Environment
 

New measurements show sea level rise swallowing Grand Isle at record rate

The red areas have the greatest vulnerability to sea level rise, according to the National Climate Assessment.

National Climate Assessment

The red areas have the greatest vulnerability to sea level rise, according to the National Climate Assessment.

Stephen Estopinal, an engineer on the board of the Southeast Louisiana Flood Protection Authority-East, says he has a concern that “keeps me up at night.”

What worries him is the speed with which land beneath the levees and floodwalls protecting the metro area from storm surge is sinking.

New data from the National Oceanic and Atmospheric Administration is not going to make it any easier for Estopinal to get to sleep.

The federal agency charged with keeping track of sea levels, NOAA says Grand Isle has lost 1.32 inches of elevation to the Gulf of Mexico in the past five years alone — a rate of subsidence about four times faster than any other coastline in the lower 48 states, and one of the highest on the planet.

The rate is so high because the entire southeast portion of the state is sinking while seas are rising worldwide due to global warming. When subsidence is combined with sea level rise, the result is called relative sea level rise.

The latest update only confirms Grand Isle’s nation-leading average of 9.24 millimeters (.363 inches) per year over the past 65 years, or about three feet per century, said Steve Gill,  a chief scientist at NOAA. By contrast, Key West, less subject to subsidence, has been succumbing to rising seas at a rate of just 2.24 millimeters per year, according to federal charts of high tides kept over many decades at selected coastal stations.

And the National Climate Assessment, a federal study released this week, indicates only more problems ahead for Louisiana’s coast. The assessment predicts sea level rise will continue at current rates if greenhouse gas emissions — which drive the warming trend — are not curtailed. Worse yet, if those emissions continue to increase, as they have for a century or more, sea level rise will also increase, the assessment said.

It was all bittersweet news for Estopinal.

“It validates everything I’ve been harping on for years,” he said. “This is a huge problem for us, because it makes it more challenging for us to keep levees and walls high enough to keep surge out.

“This landscape is always moving, and that means this job is never finished. We can’t rest on the datum (elevations measurement) we have when we start a project, because it’ll be out of date in five years.”

The National Oceanic and Atmospheric Administration study compares sea level rise at Key West (upper graph) with Grand Isle, where subsidence aggravates the problem and accelerates the rate.

NOAA

A federal study compares sea level rise at Key West — 2.24 millimeters per year — with Grand Isle (lower graph), where subsidence aggravates the problem and accelerates the rate to 9.24 millimeters per year.

For most of coastal America, NOAA recalculates mean sea level once every 15 to 20. But Southeast Louisiana is on a five-year schedule because the Mississippi River deltas it rests on are sinking and crumbling at such a rapid rate.

Levees on the river built for flood protection and to aid shipping have blocked the annual spring floods that for eons slathered the delta with new layers of land-building sediment. Meanwhile, thousands of miles of canals dredged for oil and gas development have destroyed the delta’s hydrology by allowing an influx of vegetation-killing salt water deep into coastal marshes. Just under 2,000 square miles of the state’s coastal wetlands have been turned to open water since the 1930s, a loss that continues at the rate of 16 square miles per year.

Engineers say the constantly sinking and shifting landscape makes the area one of the most difficult to build on. The Army Corps of Engineers said it took account of subsidence rates while constructing its $14.5-billion post-Katrina storm surge protection system around the metro area. And the corps vows the system will meet design specs as it is turned over to the local sponsor – the authority Estopinal sits on. But in the past 18 months the corps has been forced to raise at least two sections at a cost of millions because the sinking happened faster than it forecast.

That unpleasant surprise has confirmed the urgency of Estopinal’s calls for more frequent reevaluation of the system’s subsidence-prone underpinnings.

“They haven’t redone the datum for this system since it was started — and that was eight cotton-picking years ago,” he said. “You might start out with something that’s accurate, but the way this land moves, you’re not going to be accurate a few years later. This latest data from NOAA just validates my concerns. There’s a reason they’re redoing southeast Louisiana every five years — not 10 or 15 or 20.

“And there’s just too much at risk here to assume anything. This place moves too much and too fast.”

NOAA will update the Flood Protection Authority on these latest findings at a meeting of the agency’s Operations Committee meeting, 9:30 a.m. Thursday , in the Orleans Levee District’s lakefront headquarters, 6920 Franklin Ave.

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  • Mark Schleifstein

    Good story, Bob. This year’s findings for subsidence on Grand Isle by NOS are in line with their predictions in 2010: http://www.nola.com/news/index.ssf/2010/11/flooding_predictions_have_dram.html
    Note, however, that the subsidence rates at Grand Isle are much faster than in New Orleans. The problems with subsidence beneath the levees must factor in both the underlying sinking rate and the fact that new material has been added — and in many cases, heavy concrete structures, such as the T-walls in St. Bernard — that can increase the speed of overall subsidence. However, since the improved East Bank levees were built atop longstanding older earthen levees, they won’t be sinking as fast as those on the West Bank, where much of the levees are brand new and built on much softer underlying soils.

