Project Delivery / Construction
Royal Haskoning (2010) provides some recommendations about the construction phase of the Managed Realignment. The recommendations consider the way of creating desired elevations and establishment of vegetation. Further recommendation on the construction of managed realignments are listed in CIRIA (2004).
Creating desired elevations
If the potential restoration site is subsided below the elevation required for the desired wetland habitats, then filling may be required. This is particularly so if the desired habitat is saltmarsh, but the site is not at elevations conducive to vegetation colonization. Two different strategies can be adopted to raise the elevation of a restoration site:
1) Take advantage of natural sedimentation
In this technique the required elevations are achieved by taking advantage of the natural deposition of suspended sediments brought into the restored site on flood tides. The rate of accretion will depend on the suspended sediment concentrations carried into the site, the amount that is deposited from suspension and the amount of sediment that is eroded and carried out of the site on ebb tides. The expected rate of sedimentation at a restored site can be predicted from measurement of nearby suspended sediment concentrations, observed rates of sedimentation at similar restoration sites and/or local established saltmarsh areas.
If the desired wetland elevations cannot be achieved by natural sedimentation and artificial infill is too costly or impractical, then techniques can be adopted to accelerate sedimentation rates. This means maximizing the amount of sediment that is deposited on the flood tide and/or minimizing the amount that is eroded and leaves on the ebb tide. Options are:
- Wave breaks
- Sedimentation fields
- Re-introducing river sediment inputs
2) Fill with imported material
If the rate of natural sedimentation is predicted to be too low to reach the target
elevations, then the alternative strategy (if practical and affordable) is to fill the site with imported material. The technique could use sediment derived from various places including borrow pits on the site, nearby ponds or newly created tidal channels. Larger volume fill could be derived from the navigational dredging of ports and harbours (or other remote areas), providing a beneficial reuse for this material. The most suitable method to directly place material on the restoration site is by hydraulic pumping.
Establishment of vegetation
1) Natural Colonisation, Seeding or Planting
In situations where saltmarsh habitat is the restoration objective, natural vegetation colonisation is generally preferred over seeding or planting. This is because natural colonisation will reflect the existing species and allow the vegetation community to change over time from initial colonisation to site maturity. This will provide a range of plant species that can adapt to future change and are suited to the niche environment offered by the restoration site. However, there may be some situations where natural colonization will not take place and seeding or planting is necessary. In these cases, the plant source should be from an existing saltmarsh close to the restoration site so adaptation to local conditions can take place.
2) Soil Treatment
Vegetation colonization to restore saltmarsh requires a suitable substrate in the rooting zone in terms of its soil chemistry, particle size and bulk properties. Wetland plants are adapted to take advantage of and thrive in naturally deposited sediments. Filled sites may have unsuitable substrates, perhaps due to high acidity, low nutrients or excessive compaction. A potential strategy for dealing with this problem (in addition to fill removal)is to modify the soil substrate, by artificial addition of sediment. In general, wetland plants prefer to grow in sediment finer than sand (which is not compacted or polluted).
When tidal action is restored to a subsided tidal wetland, physical processes are set in motion that dictate the rate and manner in which the site will evolve. As long as the site is sheltered from significant wind-wave action and is at the appropriate elevations, it will evolve in response to coastal sedimentation processes, from intertidal mudflat, to initial mudflat colonization by salt tolerant marsh plants, to ultimately a fully mature vegetated marsh plain (see Figure). Subtidal (lagoon) habitats could also form across lower parts of the site, if the site is low relative to the tidal frame.