12/21/2005

Downgraded

The National Hurricane Center has downgraded Hurricane Katrina from a Category 4 to a Category 3 as it made landfall. By their best estimate because so many gauges were broken or inoperable, they have to study other indirect instruments to give basically what amounts to a best guess. The winds of Katrina along the Mississippi Gulf Coast were not the major problem. Much damage was caused by the wind far inland in Mississippi, even as far as Laurel Mississippi where some houses were damaged or destroyed. Laurel is about 100 miles inland on the eastern side of the state.

The problem that towns such as Lakeshore, Waveland, Bay St Louis, Pass Christian, Long Beach, Gulfport, Biloxi, Gautier and Moss Point Mississippi faced was the brutal storm surge. In prior posts I had stated that the storm surge was anywhere from 40 ft to 20 ft along the Mississippi Gulf Coast. The storm surge even affected Mobile and Bayou La Batre Alabama, which is around 100 miles from Katrina's landfall.

The NHC is also having to guess at the storm surge.

A precise measurement of the storm surge produced by Katrina along the northern Gulf coast is complicated by many factors, including the widespread failures of tide gauges. Additionally, in many locations, most of the buildings along the coast were completely destroyed, leaving few structures within which to identify still-water marks. Survey crews are still collecting data and conducting analyses as of this writing, without yet an explicit attempt to separate out the surge component of the high water mark measurements that are also a result of waves and, to a lesser extent, tides. An unofficial storm tide (actual level of sea water) observation of 28 feet at the Hancock, Mississippi Emergency Operations Center suggests that the storm surge produced by Katrina was as high as about 27 feet at that location. This observation provides an indication of the magnitude of the event, and comparable surge heights could have occurred along other portions of the western and central Gulf coast of Mississippi. The surge appears to have penetrated at least six miles inland in many portions of coastal Mississippi and up to 12 miles inland along bays and rivers. The surge crossed Interstate 10 in many locations. Katrina produced a lesser but still very significant storm surge along the eastern Gulf coast of Mississippi and along the coast of Alabama. Observations suggest the storm surge was about 10 feet as far east as Mobile, Alabama where Katrina caused flooding several miles inland from the Gulf coast along Mobile Bay.

Although the storm surge was higher to the east of the path of the eye of Katrina, a very significant storm surge also occurred west of the path of the eye, but the height of the surge is uncertain, in part because tide gauge observations along the southeastern coast of Louisiana were very limited and incomplete. As the level of Lake Pontchartain rose, several feet of water were pushed into communities along its northeastern shore from Slidell to Mandeville, Louisiana. The 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, although the specific times and exact causes of failure remain uncertain as of this writing. The surge overtopped large sections of the levees during the morning of 29 August 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 both the Industrial Canal east of downtown New Orleans and the 17th Street Canal northwest of downtown appear to have occurred during the early morning on 29 August, possibly even before the eye made initial landfall in Louisiana. Breaches along the London Avenue Canal north of downtown appear to have occurred later that night. Overall, about 80% of the city of New Orleans flooded, to varying depths up to about 20 feet, within a day or so after landfall of the eye. Following the setbacks caused by additional flooding associated with the late September 2005 passage of Hurricane Rita to the south, the Army Corps of Engineers reported on 11 October 2005, 43 days after KatrinaÂ’s landfall, that all floodwaters had been removed from the city of New Orleans.

The massive storm surge produced by Katrina, even though it had weakened from Category 5 intensity the previous day to Category 3 at landfall in Louisiana, can be generally explained by the huge size of the storm. Katrina had on 29 August a large (about 25-30 n mi) radius of maximum winds and a very wide swath of hurricane force winds that extended at least 75 n mi to the east from the center. Even though Hurricane Camille (1969) was more intense than Katrina at landfall while following a similar track, Camille was far more compact and produced comparably high storm surge values along a much narrower swath. Also, Katrina had already generated large northward-propagating swells, leading to substantial wave setup along the northern Gulf coast, when it was at Category 4 and 5 strength during the 24 hours or so before landfall. In fact, buoy 42040, operated by the National Data Buoy Center (NDBC) and located about 64 n mi south of Dauphin Island, Alabama, reported a significant wave height (defined as the average of the one-third highest waves) of 30 feet as early as 0000 UTC 29 August. This buoy later measured a peak significant wave height of 55 feet at 1100 UTC that matches the largest significant wave height ever measured by a NDBC buoy. Overall, Katrina's very high water levels are attributable to a large Category 3 hurricane's storm surge being enhanced by waves generated not long before by a Category 5 strength storm.

Tide gauge failures, buildings destroyed and the best scientific instruments destroyed and the people at the NHC are left with best guesses. I am sure at some point the initial guess of a 27 foot storm surge will be revised higher. For the simple reason being, in the town of D'Iberville a house that had 3 inches of water in it during Camille had 7 feet of water in it during Katrina. Hurricane Camille's storm surge was 25 feet. D'Iberville is about 30 to 40 miles from where Katrina made landfall. The size of a hurricanes storm surge reduces the further away it is from the eyewall.

It is always the storm surge in hurricanes that causes the most damage and takes the most lives. Hurricane Katrina has be labeled as Category 3. The last Category 3 to hit the Mississippi Coast was Elena in 1985. There was widespread damage and some flooding. But nothing like we saw in that b@tch Katrina. Elena killed 4 in Florida and destroyed 250 homes before hitting Mississippi. Katrina killed 238 in Mississippi and destroyed 68,000 of the homes along it's coast.

4 Comments:

Blogger MissingLink said...

Incredible force of nature.
What a terrible year this 2005 has been.

2:42 PM  
Anonymous seawitch said...

felis,

A huge b@tch of a storm. Sorry if the posts seem redundent. It is about the only news here.

7:05 PM  
Blogger Esther said...

I heard about this and thought it sounded absolutely ridiculous. Reminds me of when they downgraded the Northridge earthquake of '94. Or at least, we felt like it was downgraded. Had it been any higher, Federal govt would have to kick in money. I think many of us always felt that was the reason it was given what it was. I wonder if Katrina could be a similar sitch?

9:58 PM  
Anonymous seawitch said...

esther,

In discussing with a few people down here, some are speculating it has more to do with the insurance companies. There's a battle royale going on between homeowners and the insurance companies. Homeowners are saying that there was a lot of wind damage before the storm surge.

I've been through two Category 3 hurricanes. The wind was much stronger in Katrina. You can tell that just from all the wind damage that was done far away from any storm surge.

By their own admission, the NHC is making best guess estimates.

10:08 PM  

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