New Orleans floods because roughly half the city sits below sea level, the land is still sinking, and the region’s clay soils can barely absorb rainfall. These factors combine with its location between the Mississippi River, Lake Pontchartrain, and the Gulf of Mexico to create a city that essentially functions as a bowl surrounded by water on all sides.
A City That Sank Into a Bowl
New Orleans was originally built on natural high ground along the Mississippi River’s banks, and in its early days the entire city sat above sea level. That changed as the city expanded into surrounding marshland. Draining those wetlands for development caused the soft, organic soils underneath to compact and oxidize, and the ground began to drop. By the 1930s, one-third of the city’s land surface had sunk below sea level. By the 2000s, that figure reached roughly half.
The depth varies dramatically by neighborhood. Parts of Broadmoor sit 3 to 6 feet below sea level. Lakeview and Gentilly range from 5 to 8 feet below. Some areas of Metairie and New Orleans East are 6 to 12 feet below sea level. Meanwhile, the French Quarter and other neighborhoods along the river’s natural levee remain at or slightly above sea level, which is why they stayed relatively dry even during Hurricane Katrina.
The Ground Is Still Sinking
The sinking hasn’t stopped. Scientists call this process subsidence, and it happens at different rates across the metro area. Downtown New Orleans sinks at a relatively modest 0 to 2 millimeters per year. But the Lakeshore area loses 8 to 13 millimeters per year, and some suburban areas are dropping even faster. Near Louis Armstrong International Airport, land sank at rates up to 27 millimeters per year between 2016 and 2020. The Harvey area, across the river, lost elevation at rates up to 21 millimeters per year during roughly the same period.
Several forces drive this sinking. The thick layers of river sediment beneath the city naturally compact under their own weight over time. Groundwater pumping for drinking water and industrial use pulls water out of underground layers, causing the soil above to compress further. When nearby industrial groundwater withdrawal stopped in one area in 2016, researchers actually observed the land rebounding slightly, confirming the connection. Oil and gas extraction deeper underground contributes as well. The result is that even as global sea levels rise, the ground in New Orleans is simultaneously dropping to meet the water.
Clay Soil That Won’t Absorb Rain
Most of New Orleans sits on clay soil with very low permeability. When rain falls on clay, it pools on the surface rather than soaking into the ground. In a city built on sandy or loamy soil, a significant portion of rainfall would filter down naturally. In New Orleans, nearly all of it has to be moved mechanically.
This is why even a routine afternoon thunderstorm can flood streets. The city receives about 64 inches of rain per year, one of the highest totals of any major U.S. city, and virtually all of it has to be captured and pumped out. Green infrastructure like rain gardens and permeable pavement can help in small areas, but their effectiveness depends heavily on the surrounding soil and fluctuating groundwater levels, both of which work against drainage in most of the city.
A Pump System With Hard Limits
Because gravity can’t drain a city that sits below the water surrounding it, New Orleans relies on one of the largest pumping systems in the world. The city uses a network of canals and massive pump stations to push rainwater up and out, primarily into Lake Pontchartrain. The three main outfall canal pump stations at the lake can move a combined 24,300 cubic feet of water per second. The largest single station, at the 17th Street Canal, handles 12,600 cubic feet per second on its own.
That’s an enormous amount of water, but it has a ceiling. When rainfall rates exceed what the pumps can move, water backs up in the streets. A storm dropping 2 or 3 inches per hour across large sections of the city can overwhelm the system, even when every pump is running. Mechanical failures compound the problem. In August 2017, a heavy rainstorm flooded parts of the city in part because several pumps were offline for maintenance. The system also depends on an aging network of underground drainage canals that can develop blockages and capacity issues of their own.
Hurricanes and the Mississippi River
The most catastrophic flooding comes from storm surge during hurricanes. New Orleans sits near the mouth of the Mississippi River, along a coast that funnels Gulf of Mexico storm surge directly toward the city and Lake Pontchartrain. When a hurricane pushes water northward into the shallow lake, the surge can overtop or breach the levees and floodwalls protecting the city’s below-sea-level neighborhoods. This is exactly what happened during Katrina in 2005, when multiple levee failures allowed the bowl to fill.
The Mississippi River itself poses a separate flood threat. When heavy rainfall across the Midwest swells the river, New Orleans depends on a system of spillways to relieve pressure. The Bonnett Carré Spillway, located upstream of the city, diverts excess river water into Lake Pontchartrain. Since it was built in the 1930s, the Army Corps of Engineers has opened it repeatedly: 10 times between 1937 and 2011, with some openings lasting more than two months. At full capacity, it can divert 250,000 cubic feet of water per second to keep river flow past New Orleans below 1.25 million cubic feet per second. Without it, the river levees protecting the city could be overtopped during major floods.
The Post-Katrina Protection System
After Katrina, the federal government spent over $14 billion building the Hurricane and Storm Damage Risk Reduction System. This network of upgraded levees, concrete floodwalls, surge barriers, and pump stations is designed to protect against a storm surge with a 1-in-100 chance of occurring in any given year. The massive Lake Borgne Surge Barrier, one of the largest in the world, blocks storm surge from entering the city through the waterways that channeled Katrina’s floodwaters.
The system significantly reduces risk, but it doesn’t eliminate flooding. It’s designed for a specific level of storm, not the worst-case scenario. A hurricane stronger than the design parameters, or one that stalls and dumps extreme rainfall, could still cause major flooding. The system also has a built-in vulnerability: because the land keeps sinking, the effective height of levees and floodwalls decreases over time unless they’re raised to compensate.
Rising Seas on Sinking Land
Sea level along Louisiana’s coast is rising faster than the global average, because the land subsidence adds to the effect of rising oceans. Under intermediate projections, sea level in Louisiana is expected to rise 17 inches between 2020 and 2050. That’s roughly a foot and a half in just 30 years.
For a city already sitting in a bowl, higher surrounding water levels mean the pumps have to work harder to push water out, storm surge starts from a higher baseline, and the margin of safety in the levee system shrinks. Every inch of sea level rise effectively makes the bowl deeper. Combined with continued land subsidence and the likelihood of more intense hurricanes in a warming Gulf of Mexico, the fundamental conditions that cause New Orleans to flood are getting worse, not better.