The study, published in Science Advances, used satellite radar data to track subtle shifts in ground elevation across Greater New Orleans between 2002 and 2020. The study found that some neighborhoods, wetlands and even sections of floodwalls are sinking by more than an inch per year — with some areas experiencing up to 47 millimeters (nearly 2 inches) of elevation loss annually.

“In a city like New Orleans, where much of the land is already near sea level, even minor drops in elevation can increase flood risk,” said Simone Fiaschi, lead author of the study and a former researcher with Tulane’s Department of River-Coastal Science and Engineering, now employed at TRE-Altamira.

The findings underscore how both natural and human-driven forces are reshaping the city’s landscape. Causes of the sinking — known as subsidence — include natural soil compaction, groundwater pumping, industrial development and the legacy of wetland drainage for urban growth.

The study used a remote sensing technique called InSAR (Interferometric Synthetic Aperture Radar), which detects millimeter-scale changes in land surface elevation by comparing satellite radar images taken over time. This allowed the researchers to build the most detailed map yet of vertical land motion in New Orleans — including areas like wetlands that had previously lacked reliable data.

Among the most troubling findings: some of the concrete floodwalls and levees built to protect the city after Katrina are themselves sinking. In a few cases, parts of the Hurricane and Storm Damage Risk Reduction System (HSDRRS) are losing elevation faster than sea levels are rising, reducing their capacity to block storm surges.

“These results are a wake-up call,” said co-author Prof. Mead Allison, also of Tulane. “We need ongoing monitoring and maintenance to ensure that our flood defenses don’t lose their level of protection beneath us.”

The study also found pockets of sinking around industrial sites, the airport and newer residential developments — areas where soil compression and groundwater withdrawal are likely contributors. In contrast, some areas such as parts of Michoud showed modest land uplift, likely due to the halt of industrial groundwater pumping and recovery of the water table.

Wetlands east of the city, long known for their ecological importance, are also sinking rapidly in places. In some spots, the loss of elevation could transform marshes into open water within a decade if trends continue. This has implications not just for wildlife but also for storm protection, as wetlands help buffer storm surges.

New Orleans, much of which lies below sea level, relies on an elaborate system of levees, pumps and drainage canals to keep water out. As sea levels rise and the ground sinks, the margin for error narrows.

Experts say that without sustained monitoring, including satellite data and ground-based measurements, it’s difficult to know where to reinforce levees or how to plan for future storms.

“This research shows that land movement isn’t uniform, and understanding these patterns is crucial for protecting lives and property in a city where inches truly matter,” Fiaschi said. “However, it’s crucial to remember that our results still require careful ground-truthing. This is especially true for critical areas like the floodwalls, where on-site verification was not possible during this project.”

The study highlights the potential of satellite monitoring to guide infrastructure maintenance and urban planning, not just in New Orleans but in coastal cities worldwide facing similar challenges.