  • dimdingledon

    Mr. Estopinal must have had a change of heart lately. For years the SLFAP-E was warned about the assumptions made by the Corps of Engineers in their storm surge model used for determining out floor threat from hurricanes. In that model the Corps assumed a combined total of 1 foot for the combination of sea level rise and subsidence. It was brought to the attention of the SLFPA-E years ago that this value was too low, but the SLFPA-E just went along with the Corps of Engineers. Just look at the minutes of their meetings to verify this.

  • dimdingledon

    There are some levees on the eastbank that were built on top of existing levees that have settled faster than the Corps assumed and the Corps had to add lifts to them already.

  • dimdingledon

    Dr. Dokka is the one that came to the SLFPA-E meeting and told them how critical the restoration of our coast would be or the southern part of the state as we know it would become part of the Gulf of Mexico. He told the SLFPA-E leaders they had about a 10 year window of opportunity to get it fixed or they would reach a point of no return. I think Dr. Dokka said that in 2008. I guess the leaders of the SLFPA-E did not take him seriously.

  • dimdingledon

    Dr Dokka informed the SLFPA-E of the rate of rapid land loss years ago. He told them in 2008 that they had a 10 year window of opportunity to fix it or they would reach a point of no return and much of the southern part of the state would become part of the Gulf of Mexico. I guess the SLFPA-E just didn’t listen.

  • dimdingledon

    Every foot we loose in subsidence or gain in sea level rise is that much more we increase our flood threat and reduce our level of protection. Good thing we already locked into the 10 year certification with FEMA for flood insurance rates. I doubt the system would receive certification if FEMA actually loked at the modeling and the exisiting and predicted conditions (subsidence/sea level rise). Anyway, this gives us a 10 year window to try to fix the problems.

  • Mark Schleifstein

    Roy did indeed do that. And he was preaching to the choir. As you well know, the levee authority’s own contractors have produced an extensive report on increased risk, and it’s been the authority’s staff that identified and demanded action of the corps on the areas of levees they saw subsiding, including portions under the new T-walls in St. Bernard. Which the corps has responded to. It’s also authority members who keep pushing the corps to review bathymetry in areas east of the levee system out of concern that scouring and erosion since Katrina has also reduced the effectiveness of the new levee systems. In addition, it is the authority that got the study done that suggested alternatives for extending levee along New Orleans land bridge to St. Tammany. And it was the authority that unsuccessfully objected to the corps decision not to coat steel pilings used during construction.

  • nickelndime

    SEA LEVEL RISE eats away GRAND ISLE. We are 6 years into Dr. Dokka’s 10-year window of opportunity to fix it or lose it. What good is flood insurance if we are filing for it from another state and have to meet those blasted insurance adjusters in the Gulf and they keep asking for proof of loss.

  • dimdingledon

    Mark,
    All those things you are referring to should have been done years ago if the authority would have bothered to review the modeling and designs. Instead they waited until the construction was underway. They could have stopped the constructioon by not granting the Corps a right of entry permit, but the authority was the biggest lap dog for the Corps. As far as the coating on the sheet piles go, that is something that could have been stopped by the authority. And the landbridge in New Orleans east has been neglected for years. Part of the battle was a poor decision by Governor Jindal who wanted to use the concrete from the old twin spans for a fishing reef rather than protecting the landbridge. So we’ve had an incompetent leadership of the authority battling an incompetent governor for years over the issues of flood protection. We are so screwed.

  • fran farrell

    Sleep well at night. FEMA certifies that you won’t drown. Living up to its name Federal Emergency Making Agency.

  • nickelndime

    fran, I have another set of words for the FEMA acronym, but if I print them, they are likely to get censored and deleted.

  • mcdonlad

    I realize I’m several months late in reading the article, but there is an article in the October 2004 National Geographic
    worth reading. It gives a history of the area that explains a lot.

    http://ngm.nationalgeographic.com/2004/10/louisiana-wetlands/bourne-text

    “… the city’s natural defenses are quietly melting away,…Louisiana is losing its protective fringe of marshes and barrier islands faster than any place in the U.S.

    Since the 1930s some 1,900 square miles (4,900 square kilometers) of coastal wetlands—a swath nearly the size of Delaware or almost twice that of Luxembourg—have vanished beneath the Gulf of Mexico. Despite nearly half a billion dollars spent over the past decade to stem the tide, the state continues to lose about 25 square miles (65 square kilometers) of land
    each year, roughly one acre every 33 minutes.

    A cocktail of natural and human factors is putting the coast under. Delta soils naturally compact and sink over time, eventually giving way to open water unless fresh layers of sediment offset the subsidence. The Mississippi’s spring floods once maintained that balance, but the annual deluges were often disastrous. After a devastating flood in 1927, levees were
    raised along the river and lined with concrete, effectively funneling the marsh-building sediments to the deep waters of the Gulf. Since the 1950s engineers have also cut more than 8,000 miles (13,000 kilometers) of canals through the marsh for petroleum exploration and ship traffic. These new ditches sliced the wetlands into a giant jigsaw puzzle, increasing erosion and allowing lethal doses of salt water to infiltrate brackish and freshwater marshes.”