Advisory Number 23 is issued at 10:00 a.m.:
“... Potentially catastrophic Hurricane Katrina…Even stronger… Headed for the Northern Gulf Coast…Reports from an Air Force Hurricane Hunter Aircraft indicate that the maximum sustained winds have increased to near 175 miles per hour…with higher wind gusts…Hurricane force winds extend outward up to 105 miles from the center and tropical storm force winds extend outwards up to 205 miles…Coastal storm surge flooding 0f 18 to 22 feet above normal tide levels…Locally as high as 28 feet along with large and dangerous battering waves…can be expected near and east of where the center makes landfall. ”
At 4:00 p.m., Hurricane Advisory 24 is issued adding that:
“…Katrina is moving towards the Northwest near 13 miles per hour and a gradual turn to the north is expected over the next 24 hours… Katrina is expected to make landfall at Category 4 or 5 intensity. Winds affecting the upper floors of high rise buildings will be significantly stronger than those near the ground…Some levees in the Greater New Orleans area could be over-topped”.
At 10:00 p.m., Hurricane Katrina Advisory Number 25 is issued:
“A Hurricane warning is in effect for the North Central Gulf Coast from Morgan City Louisiana eastward to the Alabama/Florida border…Including the city of New Orleans and Lake Pontchartrain. Preparations to protect life and property should be rushed to completion.”
At 2:00 a.m., August 29th, Katrina is downgraded to a Category 4 Hurricane. At 6:10 a.m., Hurricane Katrina made her second landfall at the Category 3-4 threshold near Buras, Louisiana, with sustained winds of more in the 130 to 135 miles per hour range.
At 10:00 a.m., Hurricane Katrina makes final landfall at the Louisiana/Mississippi border classified as a Category 3 Hurricane. (Note that her category 3 status is a controversial subject, as many folks claimed that there was no viable method for actually measuring the accurate wind speed. Some interviewed have claimed that the lower wind velocity claim favored the insurance companies with respect to their pay-out requirements.) As she moved inland, her energy was quickly dissipated, no longer fed by the uncharacteristically warm Gulf waters.
Different modes of destruction occurred following Katrina’s passage as she made her journey. From her second landfall in Southeastern Louisiana, she moved northeastwardly along the eastern side of Louisiana and then easterly along the Mississippi Coastal regions.
In the areas where landfall occurred, the destruction was dominated by severe wind and storm surge damage. In areas further inland, the relative affects of wind damage became secondary to the surge-induced flooding as the levee and canal systems protecting the city of New Orleans became overwhelmed.
Katrina demonstrated higher than usual surges than would have been expected from her size, likely due to the huge area her mass encompassed as well as her forward movement being slow for a hurricane at about 11 to 13 miles per hour. Typically, hurricanes exhibit forward speeds of 15 to 20 miles per hour. Katrina’s slower forward speed enhanced her affect on the magnitude of the storm surges she generated.
A Tropical Cyclone Report regarding Katrina issued by the National Hurricane Center in December of 2005 indicated that while Katrina was a Category 3 as the center made its closest approach to New Orleans, the sustained wind speeds over New Orleans were likely weaker than the Category 3 level. The actual sustained wind speeds in New Orleans are subject to speculation since observations were sparse, due in part to the power failures that disabled the Automatic Surface Observing System (ASOS) stations in the area before peak wind conditions occurred. Measurements that were taken indicate wind strengths in Category 1 to Category 2 levels. It should be noted that wind velocities in hurricane winds increase dramatically with increasing elevation; thus, those speeds experienced on the ground are significantly lower than those experienced by high-rise buildings.
Precise measurements of the storm surge produced by Katrina along her path of destruction were complicated by many factors, including the widespread failures of tide gauges. Additionally, most of the buildings along the coast were completely destroyed, leaving relatively few structures within which to identify still-water marks.
Where reliable data was collected, it was determined that the storm surge was 24 to 28 feet along the Mississippi coast across a swath about 20 miles wide, centered roughly on the Bay of St. Louis, Mississippi. The maximum high water mark observation of the storm surge was 27.8 feet at Pass Christian, Mississippi. 17 to 22 feet surges were indicated along the eastern half of the Mississippi coast, roughly from Gulfport to Pascagoula.
It is likely that the maximum high water mark is not indicative of the true height of the surge as a water mark forms when the water quits moving. There is some evidence that suggests the surge in some places could have exceeded 50 feet.
The surge appeared to have penetrated at least 6 miles inland in many portions of coastal Mississippi and up to 12 miles inland along bays and rivers, in some cases crossing Interstate 10. Surges from 10 to 15 feet extended along the coastal waters of Alabama.
Although the storm surge was highest to the east of the path of the eye of Katrina, a significant storm surge occurred west of the path of the eye towards New Orleans and Lake Pontchartrain. High water mark data indicate the storm surge was 12 to 16 feet in St. Tammany Parish from Slidell to Mandeville, Louisiana. The data also indicate a storm surge of 15 to 19 feet occurred in eastern New Orleans, St. Bernard Parish, and Plaquemines Parish, while the surge was 10 to 14 feet in western New Orleans along the southern shores of Lake Pontchartrain.
For New Orleans wind damage, though severe, was less than the wind damage observed in those areas of the eye and her landfall points.
The National Hurricane Center reported that, “The storm surge severely strained the levee system in the New Orleans area.” Several of the levees and floodwalls were overtopped and or breached at different times on the day of landfall. Most of the floodwall and levee breaches were due to erosion on the back side caused by overtopping, but a few breaches occurred before the waters reached the tops of the floodwalls.
The surge overtopped large sections of the levees east of New Orleans in Orleans Parish and St. Bernard Parish, and it also pushed water up the Intracoastal Waterway and into the Industrial Canal. The water rise in Lake Pontchartrain strained the floodwalls along the canals adjacent to its southern shore, including the 17th Street Canal and the London Avenue Canal. Breaches along the Industrial Canal east of downtown New Orleans, the London Avenue Canal north of downtown and the 17th Street Canal northwest of downtown occurred early Monday morning on August 29th.
80% of the city of New Orleans and outlying areas were under up to 25 feet of sitting flood waters. A big difference between the storm surge in the New Orleans and those of the Gulf Coast was that the Gulf Coast surges, though far more unabated and energetic flowed inland… then back out to sea. The floodwaters in the areas around New Orleans had nowhere to go, thus remaining for up to 3 weeks.
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