Mystery surrounds floodwall breaches

[ramblin_dave]

Mystery surrounds floodwall breaches
Could a structural flaw be to blame?
Times-Picayune

One of the central mysteries emerging in the Hurricane Katrina disaster is why concrete floodwalls in three canals breached during the storm, causing much of the catastrophic flooding, while earthen hurricane levees surrounding the city remained intact.

Ivor Van Heerden, who uses computer models to study storm-surge dynamics for the LSU Hurricane Center, has said that fragmentary initial data indicate that Katrina's storm-surge heights in Lake Pontchartrain would not have been high enough to top the canal walls and that a "catastrophic structural failure" occurred in the floodwalls.

[woodsprite]

1. Wasn't there an article somewhere over the last few days

that had one of the evacuees saying they heard something like explosions coming from the levees?

Just a thought.....

I'll take my tinfoil hat off now :tinfoilhat:

[Taxloss]

6. Yes, there was.

It was not in any way reputable or reliable, and was swiftly debunked.

[SmokingJacket]

2. Hmmm ...

First article from a legitimate source that raises questions about why the concrete failed.

Hmmm. That's all I'm saying.

[rfranklin]

4. Things just seem to blow up when Bush is involved....

World Trade Center

Iraq

New Orleans

What's next?

[Taxloss]

7. Well, that proves it then! Two conspiracies for the price of one!

[rzemanfl]

3. Probably Republican concrete supplied by the company that

made the biggest donation to the Bush campaign.

[NYC]

10. Concrete has to be evenly mixed.

Combination of cement and gravel. Gravel must be evenly suspended throughout the cement.*

If this job was given to the biggest contributor, it could have been a poor job, either to save money & make more profit, or because the company wasn't qualified.

I guess we need to find out who did this.

*I'm not saying that this is the only thing that could have gone wrong. I guess a million things could have gone wrong.

[LisaM]

5. It reminds me of a documentary on the Titanic

which showed that weak materials contributed to its being punctured so easily. They likened it to a sheet of aluminum foil. I wonder if someone stinted on a contract and put in unsafe materials. If so, I'd be interested to know who that was and when it happened (and if levee underfunding contributed to it.)

[Taxloss]

8. Self-delete

[LisaM]

9. I read the article (don't know if you were talking to me)

and it seemed inconclusive.

[nashbridges]

11. The debate amongst engineers so far

Seems to be whether the walls were "topped" (the water flowed over) or whether they broke without the water coming over the top at all. Since floodwalls are constructed differently than levees, if they failed because of a design flaw than it is important to find out why.

Floodwalls take up less ground area than a levee, but because of design are supposed to hold the water back just as well. This is an oversimplification, but think of floodwalls and levees like this:

|.........................................../
|........................................../
|........................................./
|......................................../
|......................................./
|....................................../
|...................................../
FloodWall..........................Levee

Levees need to be much larger at the base because water pressure pushes outward (horizontally - it's counterintuitive) in direct proportion to the water depth. Floodwalls counteract that horizontal force by burying steel deep in the earth and attaching that steel to concrete walls above ground.

In theory, both solutions should work. Obviously, one solution didn't . Keep in mind that a concrete floodwall can fail for numerous reasons, including being hit by a large object (floating debris), bad concrete, poor design, or even crappy, crappy construction.

There are some suggestions out there that the floodwalls broke through not because of overtopping or even being hit by debris, but because of the proverbial "hole in the dam" - a small leak, or several small leaks, broke through and progressively weakend the wall to the point of failure.

This Q+A is from NPR: http://www.npr.org/templates/story/story.php?storyId=4826934

Q: What is the difference between a floodwall and a levee, anyway?

The canals walls that broke are technically called floodwalls. They are made of concrete and steel, are 6-to-10 feet tall, and about a foot wide at the top and 2 feet wide at the bottom. They stand on top of an earthen base.

A levee is a broad mound, 50 feet or more wide at the base, that rises slowly to a broad crest at the top. You could easily walk or drive up the side of one. These are far more stable than floodwalls. Water can spill over the top and erode some of the levee, but it will still function. When a floodwall fails, it fails catastrophically.

Please excuse the extra periods - I couldn't get the html to include the spaces.

[ramblin_dave]

12. Thanks, interesting comments

I guess it will take a while to determine if the water overflowed the flood walls or not.

[nashbridges]

13. It doesn't matter if there was overflow or not

The question is why the floodwalls failed.

Overflow is an easy answer, but if they failed through leaks, impact from debris, or bad concrete - it's all important, at least to us.