Parts of New Orleans, including its international airport, are sinking nearly two inches per year, according to a new study. Wetlands and parts of the city’s levee system are sinking, too.
The geography of New Orleans resembles a bowl, and it’s protected from flooding by a system of earthen levees, concrete flood walls, pumps and canals that took the Army Corps of Engineers nearly 15 years and $15 billion to build. That makes it particularly vulnerable to rising sea levels and other environmental stresses.
“Subsidence can compromise protective infrastructure over time,” said Leonard Ohenhen, an expert on remote sensing at the University of California, Irvine, who was not involved in the study. “I hope we get more work like this, quantifying subsidence over time in cities.”
As levees and flood walls sink, they can crack and accumulate structural damage. They also become shorter, making them less effective against storm surges and rising sea levels. That’s a particularly big concern in New Orleans because the Gulf of Mexico has the fastest sea level rise in the country.
To assess where and how quickly land elevation in New Orleans has changed in recent decades, the authors of the new study, published in the journal Science Advances, used radar collected by satellites over two time periods, from 2002 to 2007 and from 2016 to 2020. In each period, a satellite passed over the city multiple times, letting researchers compare elevations over time. The later time series included the levee system and wetlands. The researchers then compiled a map showing the land’s rising and sinking over time.
Much of the city was stable during the study period, but some hot spots of elevation change — around levees, wetlands, industrial sites and the airport — stood out.
Levees and flood walls are sinking up to 28 millimeters per year, with the fastest rate of levee height loss near the airport and wetlands.
“These rates may affect the flood protection system in the next few decades,” said Simone Fiaschi, a remote sensing expert who led the new study while at Tulane University.
New Orleans’s storm surge protection system was built largely in response to Hurricane Katrina, which devastated the city in 2005, killing more than 1,800 people and causing more than $160 billion in damage. Now, with funding cuts to the Federal Emergency Management Agency and a month into hurricane season, the city is concerned about its ability to respond to storms.
Some settling is natural after levees and flood walls are installed simply because they’re heavy and the ground is soft.
“In New Orleans, you’re kind of building on pudding,” said Ricky Boyett, a spokesman for the Army Corps of Engineers who was not involved in the study. “If you build anything on that ground, it’s going to sink.”
The new map is useful for identifying spots where settling is either increasing or happening faster than expected. The levees and flood walls were designed to accommodate some future settling, but the measurements will help repair teams prioritize which levees are maintained first.
The New Orleans airport, in the northwestern part of the city between Lake Pontchartrain and the Mississippi River, is one of the fastest-sinking sites, dropping nearly three centimeters, or nearly one inch, per year. Other industrial sites around the city are sinking 20 to 50 millimeters per year.
And wetlands could be sinking three to nearly five centimeters per year, although these results need to be confirmed by on-the-ground measurements, Dr. Fiaschi said. Wetlands’ elevations are notoriously difficult to measure using satellites with radar imagery because of fluctuating water levels and vegetation.
“If the wetlands are subsiding this much, they are slowly dying,” he said. They could disappear within decades, he added, taking with them important ecosystems and buffers of flood water.
Large areas of the city are stable, and some spots are even rising. For example, a former power plant site has been gaining about six millimeters of elevation per year because groundwater pumping stopped.
“One surprising thing was the uplift, which is not usually expected in a city,” Dr. Fiaschi said. “It means that some of this settling can be reversed, at least partially, just by halting the exploitation of water.”
The map from the study gives the city and the Army Corps of Engineers updated, high-resolution information on where the levees will need height added, Mr. Boyett said.
Levees are designed to be continually built up. Flood walls are harder to repair, Mr. Boyett said, but they are sinking less quickly because they are smaller and therefore lighter. Building a taller levee in the first place would require a larger footprint, taking away valuable space in the city.
Responsibility for monitoring, maintaining and repairing all of the flood walls and levees transferred to the state of Louisiana in 2022, but since then, Congress has authorized the Army Corps of Engineers to resume a shared support role and assess what it would take to protect the city from a more extreme flood than the system was originally designed for.
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