The Need for Coastal Defences - Essay Example

s Coastal Defence The term 'coastal defence' is a much used, but much abused term for referring to any feature along the coast or estuary designed to protect a beach or land. It is more correct, however, to distinguish between two forms of coastal defence. First, flood or sea defences are structures used to prevent the land from being flooded by the sea; examples include earth embankments in estuaries. Second, there are coastal protection structures, a term used to describe measures taken to prevent the land from being eroded, such as sea walls along the base of cliffs. (Cooper 9-15) In estuaries, tidal processes facilitate the accumulation of thick deposits of sand, silt, and clay, reaching to the upper limit of the tidal range. The process of land claim means that it is necessary to exclude the sea from part or all of this intertidal area, and also protect this area from reinundation. Land claim for agriculture and industry generally takes in the higher salt marsh because the higher elevation of the intertidal area claimed means, first, that the wave activity will be reduced by the lower marshes and mudflats fronting the area to be reclaimed. (Yates 235-249) In cases where there are not sufficient areas of such deposits fronting the proposed claim, then the area is perhaps not well suited. Second, less material is needed to build up the newly created dry land. Third, the higher the elevation, the lower the sea walls need to be to prevent tidal overtopping. Finally, agriculture needs good-quality farmland, and the upper marshes provide the most 'mature' sediments available in respect of the processes of soil formation. (Archer 103-120; Knecht 183-199) There are few areas of coastline in the 'developed' world which have not been subject to some form of land claim, defence works, or development. (Louisse 10-15; Fischer 437-447) It has been the tendency for coastal populations to utilise their immediate environment to the full, obtaining as much land as possible in order to increase their agricultural or industrial potential, and to defend low-lying land to increase their security from flooding by the sea. As far as port development in estuaries is concerned, one of the key requirements is that the port itself should be afforded as much shelter as possible. Traditionally, this meant that estuaries were the most suitable sites and, because of the small size of the earliest vessels, many of the early ports were built some way upstream from the estuary mouth. As ship size has increased, so these earlier ports have become uneconomic, and thus many have been relocated downstream, towards the estuary mouth. (Guy 219-248) This has meant that the majority of the world's major estuaries have some form of port development located within them. In addition, even these areas may not be suitable for the larger vessels of today, and thus many estuaries have been artificially deepened by dredging to allow for the increased draught of modern ships. This process leads to an artificial intertidal profile in which natural processes are in constant competition with the alien environment in which they find themselves. (Beatley 1-22) As a result, especially over the past few centuries, many estuaries have experienced considerable modification to their natural ecosystems, which have brought about changes in floral patterns and bird populations. From a process point of view, the dredging and land claiming which have occurred in estuaries have also produced changes in circulation patterns, tidal regime, and sediment deposition patterns, causing further knock-on effects for natural habitats and wildlife populations. (Barston 93-116) The need for coastal defences Because of the need to protect the newly claimed land from reinundation by the sea, it is also a requirement of land claim to install some form of flood defence. (Doody 275-283) This defence is largely to keep the sea out, rather than an anti-erosion structure, and so tends to be more a flood defence measure than a measure for coastal protection. Typically this defence is an earth embankment, sometimes faced with stone, although occasionally defences may be concrete walls, particularly in cases of. high hinterland value, such as land claim for industrial development, nuclear power stations, or port activity. These flood defences are often constructed using material derived from the upper salt marshes. Methods of obtaining this material can vary, but in the Severn estuary in the UK, for example, the upper marsh is scraped, and the material piled up to form the embankment. (Charlier 525-543) The result of this is an upper marsh