[UPDATE: Meteorology Ph.D. student and weather blogger Charles Fenwick believes the Army Corps’ numbers are plausible. He knows a lot more about this stuff than I do, and his analysis makes sense.]
The New York Times has an article today declaring that New Orleans is still vulnerable to severe flooding from a hurricane — hardly a surprise, though the timing of the article is interesting, since that fact could become quite relevant in five or six days’ time if the computer models keep shifting Hurricane Dean’s track to the right.
But anyway, what is surprising, to me at least, is the fact that the Army Corps of Engineers has declared that Hurricane Katrina was “a 1-in-396 [years] storm.” That is to say, according to the Corps, residents of New Orleans can expect to go almost 400 years, on average, between storms as bad as Katrina — and even longer, one presumes, between storms that are worse.
I would really like to see the study on which this conclusion is based, and examine the rationale underpinning it, because on the face of it, this conclusion strikes me as peculiar, even bizarre. As I’ve pointed out numerous times, Hurricane Katrina could have been far worse than it actually was for New Orleans: the center passed 30-40 miles east of the city, sparing New Orleans a direct hit; a last-minute bout of dry-air entrainment severely weakened the portion of the eyewall that passed over the city; and, more broadly, the whole storm weakened just before landfall from a Category 5 to a Category 3 and, by the time it reached New Orleans’s latitude, a Category 2. How is that a 400-year storm? (Read more here, re: the NHC’s official report on Katrina.)
Admittedly, Katrina’s storm surge was historic, far worse than a typical Cat. 2 or 3 (worse even than a “typical” Cat. 5, if there is such a thing), because it had been so powerful the day before and was so geographically huge. But the worst of the surge hit Mississippi, not New Orleans. The dangerous right-front quadrant of the eyewall brought a wall of water 30+ feet high into Waveland and environs, while back over in New Orleans, the weakened left-front quadrant was delivering the city what amounted to a glancing blow. The worst damage to New Orleans, by far, was from seeping water slowly filling up the city after the storm; the storm itself wasn’t that bad, except insofar as it breached the deficient levees. I have always contended that, if the track had been slightly different and that massive surge had made a direct hit on southeastern Louisiana, funneling the wall of water up the rivers and canals toward the Big Easy and breaching the levees more quickly and completely, the death toll could have been in the tens of thousands because many, many people who sought refuge on their roofs (and were eventually rescued by helicopter) would have instead drowned in the much higher water levels during the height of the storm — which, in such a scenario, would have been far more deadly, with higher winds, more debris flying around, more wave action, etc.
In any event, the Army Corps’s conclusion makes no sense to me. If Katrina was a once-in-400-years event, that would seem to imply that hurricanes of Category 2 or greater intensity, which were Category 5 less than 24 hours before and are geographically very large, will only pass within 30-40 miles of New Orleans (on either side) every 400 years on average. That conclusion seems obviously wrong, doesn’t it? They do realize that New Orleans is on the Gulf Coast, right?
As I said, though, I’d really like to see the rationale underlying the Corps’s conclusion, because maybe I’m missing something. Perhaps their conclusion is well-founded, for reasons I’m not grasping. As I try to repeat frequently, I’m not actually an expert on this stuff, so I could be wrong. But if I’m not wrong — if the Corps is engaging in some fuzzy math here, perhaps because it wants to pretend it’s gotten New Orleans better prepared than it really has — then I fear for the complacent attitude this sort of thing may cause. If New Orleans residents believe Katrina is as bad as it’s ever likely to get in their lifetimes, or their children’s or grandchildren’s lifetimes for that matter, the ones who stayed put and survived in 2005 will probably stay put again the next time a storm threatens, not realizing that Katrina was just the tip of the iceberg in terms of what New Orleans could potentially face whenever a major hurricane finally decides to make a direct hit on the city.
P.S. Much more about the Corps and its blatant failures from Time magazine. (Hat tip: Patrick Cullen.) The article includes a quote from LSU hurricane researcher Ivor van Heerden, who certainly doesn’t sound as if he believes Katrina was a 1-in-400-years storm: “Katrina wasn’t even close to the Big One,” he says. “We better start getting ready.”
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Categories: Hurricane Katrina
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August 17th, 2007 at 3:09:04 pm
I am a native Orleanian. I went through Betsy and was on the Gulf Coast two days after Camille.
My grandmother told me about the 1915 storm and the 1947 storm.
I believe that Katrina was a 400 year storm. The surge is what did the damage.
New Orleans could not survive Cat 4 or 5 winds, so it one hits head on and maintains that velocity, the surge won’t matter.
August 17th, 2007 at 3:13:37 pm
In New Orleans, I’ve been through four “once in a hundred years” floods, Betsy, Camille, and now Katrina.
