Synthetic mats like this one can avert scouring and erosion on the landward side of a grassy levee that's been overtopped. Credit: Storm Water Solutions
Bermuda is considered an ideal grass for armoring local levees because its dense root mass holds soils and it grows well in the local climate
Bermuda is considered an ideal grass for armoring local levees because its dense root mass holds soils and it grows well in the local climate Credit: Bob Marshall

How soon do you want to feel 500-year safe?

That’s a $300 million question the local flood protection authority is now struggling with.

The Army Corps Engineers has agreed to place armoring – synthetic mats that reinforce the grass  — on the landside of the new levees it built around metro New Orleans. That move, long sought by the state, would raise the levee resistance to collapse because of overtopping from the 100-year-storm level to the 500-year storm. The corps says it expects to spend about $300 million on the project.

But the Southeast Louisiana Flood Protection Authority-East is still evaluating a report that suggests the new levees are lower than they should be even for that 100-year storm.

So the authority members are weighing this decision: Should they give the corps the go-ahead to put the armoring on now, even though it might have to be ripped off and replaced if the levees prove to be too low? Or should the authority delay the project while it waits for an answer – which could save money, but might leave the area more vulnerable for years?

For Bob Turner, the engineer who is the regional director of the authority, the answer is simple: Protect now.

“At the end of the day, the purpose of the system is to reduce risk, and the longer we wait for the armoring, the longer we’re at risk of a failure during a 500-year event,” he said.

“If you’re concerned the overtopping risk is higher now because the levees aren’t high enough [for the 100 year event], then it may be even more important to get that armoring in place as soon as possible.”

In some ways this is a dilemma the board is happy to have.

Local and state officials were never satisfied with the Bush administration decision to replace the poorly built system that failed during Hurricane Katrina to one built for the 100-year storm level.  Turner and the board he serves said that level is far too low for the population and economic infrastructure at risk during an era when hurricanes are expected to increase in size and intensity.

They also maintain Congress originally ordered the corps to build a system that would repel at least a 200-year storm if not a 400-year event.

So they have long urged the corps to armor the levees. Experience has shown water rushing down the landside or “protected” side of levees can rapidly tear through grass coverings and begin eroding the levee’s soils, causing collapse.

Engineers say adding armoring  would raise the system’s “resilience” to the 500-year level. That means while the surge from such a storm would still overtop the levees and flow into the city, the levees would be much less likely to breach as they did during Katrina.

If levees were built to a repel a 500-year storm, they would be high enough to prevent overtopping now a concern with the 100-year levees.

Researchers reported that if levees along the MIssissippi River-Gulf Outlet that failed due to overtopping during Katrina had remained intact, flooding across St. Bernard Parish might have only been shin deep, rather than 12 feet.

“Preventing failures is critical to preventing another Katrina-type disaster,” Turner said. “Yes, with armoring we would still have water coming into the city. But that would last only for a few hours.”

The corps originally seemed convinced the Bermuda grasses planted on the levees would suffice. Eventually its tests showed turf sections reinforced with the synthetic mat performed better during overtopping.

Anticipating the corps’ announcement, the flood protection authority has already been working on two considerations.

The first is a cost-saving move. Built on a steadily subsiding delta, the new levees are expected to sink below heights required for residents to qualify for federally subsidized flood insurance when FEMA takes those measurements again in 10 years. Indeed, that has already begun to happen. So the board has asked the corps to delay armoring while it spends $40 million to $50 million to add enough height to the levees to compensate for the expected subsidence.  Stephen Estopinal, president of the authority board says the levee districts involved would have that funding.

Delaying the work could save the board the estimated $250 million cost of replacing the armoring, which would have to be ripped up during the levee lift.

“Once that mat is removed, it can’t be reused,” Turner said. “So, our first concern is to be able to do those lifts now. That could be a real money saver and it shouldn’t take that long.”

Secondly, board member Rick Leuttich, a University of North Carolina researcher who is an engineer and a developer of one of the most-used storm surge models, has raised another question.

What if continuing studies show that even the higher proposed levees are still not high enough because the corps made the current levees too low? Would the flood authority be required to go through the expense of increasing height even sooner, wasting the armoring anyway?

Turner said no one can say when that finding on the veracity of the corps’ original modeling might come in.

“We have our consulting engineer looking at this, but so does the CPRA [state Coastal Protection and Restoration Authority] and the corps,” he said. “No one knows when anyone will reach any conclusions – or if they’ll even agree.

“In the meantime, we need to reduce the higher risk this area is exposed to while we’re waiting to find out.”

Bob Marshall

From 2013 to 2017, Bob Marshall covered environmental issues for The Lens, with a special focus on coastal restoration and wetlands. While at The Times-Picayune, his work chronicling the people, stories...