Silted In

What do we know about Options 1 and 2?

- The Corps has been playing with information on the projects, redacting critical figures that show serious discrepancies in their cost estimates. Then, they've been attempting to scare the public into Option 1.

- The Corps claims they are only authorized to proceed with Option 1. However, we know it was the Corps that made a CHOICE to proceed with Option 1 over Option 2 just months after Katrina with no public consultation. They drafted legislation for Congress - eventually adopted unchanged from the original language - which locked Option 1 in. That "lock in" is according to the Corps' lawyers' interpretation of that Corps-drafted legislation. And the circle of legalism is complete.

- The Corps cost estimates for the Option 2 stations are likely wildly inflated. Between 2006 and 2009, the estimate ballooned over $1 billion. That is even taking into account a very generous 60% contingency on the 2006 estimate.

- The Corps' plans for Option 1, which they claim will allow adaptation to Option 2 later, are actually not as adaptable as all that. Their Option 1 stations leave out key features which will be necessary for Option 2, such as formed suction intakes (FSI's) on the pumps, as well as gearboxes, electric motors and support equipment which is sized for both Options 1 and 2.

- The Corps' plans for Option 1 also leave the outfall canal walls untouched, even though their own reports say those I-walls should be replaced with T-walls if the Corps proceeds with the plans they are currently proposing. The reports show that future pump station operating costs will eventually outstrip the cost to fix the walls now. Even so, the Corps is showing no inclination to do the right thing and fix the walls.

With all that in mind, it is valuable to examine what New Orleans and Jefferson Parish will be left with if Option 1 proceeds as the Corps wishes.

First up, the canals...

Put simply, the current canals are a mess. They appear to be filling with silt, impeding the drainage capacity of the city. The silt also raises the water level on those weak walls, making every major rainfall event riskier. As more and more water is pumped out of the city, more silt is deposited in the canals. Eventually, there will be so much silt, the canals will not be able to convey enough water without exceeding safe water elevation restrictions.

Silting as of 2006

The problem of silting is well known. It appears in diagrams in the 2009 B&V report. Those diagrams are based on other diagrams from another Black & Veatch report, this one from 2008. That 2008 report analyzed various flow scenarios, attempting to quantify how often the gates need to drop at the lakefront based on rainfall, lake level, and existing & future safe water levels.

The diagrams include hydrographic survey results from the post-K years (likely 2006), showing the "current" canal bottoms. Here they are for two of the canals (my annotations are in red):

17th Street:

London Avenue:
As you can see, in 2006 there were significant stretches of those canals with a lot of silt in them. There's no reason to think anything has changed (see "Where did the silt come from?" below).

I have a few caveats to these diagrams:

a) I'm having problems finding the bottom elevations of the Orleans Avenue canal from the early 90's. If I can get them, I'll make a diagram like the ones above and add it to this post later.

b) I pulled the old canal bottom and wall top elevations from the as-built drawings of the work done on both these canals, available at the IPET site (17th Street here, London Avenue here and here). The datum on those drawings was NGVD29. Now it is NAVD88. According to this document from the London Avenue load test, the conversion is NAVD88 = NGVD29 - 1.5 feet. I'm not sure if that is true or not, and I'm not particularly qualified to say so. However, for the point I'm trying to make - there's a LOT of silt in the canals - I don't believe it matters.

c) Notice the sections in the canals closer to the lake on the London Avenue canal that apparently have gotten much deeper than the -10 feet NGVD (or -11.5 NAVD) original depth. The section at the Robert E Lee Blvd bridge shows a great deal of scour. It is right in front of the "London North" breach. Since the London Avenue survey was done in March, 2006, it would make sense there would be a large scour hole right in front of the breach area at that point, before the area had been completely repaired. It's unclear if filling this hole was part of the Corps work done in that vicinity after Katrina - the hole may still be there.

One last thing on the data from 2006... The Corps had the London Avenue survey from March of 2006 turned into a video:



This video, which travels from south to north along the canal, backs up the graphic above. The areas in pink and orange are those that have lots of silt (water depths from -4 to -7 feet), and they are concentrated in the area to the south of Mirabeau Avenue. The large scour hole at Robert E Lee also shows up in deep purple (water depth of -20 feet). Most of the canal north of Mirabeau - shown in green - appears (as of March 2006) to be at nominal depth, which is in the range of -9 to -12 ft. It's somewhat interesting that there are some areas of scouring around the bridge foundations, shown in light blue (-13 to -14 feet).

Where did the silt come from?

There's two possibilities here:

1) There's always been a silting problem on the canals.
2) Katrina

While we know (1) to be true (it was why dredging was done on the 17th Street canal in the late 80's and early '90's leading to the breach), the possibility that much of this silt is from Katrina is quite strong.

Thousands of tons of silt from Lake Pontchartrain flowed into the city through the breaches in the outfall canals after Katrina. The city's Public Works department has spent years trying to get it out of the subsurface drainage system. Other canals all over the area were similarly clogged and had to be cleaned out.

What is interesting is that while the Federal government has picked up the tab to for all those cleanouts, there was no move to do the same for the three biggest areas of potetial cloggage: the outfall canals. Thus, it is possible the New Orleans area is now living with a huge legacy of Katrina - stopped up drains. The right thing to do would have been to do some dredging in combination with replacement of the weak sections of the walls soon after the storm, but the Corps foreclosed that possibility by eliminating Option 2 (or any remedial work on the walls) from consideration in late 2005 and early 2006 - with no public consultation.

Now the area is stuck with a problem that is very difficult to solve. Nonetheless, dozens of other culverts and canals have been cleaned out of Katrina debris at Federal expense since the storm; the right thing to do would be to address this problem - safely if possible.

Is it getting worse?

