terms of geo-chemical soil conditions. For example a former meadow area is more likely to develop into a salt marsh than a ploughed field | (French, 2006) |
| Elevation | Mean High Water Spring-Mean High Water Neap | Salt marsh | Minimum of 400-500 inundations per year. From 1 m below MHWN colonization by pioneer vegetation takes place. | (Toft & Maddell, 1995; French, 2008) |
| Mean High Water Neap-Mean Low Water Spring | Intertidal flat | |||
| Slope | Possible:0-7% (1:0-1:18) Optimal:1-2% (1:0-1:64) | Salt marsh | Slope determines the drainage. A gentle slope in seawards direction reduces flow velocity and enhances sedimentation. | (Burd, 1995) |
| Size | Fetch < 2000 m | Salt marsh Intertidal flat | The “fetch” (length or width) and depth are determining for the degree in which waves can develop and cause resuspension of sediment. A study by French et al. (2000) describes erosion of 10 mm/year in realignment areas with a fetch of 2000 m and sedimentation of 7-16 mm/year with fetch 400 m. | (French et al., 2000) |
| Shape | Length/width ratio | Salt marsh Intertidal flat | An elongated area that is far more deep than wide supposedly has a lower initial accretion, because most of the sediment has settled by the time the tide has traveled to the back of the area. | (Van Oevelen et al., 2000) |
| Suspended sediment | 1-10 mg/l survival 10-100 mg/l growth | Salt marsh | Survival of salt marshes depends on the degree of sea level rise. Besides the suspended sediment concentration the sedimentation rate also depends on elevation of the area which determines the water depth and plant characteristics/age of the salt marsh and estuarine characteristics (river discharge and tide). | (Borsje et al., 2011; Li & Yang, 2009; Temmerman et al., 2004) |
Safety standards
The coastal buffer zone can face a higher flood risk. Where flood risk standards for inhabited areas are set to 1/4000 year or even 1/10.000 year, a standard of 1/100 or even 1/50 is allowable in the buffer zone. This will reduce costs on dike maintenance and strengthening considerably.
Costs
Costs of managed realignment sites widely differ and there is no standard guideline that indicates costs. This is because the concept is still young and few data are available but also because of many variables that influence costs. Breached realignment is the most frequently used method and is therefore elaborated here. Numerous factors can influence costs of a breached realignment site, for example:
- Land of construction (costs of realignment in development countries are lower than in developed countries)
- Costs of the inland area that will be re-inundated
- Compensation to land owners
- Costs for dismantling buildings, roads etc.
- Costs for a new inland sea defence
The table below gives an indication of costs of managed realignment sites in the Humber estuary. Based on these examples costs of a breached realignment site are roughly €100.000 per hectare on average. These costs include the construction of a new inland sea defence.
Costs of breached realignment sites in the Humber estuary (UK) according to IECS (2008)
| Breached realignment sites | Area (ha) | Costs (€) | Costs per hectare (€/ha) |
|---|---|---|---|
| Chowder Ness (2006) | 18,5 | 1.253.552 | 67.759 |
| Paul Home Strays (2003) | 45 | 8.855.541 | 198.789 |
| Alkborough (2006) | 440 | 12.20.286 | 27.741 |
Salt marshes attenuate wave energy, therefore the new inland dike can be less strong and high and maintenance costs are reduced. Maintenance cost savings will vary from site to site according to wave climate, coastal topography and consequent defence works (Turner et al., 2007). The table below shows how salt marshes before the sea defence influence dike height and costs involved based on a report by the English Environment Agency (2006).
Relation between salt marshes and dike costs in the UK, based on EA (1996)
| Salt marshes before dike (m) | Dike height (m) | Costs of sea wall (€/m) |
|---|---|---|
| 0 | 12 | 6000 |
| 6 | 6 | 1800 |
| 30 | 5 | 960 |
| 60 | 4 | 600 |
| 80 | 3 | 480 |
A study by Hillen et al. (2010) describes the costs for heightening dikes in the Netherlands based on several studies:
- 9-11 M€/km for 1 metre higher in rural areas
- 13,8-21,6 M€/km for 1 metre higher in urban areas
Maintenance costs of dikes are estimated at 0,1 M€/km/year in the Netherlands (ACPM, 2006).
Benefits: ecosystem services and multifunctional use
Besides sea defence, managed realignment sites also provide other goods and services (ecosystem services) and have potential for multifunctional use as recreation, aquaculture and cultivation of saline crops. A study by De Mesel et al. (2013) describes several forms of aquaculture and cultivation of saline crops that are possible in a re-inundated saltwater tidal area. As the main goal of a realigned area is elevation increase by sedimentation, land use forms should be extensive without hampering the sedimentation process. The potential benefits of aquaculture is described in the Concept Coastal Buffer Zone.