largely devoid of vegetation, and lower by a few centimetres. The implications for the system are that vegetation needs to recolonise the area and an equilibrium state be regained before any significant vertical marsh accretion can occur. The process of vertical marsh accretion is essential if a marsh is to keep pace with sea level rise. (Day 241-257; Boorman 1-7) An alternative method can be observed on the east coast of England. In the Wash, individual pits (borrow pits) are dug to a depth of a few metres adjacent to where the new defence is to be built. Again, material excavated is piled up to form the flood defences. As compared to the earlier method, digging discrete pits means that most of the upper marsh remains undisturbed, although parts of this are lowered considerably. The methodology states that infilling with water during flood tides will input sediment, and vegetation will begin to colonise, with the end result that the pit will, after a period of around fifteen years, merge in to the rest of the marsh. While this is generally the case, recent work has indicated that in a significant number of pits there is only limited infilling, and, far from being accretional, the pits are behaving as tidal reservoirs, discharging large volumes of water on the ebb tide and causing erosion of the marsh creeks, and internal dissection of the marsh itself. (Cicin-Sain 11-43; Brooke 151-157) Clearly, the onset of erosion due to borrow pit construction indicates a failure of the borrow pit methodology. It is possible that the problems in these cases lie in the fact that the pits were dug too deep, and so lie below the level where marsh vegetation can normally colonise. Because of this, any sediment that does accumulate in the pits is rapidly removed by scour, owing to the high velocities reached during the ebb tide. It is also possible that the explanation is more complex than this, including variations in wind/wave climate or the decline in sediment availability due to coastal defences. Whichever of these explanations, or combination of reasons, is correct, poor construction methods and/or poor understanding of the system hydraulics have produced a situation which is the reverse of that naturally operating in the local environment; that is, localised areas of erosion in an environment of general accretion. Works Cited Archer J.H. and Knecht R.W. (1987) The United States national coastal zone management program: problems and opportunities in the next phase. Coastal Management 15(2): 103-120 Barston R.P. (1994) International dimensions of coastal zone management. Ocean and Coastal Management 23: 93-116 Beatley T., Bower D.J., and Schwab A.K. (1994) An Introduction to Coastal Zone Management. Island Press, Washington, DC, 1-22 Boorman L.A. and Hazelden J. (1995) Saltmarsh creation and management for coastal defence. In Healy M.G. and Doody J.P. (eds) Directions in European Coastal Management. Samara Publishing, Cardigan, 1-7 Brooke J.S. (1992) Coastal defence: the retreat option. Journal of the Institute of Water and Environmental Management 6: 151-157 Charlier R.H. and de Meyer C.P. (1989) Coastal defence and beach renovation. Ocean and Shoreline Management 12: 525-543 Cicin-Sain B. (1993) Sustainable development and integrated coastal management. Ocean and Coastal Management 21:11-43 Cooper N. (1996) Beach replenishment: implications for source and longevity from results of the Bournemouth schemes. Unpublished conference abstract, Coastal Defence and Nature Conservation, Portsmouth University, March, 9-15 Day J.W. and Templet P.H. (1989) Consequences of sea level rise: implications from the Mississippi Delta. Coastal Management 17(3): 241-257 Doody J.P. (1992) Sea defence and nature conservation: threat or opportunity Aquatic Conservation: Marine and Freshwater Ecosystems 2: 275-283 Fischer D.W., Stone G.W, Morgan J.P., and Henningsen D.E. (1986) Integrated multidisciplinary information for coastal management, Florida. Journal of Coastal Research 2(4): 437-447 Guy W.E. (1983) Florida's coastal management program: a critical analysis. Coastal Zone Management Journal 11(3): 219-248 Knecht R.W. and Archer J. (1993) Integration in the US coastal zone management program. Ocean and Coastal Management 21: 183-199 Louisse C.J. and Kuik T.I. (1990) Coastal defence alternatives in the Netherlands. In Louisse C.I., Stive M.I.F., and Wiesma H.I. (eds) The Dutch Coast: Report on a Session of the 22nd International Conference on Coastal Engineering. Delft Hydraulics, 10-15 Yates B.F. (1994) Implementing coastal zone management policy: Kepulauan-Seribu marine park, Indonesia. 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