The biblical verse Matthew 7:26 admonishes the foolish man who builds his house upon the sand, or, in the case of New Orleans, upon the sand, silt, and soft clay. The "Big Easy" sits atop a geologic foundation of soft earth, and the weight of the city literally causes it to sink about 3 feet (0.9 meters) every century. This sinking feeling is known in geological circles as subsidence, and perhaps nowhere in the United States is subsidence a more precarious issue than in the Mississippi River delta.
Before the early 20th century, delta subsidence was counterbalanced--albeit destructively--by seasonal flooding of the Mississippi River, which deposited roughly equal layers of the aforementioned sand, silt, and clay atop the delta region, rebuilding the land even as it continued to slowly compact. This flood-powered rebuilding process is known as sedimentation.
The counterbalanced effects of subsidence and sedimentation were only barely understood in 1889, when the federal government first began to finance the construction of levees to control flooding along major rivers in the United States. That levee system would prove grossly inadequate when the great Mississippi Flood of 1927 claimed the lives of 700,000 Americans throughout the Mississippi River valley.
The 1927 floods led to the 1928 Flood Control Act and related congressional measures that saw the federal government's role expand from merely underwriting levees to overseeing the design, construction, and maintenance of almost the entire national flood-control infrastructure. Unfortunately the vast and complicated levee systems built under these federal initiatives paid almost no heed to the balance of subsidence and sedimentation, especially in the Mississippi delta. The delta levees effectively cut off many "protected" lands from the supply of flood sediments that would have rebuilt the subsiding earth beneath them.
In simplest terms, protecting certain areas behind flood-proof levees actually made them more vulnerable to flooding. Denied the benefits of sedimentation, these lands dropped farther and farther below the flood plain, making them more dependent on protective levees. An "arms" race had thus begun between the U.S. Army Corps of Engineers, which endeavored to build ever-stronger levees, and the inexorable processes of nature. This precise circumstance was writ large in the city of New Orleans, most of which sat several feet below sea level on a narrow strip of land between the Mississippi River and Lake Pontchartrain.
Surrounded by high water on nearly every side, all that stood between the "Crescent City" and certain destruction were decades-old levees and pumping systems, none of which were designed to withstand a direct hit by anything stronger than a Category 3 hurricane. When Katrina made landfall just east of New Orleans as a Category 4 storm, the results were devastating. The levees protecting New Orleans were breeched, allowing the water to finally reclaim the city that for decades it had been denied.
Flood defenses put in place after the epic 1927 flood made New Orleans safer in the short term but placed the city in grave danger over the long term. In the wake of America's worst natural disaster of the young 21st century, the entire Gulf Coast, the scientific community, and the federal government must learn another hard lesson about flood control--one that enables them to build a city and levee system that work with Mother Nature, rather than against her.
