A new study by researchers at Tulane University has concluded that Louisiana will not be able to build land fast enough to keep up with how quickly wetlands are disappearing.
The study, published Wednesday in the journal Science Advances, has dire implications for the latest version of the state’s Coastal Master Plan, a $50 billion blueprint designed to save as much of the coast as possible.
The authors of the study agree with scientists who say river diversions are the best way to sustain portions of the Mississippi River Delta. But even the best-built diversions won’t deposit sediment quickly enough to save the entire coast, they say.
“The river is capable of building land,” said Elizabeth Chamberlain, who led the study, “but the rates at which it’s building aren’t high enough to offset the rates of loss we’re seeing.”
Torbjörn Törnqvist, a geology professor at Tulane and co-author of the study, said the state will have to place the diversion structures in areas with the greatest potential to build land and protect highly populated areas, rather than create multiple diversions along the entire shoreline.
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“Difficult choices will have to be made about where to locate these diversions,” he said in a news release.
The state’s plan earmarks $5 billion for diversions along the Mississippi River. They are intended to rebuild Louisiana’s disappearing coast by carrying sediment through cuts in the river's levees to silt-starved wetlands. Periodic flooding built land this way over thousands of years, but the levees blocked that process.
The 2017 Master Plan calls for building 10 sediment diversions across the central and southeastern coast in the next decade.
The state's Coastal Protection and Restoration Authority, which oversees the rebuilding plan, didn’t immediately respond to a request for comment on the study.
Researchers conducted the study to better understand how land was formed in the delta hundreds of years ago. They took sediment samples up and down Bayou Lafourche, an abandoned distributary of the Mississippi River. Using a technique called luminescence dating, they measured how fast the shoreline migrated toward the sea under natural conditions.
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By examining when minerals in the sediment were last exposed to sunlight, they determined that the delta grew at a rate of two to three square miles per year before humans reined in the river.
That growth rate is dwarfed by the current rate of land loss in coastal Louisiana, which has averaged 15 to 20 square miles per year over the past century. The latest research from the U.S. Geological Survey shows that number is now close to 10.8 square miles per year.
And seas are rising five times faster now than when sediment was deposited along Bayou Lafourche, the researchers said.
The study confirms one conclusion already made by scientists: that human decisions greatly contributed to the state’s land loss.
Canals dredged for oil and gas drilling allowed saltwater to penetrate and kill marshes, causing them to erode and making them more vulnerable to storm surge. River levees prevented moisture-rich sediment from being deposited, allowing the delta to dry out and sink over time.
Törnqvist said Louisiana’s current reality — that land can’t be built faster than it erodes, even with the best technology — will not change in the future, taking into account accelerated sea-level rise.
The idea that the state won’t be able to catch up on land loss isn’t new. When the Coastal Master Plan was revised in early 2017, scientists abandoned a long-held hope that the state could build more land than it loses by 2065.
The more dire outlook is due to sea-level rise. The worst-case scenario for human-caused sea-level rise in the 2012 plan, 1.48 feet, became the best-case scenario in the 2017 plan.
The National Climate Assessment now estimates sea levels on U.S. coastlines could rise 4 feet by 2100.
The new worst-case scenario projects that Louisiana would lose 4,000 square miles over the next 50 years if the state were to abandon all efforts to restore its coast. That’s double the loss projected under the same scenario in the 2012 plan.
Scientists now hope to restore between 800 and 1,200 square miles of wetlands in the state’s southern third, in addition to shoring up protection against storm surges.
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Natalie Peyronnin, director of science policy for Mississippi River Delta restoration at the Environmental Defense Fund, said diversions are still the state’s best tool for building new land. But it’s become even more important to build them quickly, she said.
“There’s been a growing recognition … that we can’t sustain all the land that we have today and that the delta of the future with sea-level rise is going to look different than it does today,” said Peyronnin, who wasn’t involved in the Tulane research. “But there are things we can do to build that sustainability.”
Indeed, the researchers found that Louisiana deltas have been able to grow even when constrained by limited sediment, accelerating sea-level rise and rapid subsidence.
The study is important, the researchers say, because understanding how the delta grew in the past will help them figure out how it will change in the future — especially as scientists attempt to build more land with expensive engineering.
“So many people live on the coast,” Chamberlain said, “and having a fundamental understanding of how these systems operate can be used to create models in how the coast might change in the future.”