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Storm Proofing Facilities- Katrina 4
- Thread starter Duckeng
- Start date
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- #5
How engineering (and building codes) work:
You're building a footbridge.
Next door to the footbridge lives a man that weighs 500 lbs.
You figure there is a 50% chance that he will try to cross your bridge.
Therefore, you can design the bridge for 250# load.
Seriously, one lesson learned is a lesson we knew already. Most things are designed for a certain reliability. You design a building for a windspeed such that there is a 90% chance that it won't be exceeded in 10 years. That means there IS a 10% chance that it will be exceeded. It all sounds good when you design it, but then when that 10% case comes along and destroys what you built, you don't feel so good about it.
When Rita was headed for the Texas coast, it had 175 mph winds. Stuff on the Texas coast is not designed for that speed. As it turns out, rightly so, in this case, because the wind speeds did drop as it came in. But if you tried to design everything for the worst case, no one could afford to build anything.
People criticize the gov't because New Orleans' levee system was only adequate for Category 3 hurricane, not Category 5. What would it take for a Category 5? You'd turn those levees into the Great Wall of China. They already had concrete walls sticking 12' high. Just add another 25' or so, then make them 4 times as thick for wind loading, etc.
You're building a footbridge.
Next door to the footbridge lives a man that weighs 500 lbs.
You figure there is a 50% chance that he will try to cross your bridge.
Therefore, you can design the bridge for 250# load.
Seriously, one lesson learned is a lesson we knew already. Most things are designed for a certain reliability. You design a building for a windspeed such that there is a 90% chance that it won't be exceeded in 10 years. That means there IS a 10% chance that it will be exceeded. It all sounds good when you design it, but then when that 10% case comes along and destroys what you built, you don't feel so good about it.
When Rita was headed for the Texas coast, it had 175 mph winds. Stuff on the Texas coast is not designed for that speed. As it turns out, rightly so, in this case, because the wind speeds did drop as it came in. But if you tried to design everything for the worst case, no one could afford to build anything.
People criticize the gov't because New Orleans' levee system was only adequate for Category 3 hurricane, not Category 5. What would it take for a Category 5? You'd turn those levees into the Great Wall of China. They already had concrete walls sticking 12' high. Just add another 25' or so, then make them 4 times as thick for wind loading, etc.
- Thread starter
- #6
I visited New Orleans this past weekend Oct 8-9, 2005. It is a city in the depths of a death blow grasping for breath to survive. Historically there has not been a catastrophe of this magnitude that I know of, since the Mississippi River basin flooding which occurred in the 1920s. In nature all things fail at some point in time. The challenge is good engineering is to design for some reasonable service life without failure without "over building". Some of the Roman highways and the pyamids still stand after centuries. The bridges across lake Ponchartain were not designed for upheaval. 60-80 slabs of concrete, three lanes wide were thrown off their piers. The lesson learned from the levy system failure is what is the cost of not building them strong enough verses what is the cost of having to rebuild the city. They should not have collapsed if designed properly even though the storm surge topped them. Let the water top them and then pump it out. You don't have to go up 30 feet above sea level. New Orleans has the largest pumping system in the world second to none. We need a failure analysis done right!
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There's a lesson that sometimes social issues and timing and just plain bad luck combine in a way that engineering can't fix or, if it can, it costs too much.
N'Awlins was founded as a trading post on a high spot of land, which is why the French Quarter didn't get much flooding. Everything else grew up around that. If a couple of direct-hit hurricanes had happened earlier in the history, then folks would have probably wised up and said "the heck with this place, let's move up river to Red Stick".
When I was at LSU my roommate took to me to visit his home in N.O. I got out of his car and looked up 30+ feet to the top of the levee of the Miss. River located not 50 feet from his house. That means the surface of the river was above the roofline of his house. Rising another 50 feet above the top of the levee was an ocean-going cargo ship tied up at a wharf. Even as a young invincible hotshot college kid I thought living in this place was insane.
But the opposite almost happened in the floods of 1973. Follow on a map the course of the Miss. River. Upstream from Baton Rouge the river takes a sharp turn to the Southeast. At that point the river wants to follow a course due south through the Atchafalaya Basin to the Gulf. The US Army Corps of Engineers put the Old River Control Structure to keep this from happening and keep the Miss River flowing through B.R. & N.O. This structure almost, but not quite, got washed away in '73. The crawfishing was great because of the flooding ($0.12 a pound, yum!), but NOT having water flow through B.R. & N.O. would have had crippling effects on the US economy at the time.
TygerDawg
N'Awlins was founded as a trading post on a high spot of land, which is why the French Quarter didn't get much flooding. Everything else grew up around that. If a couple of direct-hit hurricanes had happened earlier in the history, then folks would have probably wised up and said "the heck with this place, let's move up river to Red Stick".
When I was at LSU my roommate took to me to visit his home in N.O. I got out of his car and looked up 30+ feet to the top of the levee of the Miss. River located not 50 feet from his house. That means the surface of the river was above the roofline of his house. Rising another 50 feet above the top of the levee was an ocean-going cargo ship tied up at a wharf. Even as a young invincible hotshot college kid I thought living in this place was insane.
But the opposite almost happened in the floods of 1973. Follow on a map the course of the Miss. River. Upstream from Baton Rouge the river takes a sharp turn to the Southeast. At that point the river wants to follow a course due south through the Atchafalaya Basin to the Gulf. The US Army Corps of Engineers put the Old River Control Structure to keep this from happening and keep the Miss River flowing through B.R. & N.O. This structure almost, but not quite, got washed away in '73. The crawfishing was great because of the flooding ($0.12 a pound, yum!), but NOT having water flow through B.R. & N.O. would have had crippling effects on the US economy at the time.
TygerDawg
- Thread starter
- #8
I was aware of the situation with the Old River Control Structure that you wrote up and agree with your conclusions. The Mississippi in geological time has shifed it's course many times at about 10,000 year intervals to redeposit its silt where ever needed. This process is now interrupted by engineering's attempt to channel the river's silt out over the continental shelf! I've also stood in Jackson Square, the site near the orginal indian village, upon which the city was founded in the 1700's on the highest land as one sails up the Mississippi, and watched large ocean going vessels pass bye while looking up at their hulls above my head. The land is highest here because of the large bend in the river from which the city takes one of its nicknames, "the cresent city". The canals and pumping system developed to drain this subsea level area are engineering marvels used to by europeans to drain the Zider Zee (sp?) in Holland, also a subsea level city. Napoles in Italy is an ancient city that has learned to coexist below sea level although recently they are lossing it to subsidence.
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We were discussing Katrina at a Labor Day party with a neighbor originally from Holland. According to him, the same problems existed in Holland as in New Orleans, resulting in a terrible catastrophe in the 1950's. The country then embarked on a major effort to upgrade the dyke/pumping systems.
Jim Treglio
Molecular Metallurgy, Inc.
Jim Treglio
Molecular Metallurgy, Inc.
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