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(A brief pause in an ongoing series for a sidebar discussion.) The Washington Post reports that Katrina may not have been as powerful as many were led to believe:
The Army Corps of Engineers has said that Katrina was just too massive for a system that was not intended to protect the city from a storm greater than a Category 3 hurricane, and that the floodwall failures near Lake Pontchartrain were caused by extraordinary surges that overtopped the walls.It should be noted here that modeling and simulation of complex systems is not perfect, even after the fact. If such behaviors could be modeled with great precision then the decisions to evacuate large cities would not be difficult. But beyond the numbers there's additional evidence supporting the result of this study:
But with the help of complex computer models and stark visual evidence, scientists and engineers at Louisiana State University's Hurricane Center have concluded that Katrina's surges did not come close to overtopping those barriers. That would make faulty design, inadequate construction or some combination of the two the likely cause of the breaching of the floodwalls along the 17th Street and London Avenue canals -- and the flooding of most of New Orleans.
"We are absolutely convinced that those floodwalls were never overtopped," said van Heerden, who also runs LSU's Center for the Study of Public Health Impacts of Hurricanes.
The center has also completed a computerized "hindcast" of Katrina, which has confirmed the evidence before their eyes. Their model indicates that most of the surge around the lake and its nearby canals was less than 11 feet above sea level, and that none of it should have been greater than 13 feet. The Army Corps's flood-protection system for New Orleans was designed to handle surges of more than 14 feet above sea level.
On a tour Tuesday, researchers showed numerous indications that Katrina's surge was not as tall as the lakefront's protections. They showed a "debris line" that indicates the top height of Katrina's waves was at least four feet below the crest of Lake Pontchartrain's levees. They also pointed out how the breached floodwalls near the lake showed no signs of overtopping -- no splattering of mud, no drip lines and no erosion at their bases. They contended that the pattern of destruction behind the breaches was consistent with a localized "pressure burst," rather than widespread overtopping.Those results are consistent with what we noted back on 10 September regarding the strength of Hurricane Katrina as it passed close to New Orleans on it's way to Mississippi. It's good to see evidence supporting that hypothesis. (And seeing it very rapidly - a few short years ago this sort of result would take months to achieve, and longer to become "public knowledge" - if it ever did so at all.) Expect controversy to be one of the main results of the release of this information, but the first step in avoiding a "repeat of Hurricane Katrina" is to identify what Hurricane Katrina was - something that is still not completely clear (in spite of the endless media coverage to the contrary).
More to come.
I've been puzzled by the focus on whether the waters "overtopped" the Levy's. I don't know much engineering, but I lived in Sacramento for many years, of which a large part is protected by a levy system.
In times of serious flooding, the Levy's that were considered a problem by city government agencies were not so much the ones that might overflow, but rather those that shows signs of breech. Engineers from the city were always looking for "wells" alongside the levy rather than at the levy and the height of the water itself. These "wells" look like artesian springs, and can show up many yards from the actual levy itself. They look like pressurized water bubbling up through a hole in the ground.
With the storms, its not just the amount of water that is the problem, it's the extra weight it brings, along with the speed. The extra weight increases the pressure on the water at the bottom of the river to find new outlets. This with the speed makes the river (or canal) become undercut and grow deeper as well as faster. So what happens is that the water can actually force its way underneath the levy to bubble up on the other side. If left unchecked the well can widen and basically destroy the levy from the bottom up rather than the top down. They city engineers always seemed to be more worried about that happening rather than the water overflowing the top. After all, even if water overflows the levy, so long as it remains intact you are probably going to be in decent shape.
Whether this scenario can happen depends more on how deep the levy is, not how much higher it is than the level of the river water. And on what the composition and weight is of the foundation material that the levy is based on.Posted by Patrick (Gryph) at September 21, 2005 07:21 PM
This is great!
Both positions have merit and it is worthwhile to understand what happened here for a number of reasons.
The main problem is "below sea level".
Dump stones and dredge into the city until it is 20 feet above sea level. We are in the position if opposing irresistable force here. If the city continues to subside it will be at the bottom of a 50 foot well.
Or the city can be moved.
25 years after it is done no one will remember and it will be politically irrelevant anywayPosted by Mike Johnson at September 21, 2005 07:44 PM
Same data, different interpretation: On the Levees of New Orleans (Update 11)Posted by Solomon2 at September 21, 2005 07:52 PM
There is yet another set of factors, that none of these reports have addressed yet. Maintenance and nearby construction. A good friend of mine who aside from being an infantry platoon sergeant is a civil engineer. His firm does flood control projects almost exclusively for the Army Corps of Engineers. He pointed out that the money spent on levee maintenance in New Orleans has been below Corps recommendations for some time. And on top of that, the city has long allowed construction along the levees much closer than Corps engineers, or most competent engineers advise. Lack of maintenance and nearby construction certainly weakened the levees.Posted by John Byrnes at September 21, 2005 08:23 PM
Somewhere in the first few days after Katrina, I thought I read that the breach was caused by a loose barge. I never saw anything about it again, and can't help to wonder if I actually _did_ see it - or heard it, perhaps, and if so, if it was known or theorized. Are there barges in Lake Ponchetraine? or just in the Mississippi?Posted by suek at September 21, 2005 09:37 PM
I also saw the story about the barge. It seems quite likely.Posted by Don Cox at September 22, 2005 10:28 AM
Re: Lake Ponchetraine & debris. Why do we need the "big lake"? Drain it into the gulf and use the hole as a landfill in which to put all the destroyed houses, school busses,cars, etc. What else can you do with thousands of refrigerators? No more levy problems, no cars sold to unsuspecting buyers, many problems solved.Posted by Lucille at September 22, 2005 01:49 PM
I have posted correspondence between Dr. Kemp of LSU and myself on the levee break issue. Dr. Kemp reports no evidence of scouring that would indicate the levees were overtopped. It is now up to the Corps of Engineers to prove otherwise:Solomon2 at September 22, 2005 02:25 PM
Latest update, w/news & analysis by guest G.B.:Solomon2 at September 28, 2005 12:49 PM Hide Comments | Show/Add Comments in Popup Window(9) | (Note: You must refresh main page to view newly posted comments here)