2005 was a record year for tropical storms in the Atlantic basin. There were so many that the list of 21 names ran out and 6 had to be named as letters of the Greek alphabet: Alpha, Beta, Gamma, Delta, Epsilon, & Zeta. Twelve were major hurricanes. more
On the morning of August 29, 2005, Hurricane Katrina came ashore with her eye tracking just East of the City of New Orleans. She was the third most powerful storm in the recorded history of the Atlantic basin (based on a central pressure of c908 mB), and she was huge. A storm surge of about 30' devastated the coast of Mississippi, taking out many structures including highway and rail bridges. New Orleans mostly weathered the blow and its 16" of rain. But a number of levees failed, flooding about 80% of the city. 1300 lives were lost, and $100-$200(10)^9 property damage ensued.
On September 24, Hurricane Rita made landfall on the border between LA and TX. Rita wiped a number of communities off the map in Cameron parish, and in New Orleans reflooded some areas through unrepaired levee breaches with its storm surge. The combined event was called locally "Katrita."
The flooding by Katrina was by far the worst aspect of the storm. The New Orleans Times-Picayune map shows flood depths. Our house is on piers, and is located on a local high spot approximately at the base of the first "L" in "CARROLLTON." So we had no flood damage, but friends just a few blocks away lost cars and appliances.
So why is the flooding distributed as it is? The Mississippi River is built its delta by depositing soil. During spring high water, historically it would overbank and drop the heavy stuff nearest itself and the lighter, finer stuff further away. So a natural levee is formed making it the highest land around. The French Quarter, center of the original city, was conveniently high. Behind the natural levee was wetland ("swamp"). (There are also "ridges" scattered throughout the area which are the natural levees of abandoned, former channels of the river.)
Lake Ponchartrain is actually a tidal, brackish bay of the Gulf of Mexico. It is basically at sea level plus a bit because a number of rivers drain into it. To drain New Orleans, canals were dug to the lake, and drainwater is pumped into these canals. The drained land subsided sometimes to several feet below sea level. So the drainage canals had to be walled with levees. It is these levees that broke and allowed the lake to fill the city. So there were weeks that the floodwater stood at sea level. The levees failed well below their design loads by still uncertain mechanisms.
While we were evacuated we were able to get satellite photos of our house before and after Katrina. After showed diminished tree canopy and a carport in the yard gone. Also, a solid white fence fell over part of our garden. Our early returning friend Rene took this photo that shows brush on our front porch. The major part is an 8" diameter limb from the sweet gum in the foreground.
Why the levees failed is still under investigation. Many details are available from Tulane University Professor Steve Nelson's Katrina Page. Also available is the NSF sponsored Preliminary Report.
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