What you quote are mere statistics. Mark Twain once said there were three kinds of liars. Liars, damn liars, and statisticians.
August 17th, 2007 at 3:42:45 pm
Greg, with all due respect, belief is one thing, and facts are another. What “did the damage” in New Orleans isn’t the “surge” per se, but the flooding that occurred as a result of the faulty levees, which were overwhelmed by the surge — which, according to everything I’ve read, shouldn’t have been enough to overwhelm the levees, if they’d been constructed properly. To cite the flooding damage as evidence that Katrina was a 400 year storm is to beg the question, in a way. The question isn’t whether New Orleans suffered a 400-year flood; the question is whether Katrina was a 400-year storm. Because the flood was partly caused by the storm and partly by human error, the two questions are distinct.
The central issue is whether the levees’ failures were exposed because Katrina was so much more powerful than other storms, or for some other reason… like, for example, because the levees were faulty and bound to fail sometime. The latter has been convincingly argued by others — that “New Orleans was doomed with or without Katrina, we just didn’t know it.” In other words, the levees were inevitably going to fail at some point, due to their blatant engineering failures, and Katrina just sped up the process.
Even if that’s overstating the case, the fact remains that Katrina’s strength was what it was, its surge was what it was, its winds were what they were. That they happened to cause a horrible flood isn’t evidence that they were stronger than the objective data indicates. So I’m really not sure how the “400 year storm” conclusion can be supported.
August 17th, 2007 at 4:12:07 pm
Personally I’ve never been a fan of the X year whatevers. It leaves the less analytical types believing that the worst thing has happened and we have nothing to fear for a long time. Mean while the X*10 storm comes along and we haven’t even bothered to purchased trousers to be caught down with.
August 17th, 2007 at 4:12:26 pm
With similar respect, I think you are another victim of lawyers, devastated Orleanians who are seeking someone to blame, the media, and many politicians.
No doubt there were engineering problems all along the 300 miles of levees. The levees that were overcome were east of the Industrial Canal. These levees were built to withstand a so-called Cat 3 storm and surge. Katrina’s surge was plus 5. These levees, perfectly built, were not built to withstand Katrina’s surge.
The engineering failures most attacked were were with the walls atop the levees along two outfall canals. Once again, there were built to withstand the so-called Cat 3 surge. That is not what we got with Katrina. Those walls did not exist for Betsy in 1965, yet there was no topping of the shorter levees.
This does not mean that there were not problems throughout the 300 miles of levees. There were, but even with problems, all were stronger than in 1964 when Cat 3 Betsy hit.
Some of the analysis for the 400 year figures was geologic, looking at how often certain factors have happened in the area.
The only way to guarantee protection of New Orleans is to overengineer that protection similar to the way the Dutch do it, and I suspect the money for that will never come out of Congress.
One look at the Mississippi Gulf Coast after Cat 5 Camille compared to the aftermath of Cat 3 Katrina is instructive. There is no comparison between the damages.
August 17th, 2007 at 4:17:17 pm
Brendan,
My company performed damage assessments for all three major hurricanes in 2005. While the flood damage was severe, I find it hard to believe that Katrina would be classified as a 1 in 400 year storm. The reason being–the wind damage was minor. We found few roofs destroyed or even moderately damaged in New Orleans, certainly nothing compared to what Wilma did in South Florida.
If a relatively weak storm like Katrina could cause so much flooding, I shudder to think what a major cat-4 or 5 storm would do. Instead, the 1:400 designation seems to based upon the location of impact rather than the intensity of the storm.
August 17th, 2007 at 5:30:58 pm
Katrina’s surge was plus 5.
Can you back this up with some sort of data? Because if it were true, Greg, then it would certainly change my opinion. However, as far as I’m aware, Katrina’s surge was only Category 5 strength in Mississippi. In New Orleans, which was hit by the left-front quadrant of the storm (whereas the right-front quadrant always has the biggest surge), my understanding is that the surge was on the level of a borderline Cat. 2-3 storm. I’ve read the NHC’s official reports and various other documents and articles relating to the storm (none of them written by “lawyers” or “devastated Orleanians”), and nowhere, except in your comment, have I read any suggestion that Katrina’s surge in New Orleans was a “plus 5.” And since every bit of meteorological data regarding the storm’s path and strength would seem to fly in the face of this statement, I will assume that you’re incorrect until I see some data to back you up.
Some of the analysis for the 400 year figures was geologic, looking at how often certain factors have happened in the area.
But if those factors were based on the extent of the flood, then, again, they beg the question and prove nothing.
One look at the Mississippi Gulf Coast after Cat 5 Camille compared to the aftermath of Cat 3 Katrina is instructive. There is no comparison between the damages.
