Background
Previously, the Corps has told the public the London Avenue canal Safe Water Elevation (SWE) is 5 feet. That was based on a load test they did in 2007. What that means is the water shouldn't get above 5 feet in any circumstance, or a failure could occur, due to any one of a number of mechanisms. Most of the failure mechanisms trace back to the totally permeable sand in and above which the walls sit and through which Katrina's surge swept in 2005, taking out two sections of the canal walls and nearly destroying a third. Those failures happened with water far below the tops of the walls.
So with a five foot limitation if water starts getting high from surge, stormwater pumping, or a combination, there are two options to keep the water below 5 feet:
a) the Corps has to close the gates at the lakefront (to shut off water from the lake) or
b) the Sewerage & Water Board pumps have to cut back on pumping.
Both have happened repeatedly in the past.
The new stuff
What the Corps hasn't told the public since they announced the 5 foot SWE in September 2007 is that they have continued studying the canal, paying for tons of soil sampling and geotechnical analyses of the canal. Report after report has piled up. I have what I believe to be a near-complete collection of these reports.
But this latest version of the SWE report - Version 5 - is the one that counts, and is the scariest yet.
It reveals that the Safe Water Elevation of the canal is actually 1.5 feet, not 5 feet.
In fact, there are 7 sections of walls along both sides of the canal with SWE's below 5 feet, ranging from a high of 4 feet all the way down to 1.5 feet. The reason given in the report is that the silty layer at the bottom of the canal is so thin - less than 2 feet - that it provides no defense from water travelling through it. That's a problem, because below that silt is the sandy soil that also underlies the levees and walls. So water just goes through the silt, into the sand, and spreads under the walls, eventually pushing up the soil on the "dry," protected side of the walls. This is known as "seepage."
The report also reveals that apart from a few select areas, the Corps did not conduct a comprehensive survey of the thickness of the canal bottom until February and March of 2010, despite evidence dating back years of water penetrating directly through silt to the sand below and seeping past the walls to the neighborhoods beyond. That's troubling.
The Corps has included a considerable "pay no attention to this report" section at the end of the report, written with a future press release in mind. This is weird beyond belief, because it basically says the report and all its work isn't really right, the walls won't fail because they haven't failed yet, and that the Corps will keep New Orleans safe, blah, blah. They even crazily suggest a breach wouldn't be so bad, because it could only happen if the lakefront gates were closed, and then only the water trapped in the canal would pour out. It's garbage. Just ignore it.
The numbers are there in black and white starting on PDF page 193. And to make it simpler, they included a picture, which I have annotated:
The 1.5' SWE section - aka "Reach 13"
Burrowing into the appendices to find the precise analysis that led to the 1.5 foot number (note that it is just one of seven sub-5-foot SWE's along the canal), I discovered that along this section of the canal, a depth of 1.5 feet doesn't even put water against the wall:
[Note: this graphic is taken from a seepage analysis, but it may not be the seepage analysis that generated the 1.5 foot number. Nevertheless, that doesn't change the height of the 3.4 foot embankment relative to the 1.5 foot SWE.]
So just normal operation of the canal would likely cause seepage.
This unsafe section of wall - termed "Reach 13" by the Corps - is on the west side of the canal in the 5500 block of Pratt Drive, just north of Filmore Ave. It is about 300 feet long:
The map above is from Appendix A to the report. It is taken from images shot in late 2005 or early 2006. Of the five houses behind Reach 13 shown in that photo, all have been torn down since the photo was taken. Two - 5543 and 5551 - have been replaced.
Here's the Reach 13 neighborhood in late 2009, courtesy of Google:
The large amount of sand on the lot at 5525 Pratt was placed there by a Louisiana Land Trust demolition contractor after the October 26, 2009 knockdown of a shed on the lot. The home at 5535 Pratt was demolished December 19, 2009, placing this photo sometime between those two dates.
What is notable about this 1.5' SWE finding is that in previous London Avenue canal analyses, including one published in June, 2009, the calculated Safe Water Elevation for this same stretch of wall and levee (then known as Reach 14) was 10 feet. Now, with the improved data showing the extreme permeability of the canal bottom (the Corps' contractors actually call it an "open canal"), the idea of protection has been turned on its head. Now, it's not the walls and levees that matter, it's the bottom of the canal.
Summing up
I'm kind of in disbelief at these developments. How many chances has the Corps had to tell the truth to the public since last October, or even since they got this result (likely months before that)? They announced a contract to remediate the canal in December, 2010, but they had nearly complete designs for that remediation way back in June, 2010. The remediation started in January, 2011 and piles are being driven today. Reach 13 is getting piles driven to -56 feet, well below the sand. But the reaches north and south aren't getting anything. Such is the patchwork pattern of the remediation, following the same pattern as the analysis effort over the last five years. We can only hope the data on the canal bottom outside those sections getting sheetpile cutoffs is correct, now and forever.
And what about those last five years, when they were doing nothing on the canal while people have rebuilt? They can try to spin this however they like, but it just plain looks like suppressing bad news to me. Do you know how often the water in the canal gets above 1.5 feet? All the damn time, including every time there's a rainstorm and the city pumps turn on. How much risk has the city unknowningly accepted since Katrina? How many times have the walls been in danger of failing? How can the Sewerage & Water Board now send water down the canal from their pump stations knowing that many sections of the walls (or just the embankments below the walls, like in Reach 13) cannot have water put against them? Whoever decided to withold this from the public needs to get up and publicly apologize.
Here's the report, including nearly all the appendices.
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