Text 8750, 67 rader
Skriven 2005-12-19 15:52:42 av John Cuccia (1:379/45)
   Kommentar till text 8749 av Frank Haber (1:379/45)
Ärende: Re: NO Levee Failures not so simple?
============================================
From: John Cuccia <jcuccia@bigfoot.com>

On Mon, 19 Dec 2005 14:31:37 -0500, "Frank Haber"
<frhaber@N0SPMrcn.com> wrote:

>Any civil engineers here?  That article is fabulous, but doesn't give a
>picture of the standard calculation of safety factors.  Is it an RMS thing,
>with outlying data ignored?

Well, according to the article, designers are supposed to find the weak spot(s)
and apply a safety factor of 1.3 to the design at those locations.  Doing that
should yield higher safety factors at most other places, but in this case did
not.  That's why I think this is the money quote from the article:

===========
"If I can do a hundred cross sections of a levee and a few are 1.3, that's OK.
But here all the factors are around 1.3," Rogers said. "From the outside,
that's suspicious. If you come across something like this, you say maybe the
answers they want to get are driving the analysis, not the analysis driving the
answers."
===========

The Times-Picayune has been really good a covering this story from the
beginning. If you do a search on their site for either "levee design" or
"floodwall" you get the whole list of articles from their post-Katrina
archives.

>I've no problem with the implied ignoring of surveys and improper basic
>assumptions.  That soil looks like really slippy-slidey stuff, and it seems it
>wasn't properly surveyed.

They knew the soil was weak long before they designed the floodwalls, as
indicated in the first paragraph of the article.  Note that the floodwalls
weren't designed and built until the 90s.

===========
After a 1980 flood caused a stretch of the city's London Avenue canal levee to
collapse, the Army Corps of Engineers proposed replacing it with a fortified
design called a T-wall, with sheet pile foundations driven 26 feet deep. And in
1981, a study by Metairie design firm Modjeski & Masters found that proposed
higher levees along New Orleans' 17th Street Canal likely would fail in high
water because they were built on "very soft clays with minimal cohesion."
===========

>And then we come to the political and costs/bidding pressures.  I wonder
>whether there is a "crisis in morality" in the civil trades, mirroring the
>almost total abdication of probity by the accountants over the last thirty
>years?  Bet that last one gets a rise out of someone here (g).

It's entirely possible that the Corps, who have a long history of NIH syndrome
(try John Barry's "Rising Tide: The Great Mississippi Flood of 1927 and How It
Changed America" for a taste) , and who initially recommended floodgates at the
canal/lake interface, were pissed that locals did not accept that proposal and
proceeded to do sloppy engineering because their noses were out of joint.

There were two reasons for the lack of acceptance by locals.  First, the people
responsible for pumping water out of the city (the Sewerage and Water Board, I
believe) would not be able to pump rainwater out during a hurricane if the
floodgates were closed.  Second, maintenance costs are local and the local pols
didn't want the expense of maintaining floodgates.

It's also possible that there were budgetary constraints, but in that case it
would have been better to ask for more money than to build something that had
little chance doing its job?

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