Restoration of salt marshes and mudflats
This article deals with technical aspects of restoration projects. Cultural, social and economic aspects also play a crucial role. For these aspects the reader is referred to Introduction of public participation and other articles in the categories Participation and governance in coastal management and Evaluation and assessment in coastal management.
Current efforts to restore estuarine salt-marsh systems in Europe and elsewhere illustrate the political and administrative value placed on the goods and services provided by this ecosystem.
The principle of managed realignment and managed retreat comes down to allowing salt-marsh areas, that were historically converted to alternative use for anthropogenic purposes (e.g. agricultural land or tourist development), to return to their natural state and area cover (Garbutt, et al., 2006). This can be done in a number of ways, but typically involves making a breach in the historically erected barrier (seawall, dike) rather than removing the whole structure. This approach reduces the costs involved, as well as the wave action depressing the development of the vegetation. Cost benefit analyses typically show a net advantage of managed realignment over other constructed defence options (Turner et al., 2007). An overview of managed realignment projects realized until 2014 has been compiled by Esteves (2014).
Full restoration of natural ecosystem function has met some complications. The substrates and biodiversity of pristine salt marshes is often markedly different from an artificial or restored system, even 100 years after natural processes have been allowed to operate (Hazelden and Boorman, 2001). Experience of estuarine restoration by managed realignment projects during the past decades has shown that the estuarine character of newly created mudflats and marshes is easily lost (Mazik et al., 2010). Marshes reactivated by managed realignment do not provide habitats and species in comparable proportions to natural marshes and do not have equivalent biological characteristics (Mossman et al., 2012). It is even questionable whether restored marshes satisfy the requirements of the EU Habitats Directive.
Two aspects, both related to sediment management, deserve particular attention when designing a marsh restoration scheme. The first aspect concerns the design of the tidal inlet structure. An adjustable inlet makes it possible to regulate the sedimentation rate in the restoration area and in this way influence the development of the creek system (Oosterlee et al., 2020). The second aspect concerns the origin of the sediment that is deposited in the restoration area. In estuaries with a small sediment supply (e.g. estuaries with strong ebb dominance), this deposition can alter the sediment balance of the estuary and cause or enhance erosion of flood protection barriers and intertidal areas elsewhere (Morris, 2012).
Because embanked land was often used for livestock farming, the soil has become compacted and poorly permeable. The freshly sedimented top layer of the restoration area has high hydraulic conductivity, organic matter content, macroporosity and a low bulk density, while the underlying relict polder soil has a very low hydraulic conductivity, a low micro- and macroporosity, a low organic matter content and a high bulk density (Van Putte et al., 2020). This hinders drainage, which supposedly affects both vegetation development and nutrient cycling. Measures to be considered are (Brooks et al., 2015; Lawrence et al., 2018): (1) deep plowing, (2) amending the soil with organic wastes to induce the development of macropore networks, (3) the creation of small creeks and hillocks. When formerly embanked land has changed too much, restored salt marshes may be considered novel ecosystem types, existing somewhere between unrestricted and tidally restricted systems (Gerwing et al. 2020).
Mudflats are the unvegetated or sparsely vegetated intertidal transition zones between the tidal channel and the higher vegetated salt marsh. They are home to a large number of organisms that provide a rich food source for fish and birds and fulfill an important nursery function for the estuarine ecosystem (Fig. 2). In addition, strong microbial activity in the sediment top layer fulfills an important water purification function. Estuarine mudflats are typically low diversity/high biomass/high abundance systems, hosting organisms adapted to alternating wet-dry and fresh-salt conditions. According to reports under the European Commission's Habitats Directive, the mudflats (habitat 1140) along the European Atlantic and Mediterranean coasts are in a poor state, especially due to the many land reclamation projects that have taken place in the past (EEA, 2015).
