Coastal erosion in Louisiana is the process of steady depletion of wetlands along the state's coastline in marshes, swamps, and barrier islands, particularly affecting the alluvial basin surrounding the mouth of the Mississippi River. In the last century, coastal Louisiana has lost an estimated of land, approximately the size of Delaware's land area.Couvillion, B.R., Beck, Holly, Schoolmaster, Donald, and Fischer, Michelle, 2017, Land area change in coastal Louisiana 1932 to 2016: U.S. Geological Survey Scientific Investigations Map 3381, 16 p. pamphlet, https://doi.org/10.3133/sim3381. Coast wide rates of wetland change have varied from to annually, with peak loss rates occurring during the 1970's. 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, but 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 in Louisiana is the result of various factors, including sea level rise; ordinary subsidence of organic materials; deprivation of periodic sediment due to flooding prevention measures; tropical cyclones; oil and gas extraction and infrastructure; navigation infrastructure; and saltwater intrusion. While land subsidence is dominated by Glacial Isostatic Adjustment (GIA),Wolstencroft, M.; Shen, Zh.; Törnqvist, T.; Milne, G.; Kulp, M.; 2014. Understanding subsidence in the Mississippi Delta region due to sediment, ice, and ocean loading. Insight from geophysical modelling" [in:] Journal of Geophysical Research: Solid Earth, 119, 3838-3856, 31 Mar 2014; published online: 28 Apr 2014, http://www.tulane.edu/~tor/documents/JGR2014.pdfTörnqvist, T.; Wallace, D.; Storms J.; Wallinga, J.; van Dam, R.; Blaauw, M.; Derksen, M.;, Klerks, C.; Meijneken, C.; Snijders, E., 2008. Mississippi Delta subsidence primarily caused by compaction of Holocene strata, [in:] Nature Geoscience 1, 173 - 176 (2008), Published online: 17 February 2008 | doi:10.1038/ngeo129 http://www.nature.com/ngeo/journal/v1/n3/full/ngeo129.html sediment compression is next factor further compounding the problem.Thomas, D. et al., 2016. The Dictionary of Physical Geography, Fourth Edition, Wiley Blackwell, p. 471 Sea level rise attributed to global warming, though not a root cause, is also considered a contributing factor and future concern.
Causes and factors
+Estimated Wetland Area Change in Coastal Basins, 1932–2016BasinEst. ChangePct. Starting AreaTerrebonneBaratariaCalcasieu–SabineMermentauPontchartrainBreton SoundMississippi RiverTeche–VermilionAtchafalaya River
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 prevent it from being distributed into the marshes downriver. With no new accretion and steady subsidence, the wetlands slowly are replaced by encroaching saltwater from the Gulf. As a result of this apparent dilemma, 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 filled the area with sediment, and subsidence, the sinking of land. These man-made levees contribute to extensive down-stream flooding and sediment pollution in the Mississippi River Delta. 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.Tidwell, Mike. The Ravaging Tide: Strange Weather, Future Katrinas, and the Coming Death of America's Coastal Cities, Free Press, 2006. 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, known as fracking. They believe the removal of subsurface materials, such as oil, hastened the rates of land subsidence. They contend that, as billions of barrels of oil and saltwater and 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 the rising sea levels associated with global warming.
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." 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.
The introduction of nutria, an invasive wetland rodent from South America, in the 1930s provided an entirely new species of grazing mammal. Although only a few escaped, there are now millions. Natural grazing by muskrat was now accelerated by grazing from nutria. By removing plants, nutria cause both 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 predation by large alligators, which may provide a useful tool for biological control of nutria, thereby reducing their impact on marshes.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.
Terrebonne and Lafourche Parishes in Southeast Louisiana, with a combined population of 209,136, are at great risk of going underwater due to coastal erosion. It is estimated with the current rate of erosion, 75 square kilometers a year, these areas and their surrounding parishes will be underwater within fifty to eighty years.
Erosion from heavy storms, climate change, and human interference with the environment contribute to erosion in South Louisiana. The Gulf of Mexico brings heavy rains and hurricanes to this region. This loosens the sediments in the marshes, and along the Mississippi River, allowing them to be carried away by the water.
Oil and gas industry Oil and gas extraction in the Louisiana coastal zone began in 1926 and peaked in 1970 with 72 million barrels of oil. Although most oil and gas extraction has shifted offshore to the outer continental shelf, the construction of channels and pipelines continues in the wetlands. Infrastructure, such as canals, pipelines, and other features, contributes to wetland loss not only through direct removal of material but also by altering hydrologic flow: salt water may more easily intrude, and wave action may propagate through canals. For example, 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. <http://lacoast.gov/new/Data/Reports/ITS/MRGO.pdf> In addition to the canals themselves, spoil banks also alter local hydrology.