Environment
 

Sinking levee shows difficulty of protecting New Orleans from flooding

Shown here during construction, this section of levee in the new surge protection system was recently discovered to be below design height by as much as 6 inches. The U.S. Army Corps of Engineers says repairs will cost about $1 million and could take two and a half months.

U.S. Army Corps of Engineers

Shown here during construction, this section of levee in the new surge protection system was recently discovered to be below design height by as much as 6 inches. The U.S. Army Corps of Engineers says repairs will cost about $1 million and could take two and a half months.

What’s more worrisome?

  • A. The Army Corps of Engineers is repairing a newly-built levee that has subsided as much as six inches — the second such repair in a year and just as the state is about to take ownership of the $14.5-billion system.
  • B.No one with the regional Flood Protection Authority was surprised. In fact, they’ve been expecting this news.

Welcome to life on a starving, sinking delta – in the middle of Hurricane Alley.

Officials recently discovered that a 1.1-mile long stretch of the levee along the Gulf Intracoastal Waterway had sunk 3 to 6 inches below its design height of 25 to 27.5 feet. It will cost about $1 million to repair, according to the Corps of Engineers.

“Actually the corps has estimated it will probably cost about $35 million over the next 10 years to keep this system certified for flood insurance,” said Tim Doody, president of the Southeast Louisiana Flood Protection Authority-East.

“I mean, anyone who lives here knows this is a problem. We certainly do, and it’s one of the biggest challenges for us going forward.”

Delta sinking all the time

The cause of the problem is simple and well-known. All of southeast Louisiana consists of deltas made of sediment laid down by Mississippi River floods over the past 7,000 years. The sheer weight of that massive layer of soft mud, 400 feet thick in some places, causes it to constantly compress against the bedrock far below. If new layers of sediment are not added, subsidence begins.

New sediment layers were blocked in the 1930s when the Corps of Engineers completed a system of world-class levees to keep those river floods from damaging communities.

But if the cause of the problem is simple, the way it unfolds locally has a complexity that makes fighting subsidence more challenging here than almost  anywhere on earth, engineers say.

As our delta was built over the millennia, different parts were covered at different times by a variety of surface features, ranging from marshes to beaches to swamps to forests. Over time, more sediment buried these surface features and new ones emerged.

The result is a complex weave of different types of soils — soils of varying thickness, compressing at varying rates.

So not only is the entire landscape settling at one of the highest rates on the planet, it’s happening at different rates across the landscape. Worse, for builders, those different rates can take place over a few yards, not a few miles.

“We know there will be subsidence, but there is no predictability about the rate of subsidence over distances here,” said Stephen Estopinal, a civil engineer on the board of the Flood Protection Authority. “We might have one section of levee sink rapidly over a period of months and another next to it sit perfectly stable during that time. Then the reverse will happen.

“You can make your best guess at what’s going to happen – but that’s all you’ve really got: guesswork,” Estopinal said. “This place is moving in three dimensions all the time, so you’ve got to stay vigilant.”

J. David Rogers, a forensic engineer who worked on the postmortem analyzing why and how levees collapsed after Hurricane Katrina, famously summed up the challenge of levee-building here this way: “It’s like putting bricks on Jell-O.”

The analysis concluded that Corps of Engineers of the 1980s hadn’t studied the Jell-O thoroughly enough and chose the wrong kind of bricks to put on it.

Post-Katrina vigilance

Unlike the levee boards that it superseded, the Flood Protection Authority was required to put engineers and other professionals on its board, and it has monitored construction of the new system to make sure the same mistakes are not repeated.

The corps, too, is more vigilant now. The agency has done extensive work to get a better profile of soils under the entire system. And it builds levees taller than required in anticipation of their predictable subsidence.

But both the corps and the Flood Protection Authority know that local subsidence can outfox even the best-laid plans.

The levee along the Gulf Intracoastal Waterway is a case in point.

