Accueil Events Actes Risque cyclonique Les conséquences du cyclone Katrina sur les côtes du golf du Mexique et les îles Barrières
Les conséquences du cyclone Katrina sur les côtes du golf du Mexique et les îles Barrières
Written by Hermann M. FRITZ   
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Figure 1a. Overland flow depth measurements: bark off tree on East Ship Island (MS, USA).             Figure 1b. Overland flow depth measurements:  wash-out damage on the steel frame structure of the lighthouse at Beau Rivage Casino in Biloxi (MS, USA).
Figure 1a.                                Figure 1b.

Figure 2. Storm tides of Hurricanes Camille (1969) and Katrina (2005) along the US Gulf of Mexico Coast.  Storm tracks and surge elevations from Camille (USACE, 1970) and Katrina are shown, along with areas inundated by Katrina (grey shaded areas).  Katrina maximum sustained wind speeds and direction indicators at the time of final landfall (AOML, 2005).  Although wind speeds were lower during Katrina, the storm’s massive size caused the higher storm tides. Satellite Imagery modified from Earth Satellite Corporation (EarthSat), ESRI.
Figure 2.

 

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Hurricane Katrina (23-30 August 2005) was the costliest and one of the five deadliest hurricanes to ever strike the United States. Katrina made landfall, at the upper end of Category 3 intensity (central pressure 920 mb, the lowest pressure measured in an Atlantic basin storm of equal intensity) with estimated maximum sustained winds of 200 km/h, near Buras, Louisiana at 11:10 UTC 29 August 2005 and its subsequent final landfall at the Louisiana/Mississippi border (Knabb et al., 2005).

Katrina struck low-lying coastlines particularly vulnerable to storm surge flooding. The widespread failure of tide gauges along the Mississippi and Louisiana shores required survey teams to collect high water marks to document the surge height. Reconnaissance teams led by the author surveyed the effects of the storm in Florida, Alabama, Mississippi and Louisiana during September and October, 2005. Maximum storm tides, overland flow depths, and inundation distances were measured. Sediment samples from storm deposits were collected and erosion documented. Perishable infrastructure damage was recorded at various scales. The elevations of water marks on buildings, scars on trees, and rafted debris were measured as indicators of the maximum storm tide (Figure 1).
Figure 2 shows the measured Katrina high water marks along with the high water profile from Hurricane Camille in 1969 (USACE, 1970). The storm surge was the primary cause of the high water marks. However wave action and wind also contributed and are not separated out. The storm tide peaked to the East of Katrina’s path and was consistently measured between 7 and 10 meters along a 60 km stretch of Mississippi coastline from Lakeview (20 km east of center) to Ocean Springs (80 km east of center). Even along the hardest hit coastline, buildings designed to resist peak hurricane winds were only marginally damaged on upper floors, while walls were blown out on the lower floors. The surge penetrated to at least 10 km inland near Waveland and up to 30 km inland at Pearl River, Mississippi, where the center made landfall.  Further to the east along the Alabama coast the high water marks dropped below 5 meters. Nevertheless, more than 2 meter high water marks were measured 240 km east of the Katrina’s track along Florida’s panhandle.  The barrier islands 10 to 20 km offshore the Mississippi and Alabama coast were completely over washed by storm tide elevations as measured directly onshore. The high water marks dropped more quickly to the west reaching 2 meters along Lake Maurepas (80 km west of center) resulting in a significant east-west storm tide gradient across Lake Pontchartrain. The high water marks along the intact levee system in New Orleans’s Lakeshore adjacent to the 17th Street Canal did not exceed 5 meters indicating that the 17th Street Canal levee failed prior to overtopping.
Hurricanes Katrina’s storm tide exceeded the corresponding envelope of Hurricane Camille at all locations (Figure 2). The massive storm tide produced by Katrina is primarily attributed to the huge size of the storm at landfall with a 50 km radius of maximum winds and hurricane force winds extending 140 km to the east from the center (Knabb et al., 2005). In addition Katrina had already generated large northward-propagating swells as a Category 5 storm in the hours before landfall. Hurricane Camille (1969) was more intense than Katrina at landfall in terms of peak wind velocities (ESSA, 1969). However Camille was far more compact with hurricane force winds extending only 100 km to the east of the center resulting in a narrower storm surge distribution (USACE, 1970). The 6.9 meter maximum high water mark recorded in the aftermath of Hurricane Camille was likely exceeded at Pass Christian (MS) according to eyewitness estimates of 8.5 meters (Hearn, 2004). Another Katrina storm tide enhancing factor is the significant reduction of buffering land area in the Gulf Islands caused by Camille. The combination of land loss due to storms that are increasing in magnitude (Emanuel, 2005) and increasing sea level will likely expose the Gulf of Mexico mainland to higher storm surges than have been recorded in the past.