The removal of embankments creates new space for the expansion of mudflats. However, the high turbidity that occurs in many estuaries causes rapid sedimentation. In sheltered restoration areas, mudflats quickly evolve into salt marshes. Even in less turbid estuaries, this happens over time. Preservation of mudflats therefore requires active management involving, e.g. bed leveling, dredging and flushing (Pontee, 2014).
- Dynamics, threats and management of salt marshes
- Natural shore protecting barriers
- Spatial and temporal variability of salt marshes
- Garbutt, R.A., Reading, C.J., Wolters, M., Gray, A.J., Rothery, P. 2006. Monitoring the development of intertidal habitats on former agricultural land after the managed realignment of coastal defences at Tollesbury, Essex, UK. Marine pollution bulletin. 53(1-4): 155-164. DOI: 10.1016/j.marpolbul.2005.09.015.
- Turner, R.K., Burgess, D., Hadley, D., Coombes, E. and Jackson, N. 2007. A cost-benefit appraisal of coastal managed realignment policy. Global environmental change-human and policy dimensions. 17(3-4): 397-407. DOI: 10.1016/j.gloenvcha.2007.05.006.
- Esteves, L.S. 2014. Managed realignment:A viable long-term coastal management strategy? Springer Briefs in Environmental Science. New York, Springer, 143 pp.
- Hazelden J. and Boorman L.A. 2001. Soils and 'managed retreat' in South East England. Soil use and management 17(3): 150-154. DOI: 10.1079/SUM200166.
- Mazik, K., Musk,W., Dawes, O., Solyanko, K., Brown, Su., Mander, L. and Elliott, M. 2010. Managed realignment as compensation for the loss of intertidal mudflat: a short term solution to a long term problem? Estuar. Coast. Shelf Sci. 90: 11-20
- Mossman, H.L., Davy, A.J. and Grant, A. 2012. Does managed coastal realignment create saltmarshes with ‘equivalent biological characteristics’ to natural reference sites? J. Appl. Ecol. 49; 1446-1456
- Oosterlee, L., Cox, T.J.S., Temmerman, S. and Meire, P. 2020. Effects of tidal re-introduction design on sedimentation rates in previously embanked tidal marshes. Estuarine, Coastal and Shelf Science 244, 106428
- Morris, R.K.A. 2012. Managed realignment: A sediment management perspective. Ocean & Coastal Management 65: 59-66
- Van Putte, N., Temmerman, S., Verreydt, G., Seuntjens, P., Maris, T., Heyndrickx, M., Boone, M., Joris, I and Meire, P. 2020. Groundwater dynamics in a restored tidal marsh are limited by historical soil compaction. Estuarine, Coastal and Shelf Science 244, 106101
- Brooks, K.L., Mossman, H.L., Chitty, J.L., Grant, A., 2015. Limited vegetation development on a created salt marsh associated with over-consolidated sediments and lack of topographic heterogeneity. Estuaries Coasts 38: 325–336
- Lawrence, P.J., Smith, G.R., Sullivan, M.J.P. and Mossman, H.L. 2018. Restored saltmarshes lack the topographic diversity found in natural habitat. Ecological Engineering 115: 58–66
- Gerwing, T.G., Davies, M.M., Clements, J., Flores, A-M., Thomson, H.M., Nelson, K.R., Kushneryk, K., Brouard-John, E.K., Harvey, B. and Plate, E. 2020. Do you want to breach an embankment? Synthesis of the literature and practical considerations for breaching of tidally influenced causeways and dikes. Estuarine, Coastal and Shelf Science 245 (2020) 107024
- Elliott, M. and Whitfield, A.K. 2011. Challenging paradigms in estuarine ecology and management. Estuarine, Coastal and Shelf Science 94: 306-314
- EEA 2015. Report under the Article 17 of the Habitats Directive Period 2007-2012 1140 Mudflats and sandflats not covered by sea water at low tide. European Environment Agency European Topic Centre on Biological Diversity
- Pontee, N. 2014. Accounting for siltation in the design of intertidal creation schemes. Ocean and Coastal Management 88: 8-12
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