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Salt marsh as a Coastal Defence Salt marshes make an important contribution to coastal defences and are beneficial as they are natural and very economical when compared to hard defences. A salt marsh is a vegetated store of mud and silt. There are times when material is added to this store i.e. when the salt marsh is accreting, and there are times when this store is depleted i.e. when the coast is eroding. This can be part of a natural cycle of accretion and erosion. For details of salt marsh development click here. Beaches, mudflats and salt marshes are the most efficient method of wave energy dissipation and hence protection of the land behind the marsh. The salt marsh vegetation can further reduce the wave energy as :- 1. the roughness of the plants produce greater energy loss from waves 2. the vegetation binds the sediment together, thus increasing its stability 3. the leaves and stems increase the rate of vertical accretion by acting as wave baffles. (French, 2001) As a wave moves across a vegetated surface, the energy levels and wave height decrease exponentially. The amount and rate of decrease is a function of wave height approaching the marsh, distance travelled through the marsh, depth of water and density and size of plants. The vegetation composition will relate to the topographic elevation (frequency of tidal inundation) and sedimentation types and rates (Carter, 1999). If the area of salt marsh is large enough it can remove all of the energy of the wave under certain conditions. Approximately 50% of the wave energy and 40% of the wave height can be removed by the first 2.5m of salt marsh . However, where there is a sufficient depth of water the waves will travel above the vegetation and not be significantly affected by it (French, 2001). Threats to salt marshes include:- land reclamation for farming, recreation, housing and industry. In these circumstances the defences are usually replaced with hard structures. management works on river channels such as dredging and construction of training walls, these can affect the sediment budget and distribution. Sea level rise could have significant impacts on the amount of salt marsh especially if the back edge has been fixed by human development. In these circumstances the marsh is unable to respond by moving backwards and is eventually eroded back to the fixed line. Where there is space the salt marsh will realign its self further back. References Carter, R.W.G (1999)Coastal Environments. An introduction to the physical, ecological and cultural systems of coatlines. Academic Press, London. French, P.W (2001) Coastal defences. Processes, Problems and Solutions. Routledge, London. Home Page - Top www.seftoncoast.org.uk

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