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A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation and leisure activities from the action of tides and waves.〔Kamphuis, W J. (2010) Introduction to Coastal Engineering and Management. World Scientific Publishing Co Ltd. Singapore.〕 As a seawall is a static feature it will conflict with the dynamic nature of the coast and impede the exchange of sediment between land and sea.〔Shipman, B & Stojanovic, T. (2007) “Facts, Fictions, and Failures of Integrated Coastal Zone Management” in Europe Coastal Management. Vol. 35, Issue 2, p375 - 398 .〕 The coast is generally a high-energy, dynamic environment with spatial variations occurring over a wide range of temporal scales.〔Allan, J C, Kirk, R M, Hemmingsen, M & Hart, D. (1999) Coastal Processes in Southern Pegasus Bay: A Review – A Report to Woodward-Clyde New Zealand Ltd. and the Christchurch City Council. Land and Water Studies Ltd. Christchurch.〕 The shoreline is part of the coastal interface which is exposed to a wide range of erosional processes arising from fluvial, aeolian and terrestrial sources, meaning that a combination of denudational processes will work against a seawall.〔Fletcher C H, Mullane R A & Richmond B M. (1997) “Beach loss along armored shorelines on Oahu, Hawaiian Islands” in Journal of Coastal Research. Vol. 13, No 3. P 209-215〕 Given the natural forces to which seawalls are constantly subjected, maintenance (and eventually replacement) is an ongoing requirement if they are to provide an effective long-term solution. The many types in use today reflect both the varying physical forces they are designed to withstand, and location specific aspects, such as: local climate, coastal position, wave regime, and value of landform. Seawalls are classified as a hard engineering shore based structure used to provide protection and to lessen coastal erosion. However, a range of environmental problems and issues may arise from the construction of a seawall, including disrupting sediment movement and transport patterns, which are discussed in more detail below.〔Kraus, N & McDougal. (1996) “The Effects of Seawalls on the Beach: Part I: An Updated Literature Review” in Journal of Coastal Research. Vol. 12, No. 3.〕 Combined with a high construction cost, this has led to an increasing use of other soft engineering coastal management options such as beach replenishment. Seawalls may be constructed from a variety of materials, most commonly: reinforced concrete, boulders, steel, or gabions. Additional seawall construction materials may include: vinyl, wood, aluminium, fibreglass composite, and with large biodegrable sandbags made of jute and coir.〔Clarke, J R. 1994. Integrated Management of Coastal Zones. Fao Corporate Document Repository, USA.〕 In the UK, ''sea wall'' also refers to an earthen bank used to create a polder, or a dike construction. ==Types== A seawall works by reflecting incident wave energy back into the sea, thereby reducing the energy and erosion which the coastline would otherwise be subjected to.〔Kajendra, R. (2011)〕 In addition to their unsightly visual appearance, two specific weaknesses of seawalls exist. Firstly, wave reflection induced by the wall may result in scour and subsequent lowering of the sand level of the fronting beach.〔Masselink, G and Hughes, M J. (2003) ''Introduction to Coastal Processes and Geomorphology''. Oxford University Press. New York. Ch 11.〕 Secondly, seawalls may accelerate erosion of adjacent, unprotected coastal areas because they affect the littoral drift process.〔NOAA. (2007) Shoreline Management: Alternatives to Hardening the Shore. Retrieved online 15 April 2011 from: http://coastalmanagement.noaa.gov/shoreline.html〕 The design and type of seawall that is appropriate depends on aspects specific to the location, including the surrounding erosion processes.〔GeoResources. (2001) Coastal management. Retrieved online 18 April 2011 from: http://www.georesources.co.uk/coastman.htm〕 There are three main types of seawalls: vertical, curved or stepped, and mounds, as set out in the table: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「seawall」の詳細全文を読む スポンサード リンク
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