How long should we keep fixing up New Orleans? The geology of New Orleans and much of the surrounding area suggests it is no longer reasonable to try to keep restoring structures because the area will be increasingly vulnerable in the future.
The Gulf of Mexico used to extend much farther north, but over time, the Mississippi has transported sediment -- clay, silt, sand and gravel -- that has built a huge delta into the Gulf of Mexico leading to the current shoreline. The river would fill one area with sediment, then change course and fill another lower spot.
Downtown New Orleans is located on coarser sediment that spilled out of the river during floods and is where the initial settlers chose to locate because it represented a local high point. As the city grew, a marsh was selected for expansion, which required the building of protective levees as the site was below normal river levels.
Any rain falling here has to be pumped out into the river. During one major flood, the river upstream started taking an entirely new course, which would have left New Orleans high and dry. Engineers built a dam to block the river from its new path and forced it back into the channel that goes past New Orleans. Low spots and high spots are common in deltas as the river changes course and sediment is deposited.
If river’s course changes were the only process going on, it might pay to fix disaster damage, but there are other processes at play that are very hard or impossible to correct. The clay-size sediment drops out of suspension in slow or stagnant water (often associated with vegetation) and is not packed.
Clay minerals resemble very small flat plates. Picture a large pile of clay plates dropped at random. There are openings (pore spaces) amongst the plates, and if vegetation is present, there is potential for more pore spaces. While clay-sized sediment in deltas has small pore spaces between individual mineral grains, the total volume of pore space in clay layers is large and filled with water.
As more sediment is piled on top of the clay layers, the added weight causes the clay layers to compact, leading to smaller pore spaces. The process is slow but over time causes the overlying land to sink -- more over thick clay layers -- even if no new overlying sediment were added.
As the Mississippi River is mostly channeled now, river-transported sediment is now carried out into the gulf. Regardless, compaction of existing underlying clay layers continues. In the next 100 years, there will be double-digit feet of compaction, and New Orleans will be way below sea level.
When groundwater is pumped, water pressure and buoyancy supporting the pores in clay’s decreases and compaction occurs. The same is true when oil is pumped. The land surface around Fresno, California, has dropped more than 40 feet since groundwater pumping started. In Mexico City, well casings project dozens of feet above ground as clay compacts and the land surface drops with pumping. Venice is a prime example. These processes cannot be reversed as the clay minerals deform.
Some estimates are that New Orleans will be 80 feet below sea level by the end of the century. Couple this with global warming and sea level rise, should vulnerable areas like New Orleans be fixed up every time there is a “natural disaster?” I use the quotes because more people are becoming increasingly vulnerable to catastrophic damage because of issues discussed above and their actions.
It is natural for rivers in flood to occupy their flood plain, that is how it was formed. Any tinkering with this is opening new issues, for instance, what if the levees break? Nature will win over time.
Darryll Pederson is an emeritus professor of hydrogeology in the Earth and Atmospheric Sciences Department at the University of Nebraska-Lincoln.