So is the problem getting worse? According to data collected earlier this year by the Corps, it appears at least one canal may be silting up somewhat quickly.

In May, 2007, the Corps issued a study to determine the safe water level for the 17th Street canal. The complete version (just released this year as part of the IER #5 comment process and which includes graphics in the back) is here. It concluded that 6 feet was okay, if some work was done in certain spots near the I-10 and Veterans Blvd. bridges. That work was completed shortly afterward, and the safe water elevation has been 6 feet ever since.

Of course, that study was completed before new hurricane protection design guidelines were promulgated in October, 2007. Two years later, the Corps is still studying safe water elevations on the canals, using those new guidelines. Results are not expected until the beginning of the 2010 hurricane season, or almost five years after Katrina and two years after permanent protection was originally promised to be complete.

Part of the 2007 17th Street canal study was a section on silt build up in the canal:

"(3) Sedimentation. The post Katrina surveys show that the accretion has occurred in the canal. Between I-10 and the railroad bridge the lowest canal bottom elevation is El. -10 NAVD or over 7 ft. of sedimentation according to post Katrina surveys. Between Veterans Blvd and I-10 the canal bottom elevation is El. -14 to -18 NAVD or between .5 to 3 feet of sedimentation according to post Katrina surveys. Between the B/L Station 583+00 and Veterans Blvd the canal bottom elevation is El. -15 NAVD or higher about 4 feet of sedimentation according to post Katrina surveys. Between Hammond Highway Bridge and Station 583+00 the canal bottom is between El. -17.5 NAVD and El. -19.5 NAVD (at one location 120 ft south of Hammond the bottom elevation is -20.0 NAVD)about .5 ft to 2.5 ft of sedimentation. Since the surveys were taken 18-inches of riprap has been placed on the canal bottom from Hammond Highway to the south end of the breach."

As part of the ongoing, neverending process of evaluating the safe water levels that I mentioned above, the Corps had a new survey done of the 17th Street canal this spring. The report for that survey is here, and the raw data is here.

After reading through that data, the above paragraph could be rewritten to read:

"(3) Sedimentation. The post Katrina surveys show that the accretion has occurred in the canal. Between I-10 and the railroad bridge the lowest canal bottom elevation is El. -10 NAVD or over 7 ft. El. -7 NAVD or over 10 ft. of sedimentation according to post Katrina surveys. Between Veterans Blvd and I-10 the canal bottom elevation is El. -14 to -18 NAVD or between .5 to 3 feet El. -10 to -15 NAVD or between 2 to 7 feet of sedimentation according to post Katrina surveys. Between the B/L Station 583+00 and Veterans Blvd the canal bottom elevation is El. -15 NAVD or higher about 4 feet El. -13 NAVD or higher about 6 feet of sedimentation according to post Katrina surveys. Between Hammond Highway Bridge and Station 583+00 the canal bottom is between El. -17.5 NAVD and El. -19.5 NAVD (at one location 120 ft south of Hammond the bottom elevation is -20.0 NAVD)about .5 ft to 2.5 ft of sedimentation between El. -14 NAVD and El. -18 NAVD about .5 ft to 4.5 ft. Since the surveys were taken 18-inches of riprap has been placed on the canal bottom from Hammond Highway to the south end of the breach."

So that is saying that in the space of about three years, some sections of the 17th Street canal have had 3 to 4 feet of silt build up in the canal. Considering that those particular sections have a design depth of approxmately -18.5 feet (see notes below about the canal bottom elevations), and have gone from 7 feet to 10 feet of sediment, the canal is clearly getting clogged up to the point where drainage capacity will soon be impeded.

What is the upshot of all this?

It means that the problem which led in part to the failure of the 17th Street walls during Katrina - the need to dredge the canals due to the buildup of silt - will not go away with Option 1, the Corps' preferred alternative. It also means that at the rate the silt is apparently building up in the canals, the need to dredge may come sooner rather than later. That should be worrying, considering what happened the last time the 17th Street canal was dredged.

In addition, the Corps might put forward a rather bizarre argument - silting of the canals is good. In Appendix B to the London Avenue Load Test report, the Corps paid consultants studied how the permeability of silt on the canal bottoms can prevent the walls from getting undermined like in Katrina. They concluded,

"Analyses have shown that when there is direct hydraulic communication between the canal water and the sand layer, high toe exit gradients and large uplift pressures can exist, even for modest canal water levels. If the canal was 'silted in,' then these exit gradients and uplift pressures are substantially reduced."

In non-engineerese, that means the silt is blocking water from getting under the walls.

These are the kinds of crazy situations and arguments that these faulty walls create - is it more valuable to silt in the canal, reducing drainage capacity, just so you can save some crummy walls? Or should the canals be dredged, possibly opening up seepage paths under the walls, and maybe even damaging them in the process? It's ridonkulous.

What would solve this? In the short term, some federally funded cleanups would seem to be in order. However, in the long term, concrete lined canals with a nice gentle slope that would keep the silt moving during drainage operations are the best choice. Such canals already exist all over the metro area. The Palmetto Canal - which leads to the 17th Street canal - as been concrete lined for a very long time and does not appear to have a silting problem along its length. It also doesn't have any walls, since it is subsurface. Here's a picture:


(image courtesy user "Infrogmation" at wikimedia.org)

Now, even with a concrete channel, the silt doesn't magically disappear. It builds up wherever there is an obstruction in the canal. This would include bridge foundations and the inlets of the lakefront pump stations. However, installing lowered concrete canals would significantly reduce the current problem to just those spots, and would also keep future dredging operations from the weakened walls and levees - since those walls and levees would not exist.

This is just one of the unannounced problems that will remain for the New Orleans area if Option 1 is allowed to proceed. There are others...