A flood barrier across the mouth of Lake Pontchartrain — a 50-year-old idea to protect New Orleans and communities along the North Shore from storm surge — could cause millions of dollars a year in flooding damage to vulnerable parts of coastal Louisiana and Mississippi, according to a new study.
The barrier, included in Louisiana’s plan to rebuild and protect the coast, could cost St. Bernard Parish an average of $14 million a year in severe storms.
In Mississippi’s three coastal counties, the barrier could cause an average of about $22 million annually in additional flood damage.
But those estimates are much smaller than the $1.4 billion in potential savings in flood damage in southeastern Louisiana.
“We see huge benefit,” said Jordan Fischbach of the RAND Corporation, which published the report. “But of course it’s always a tradeoff. The water needs to go somewhere.”
The latest iteration of Louisiana’s Coastal Master Plan calls for the set of closable flood gates at the Rigolets and Chef Menteur passes, which connect Lake Pontchartrain to Lake Borgne and the Gulf of Mexico.
The project, to be built within 30 years, would cost $2.4 billion in today’s dollars.
The Master Plan forecasts a wet future for Louisiana’s coastal communities due to rising sea levels, sinking coastal land and erosion of wetlands.
The plan relies on pipelines, massive river diversions, levees and floodwalls to protect and rebuild the coast. It also calls for elevating, flood-proofing or buying out homes.
However, even if all the restoration projects work as they should, scientists predict 2,800 square miles of the state’s coast could still erode in the coming decades.
The new report analyzes the costs and benefits of the barrier at the mouth of the lake, looking at communities from Ascension Parish, southeast of Baton Rouge, to the Mississippi-Alabama line.
It was written by researchers with the Infrastructure Resilience and Environmental Policy Program of the RAND Corporation, which is working with the state on its 50-year coastal rebuilding plan.
The barrier would include about a mile of earthen levee as high as U.S. 90 and gates at the two passes, each two feet high and 150 long. The gates would rise vertically out of the water and would allow some water to pass over.
Communities on the North Shore, including Covington, Mandeville and Slidell, would primarily benefit. Those areas would still be at risk from flooding caused by water already in the lake.
During Hurricane Isaac in 2012, storm surge from Lake Pontchartrain caused up to nine feet of flooding along the North Shore.
The barrier would put St. Bernard Parish at greater risk to storm surge. Coastal counties in Mississippi would fare even worse.
A proposal to build a barrier at the mouth of Lake Pontchartrain goes back to the late 1960s, in the wake of Hurricane Betsy. The U.S. Army Corps of Engineers scrapped the idea.
It was reintroduced as a potential project in the 2012 version of the Master Plan. The latest plan, released in January, includes a scaled-down version to be built by 2047.
Previous versions of the barrier considered a levee as high as 24.5 feet along U.S. 90, but the state Coastal Protection and Restoration Authority decided that would cause too much flooding in Mississippi and St. Bernard Parish.
The RAND research supports the decision to go with a relatively short barrier. It would provide nearly as much protection to Orleans Parish and other areas around the lake as a higher levee, and it would cause the least damage to coastal Mississippi.
“We’re really trying to balance water flowing into the lake and water being pushed off into areas adjacent to the lake in a storm,” Fischbach said.
Without any barrier, areas around Lake Pontchartrain face extreme flooding in the case of a 100-year storm. At the mouth of the Pearl River and in parts of Plaquemines Parish, water could rise more than 18 feet.
The barrier would reduce flood depths by one to two feet in most areas around the lake during a 100-year storm, according to the study.
St. Tammany Parish would benefit the most. About $620 million a year could be saved in potential damage from storm surge.
St. John the Baptist Parish would also benefit greatly, saving $209 million annually. In Orleans Parish, it would be $142 million. Jefferson would save $150 million in damages.
The damage estimates for Mississippi were calculated by averaging risk posed by storms ranging from a so-called 10-year event, which has a 10 percent chance of occurring in any year, to a 2,000-year event, which has less than a 0.05 percent chance of happening.
St. Bernard’s damage projections only account for more severe — and rarer — storms because the barrier would not affect flooding in lesser storms.
Fischbach was careful to note that Mississippi’s annual flood damage is already projected to be $1.6 billion a year without the Lake Pontchartrain flood gates.
Still, some officials are not happy that their communities would face more flooding from the barrier.
“Any deviance to the normal flow of water — yes, I have concerns,” said Rupert Lacy, the emergency management director of Harrison County. “The water’s got to to go someplace. You stop it from moving west, it pushes it north or south.”
“We’ll just have to do our homework, and do our job,” he added.
Hancock County, just across the Louisiana border, could incur an average of $14 million more flooding damage each year due to the barrier.
The annual damage estimates are $5 million in Harrison County, where Gulfport and Biloxi are located, and $2 million in Jackson County, which borders Alabama.
But Fischbach noted that those figures are only slightly higher than the damage estimates without a barrier. Hancock’s increase is just 3 percent greater than without a barrier; Jackson’s is less than 1 percent.
It’s far from certain that the surge barrier will actually be built. The Master Plan is estimated to cost more than $90 billion and funding hasn’t been secured for projects years down the road.
Bren Haase, chief of Coastal Protection and Restoration Authority’s research division, said the plan for the barrier could change. “There’s a lot of work that’s going to need to go into the details of this thing,” he said.