[StBernard] Levee Design

[Westley Annis]

This is a follow up on an email I wrote previously about levee design. 
I am now able to compare the 1965 Corps of Engrs.' project storm 
criteria to the National Weather Service's Saffir-Simpson scale which 
came out later.  The Standard Project Hurricane Parameters for Coastal 
Louisiana that the COE uses for their designs are: a central pressure 
of 27.6 inches of mercury (falls within cat 4) , max. average wind 
speed in a five minute time period of 100 mph (falls within Cat. 2, but 
the Saffir Simpson scale is the average over a one minute time period 
so it's not quite the same thing), storm surge 9 to 12 feet high (falls 
within cat 3).  This Standard Project Hurricane has a theoretical 
probability of happening once in 200 years and is based on a 
fast-moving storm on a certain, critical path.

However, keep in mind that the weather database doesn't have 200 years 
of actual scientific data. Maybe in 2125 they'll be able to look back 
on 200 years of data and have a better understanding of probabilities.

By comparison, Hurricane Katrina was reported at landfall  at a 
location of 60 miles southeast of N.O. to have a central pressure of 
27.17 inches of mercury (cat 4) and a max sustained wind speed of 140 
mph (cat 4). The storm surged varied a lot with Katrina, supposedly 
anywhere from  11. 2 feet in the Industrial Canal and 15 to 25 feet in 
the Miss. River and MR-GO. Many gages were destroyed in the storm so 
there isn't a definite answer for all locations, or at least I couldn't 
find it published anywhere with certainty.

So in summary, Katrina on the Saffir Simpson scale was a category 4 for 
pressure and wind speed and a cat 4 or 5 for storm surge, depending on 
location. By comparison, Katrina's pressure was within the COE design 
criteria, the wind speed was higher, and the storm surge was within 
theCOE's design range if you look at the interior canals like 17th 
Street, London, and Industrial, but supposedly much more for the other 
areas.

Sounds confusing, but it's to focus our attention that changing the 
COE's criteria to Saffir Simpson Cat 4 or Cat 5 is an engineering 
challenge and if they do this, in my opinion, they should keep it 
simple and use the Saffir Simpson criteria, now that there is Saffir 
Simpson. This way whatever we have is a true Category 4 or 5.

Deborah Keller