17th St and Orleans Ave SWE = 0-1 feet
[Despite repeated attempts to obtain comment, the Corps has refused to offer any information about this article.]
SWE = Safe Water Elevation, the term of art the Corps of Engineers has applied indicating the maximum amount of water canal walls and levees can see before failure.
I have obtained the October, 2010 iterations of the 17th Street canal and Orleans Avenue canal Safe Water Elevation reports, including all their appendices:
17th Street: just the report and everything including appendices
Orleans Avenue: just the report and everything including appendices
And while I was stunned to find out that at least 7 sections of the London Avenue canal actually have Safe Water Elevations below the publicly disseminated 5 foot level, the findings for the 17th Street and Orleans Avenue canals are even more startling.
Since 2007, the Corps has claimed the SWE for 17th Street was 6 feet; they said it was 8 feet for Orleans Avenue. They have made operational decisions to close or not to close the lakefront gates during high water events based on these numbers, specifically during Gustav and Ike in 2008. The 17th Street gates were closed during both those events, and the Orleans Avenue gates were not. In fact, the Orleans Avenue gates have never been closed during a storm event.
As it turns out, the city may have just gotten lucky since 2005. Because these reports reveal there are sections along both canals which are so weak, they cannot even stand normal tidal motions throughout the year, never mind storm surge of 6 or 8 feet.
The reports provide handy charts indicating the overall SWE for each bank of each canal. First, the east bank of the Orleans Avenue canal:
And the west bank of Orleans:
Yes, there are sections along both banks with Safe Water Elevations of 1 foot (Actually, their SWE's are so low, they've just been assigned "1 foot" to keep things simple). Overall, there are 10 sections, or "reaches" with SWE's below the "official" 8 foot level.
Over at 17th Street, the picture's just as bad. First the east (Orleans Parish) bank:
And the west (Jefferson Parish) bank:
The reaches marked in red (12 of them) are those below the currently publicized SWE of 6 feet. As along the Orleans Avenue canal, there are also reaches with SWE's of 1 foot at 17th Street. The reaches in purple (7 of them) are the ones above 6 feet but below the target SWE of 8 feet, which is what the current remediation project is supposed to deliver. All 19 of these reaches should be included in the remediation project, theoretically (the Corps is running remediation projects along all three outfall canals currently, with the stated objective to raise the SWE to 8 feet along all stretches of all three canals).
So let's look at that. Here's the SWE's from the report overlaid on a map of the remediation project along the 17th Street canal (map taken from a March 31, 2011 Corps presentation included in the April monthly update from the Orleans Levee District, found at the website of the Southeast Louisiana Flood Protection Authority - East here):
The Corps is remediating nearly all the troubled stretches. I can't speak to the quality or propriety of their chosen method of remediation (mostly deep soil mixing, combined in some streches with increases in the height of the levee embankment), but I can speak to the location of that remediation.
18 of the 19 reaches of concern are included in the 17th Street remediation project. But one stretch on the east bank, Reach 30, is not included, despite its published safe water elevation of 7 feet, one foot below the target SWE of 8 feet. Reach 30 extends 736 feet south from the Vets bridge, not quite all the way to the I-10 bridges. The 7 foot SWE in this reach comes from inadequate "global stability," meaning Reach 30 is susceptable to sliding of the entire levee and floodwall horizontally at water levels greater than 7 feet. I've gone through the report and can't really find any caveats around the global stability analysis of Reach 30 to explain why it was left out of the remediation project. The Corps should tell the public what's going on.
Over at Orleans Avenue, reaches have also been left out of the remediation project for that canal:
You can see there are 8 (!) reaches with SWE's below 8 feet which are not part of the remediation efforts along the Orleans Avenue canal. This includes facing stretches of the canal just north of the Robert E Lee Blvd bridge which have SWE's so low, they cannot tolerate normal tides. These reaches - 10B and 20B - are bare levee and do not have floodwalls.
I'm no geotechnical engineer, but leaving stretches of canal bank unremediated when they have a Safe Water Elevation of 1 foot deserves an explanation. Never mind the other 6 stretches with sub-8 foot SWE's which are also left out of the project.
