SUBSIDENCE
per part of the soil column. When the soil is drained, it and its contained organic matter (peat and organic-rich clay, depicted as black lenses in the [magnified portion of figure 7]) lose a substantial percentage of their original volume and are exposed to atmospheric oxygen. This process may result in a dramatic volume reduction of the organic matter in a short time, which at the surface appears as sinking of the ground. (In the main cross section [of figure 7], black arrows represent a decrease in the thickness of the sedimentary section over time, preferentially in the clay-rich layers. In the [magnified portion of figure 7], blue arrows represent the seepage and flow of water into a man-made excavation—in this case, a drainage canal—and black, dashed arrows represent the downward movement of the ground surface over time from volume loss caused by drainage and oxidation of organic-rich wetland soils).22
According to Christine Anderson et al., in a report by the American Society of Civil Engineers (ASCE): Large portions of Orleans, St. Bernard and Jefferson parishes are currently below sea level — and they continue to sink. New Orleans is built on thousands of feet of soft sand, silt, and clay. Subsidence, or settling of the ground surface, occurs naturally because of the consolidation and oxidation of organic soil and also the intervention of pumping out local groundwater. In the past, flooding and deposition of sediments from the Mississippi River counterbalanced the natural subsidence, leaving southeast Louisiana at or above sea level. However, because of the major flood control structures built upstream on the Mississippi River and levees built around New Orleans, fresh layers of sediment are not replenishing the ground lost by subsidence.20
Anderson, Campanella, McCulloh, Heinrich, and Good highlight how human intervention has accelerated the deterioration of New Orleans’ natural resilience. Artificial levees have inhibited fresh layers of sediment from replenishing the ground lost by subsidence. Pumping infrastructure has destabilized soil and also increased the rate of subsidence. The state of Louisiana is currently trying to figure out a way to deposit silt in a controlled fashion to replenish the soil and natural levee system.23
Figure 6 is a three-dimensional representation that illustrates the natural process of subsidence. It shows how soft sediment beneath Louisiana’s coast is sinking, due to water and gases being squeezed out by the soil’s own weight.21 McCulloh, Heinrich, and Good further explain the soil mechanics processes: [Figure 7 shows] schematic cross-sectional diagrams depicting subsidence at subregional and superficial scales in the New Orleans area. As shown in the cross section at the bottom, thick clay-rich layers in the subsurface compact more than sand layers and discrete sand bodies; the clay-rich layers expel water into the more permeable sands over time and further compact by concurrent mechanical rearrangement of the clay particles. In the New Orleans area, the resulting subsidence from this natural process has been accelerated by groundwater pumping that removes water from the sands, which are replenished with water expelled from the clayrich layers. At and near the surface [as show in the magnified portion of figure 7], human-induced alterations of the land greatly accelerate drainage of the up14