GEOLOGIC FACTORS PROMOTING GROUND SUBSIDENCE AND COASTAL LAND LOSS IN THE MISSISSIPPI DELTA AND THE GREAT DEBATE ABOUT WHAT TO DO ABOUT IT

Coastal lands loss is a high visibility problem along the Gulf Coast, especially in the Mississippi Delta. The approximate rate of subsidence has varied between 8 and 13 mm/yr; averaging close to 1.3 m/100 yrs, with sea level rise being about 0.30 m/100 yrs. The rate of coastal land loss has varied between a minimum figure of 65 km2 and a maximum of 306 km2 (in 2005).

Much of this land loss emanates from channelization and emplacement of jetties along the lower Mississippi River, beginning in the late 1870s. These measures, and instream storage and flood control, have combined to reduce the river's annual average bedload from ~700 million to ~230 million tons/yr, since 1927.

The mechanisms promoting ground settlement in the Mississippi Delta region include the following factors: 1) Elastic deformation of Mississippi Delta from silt deposition (isostasy); 2) Tectonic compaction caused by formation of pressure ridges and folding; 3) Subsidence on seaward side of lystric growth faults; 4) Drainage of old swamp and marsh deposits increasing stress on underlying clays; 5) Biochemical oxidation of peaty soils due to drainage; 6) Consolidation of compressible soils caused by surcharging with fill; 7) Surcharging by structural improvements; 8) Reduced groundwater recharge because of increase in impermeable surfaces; 9) Extraction of oil; 10) Extraction of gas; and 11) Extraction of groundwater (all of which cause pressure depletion); and, 12) Solutioning of salt domes and seaward migration of low density materials (salt and shale).

The 2050 Reclamation Plan would seek to restore 65 to 78 km2 of coastal land over the next 40 to 50 yrs, at a cost of $14 billion. The scientific community is not in harmony about what mechanisms are most responsible for land loss, nor what measures can realistically be undertaken to mitigate the impacts of the land loss, which will engender greater risk from flooding induced damage and casualty loss with time. Since Hurricane Katrina, public opinion polls have become increasingly wary and/or dubious of making the necessary investments to mitigate theses natural hazards.