Saltmarsh and mudflats are usually located together with mudflats in front of the saltmarsh. Saltmarsh vegetation and saltmarsh creeks help manage floods by dissipating wave and tidal energy. They are valuable barriers to the risks of flood, as they dissapte wave and tidal energy. Saltmarshes used in combination with other measures can have beneficial outcomes to managing climate change impacts. Even a small width of fronting saltmarsh can significantly reduce the height of sea walls required to achieve the same level of protection and thus also reduce initial construction costs. Having saltmarsh fronting will also significantly reduce maintenance costs due to the reduced exposure to wave and tidal energy.
Where is restoration of saltmarsh appropriate?
Saltmarsh and mudflats are unique habitates comprised of fine grain sediments (silts and clays) that settle out of the water colomn in calm areas or where there is low water speed. Saltmarsh is usually found in estuaries or sheltered areas such as bays or at the head of sea lochs. The overall shape of the estuary or bay determines the location and extent of saltmarsh and mudflat. Development of mature saltmarsh typically takes between 40 to 80 years. However, this will not be possible in all locations, particularly where existing protection structures restrict the establishment of higher zones.
Determining whether conditions are good for the development of saltmarsh is difficult. In addition to naturally occurring physical processes, the nutrients determine if conditions are favourable for the establishment of saltmarsh and chemical pollutants can affect whether saltmarsh can colonise mudflat. Therefore, it is generally better to increase the extent or facilitate the relative stability of existing saltmarsh, rather than attempt to establish this habitat in new areas where it has not been present historically.
The size of the saltmarsh habitat is often determined by the tidal range. To increase the size or space for a saltmarsh habitat, it can be extended seawards or re-aligned by moving existing coastal structures inland.
According to Forbes et al. (2015), sediment can be placed at various levels in the morphological profile:
- a thin layer of sediment can be sprayed over existing habitat to increase existing intertidal elevation; or
- sediment can be placed in the intertidal zone to artificially increase the intertidal area; or
- sediment can be placed in the sub-tidal zone to reduce erosion from intertidal margins
To encourage sediment to settle and saltmarsh to establish the following techniques can be used:
- Brushwood fascines/groynes: Small wooden posts erected in parallel rows and in-filled with brushwood to create a small fence. Other materials can be used but brushwood has been found to be the most durable. The best orientation is generally at right angles to the foreshore.
- Polders: Brushwood fences or fascines are erected that enclose a width of mature marsh with a similar sized seaward extent of mudflat. Ditches are dug to collect deposited sediment, which is then piled onto banks between the ditches.
Saltmarsh can be left to naturally colonise the mudflats. However, unless there are good natural sources of local seeds, planting or sowing will be needed. Planting has generally been shown to be more effective than sowing.
The majority of the costs associated with establishing saltmarsh will be the associated costs of re-charge or managed realignment required. If polders are used, the costs of establishing and maintaining these can also be significant.