Why is New Orleans sinking?
Until the 20th century, New Orleans lay above sea level. Today large areas of the city have fallen below the water levels of the Mississippi River and Lake Pontchartrain.
This unusual circumstance is the product of New Orleans’s situation in the Mississippi Delta. The city’s location has been its blessing and its curse. The city owes its existence to its location between the Mississippi River and Lake Pontchartrain: that proximity made it an ideal transfer point between the North American interior and the Gulf of Mexico. However, this situation in the dynamic landscape of the Mississippi River Delta has made New Orleans vulnerable to sinking.
When New Orleans was founded in the early 18th century, it occupied the high, dry land along the river’s natural levees. The passage to the lake could be made by boat with a short portage; and the lake offered access to the gulf via Lake Borgne.
The cypress swamps between the river and the lake lay just above sea level, but they were too wet to occupy permanently, and until the early twentieth century, the city’s development was confined to the stable high ground along the Mississippi.
The sinking city
In the first half of the twentieth century, mechanical pumping technology enabled the draining and subdivision of the city’s back-of-town swamps. The reclamation of these soggy areas had an unexpected consequence: it made ground levels fall.
This process, called subsidence, occurred through different mechanisms. Organic matter in the soil oxidized, so soil volume was reduced. As pumping extracted water from the ground, soil particles collapsed onto each other. The removal of the cypress swamps brought an end to soil creation through organic decomposition. Finally, the levees that had been constructed along the length of the Mississippi to stop flooding prevented the replenishment of soil by alluvial material.
By the turn of the twenty-first century, the city had become a giant sink. Ground levels had fallen to as low as twelve feet below sea level; the city was completely surrounded by levees; and the only way to remove water from drains and sewers was by pumping it over the levee to Lake Pontchartrain.
The low ground in New Orleans continues to sink, and even the high ground that constitutes approximately half the city must be drained mechanically. These circumstances created havoc during and after Hurricanes Katrina and Rita, when both pumps and levees failed.
They demand a new approach to water management, one that recognizes and adapts to the dynamic ecology of New Orleans’s location between the river and the lake, in the world’s fourth-largest delta.
The city’s surface has subsided, but not at equal rates: some soil types are more susceptible than others. Beyond that, different types of structures respond differently to subsidence. For instance, slab on grade foundations tip and tilt as the ground sinks, but pile foundations remain in place as the earth around them disappears.
Photograph by Erin Dorr, Gutter to Gulf Studio 2010.