Subsidence Louisiana

A rise in sea level would further accelerate the rate of land loss in coastal Louisiana. Even a 50-centimeter rise in sea level (in combination with land subsidence) would inundate almost aU of the delta and would leave New Orleans, most of which is below sea level and only protected with earthen levees, vulnerable to a hurricane. 

Hatton, R.S., Delaune, R.D., and Patrick, W.H. (1983) Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana. Limnol. Oceanogr. 28, 494-502. 

Germany demonstrated commercial-scale operation of DCL during World War II, employing the Bergius process, which they had developed. After World War II, the United States constructed a 200-300 bbl/day DCL plant at Louisiana, Missouri, based on the German technology. This plant was operated by the Bureau of Mines from 1949 to 1954. However, as the extent of petroleum deposits in the Middle East and other locations became apparent and fears of an oil shortage subsided, interest in DCL diminished. 

The relative rates of vertical marsh accretion and submergence determine the long-term stability of Louisiana coastal marshes. Coastal marshes are highly susceptible to submergence associated with a rise in relative sea level (Penland and Ramsey, 1989). Louisiana coastal marshes are undergoing rapid subsidence and currently experiencing rapid increases in the water level. Research conducted over the past quarter century in coastal Louisiana has shown that marsh accretion at many... 

Compaction of Holocene sediments includes dewatering of sediments (primary consolidation), rearrangement of mineral structure of the sediment and subsequent loss of volume (secondary consolidation), and the decomposition of organic matter in the sediment. This is the most important process affecting subsidence in Louisiana coastal areas however, it is a very difficult process to evaluate. 

A series of diversion projects have been implemented to reintroduce freshwater and sediment from the Mississippi River into Louisiana coastal wetlands (Figure 18.9). A recent study examined the impact of Mississippi River freshwater diversion on enhancing vertical marsh accretion (mineral and organic matter accumulation) in Breton Sound estuary, a coastal wetland experiencing marsh deterioration as a result of subsidence and saltwater intrusion (DeLaune et al., 2003). The Caernarvon diversion has positively impacted marsh accretion in Brenton Sound estuary helping to slow or reverse wetland loss. Several hundred hectares of new marsh have been created by the introduction of Mississippi River water into the system (Villarrubia, 1998). 

Much of the wetland loss occurring in coastal Louisiana is due to the deterioration of highly organic marsh soil. As discussed earlier, conversion of coastal marshes to inland open water is associated with plant stresses such as saltwater intrusion into nonsaline marshes and increased soil waterlogging as a result of subsidence. Marsh elevation decreases rapidly following plant mortality because of the structural collapse of the living root networks (DeLaune et al., 1994). The peat collapse and the associated erosion result in the conversion of marsh into open water. Conversions to open water system releases a considerable amount of carbon into the estuary where it is either decomposed or... 

The Louisiana Mississippi River deltaic plain is undergoing rapid subsidence with parallel increases in water level and saltwater intrusion. As a result there is rapid wetland loss. 

Reed, D. J. 1989. Patterns of sediment deposition in subsiding coastal salt marshes, Terrebonne Bay, Louisiana the role of winter storms. Estuaries 12 222-227. 

Lowering of a barrier island by subsidence is compounded as barrier sand is transported into the bay by overwash diu ing storms. The existing barrier elevation is reduced, making future overwash more hkely, and the overwash deposit ( washover ) begins to load the previously imconsolidated substrate. The newly loaded sediment base then begins the primary consolidation process. Over long periods of time, these barrier islands are eroded and distorted by successive storms, potentially migrating into the bay. They ultimately may become subaqueous, such as Ship Island Shoal in Louisiana. ... 

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