[StBernard] Levee Design

[Westley Annis]

To further add to Westley's comments about what height levees are built 
to-

The federal govt. prepared its own study years ago for the "project 
storm" that levees would be constructed to protect against. It is not 
an exact fit for any Saffir-Simpson category. Below is Saffi-Simpson 
criteria for storm strength that you can find on the internet:

Saffir-Simpson Category Hurricane Criteria:

  Central Pressure (mb) Maximum Sustained Wind Speed (mph)  Storm Surge 
(ft)    Damage Category   Wind Pressure (psf)
1   >980                        74-95

   4-5                  Minimal                     19-30
2   965-979                    96-110                                       
    6-8               Moderate                  30-40
3   945-964                    111-130                                     
    9-12                 Extensive                 41-57
4   920-944                    131-155                                     
    13-18                Extreme                  58-81
5   <920                         >155

  >18                 Catastrophic           >81


The Corps of Engineer's project storm for design of a federal hurricane 
protection levee is based on a certain central pressure, wind speed, 
storm surge, wind pressure, and most importantly, a PATH and SPEED that 
it would cross land.  I would prefer for someone from the Corps to 
provide that criteria/data as it has been many years since I last saw 
it published and I don't have it available to me now.  To make it 
easier for the general public, the Corps and others call our area's 
federal levee design as being a Category 3. BUT MAKE NO MISTAKE, it is 
NOT the maximum winds, surge, or pressures of a Cat 3. In some of these 
design elements, it may be closer to a Cat. 2 or a weak 3; I'd need to 
see the data again.

Also a factor in levee height is wave run up. In some geographic areas 
the Corps assumes that the water will be still, such as an inland 
canal, and in other areas there would be a wind driven wave running up 
the bankline or shoreline, so the levee protection has to be a little 
higher to take that into account and stronger to take the force of 
impact from a wave.

As a civil engineer, I'd prefer if we would all stop talking about 
categories of Saffir-Simpson and be specific about the actual design 
central pressure, max. sustained wind speed, storm surge, wind 
pressure, STORM PATH, and SPEED OF THE STORM ACROSS THE AREA.

Also, the creators of Saffir-Simpson said after Katrina, that these 
were approximations and that perhaps they need to re-think the numbers 
that fall in each category. In other words, maybe a Cat 3 storm 
produces storm surges greater than 18 feet.

The way engineers and hydrologists decide on a design is to take the 
actual reported data AFTER a storm, plug the numbers into the computer 
models and physical models and see if the models produce the same 
results as what actually happened with a real storm. As with any 
science, it's trial and error to correlate the assumptions and computer 
equations with real life and it takes several Betsys, Ivan, Georges, 
Andrews, and Katrinas to refine it.

And one more thing that needs to be said.  Typical residential 
construction cannot withstand the 155 mph sustained winds and the 
pressures of a Cat. 5 hurricane. Our homes might all be dry behind the 
flood protection, BUT IT WILL BLOW APART. Evacuation should always be 
recommended if you are in the eye wall  of a Cat 3, 4, or 5. Keep in 
mind that by the time you know the landfall path and the probable wind 
speed and pressures, it will be too late to leave.  And I say this as a 
person who never had evacuated for a hurricane in nearly 50 years, 
until Katrina.

Deborah Keller