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Comment by tda | Hacker News Reader

Concrete seawalls are almost always inferior to well engineered earth structures. Only when space is at a premium it may be better to use concrete, but in roughly all other cases a concrete structure is more expensive and more likely to fail catastrophically. But somehow a lot of people have the "something substantial needs to be done" mindset and just don't think moving earth is substantial enough. Or at least that is my tale when I see the pictures. Maybe someone knowledgeable on the local situation can chime in on why concrete was chosen?

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New Orleanian here - the areas in the article are generally on top of swampland and structures sink/erode quite substantially. They’re also outside of the levee protection zone which are primarily earthen levees. When following the road all the way to the end[0] roughly halfway it’s a real trip when you pass the massive concrete structure with a flood gate and suddenly see water bordering each side of the road. It’s also quite dystopian the amount of giant oil facilities that you pass by.

There are non-earthen levees in the city as well. For instance in the 9th ward. These are the levees that failed during Katrina. It’s the Army Corps of Engineers that constructs these levees and have become more complex (better?) over time; imagine an upside-down “T” structure buried in the ground.

If I had to guess, I suppose the reason may be that it’s because they’re outside the protection zone, and so, when it floods (not if) their infrastructure would be surrounded regardless. Also, a concrete structure surrounding only their plant I assume is orders of magnitude cheaper than moving the amount of earth it would take to build 100+ miles of earthen levees.

[0]: https://commons.wikimedia.org/wiki/File:Venice,_Southernmost...

I had the privilege of rocking up in NOLA in a sodding huge cruise ship some years ago after whizzing across the Atlantic. I am well aware that it wasn't the "greenest" action I have ever performed.

It takes nearly a day to crawl up the river M from the Gulf of Mexico. A 18 deck monster at around 180,000 tonnes has to take it easy to avoid damaging the levees with its wash, scouring out the river bed etc. At one point, over the course of 45 minutes, our captain decided to overtake a tanker (showboating). A few minutes later we had to park up in a herringbone formation to wait our turn to dock, which took several hours.

I think it is around 70-80 miles from the sea to NOLA. From 18 storeys up a river as huge as the Mississippi still looks huge. The levees don't look very large though from up there though, they look very flat and fragile.

I studied Civil Engineering in Plymouth (Devon, not MA) and that trip on a cruise ship gave me quite a perspective on engineering works and nature.

@pjot - if you get a chance, take a run up the Mississippi on a really big ship. Its quite an eye opener.

I live in a neighborhood right on the river - one of my favorite things is watching these massive boats go by; especially the Chiquita and Dole container ships filled with bananas.

Fun fact, a ships captain is not the one piloting the boat, they instead must relinquish all control to specialized Mississippi River pilots at the mouth of the river.

https://www.crescentpilots.com/

Yeah it’s always crazy looking down some street perpendicular to the levee and seeing something the size of a skyscraper on its side slide by at 10 mph.

The cruise ships are the really fun ones to watch (sadly I moved to Mid-City)

Same happens in the St Lawrence river / seaway in Canada.

"Fun fact, a ships captain is not the one piloting the boat,"

All ports and many transits require a Pilot.

I've designed similar structures locally. Earthen, steel, and RC.

There isn't a single reason for RC, but the biggest reason is that our local soil sucks and good soil to make 'well-engineered' earth structures is a limited resource.

Additionally, there's differences in engineering philosophy between public-works and the oil field (I do both), and it's probably the opposite of what most think. Especially wrt capitalization/ finance. Almost nothing is done cheapest, let alone cheap in the coastal oil field.

Oil wants it done for a known cost, even if higher up front, to have higher certainty in long-term risk/ or lower maintenance costs. (Also soil takes months longer to settle, and time might be big money, depending on the project.)

If that higher cost tanks the project numbers, fine; they have 100 other projects lined up to make that IRR without resorting cost/corner-cutting.

Other thoughts:

Stone that is suitable for armoring earthen structures isn't sourced locally. No mountains. Has to all be barged in.

And still wouldn't survive the constant battering of wind/waves (in both a long-term seasonal sense, as well as during an acute storm event) as well as concrete.

Subsidence of a pile supported concrete structure will match the adjacent pile supported facilities, and be minimal compared to an earthen structure, which will also have expected settling on top of subsidence.

Floodwalls can double as containment berms, and that adds many design considerations not typical for stand-alone flood structures, including chemical resistance and fire suppression.

But even something as simple as grass cutting might tip the scales. Lots of heli- or boat-only access facilities, and grass needs to be cut at least once per week during summer here. Maintaining grass/earth isn't necessarily cheaper when your yard guy needs a stack of govt twic/osha cards, a private ferry ride out, and probably transport to a private dock a few hours from town.

All of which just barely gets into the logistics involved in the industry.

>more likely to fail catastrophically

Maybe catastrophically, depending on how you want to define it, but less likely to fail overall. Concrete is more likely to fail in a predictable manor.

Even the Katrina concrete/steel 'floodwall' failures were actually soil failures, not RC or steel ones.

Thanks for the insights. That cutting the grass would be a huge cost did not occur to me but with enough red tape I see that could be the case.

Even though soil failure is the direct cause, if that fails the structure has failed. The reflex of adding more concrete does not always help. A hard, vertical structure is basically a concentrator for both currents and wave energy. So you need a huge scour protection or else the wall will only work very temporally. And a narrow structure is more susceptible to piping (where water finds a path under the structure and then erodes that away). This is of course also a soil failure, but unless you are building a boat you have to interface with the soil somewhere. And that is usually where your structure will fail...

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