History:

• New Orleans/ most of southern Louisiana is built on alluvial sediment deposits and therefore has a foundation of mostly porous, moist soil with a lot of organic matter
• When the land was first settled, most of it had to be drained in order to be developed; which resulted in lowering of the water-table and thus drying out and compacting of the once moist soil (Fischetti, 2001)
• Today, the land is still subsiding at a debatable but significant rate (predicted rates range from 1-15mm per year (McCulloh, Heinrich, and Good, 2006))

Causes:

Natural:

• Large amounts of sediment are being transported down the Mississippi River from the erosion of land along the way and deposited in a stack at the edge of the continent. The great volume of this load causes depression of the layers of the Earth’s crust creating a subsidence effect at the surface. These large piles also tend to slope southwards (because of their enormous weight), magnifying the sliding fault effect (McCulloh, Heinrich, and Good, 2006; Dokka, 2005).

Human:

• Building levees along the course of the Mississippi River restricts the natural floodplain allowed by a large amount, leaving the sediment transported by the river with nowhere to go but be deposited on the riverbed (causing it to rise) or be carried down to the continental shelf to contribute to the large volume of ancient sediment there weighing down on and compressing the crust layer. In addition, the periodic floods that would have occurred (without the levees in place) could have helped balance the subsidence rates in the land nearby the river by depositing fresh sediment to replenish the compacted soil (Dokka, 2005; McCulloh, Heinrich, and Good, 2006).
• Drainage projects that occur before development (such as buildings or canals) lower the water table and dry out some of the naturally moist soil, causing it to compact. Some housing projects have been reported to experience 1 foot of yard subsidence over the course of a year because of this effect (Heerden 2006). Also, the high subsidence rates that occurred when groundwater extraction was more prevalent in the southern Louisiana area decreased significantly during the 1980s when the process was switched to surface water collection (Dokka 2005).
• Any sort of building project that involves digging also exposes deep layers of organic-rich soil to the atmosphere, causing it to oxidize and decrease in volume (McCulloh, Heinrich, and Good, 2006)
• The canals that are built in and around the city of New Orleans to collect rainwater are currently pumped regularly to remain empty because water from the naturally moist surrounding soil can seep into these canals. As a result of this pumping, the soil is drying out and consequently compacting as a result of loss of water (Fischetti, 2001).

The Debate:

• Is there really anything we can do? Some would argue that the natural causes (especially tectonic activity such as faulting due to sedimentation) play such a large role in the process that subsidence cannot be significantly slowed by man.
• According to a study conducted by geologist Roy Dokka on the Michoud fault in southeastern Louisiana (deep enough below human oil and water extraction in the area to effectively ignore its influence), significantly higher subsidence rates were measured along the hanging wall portion (sliding downwards) than on the footwall (shifting upwards); indicating that the fault is a major factor of subsidence in the area. Between 1969 and 1971 (when the fault was very active), subsidence rates were measured to be -16.9 mm/year, accounting for about 73% of the total subsidence measured in the area (Dokka, 2006). Yet, a recent study appearing in Geology has concluded that although tectonic activity may have been greater in the past (possibly strongly influencing subsidence), it is currently at a stable state and therefore accounts for little of the current subsidence rates (Tornqvist et al 2006).
• If human activities (such as canal digging, resource excavation, and modern development) were to be the major cause of such high subsidence rates, it is again debatable that there is little we can do to stop it. New development must occur if there is to be revival in the area post-Katrina. Also, coastal Louisiana is the source for around 1/5 of the nation’s oil and 1/4 of the nation’s natural gas, implying that to get rid of part of those industries would be a major blow to the region’s economy as well as to the country’s economy (Fischetti, 2001).