Yes… on the Mississippi Gulf Coast. Katrina may indeed have been a 400-year storm for the Mississippi Gulf Coast. But we weren’t talking about the Mississippi Gulf Coast. We were talking about New Orleans. Katrina didn’t make a direct hit on New Orleans; she didn’t even hit New Orleans with the strong side of her eyewall. She hit it with her weak side, which was even weaker than would normally be expected because of some extraordinary last-minute weakening. If that’s enough for a “400-year storm,” then I think a lot of hurricane experts are going to be scratching their heads at what sort of magical charm the Army Corps of Engineers thinks is protecting New Orleans from direct hits from major hurricanes!
August 17th, 2007 at 5:38:33 pm
Also, Greg - if Katrina truly brought a “5 plus” surge into New Orleans (or into the rivers, canals, etc. that lead to New Orleans), then why did it take so long for the flood to “fill the bowl”? All the modeling had long suggested that a flood would happen very quickly in a Cat. 4 or 5 event. If Katrina was, as you claim, a Cat. 5 event, it shouldn’t have taken upwards of 12-24 hours for the city to flood. It should have happened in a matter of hours . And that’s precisely my point: in the true “Big One,” the true “400 year storm,” it will happen much faster, the waters will rise much higher, and the result will be much deadlier. Heaven help New Orleans and its residents if they adopt the Greg/Army Corps view, and complacently assume that it will never get worse than Katrina — when in reality, Katrina itself was almost very much worse than Katrina.
August 17th, 2007 at 8:19:15 pm
If you’re looking for a once-in-whatever years storm, you have to look to Galveston in 1900, a confluence of rain, sea and wind about as likely as catching the Royal Flush, runner runner, on the turn and river. What was necessary was 1) a hurricane of historical proportions, accompanied by 2) savage winds from the north that met the storm and piled up water just off (and directly parallel to) the shoreline for eight hours, followed by 3) said winds breaking ninety degrees, and all this happening at 4) the exact point of greatest vulnerability, a coastal island town whose highest point (pre-seawall) was four feet above the shoreline at high tide.
(I know there are five cards in a flush, but bear with me.)
As recounted in the brilliant book “Isaac’s Storm,” what came ashore was not rain or floodwaters but the Gulf of Mexico itself, twenty blocks inland and twenty feet deep. For one evening, Galveston became Atlantis.
Now here–so many things at extermis at once–is your four hundred- or thousand-year storm.
August 17th, 2007 at 8:23:30 pm
Yes. I would say the Galveston Hurricane of 1900 and the New England Hurricane of 1938 (which actually rode the jet stream — totally unheard-of for a warm-core tropical system — with a forward speed of 60 mph, bringing what amounted to Category Five winds to areas in the right-front quadrant) are the best candidates for 400+ year storms in the last century-plus.
Katrina, not so much.
August 17th, 2007 at 9:16:26 pm
On second thought, Charles Fenwick thinks the 400-year-storm estimate makes sense, as I just noted in an update at the top of the post. Well, I bow to his superior expertise. In other words, in the words of the immortal Gilda Radner, “Oh… nevermind.”
August 18th, 2007 at 9:47:14 am
Filling the bowl: The bowl took its time to fill because the breaches in outfall canal walls and levees were relatively small.
There is other evidence of the strength of the surge outside of New Orleans. On the Northshore of Lake Pontchartrain, house that stood for 100 years were destroyed or damaged in ways that never happened in recorded history. The surge flooded old town Slidell, and that had never happened in recorded history.
There is a criminal case going on right now about the death of nursing home patients in St. Bernard.
The levees were supposed to be 17.5 feet high. The engineering failure is that they had subsided to 14 feet, but the surge was over 23 feet. The levees breached because of backwash from overtopping and put 7 feet of water into this nursing home. Without the breach, there would have been 6 feet of water in the nursing home.
Whether it was a 400 year storm or 200 or 50, the surge was the worst recorded in the New Orleans area.
The loss of wetlands exascerbated the effects of the surge and will get worse as more wetlands diappear.
August 19th, 2007 at 9:56:16 am
The Sunday before Katrina hit, water was already being built up along the Mississippi Gulf Coast. When it starts building up like that and with Katrina forcing ever more water, it becomes bottlenecked at the Louisiana/Mississippi state line where the moth of the Pearl River and the Rigolets are. The Rigolets is the entrance to Lake Ponchatrain. The storm water forced into Lake Ponchatrain, Lake Borgne, Bay St. Louis, and the mouth of the Pearl River.
The storm surge cannot be discounted as a factor in the levee breaches. Even though New Olreans had a glancing blow, there’s all that storm surge in Lake Ponchatrain. Once Katrina moved off, all that water could not escape through the very narrow Rigolets. It had to go somewhere.
The Times-Picayune has a very good graphic that shows the effects of the storm surge.