Originally Posted by sammi jo
I'm a total layperson on this; I have a basic understanding of the atmosphere and how it works, but I guess a lot of us do. Here's the link to the computer models' track and intensity forecasts I've been looking at.
You've probably already got it perhaps?
omg, you can say that again. If this one took a track slightly west
of N.O., at this evening's forecast intensity of 110 kt sustained winds, N.O. would catch the bullet she missed
Shortly after Katrina I was looking at tidal and other data which showed that the storm surge in Lake Ponchartrain was pretty much a non-event.. this bears out with the wind records in the metropolitan N.O area during Katrina.... cat. 1 hurricane force winds were only occurring locally, and for short periods... mostly at higher elevations (high rise buildings caught some really rough weather), but the sustained winds at sealevel were for the most part strong tropical storm force. The city was in the "weak" sector of the storm (40+ miles west of the western eye-wall), thus the winds backed from north east, to the north, to northwest, and then to the west as the storm tracked northwards well to the east.. and were pushing the water back out into the GOM, as opposed to what happened east of the eye... which was a huge storm surge that went inland for miles. Despite the relative lack of storm surge in N.O., the levees failed, in multiple places. Horrendous as it was, it *could*
have been far, far worse... if Katrina had tracked a little west of N.O. and hadn't weakened considerably in the hours prior to landfall (from cat 5 to cat 3)... it would have been a catastrophe with many thousands more fatalities than the approx. 1850 who died that day.
I don't blame you. I would. A really close friend of mine flew to N.O. the day after the storm hit to volunteer in rescuing hundreds of horses in the area. What she witnessed was so much worse than what we saw on television. She took her own videocamera and shot many hours of material... utterly horrific... I can't even begin to describe it.
I hope this thread is short, and Gustav falls apart somewhere where no harm can be done.
Please don't quote me on this, since it's been a while, and I may not have all the facts stated accurately.
The original design storm was a CAT3 for the levees, meaning basically a water level or a storm surge representative of a CAT3 storm.
Now I think there was agreement that the levees failed at a storm surge below the CAT3 storm event.
How do we know that?
We know that because technically an over topped levee in not a failed levee, since it withstood the design water level, was over topped but not breached.
In 2005 no levee structure was over topped (if I recall correctly), but several were breached, at water levels below the levee crest elevation (or superstructure or crown elevation).
Thus the levee system failed at several locations below the design water level, in other words below their respective maximum crown elevations.
Now, from my limited understanding of the current design, the USACE was to restore the entire levee system to at least it's prior design crown elevations. Some sections have undoubtedly been strengthened, while others that have subsided over time due to subsoil consolidation have been raised to at least their prior design elevations.
So technically it is not strictly the "CAT3" type criteria, but whether the levees withstand water levels up to and above their maximum crown elevations. And if water levels exceed the crown elevation, a properly designed levee will not be breached, but will act like a broad crested weir.
Now with respect to over topped levees, the NO situation was to have new pumping stations capable of handling
overflow. These would likely take the longest to complete, and may not be complete to date, but don't quote me, because I haven't been paying attention to the latest status quo of the NO levee system design/build phases.