News•August 26, 2015

Katrina: Lasting Climate Lessons for a Sinking City

This week marks a decade since Hurricane Katrina spun violently toward the coasts of Louisiana and Mississippi, ravaging both states when it barreled ashore on Aug. 29, 2005.

Katrina taught New Orleans and the Gulf Coast many lessons about how vulnerable the region is to natural disaster, especially to sea level rise and storm surge made worse by climate change. But a more complex, man-made problem also threatens New Orleans and it was captured in the indelible images taken in the aftermath of the hurricane, when miasmal flood waters submerged up to 80 percent of the city: as sea levels rise, the Crescent City is sinking.

About 80 percent of New Orleans flooded when Katrina barreled ashore in 2005.
Credit: Kelly Garbato/flickr

New Orleans flooded because the levees protecting it broke after the hurricane struck. The water stayed put, however, because the city is in a bowl dipping below sea level — and that bowl is getting deeper, sinking at a rate of up to 4 feet a century, primarily because the surrounding swamps were drained so the metro area could be expanded.

Accounting for the land’s subsidence, the sea level in southeast Louisiana is expected to rise by more than 20 inches by 2050. That, coupled with increased tropical storm intensity driven by climate change — and the inexorable disappearance of the coastal wetlands that act as a storm surge buffer — has put New Orleans in a precarious position in a warming world.

“As sea level rises, the vulnerability of the land that is exposed to the ocean is higher if the land is sinking,” Virginia Burkett, a lead author of the Intergovernmental Panel on Climate Change’s Fifth Assessment Report and the chief scientist for global change at the U.S. Geological Survey, said. “The rate of subsidence of the land’s surface here in Louisiana is two to three times the global rate of mean sea level rise.”

In other words, it’s as if the sea level in southeastern Louisiana is rising three times as fast as the global average.

Much of New Orleans, except for the oldest areas situated on higher ground, was built around the turn of the 20th Century on swampy soil which was drained by engineers. In the long run, that caused the soil to compact, or subside, forcing anything on the surface — houses, for example — to sink.

“As soon as you start draining these wetlands, they start sinking like crazy,” Tobjorn Tornqvist, a geology professor specializing in sea level and climate at Tulane University in New Orleans, said. “This added a lot to the (Katrina) disaster. If you imagine a situation where the land had not subsided due to this artificial drainage, there would still be a lot of flooding, but the water would have drained relatively fast.”

Another reason the region is sinking is that humans re-engineered the Mississippi River, shutting off its natural land-building process in southeastern Louisiana.

It worked like this: For millennia, the Mississippi River flooded, depositing layers of sediment in the wetlands of the Mississippi Delta, creating new land by layering the region with new soil. That soil naturally compacts and sinks under its own weight, but with each flood and new layer of sediment, the land was replenished and its elevation stayed more or less the same.

That process ended when the U.S. Army Corps of Engineers built levees and dams along the river to preserve and solidify the Mississippi’s famously meandering main channel for shipping, trapping some of the river’s rich sediment upstream and sending the rest into the deep waters of the Gulf of Mexico, forcing it off the edge of the Continental Shelf.

All of those local factors are aiding and abetting the submergence of southern Louisiana along with global factors such as accelerating sea level rise and increasing storm intensity because of climate change, Burkett said.

“Overnight, during hurricanes Rita and Katrina, about 215 square miles of land were lost — natural lands eroded away overnight,” she said. “That would leave the coast more vulnerable to the next storm. Meanwhile, 10 years later, there has been a tremendous effort by the state to restore the barrier islands that attenuate the storm surge, to restore some of the marshes, to get the sediment out of the river into to the marshes to make them more resilient.”

Since Katrina, the U.S. Army Corps of Engineers has rebuilt and strengthened the levees around New Orleans, and the state has begun restoring some of the barriers islands that protect the region. It is also considering a $4 billion plan to divert sediment from the Mississippi River as a way to restore some of the river’s land-building capabilities.

The diversions, likely accomplished with gates installed in levees along the river’s shore, is still in the design and approval phase and is facing pushback from fishing communities and oyster farmers, said Chip Kline, director of coastal activities for Louisiana Gov. Bobby Jindal and chairman of the Louisiana Coastal Protection and Restoration Authority.

Credit: Climate Central

“The logic behind these diversions is if they built this state, then they should absolutely be used in restoring the state,” Kline said. “Every single credible study (and) planning effort has called for reconnecting the river to coastal wetlands. The Mississippi River is really the lifeline to the land loss crisis down here.”

However effective higher levees, coastal restoration and sediment diversions from the Mississippi River may be, the endgame for New Orleans likely rests in humanity’s ability to slow the progress of climate change.

“Ultimately, climate change is our biggest problem,” Tornqvist said. “You can divert sediment as much as you want. If sea level starts ramping up to rates that approach five to 10 millimeters per year, well, it’s going to be an incredibly difficult situation.”

Large river diversions that help create new wetlands are likely to take decades to be effective against future storm surges.

“In the meantime, if sea levels start to ramp up, all these diversions are going to do is delay the point of no return basically by a couple decades,” Tornqvist said. “It will be increasingly clear that the city is not going to survive. Then you will get into issues like, what are we going to do? Are we going to relocate the city? How that’s going to play out, it’s hard to predict. What’s really critical is what kind of action will be taken within the next decades.”