Q&A: Which Levees Will Hold Back Tropical Storm Barry, And Which Won't?

Jul 11, 2019

A few sections of levee along the Mississippi River are at risk of being overtopped in Southeast Louisiana. The reason: storm surge from the Gulf of Mexico. WWNO's Travis Lux spoke with Ricky Boyett from the U.S. Army Corps of Engineers about why this is happening and which areas are the most at risk. 

Responses have been lightly edited for clarity.

Q: The Mississippi River is already very high right now. It's been flooding for months - close to a year. Now, the storm surge from Tropical Storm Barry could push the river levels up a few additional feet. So which sections of levee are most at risk right now in the Greater New Orleans area and Southeast Louisiana?

Boyett: You know, fortunately with the river forecast that came out today they actually dropped the forecast a foot. So we were looking at a potential of 20 feet at the Carrollton gauge, but today we're looking at a potential of 19 feet. That's great news because our system in New Orleans is between 20 and 25 feet high. So that further kind of helps to prevent overtopping of the system. Now we understand that there is always potential for that to change again. It could go higher, so we had to plan for the worst case. But right now we're seeing good news that the river will be below levee elevations. 

Q: What about in Plaquemines Parish? The levees over there are a little bit shorter right, and that's why they've announced these mandatory evacuations. So are those areas at risk of being overtopped?

Boyett: Right. When you're starting to see the surge come up into the Gulf, and it's backing up the river water you get a little higher elevations down there in Plaquemines. You know we say 20 feet at the Carrollton gauge, but that's the water level in the New Orleans area. And that does still mean there's really good risk of overtopping in the Plaquemine's Parish. And that's why it's so critical that the residents do listen to their local officials and evacuate. 

Q: As the current forecast stands, if the water from the river were to overtop in say Plaquemines Parish, what's that going to look like? Is it gonna be rushing? Is it gonna be a trickle? And how likely is it? 

Boyett: When we say "overtopping," we are talking about water freely flowing over the top of the levee. Really, it is going to matter on what that ultimate surge is as well as the duration of the event. So we don't have enough information to be very specific on how much water will flow over the levee. That's kind of less important than, "there will be water coming over the levee as this forecast holds." And you know, water can fill up behind the levee very quickly, so residents need to heed those warnings and pay close attention to what could occur out there. 

Q: If a levee is overtopped, does that make it more at risk of breaking? 

Boyett: That's always a big concern. One of the lessons we learned from Hurricane Katrina is overtopping of the levee can cause erosion on the back side. So you armor the levees. Our Mississippi River levees are extremely robust, and the armoring that we're using for the hurricane system that we built after Katrina is very similar to what's on the levees and the river levees. So we're confident that we're going to be able to handle the overtopping in that area without breaching. 

Q: How confident are you about the New Orleans area and the risk of overtopping in the New Orleans area?

Boyett: As we were running models yesterday, even looking at the first estimate of a potential of a 20 foot reading at the Carrollton gauge, we were not seeing any overtopping in any New Orleans proper area. 

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