SWE = Safe Water Elevation, the term of art the Corps of Engineers has applied indicating the maximum amount of water canal walls and levees can see before failure.
I have obtained the October, 2010 iterations of the 17th Street canal and Orleans Avenue canal Safe Water Elevation reports, including all their appendices:
17th Street: just the report and everything including appendices
Orleans Avenue: just the report and everything including appendices
And while I was stunned to find out that at least 7 sections of the London Avenue canal actually have Safe Water Elevations below the publicly disseminated 5 foot level, the findings for the 17th Street and Orleans Avenue canals are even more startling.
Since 2007, the Corps has claimed the SWE for 17th Street was 6 feet; they said it was 8 feet for Orleans Avenue. They have made operational decisions to close or not to close the lakefront gates during high water events based on these numbers, specifically during Gustav and Ike in 2008. The 17th Street gates were closed during both those events, and the Orleans Avenue gates were not. In fact, the Orleans Avenue gates have never been closed during a storm event.
As it turns out, the city may have just gotten lucky since 2005. Because these reports reveal there are sections along both canals which are so weak, they cannot even stand normal tidal motions throughout the year, never mind storm surge of 6 or 8 feet.
The reports provide handy charts indicating the overall SWE for each bank of each canal. First, the east bank of the Orleans Avenue canal:
And the west bank of Orleans:
Yes, there are sections along both banks with Safe Water Elevations of 1 foot (Actually, their SWE's are so low, they've just been assigned "1 foot" to keep things simple). Overall, there are 10 sections, or "reaches" with SWE's below the "official" 8 foot level.
Over at 17th Street, the picture's just as bad. First the east (Orleans Parish) bank:
And the west (Jefferson Parish) bank:
The reaches marked in red (12 of them) are those below the currently publicized SWE of 6 feet. As along the Orleans Avenue canal, there are also reaches with SWE's of 1 foot at 17th Street. The reaches in purple (7 of them) are the ones above 6 feet but below the target SWE of 8 feet, which is what the current remediation project is supposed to deliver. All 19 of these reaches should be included in the remediation project, theoretically (the Corps is running remediation projects along all three outfall canals currently, with the stated objective to raise the SWE to 8 feet along all stretches of all three canals).
So let's look at that. Here's the SWE's from the report overlaid on a map of the remediation project along the 17th Street canal (map taken from a March 31, 2011 Corps presentation included in the April monthly update from the Orleans Levee District, found at the website of the Southeast Louisiana Flood Protection Authority - East here):
The Corps is remediating nearly all the troubled stretches. I can't speak to the quality or propriety of their chosen method of remediation (mostly deep soil mixing, combined in some streches with increases in the height of the levee embankment), but I can speak to the location of that remediation.
18 of the 19 reaches of concern are included in the 17th Street remediation project. But one stretch on the east bank, Reach 30, is not included, despite its published safe water elevation of 7 feet, one foot below the target SWE of 8 feet. Reach 30 extends 736 feet south from the Vets bridge, not quite all the way to the I-10 bridges. The 7 foot SWE in this reach comes from inadequate "global stability," meaning Reach 30 is susceptable to sliding of the entire levee and floodwall horizontally at water levels greater than 7 feet. I've gone through the report and can't really find any caveats around the global stability analysis of Reach 30 to explain why it was left out of the remediation project. The Corps should tell the public what's going on.
Over at Orleans Avenue, reaches have also been left out of the remediation project for that canal:
You can see there are 8 (!) reaches with SWE's below 8 feet which are not part of the remediation efforts along the Orleans Avenue canal. This includes facing stretches of the canal just north of the Robert E Lee Blvd bridge which have SWE's so low, they cannot tolerate normal tides. These reaches - 10B and 20B - are bare levee and do not have floodwalls.
I'm no geotechnical engineer, but leaving stretches of canal bank unremediated when they have a Safe Water Elevation of 1 foot deserves an explanation. Never mind the other 6 stretches with sub-8 foot SWE's which are also left out of the project.
posted by mcbrid35 at
2:30 PM
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