Chris Gilmore, the corps’ senior project leader for this section, said not only were extensive samples taken to discover what types of soils underlay the levee, but the entire stretch was built on a type of cement pilings. The corps injected concrete into holes in the soil and built the levee on top.

“It’s what we call deep-soil mixing,” Gilmore said.

And yet, a final inspection before handing over responsibility for the levee to the Flood Protection Authority revealed that a 6,000-foot stretch tied into the Lake Borgne Surge Barrier was 3 to 6 inches lower than the 25 to 27.5 feet it was designed to be.

The predicted rate of subsidence had been exceeded, and no one was really surprised. A year earlier, the corps discovered that a stretch of new levee between U.S. Highway 11 and Interstate 10 had dropped a startling 3 feet more than expected. There, design elevations range from 16.5 to 25 feet.

“As most folks who live in the area know, the ground in Louisiana is always sinking,” said Gilmore. He estimated the 6,000-foot fix will cost about $1 million and take two to two-and-a-half months to complete.

The impending change in ownership means the local flood authority – funded by property taxes – will now be responsible for the cost of shoring up sinking levees. The tab won’t be chump change.

Corps of Engineers spokesman Ricky Boyett said the agency estimates the cost for “Operation, Maintenance, Repair, Replacement and Rehabilitation” will be about $38 million over the next 10 years. He said that figure was for the entire 350 miles of levees, floodwalls and associated structures such as floodgates.

The contract between the Corps of Engineers and the state seems to offer a loophole to local agencies. It states that the agreement is not intended “to require the Non-Federal sponsor to perform future measures to restore the New Work to the authorized level of protection to account for subsidence or sea level rise,”

This section of levee between Bayou Savauge National Wildlife Refuge and Lake Pontchartrain rapidly subsided three feet after construction. The corps made the repairs last year.

U.S. Army Corps of Engineers

This section of levee between Bayou Savauge National Wildlife Refuge and Lake Pontchartrain rapidly subsided three feet after construction. The corps made the repairs last year.

But Doody said the Southeast Louisiana Flood Protection Authority-East wasn’t counting on that as an escape clause. First, the agreement doesn’t require the corps to do the work. Second, Congress hasn’t appropriated the money.

“We can’t sit around and wait for someone to settle the issue; we have to make sure we keep this system certified,” Doody said.

There is one thing they can be certain of: The flood authority’s new levees, like the rest of southeast Louisiana, will continue to sink.

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  • Roy Arrigo

    This levee board president has no expertise in any field related to levees or flood protection, he is an accountant. Yet, it is he and he alone who calls the shots of this board and what they do. Under his direction, Mr. Tim Doody allowed this entire system to be built without having provided any of the oversight that this board was supposed to provide. Because of this lack of oversight by Tim Doody’s board, the entire system was allowed to be built without it meeting the 100 year standard that it was supposed to meet. Watch:
    http://www.youtube.com/watch?v=VOt9luOHHw8
    Though subsidance may have occurred, Doody is now trying to hide his board’s lack of oversight behind this cloak of subsidance. Where is the oversight that we were supposed to get with this board?

  • dimdingledon

    The biggest poblem with the levees is the designs were not adeqautely reviewed by the board tasked with oversight, the SLFPA-E. The 1 foot the Corps assumed for combined settlement and sea level rise was way too low. This was commented to the top leadership at the SLFPA-E, the Corps, and the CPRA, but no one at any of those agencies did anything to correct it when they had a chance to. An even bigger problem that has yet to be inmasked is the modeling which the entire system was designed around is even more suspect than the levee designs. Once probing eyes look into the modeling, we will see just how unprotected and vulnerable we really are and how those tasked with oversight never put a single sight on the designs or modeling used by the Corps.

