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Coastal erosion in Southeast Louisiana is the process of the steady depletion of wetlands in the region, such as marshes, swamps, and barrier islands, affecting the alluvial basin around the mouth of the Mississippi River at the foot of the Gulf of Mexico. In the last century, Southeast Louisiana has lost a large portion of its wetlands and is expected to lose more in the coming years, with some estimates claiming wetland losses equivalent to up to 30 football fields per day. One consequence of coastal erosion is an increased vulnerability to hurricane storm surges, which affects the New Orleans metropolitan area and other communities in the region. The state has outlined a comprehensive master plan for coastal restoration and has begun to implement various restoration projects such as fresh water diversions, however the certain zones will have to be prioritized and targeted for restoration efforts, as it is unlikely that all depleted wetlands can be rehabilitated. The process of coastal erosion is the result of various factors, particularly the blockage of traditionally occurring deposits of fresh water and silt from the river caused by man-made levees which have been built up and down most of the river over the last century, which now impede the river's ability to replenish its southernmost alluvial plains which are constantly dependent on the infusion of the river's once plentiful deposits which usually occurred during annual high stage floods in the springtime, the kind of which the river-levees now serve to buffer against for the protection of residents, livestock, and property residing in regions adjacent to the river throughout the Mississippi River valley. The deterioration results in the death of fresh and brackish water plants historically part of the ecosystem, which are not only a vital feature of the wetlands' topography, but also serve to capture silt, and are thus are needed to build up and sustain marsh structures. As fresh and brackish water plant habitats recede, salt water from the Gulf of Mexico further encroaches, killing off more non-saltwater plants, thus further eroding pre-existing mud formations which these plants had once supported. Other factors acerbating coastal erosion in Southeast Louisiana include the presence of canals and navigational routes dug though marshes and swamps, often to accommodate logistical needs of the petrochemical industry, as well as the previous practice of logging, all of which have allowed the incursion of saltwater (saline) from the Gulf into previously fresh and brackish water plant habitats. Land subsidence, is also a factor, which may be partially a result of this process and/or other factors, further compounding the problem. Sea-level rise attributed to global warming, though not a root cause, is also increasingly a contributing factor. ==Causes and factors== Man-made levees, which were designed to protect residents and property adjacent to the river, block spring flood water that would otherwise bring fresh water and sediment to marshes. Swamps have been extensively logged, leaving canals and ditches that allow saline water to move inland. Canals dug for the oil and gas industry also allow storms to move sea water inland, where it damages swamps and marshes. Rising sea levels attributed to global warming have exacerbated the problem. Some researchers estimate that the state is losing a land mass equivalent to 30 football fields every day. An extensive levee system aided by locks and dams has been developed in the waterways of the lower Mississippi River.〔Reuss, M. (1998). Designing the Bayous: The Control of Water in the Atchafalaya Basin 1800–1995. Alexandria, VA: U.S. Army Corps of Engineers Office of History.〕 The levees, designed to prevent flooding along the waterways, on one hand, prevent silt from draining into the river yet also from being distributed into the marshes downriver. With no new accretion and with steady subsidence, the wetlands slowly are replaced by encroaching saltwater form the Gulf. As a result of this apparent dilmma, large areas of marsh are being lost to the ocean. Since 1930 water has consumed more than 1,900 square miles (4,900 km2) of the state's land. This loss equates to the disappearance of 25 square miles (65 km2) of wetlands each year or a football field sized area every 30 minutes. This loss can be reversed, at least in some areas, but only with large scale restoration, including the removal of levees to allow the Mississippi River to carry silt into these areas.〔Turner, R. E. and Streever, B. 2002. Approaches to Coastal Wetland Restoration: Northern Gulf of Mexico. The Hague, the Netherlands: SPB Academic Publishing.〕 Prior to the building of levees on the Mississippi River, the wetlands were kept in balance by occasional floods, which fill the area with sediment, and subsidence, the sinking of land. After the levees were built, however, flood sediment flowed directly into the Gulf of Mexico. This subsidence along with the recent sea level rise tipped the balance toward subsidence rather than marsh growth. This, along with the canals built in the area, caused decline of the wetlands and also caused less weakening of and less protection from recent hurricanes such as Hurricane Katrina.