Study raises flood concerns due to New Orleans land sinking

By Fritz Esker
Contributing Writer

A new study from Tulane University researchers suggests that the $15 billion post-Katrina flood protection system for New Orleans and the surrounding areas may need regular upgrades to keep up with long-term land sinking.

The study, published in Science Advances, utilized satellite radar data to track small shifts in ground elevation across the Greater New Orleans area between 2002 and 2020. It used a remote sensing technique called InSAR (Interferometric Synthetic Aperture Radar), which is capable of detecting millimeter-scale changes in land surface elevation by comparing satellite radar images taken over time. According to the study, some neighborhoods, wetlands and parts of flood walls are sinking by over an inch a year.

“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, in a press release. Fiaschi is now with TRE Altamira.

The study said the most important factor of the sinking is sediment compaction in the deltaic-coastal geomorphic setting of New Orleans. But other factors, like groundwater pumping for increased drinking water supply, oil and gas withdrawals and other land-use changes have contributed to the increased sea level rise rates occurring in densely populated coastal areas. The sea level rise can degrade drainage and sewage systems and rainfall runoff. It also progressively degrades the level of storm surge protection by the floodwalls and levees.

The study added that reductions in ground elevation relative to sea level due subsidence (sinking) likely played a part in both the levee failures during Hurricane Katrina and the extreme post-storm flooding depths in parts of the city.

In a few cases, the study noted parts of the protection system are losing elevation faster than sea levels are rising. There were also pockets of sinking around industrial sites, the new airport and newer residential developments. A contrast was Michoud, which showed modest land uplift, probably because industrial groundwater pumping was halted.

Wetlands east of the city are sinking rapidly in places. Some marshes could be transformed into open water within a decade if trends continue. These marshes help provide a buffer against incoming hurricanes and tropical storms.

So, what do the study’s findings mean going forward regarding New Orleans’ ability to protect itself from hurricanes and flooding?

Mead A. Allison, professor and chair of Tulane’s Department of River-Coastal Science and Engineering, participated in the study. He spoke of the challenges of predicting a future for the efficacy of the flood walls surrounding New Orleans.

“Forecasting the future is difficult for two reasons,” Allison said via email. “Our survey period (2016-2020) was immediately following the completion of the HSDRRS (Hurricane & Storm Damage Risk Reduction System) and what we may be observing is structure settling that will tail off in the future (or not). A second complication is that, due to the spatial resolution of the space-based system, we can’t resolve the narrow flood walls separate from the apron (concrete and earthen). So, it’s unclear if the high rates we have observed in some areas are the apron (settling) or the walls themselves. It’s the height of the walls themselves that is critical for storm surge protection.”

Going forward, Allison recommends continued monitoring (it is currently five years past the period surveyed in the study) to answer the important questions about the flood walls.

“Mitigation is relatively easy – raise walls and levees at intervals in time to mitigate the incremental elevation loss,” Allison said.

Allison also emphasized that while the study highlighted areas of concern that should be monitored carefully and mitigated going forward, it is not a reason for New Orleanians to panic.

“It’s clear that most of the city and even most of the HSDRSS is relatively stable with vertical motions within the error (-2 to +2 mm/y). That should encourage folks not to panic – the results could have been a lot worse,” Allison said.

This article originally published in the July 21, 2025 print edition of The Louisiana Weekly newspaper.