What the ‘big, big lessons’ learned from Hurricane Katrina mean now for New Orleans as Barry moves inland

The rain and flooding occurring with Barry has brought stark reminders of Hurricane Katrina for New Orleans area residents. Mandatory and voluntary evacuations have already been enacted for Plaquemines Parish and parts of the West Bank.

The National Weather Service (NWS) initially forecast the Mississippi River to crest at 20 feet Friday into Saturday before lowering that expectation to an expected crest of 17 feet, which is right at minor flood stage. Levees in New Orleans are between 20 and 25 feet high at different points along the river, according to what Ricky Boyett, the spokesman for the Army Corps of Engineers in the New Orleans District, told The New York Times. The highest the river has crested since 1983 is 17.38 feet in 2011, and the river hasn’t risen above 19 feet since 1950.

To understand how the situation in New Orleans has changed since Katrina and what may be ahead for the city as a result of the upcoming storm, AccuWeather spoke to J. David Rogers, the lead author of a definitive 2015 study on the canal wall failures and catastrophic flooding of New Orleans in 2005. Rogers is a professor at Missouri University of Science & Technology, as well as the Karl F. Hasselmann Chair in Geological Engineering.

AccuWeather: How are things different in New Orleans now since Hurricane Katrina?

David Rogers: Well, they’re in a different century now in terms of the amount of effort and energy and forethought and site characterization that has occurred. You can’t even compare pre-Katrina to post-Katrina; it’s like comparing a biplane to a 747. It’s a much more robust defense system that they have today with probably a 100-fold better site characterization than they had going into Katrina.

AW: Knowing what you know from your study, are you concerned with what’s headed their way?

David Rogers: No. No, I’m not. The Mississippi hasn’t flooded that city since before the Civil War, so that’s not their biggest worry. Their biggest worry has been other things that are lower level that can come in and get them from the back or the sides.

AW: What would those concerns be?

David Rogers: The coast is subsiding … and the whole delta is sinking, so that’s why they put in the huge seawall infrastructure [the Inner Harbor Navigation Canal (IHNC) Lake Borgne Surge Barrier] across the Mississippi River-Gulf Outlet (MRGO) channel. That was their biggest single vulnerability to surviving in the 21st century. It was the most expensive structure the Corps of Engineers has ever built [at $1.1 billion] and people were calling it all sorts of names, but it was a correct move on their part. And they also moved into the 21st century by taking on probabilistic hazard assessment, looking at all of the different kinds of failure modes and considering all of them, not just the ones that their brethren did back in 1933, which is kind of where they were operating pre-Katrina.

AW: So when the winds come in from the Gulf with this storm, that should mitigate the storm surge?

David Rogers: Right, that will keep the storm surge from getting too high in the IHNC. The MRGO channel allowed storm surges to travel unimpeded into the Inner Harbor area in the heart of New Orleans prior to Katrina, overwhelming the levees along either side of the IHNC in 1965, 1969 and 2005.

And then you also have the vulnerabilities coming off Lake Pontchartrain because it’s 635 square miles of land area but it’s only 15 feet deep at the deepest point. So it’s like a big glob of glycerine on a glass table. If you bring a hurricane in and you blow on it from any given direction, all that water just goes right up onto the shore.

AW: So those worries won’t be a concern with this particular storm?

David Rogers: Well, it would have to be something similar to Katrina in terms of the path it takes and the likelihood of that is lower than it would be if Katrina hadn’t occurred in 2005. If you just look at historic hurricane tracks back to 1900, they generally take similar paths but very seldom take identical paths. And it would have to be something close to identical to Katrina to really load the system to what it’s designed for. I think the existing infrastructure that they’ve built for flood control would survive a Katrina event.

AW: Do you think because of Katrina, the study and all of the changes, that this is a safe place to be with this storm approaching?

David Rogers: It’s certainly much safer than it ever was previously.

AW: At what level of the Mississippi River would you think things would be dangerous? 22 feet? 25 feet?

David Rogers: There are too many variables to say what depth of overflowage could potentially cause a breach because the Army Corps of Engineers can employ sand “push-ups” to increase levee heights by as much as three feet. In the old days they employed sand bags and/or timber flashboards, which are both temporary measures.

AW: They’re talking about the Mississippi River getting to 19 or 20 feet and since the levees are only 20 feet at spots, you’re looking a potential problem, right?

David Rogers: Yeah, but they’ve looked at making the levee systems down there more robust, even for overtopping. They do consider overtopping one of the failure modes they designed against. They can take water coming over levees if the levees have enough clay in the surface of the levees. And if you put splash pads at the base on the protected side of the flood walls, you can survive overtopping.

Those were two key things that came out of Katrina. Because the Corps kept saying, ‘We’re not to blame because the water was higher than what we designed for.’ And what everybody like me said was, ‘You can build a splash pad for 1/10th of 1% of the cost of that wall and you can survive something much higher than your wall. That makes good cost-benefit sense.’ And they admitted, yeah, that makes good cost-benefit sense and they went with it.

So if the water comes over the wall, it hits a splash pad and dissipates as energy on the concrete splash pad. If it comes over a levee, they either have riprap [see a photo of riprap here] or you have clay that you put into the levee and the clay has cohesion – it’s like glue, a binder. Clay doesn’t erode like sand or silt erode; they erode like crazy if you put a fire hose on them. They disappear. The levees that disappeared due to overtopping in Katrina were all 100% silt-and-organics levees. That was one of the big, big lessons from 2005. That was then enunciated even more profoundly in the 2008 and 2011 floods.

AW: So if someone is in New Orleans now, because of the lessons from Katrina, they can feel that they’ll be protected?

David Rogers: Yes, yeah you are. We spent a colossal amount of money there. We may need to do the same thing for other areas like Hawaii and the California Bay delta -- they’re probably even more at risk in terms of the value of the real estate.

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