  • Roy Arrigo

    J.D Rogers who is mentioned in this story is also featured at exactly 6 min, 13 seconds into this video. Mr. Marshall, what would you do if you became aware that these individuals on this Southeast Louisiana Flood Protection Authority-East board to whom you are friendly with were knowingly stealing people’s private property. Would you spin it in their favor? Would you ‘out’ these individuals that you are friendly with, or would you just ignore it, and not report on it at all?
    http://www.youtube.com/watch?v=nUSveOKptUU

  • Chris McLindon

    Great article. Check out this slide show by David Rogers, a geological engineer at the University of Missouri.

    http://web.mst.edu/~rogersda/levees/Geology%20New%20Orleans-Chapter%203.pdf

    Slide 6 shows that the area of high subsidence on the levee in question is between the relative stability of the natural levee of the river and the buried Pine Island barrier island. Before the St. Bernard Delta covered this area about 2800 years ago, a chain of barrier islands extended across what is now the southern border of Lake Pontchartrain. Two of these buried islands underlie what are now the Gentilly and Metairie Ridges (the areas that tended to have the least flooding in Katrina). The thick sand deposits of both the islands and the natural levee are more rigid, and subside more slowly than the surrounding swamp and marsh deposits. By contrast, slide 14 shows that the area between these sand bodies is filled with up to 16 feet of peat. This is some of the most highly compressible material that could have been deposited there. This article shows a map of subsidence rates in New Orleans:

    http://www.geodesy.miami.edu/articles/2006/Dixon_etal_nature_2006.pdf

    If you overlay the subsidence map over Rogers maps, you can see the effect of the barrier islands and the area of thick peat deposit under the MRGO and Bayou Bienvenue. Measurements for this map were only taken on dry land, so you have to look carefully, but you can see that the highest rates of subsidence are directly over the thick peat under Bayou Bienvenue.

  • Tommy Pinion

    Why not install helical or resistence piles to the bedrock

  • Saffa

    400 feet down???

  • Tommy Pinion

    I’ve been past 100′ several times with large and small piles with some in this area of Louisiana. Just thinking out of the box because what their doing now isn’t working. We even have a pressure grout helical which could give point and friction bearing.

  • Sandy Rosenthal

    Last June, we at Levees.org did discuss this very issue. In our oped submitted to NOLA.com, we observed:

    “…Because the newly built levees subside naturally, additional layers of earth, called lifts, will be necessary. To do a lift, all the concrete armoring will have to be removed and then replaced. If including the cost of levee lifts for the mainline river levees, costs will easily run into the billions of dollars…”

    http://www.nola.com/opinions/index.ssf/2013/06/with_new_orleans_new_14_billio.html

  • Sandy Rosenthal

    Clever observation by Bob Marshall:

    “The (post-Katrina levee) analysis concluded that Corps of Engineers of the 1980s hadn’t studied the Jell-O (soil) thoroughly enough and chose the wrong kind of bricks (I-walls) to put on it.”

    J. David Rogers was co-author of the Independent Levee Investigation Team, U of Cal Berkeley, funded in part by the National Science Foundation.

  • Tommy Pinion

    Can anyone provide soils report?

  • Chris McLindon

    Levee maintenance in the Greater New Orleans area is going to take up every dollar we can get from any source we can get it from. This is exactly why we need to stop wasting money on coastal restoration projects that have no hope of providing any meaningful measure of flood protection. The mid-Barataria sediment diversion project is nearly 15 miles south of New Orleans. It is estimated that it will cost nearly half a billion dollars ($22 million of the 2015 CPRA budget just to study it some more), and it is very likely that it will provide no flood protection to anyone.

  • Sandy Rosenthal

    It’s difficult but not impossible to build on New Orleans’s soils. As noted by the authors of Team Louisiana (van Heerden et al., 2006):

    “…the principles of foundation design, which rests primarily on the interpretation of subsurface information derived from suitably detailed boring programs, and from long experience with the response of soils and structures to the forces developed under critical conditions were well understood by practicing civil engineers of the late 1980s and early 1990s…”

  • Jan de Goei

    This is normal. When you are building dikes where the underground is weak the dike will be sunk. That is the experience of the Dutch. Every six years the dikes were controlled and then – wenn he is sunk or another problem appear – the dike will be strenghed in the next six years. it’s an ongooiing proces that will never ending.