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February, 2013
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A $5.8 million underwater barrier completed in mid-September to block the upriver flow of saltwater in the Mississippi River from threatening 
area water supplies has experienced some erosion but is still doing its job, a spokesman for the Army Corps of Engineers said Monday.
The 1,700-foot-long underwater dam, called a sill, at Alliance in Plaquemines Parish, keeps saltwater flowing upstream from the Gulf of Mexico from reaching the water intake pipes for New Orleans and Jefferson Parish.
On Dec. 12, the leading edge of the saltwater was at river mile 63.8 above Head of Passes, and was expected to retreat southward over the next few days, the result of additional rainfall that has fallen in the Midwest, combined with the release of water from several upriver dams that was required to allow barge traffic to continue moving on the equally low Missouri River.
A survey of the sill a week ago found a small amount of erosion in the center of the river channel, possibly the result of ship traffic above it, said Mike Stack, chief of emergency management for the New Orleans District office of the corps.
“We will survey the sill every two weeks and keep an eye on the erosion,” Stack said. If the erosion gets worse, the sill may have to be replenished with sediment dredged from an area just upstream, he said.
Unusually long drought conditions in the Midwest have resulted in the extremely low river levels in New Orleans this year: since June 1, the level has been at 3 feet or above at the Carrollton Gauge on only 28 days.
Four of those days were before, during and after Hurricane Isaac, when the river reached 9.6 feet in New Orleans because of storm surge moving upriver from the Gulf, before dropping back to near 2 feet.
State Climatologist Barry Keim said the relaxing of La Nina conditions, marked by lower than average water temperatures in the eastern and central Pacific Ocean, should mean more rainfall in the Midwest in coming months. The ocean temperatures are now in a “neutral” pattern, about average, and are expected to stay that way through the Spring, according to a joint forecast of the National Weather Service’s Climate Data Center and the International Research Institute for Climate and Society.
New Orleanians have been feeling a bit smug since Hurricane Isaac. After all, the Corps of Engineers’ $14.6 billion levee system worked just fine.
Louisiana’s wetlands are disappearing at the rate of a football field per hour. Photo by John Hazlett
Not so fast, says wetlands journalist Bob Marshall, who leads groups for Lost Lands Environmental Tours, a company started in March by his wife, Marie Gould, and Lindsay Pick.
Those urban levees, he explains, do repel water from storm surges, but they are not meant to hold it permanently. They’re built with mud, and eventually water will seep through. And at the current rate of wetlands loss, water will be lapping against the levees 24/7 by the end of the century.
“Locals don’t understand the danger they’re in,” he says. “We live on a starving, sinking landscape that suffers both subsidence and global water rise. Every storm has greater impact.”
Here’s the backstory. Over the past 6,000 years, the Mississippi River picked up sediment from two-thirds of the continent and deposited it as it slowed near its end. That’s how deltas are built, as water spreads and overflows, then subsides and leaves sediment behind. When Bienville arrived in the early 18th century, the sediment was 400 feet deep at the river’s mouth.
But spring floods and river course changes, so vital to delta creation, aren’t too compatible with human habitation and cultivation. When the great flood of 1927 devastated most of the Mississippi River valley, Congress took action. Levees were built from Illinois to Venice, La., the largest, strongest levee system in the country.
“They built a straight jacket,” Marshall says. “With levees, the river sediment compresses and compacts and sinks.”
Still, levees alone wouldn’t have destroyed intact wetlands. Dredging upped the ante.
“When oil and gas was discovered in the coastal zone, we went after it in the fastest way possible,” Marshall explains. “We began dredging canals for pipelines and supply access.”
Some 10,000 miles of canals have been dredged in south Louisiana since the 1972 Clean Water Act, which required permits for it. Probably another 10,000 miles had been dug before permitting began.
Canals created two problems for wetlands: They brought in salt water from the Gulf, which killed plants. And the soil dug by the dredges was deposited canal-side, creating “spoil levees” that inhibited wetlands regeneration in the same way that river levees do.
“We have a tour called the Two Worlds of Plaquemines Parish that shows the striking difference made by spoil levees,” Marshall says. “On the east side, where there are no levees, it looks like the Amazon. On the west side, where they deposited the dredging soil, there’s practically no vegetation left.”
The final nail in the wetlands coffin arrived with global warming and concurrent sea level rise.
“This is not a computer model or speculation,” Marshall says. “This is being measured every day at tidal gauge stations. The sea level is rising 9.24 millimeters a year at Grand Isle, which is four times the rate of Key West. That’s because south Louisiana is sinking at the same time the water level rises.”
If nothing is done, Marshall warns, sometime between 2060 and 2090 everything outside the levee system will be gone. Waves lapping at the levees.
The ramifications are not merely local, but national – even global.
“Why shouldn’t we just pack up and leave?” Marshall asks. “Because, among other things, 70 percent of the country’s migratory waterfowl winter here, we have the top tonnage of seafood in the nation, we provide Americans with 40 percent of their oysters, and 90 percent of U.S. energy production comes through our coast.
“If we have to shut down any of this, it will affect you. Whether you’re a taxi driver in Detroit or a farmer in the Midwest.”
While all the world’s delta systems are facing similar issues, no place in the western hemisphere, says Marshall, is as imperiled as south Louisiana. But the situation isn’t hopeless.
“We need to get the sediment back into the land. We can build marsh, at the rate of 500 acres in two months. But it will sink unless river diversions are used to add water to the wetlands.”
National environmental groups have woken up to the problem, and since Hurricane Katrina have been actively involved in coastal restoration. Recently, the Louisiana Coastal Protection and Restoration Authority released a $50 billion 2012 Master Plan. Now corporate and political types are beginning to realize the cost-benefit side of the equation – that there are not merely moral ramifications, but economical ones.
“This is not just a bumper sticker,” Marshall says. “We can’t build what we’ve lost. But we can maintain what we have left and still be here in 40 or 50 years.”
