NEW ORLEANS - Parts of New Orleans and its surrounding wetlands are gradually sinking, and while most of the city remains stable, a new study from Tulane University researchers suggests that sections of the region’s $15 billion post-Katrina flood protection system may need regular upgrades to outpace long-term land subsidence.
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.
Interferometric Synthetic Aperture Radar
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.
"We used multiple radar images collected by two different satellites from 2002 to 2020 to detect and measure ground motion in the greater New Orleans area," said Fiaschi. "Thanks to a technique called Interferometric synthetic aperture radar, or InSAR, it is possible to measure motion of objects on the Earth surface with very high precision, up to a few millimeters per year."
Findings - Flood Protection Infrastructure
The findings demonstrate 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.
Among the most troubling findings is that 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.
"There are two issues here—elevation of places within the floodwalls that protect the city and elevation of the floodwalls/levees themselves," said Mead Allison, co-author and Professor at the School of Science and Engineering at Tulane University. "For the former, anything that decreases ground elevation for a structure, regardless of mechanism (geological subsidence, groundwater withdrawal, etc.) means that flooding depths will increase incrementally in a given event through time. Those floods could be rainwater precipitation, or water exceeding the containment of the protection system in a future hurricane. For the latter (Hurricane and Storm Damage Risk Reduction System or HSDRSS-level of protection), any decrease in elevation of the floodwalls/levees means its easier for a given storm surge to exceed the protection level."
The upshot is that New Orleans needs to monitor the elevation of its levees and floodwalls regularly. "Continued regular monitoring of HSDRSS elevation is needed for the periodic recertification of the 1 in 100 y level of protection," said Allison. This refers to the statistical probability of a flood of a certain size occurring in any given year. A “1 in 100 year” flood has a 1% chance of happening in any single year.
“These results are a wake-up call,” said Allison. “We need ongoing monitoring and maintenance to ensure that our flood defenses don’t lose their level of protection beneath us.”
Allison stated that there is one caveat in relation to the HSDRSS elevation. "Our data is for 2016-2021, immediately after the completion of construction of the HSDRSS. Were the high rates of elevation loss we saw in certain sections (the whole system is not rapidly subsiding) due to immediate, post-construction settling, or will they continue post 2021? This goes back to the need for regular monitoring of their elevation," said Allison.
While the full period of analysis was broader, between 2002-2020, for this particular report the team focused on the more recent ground motion within the 2016-2020 dataset.
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.
"I don't think there's any imminent implication related to insurance rates or livability of the city. Our work suggests the need for continued and comprehensive monitoring of critical areas and assets such as the floodwalls. This is an attempt to raise awareness, not to spread concerns," said Fiaschi.
The work of Fiaschi and Allison provides unprecedented spatial coverage of land motion rates in the greater New Orleans region where, previously, there was a lack of data. The report also provides a more detailed examination of the underlying drivers of ground deformation patterns which is crucial for making informed decisions about the region's stormwater managements systems including levees, pumps, and drainage canals.