〔 The Lake Pontchartrain Basin Foundation has developed a (comprehensive management plan ) for the eastern regions of the Louisiana coast, placing emphasis upon restoration of riverine habitats, cypress swamps and fringing marsh. This could be a model applied to other coastal regions. Subsidence may be due to other factors as well. Some observers blame the direct effects of oil and gas extraction. They contend that, as billions of barrels of oil and saltwater and as trillions of cubic feet of gas were removed from the subterranean structures in which they had accumulated over millions of years, these structures lost their ability to support the weight of the earth above. As these structures slowly collapsed, the soil above gradually subsided. The wetlands on the surface began to sink into the gulf waters. Others argue that subsidence is a natural process in deltas, as sediments compress, and that the real problem is the lack of flood waters that would normally deposit new layers of sediment.〔Boesch, D. F., Josselyn, M. N., Mehta, A. J., Morris, J. T., Nuttle, W. K., Simenstad, C. A., and Swift, D. P. J. (1994). Scientific assessment of coastal wetland loss, restoration and management in Louisiana. Journal of Coastal Research, Special Issue No. 20.〕 The role of hurricanes is also a matter of disagreement; some studies show that hurricanes actually build elevation in marshes.〔Liu, K. and Fearn, M. L. (2000). Holocene history of catastrophic hurricane landfalls along the Gulf of Mexico coast reconstructed from coastal lake and marsh sediments. In Current Stresses and Potential Vulnerabilities: Implications of Global Change for the Gulf Coast Region of the United States, eds. Z. H. Ning and K. K. Abdollhai, pp. 38–47. Baton Rouge, LA: Franklin Press for Gulf Coast Regional Climate Change Council.〕 A new and important factor is rising sea levels associated with global warming. In addition to subsidence is the presence of canals which were dug through the marshes and swamps to service oil and gas wells to facilitate oil and gas exploration, which have enabled salt water intrusion from the Gulf of Mexico. In similar fashion, the construction of the now-closed Mississippi River Gulf Outlet (MRGO) introduced salt water into freshwater and intermediate marshes in St. Bernard Parish, which is adjacent to New Orleans, and facilitated significant erosion.〔"Closing the Mississippi River Gulf Outlet: Environmental and Economic Considerations." Coastal Wetlands Planning Protection and Restoration Act. Louisiana State Government. Web. 27 Feb 2013. Another factor that damaged wetlands was large scale logging, particularly the extensive logging of cypress forests in the early 1900s. One early logger described it this way: "We just use the old method of going in and cutting down the swamp and tearing it up and bringing the cypress out. When a man's in here with all the heavy equipment, he might as well cut everything he can make a board foot out of; we're not ever coming back in here again"〔Keddy, P.A. 2008. Water, Earth, Fire: Louisiana's Natural Heritage. Xlibris, Philadelphia. 229 p. P. 122.〕 This logging often required construction of canals, which, once the logging was finished, allowed salt water to enter the wetlands and prevent regeneration of the cypress.〔 As if these problems were not enough, the introduction of nutria an invasive species from South America in the 1930s provided an entirely new species of grazing mammal. Although only a few escaped, there are now millions.〔Keddy, P.A. 2008. Water, Earth, Fire: Louisiana's Natural Heritage. Xlibris, Philadelphia. 229 p. P. 148-150.〕 Natural grazing by muskrat was now accelerated by grazing from nutria. By removing plants, nutria both cause loss of vegetation, and, perhaps more seriously, a loss of dead organic matter which would otherwise accumulate as peat and raise the level of the marsh〔Keddy, P.A. 2000. Wetland Ecology: Principles and Conservation. Cambridge University Press, Cambridge, UK. 614 p. P. 163-165.〕 One of the most important natural controls on nutria is large alligators, which may provide a useful tool for biological control of nutria, and therefore for reduced impacts of grazing.〔Keddy, P.A., L. Gough, J.A. Nyman, T. McFalls, J. Carter and J. Siegrist. 2009. Alligator hunters, pelt traders, and runaway consumption of Gulf coast marshes: A trophic cascade perspective on coastal wetland losses. p. 115-133 in B.R. Silliman, E.D. Grosholz, and M.D. Bertness (eds.) Human Impacts on Salt Marshes. A Global Perspective. University of California Press, Berkeley, CA.〕 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「Coastal erosion in Southeast Louisiana」の詳細全文を読む スポンサード リンク
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