After examining the trenches
, the needles, and the guide columns
at 17th Street, London Avenue, and Orleans Avenue, it's time to move on to the spaces between all those elements and what should be filling those spaces - seals.
Without sealing, the gate structures will leak storm surge into the canals behind them. What I've found is that at two of the sites - London and Orleans Avenues - that is likely exactly what happens. But we'll begin with the site that actually has seals, 17th Street...
Seals and gaps at 17th Street
[This section of the post will include excerpts from the following as-built drawings for the 17th Street structure:
"Typical Gate Plan, Elevation, Section and Details, drawing 21 of 34"
"Typical Gate Elevation and Details, drawing 22 of 34"
"As built" refers to the final versions of the drawings - they are what is in the field.]
Looking closer at how the needles travel along the guide columns, here's the detail of one of the 17th Street needles between two guides, looking from above:
There are a lot of design features in these gates that are not found at the Orleans Avenue or London Avenue sites. Let's zoom into the interface between the needles and the guide columns to take a closer look:
First, there's a pair of welded angles on either side of the front (lake) side that prevents movement of the gates from side to side as they are raised and lowered. Such motion, if it was too large, could lead to the needles getting stuck. The gap between the angle and the guide columns is only a half inch. Here's a photo of the angles:
Then there's the seals. On the 17th Street needles the seals - known as "j-seals" from their shape (see two photos up) - are installed on the back (city) side of each needle, along the sides and the bottom of each segment. Here they are on the drawings:
Here's a picture of the seals from the 2007 Corps Infrastructure Conference presentation
that I excerpted from earlier
It's important for those seals to get compressed between the surfaces of the needles and the guide columns. Otherwise, they're not really sealing. Storm surge alone does not do it. One needs a constant static pressure along the entire length of the needles, which cannot be provided by wind-driven, wave-topped water during a storm. Hurricane storm surge is not the same as a large snow bank.
The 17th Street design accounts for this with mating sets of wedges installed on each needle and guide beam. On the needles, the wedges - which are steel - are installed at the top corners:
And the mating neoprene wedges on the guide columns are installed at the elevation where the tops of the needles come to rest (photo
from SCPR's Molly Peterson as part of her Flickr stream attached to her "Pumps Under Pressure"
series from last August):
When the gates are dropped into place, these wedges force the needles back against the guide beams. Here's details of the wedges from the drawings (I've adapted the drawings to show the wedges "in action"):
In concert with the mating sloped surfaces between the bottom of the needles and the trench...
...the needles are pushed back toward the city ("protected") side of the guide columns, scrunching the seals. This assumes the trenches are cleaned out and are in good condition, which is a questionable assumption
[Note that the design around the seals even includes a small stop block to ensure the seals are not completely crushed when compressed. That block serves a dual purpose of not allowing the gates to swing front-to-back so far toward the city (protected) side that the welded angles on the front cannot do their job preventing side-to-side motion.]
Not everything is great with this design though. There is still some room for the needles to swing front-to-back when being lowered or raised, which can lead to damage to the needles or the guides:
With W21x101 guide columns, a 15.375" thick needle, a 1.75" thick wedge and a 1.5" stop block, there is 1.13" front to back for the needle to swing. While the needle is swinging in that gap, it could bind in the guide columns, which can cause damage to needles or guides or both.
I realize that without rollers along the guides, some clearance is required. The problem is the lack of rollers, and the resultant swinging of the needles. That movement is most prominent during storm events, when storm surge will be raising the lake level and rainwater could be flowing out to the lake. In that case, the needles will see a "differential head" when they enter the water. That means that there will be water pressure on the city side of the gates which is different than on the lake side, due to the flow of the outgoing rainwater. Having the gates simply swinging in the water, banging against the front and back surfaces of the guide beams (side-to-side movement is limited - but not eliminated - by the angles on the fronts of the needles), and being lowered only by gravity, is really against almost all the design tenets laid out for vertical gates in the Corps' own standards
In addition, those standards also call for seals on the front, sides and back of each needle. The 17th Street ones only have them on one side. Such a sealing scheme would go hand-in-hand with reinforced guide columns, a topic I discussed in my previous post
The Corps consultant on the outfall canals, Black and Veatch, wrote about these very problems at the time the structures were under construction in their 2006 Alternative Considerations Report
, which looked at ways to reduce the costs of the permanent pumping stations. One of the cost containment strategies was cannibalizing the Interim Closure Structures, so B and V evaluated the ICS's. They had this to say about the gates and their supports:
"The gate side of the end supports consists of a metal-to-metal bearing and sliding surface with a bearing bar. This type of bearing surface is generally used for only emergency gates and does not operate well against a differential head. While all components are relatively new, the gate lifting/lowering system may function adequately against a differential head, but as sliding components wear and corrode, operability under differential head may become an issue. Permanent gates if equipped with wheels or rollers, although also requiring maintenance, will last longer and be a more reliable installation."
B and V brought this up again in the 2009 Black and Veatch evaluation of the gates
"most emergency scenarios for closing the gates will require that the gates move through differential head. Gates with rollers are required to move through differential head."
The 2009 report (which was mostly written in 2007) goes on to recommend replacement of the guides and needles with a roller gate system. Rollers would keep the needles perfectly aligned rather than banging around against the guidebeams.
The Corps even recognizes the problem of gates getting "hung up" due to inadequate guiding as part of the instructions for lowering the gates. In the Corps' Operation & Maintenance Manual for the gates
, it says,
"The process of putting the gate in the closed position should be closely observed. If during this process it is observed that as the gate is lowered it does not continue at a constant rate, the top elevation of the gate where the interruption occurred shall be noted and the gate shall go through the operation procedure a second time. If an interruption occurs again at the noted location then an inspection of the guide columns along their full height, as well as the portion of the gate within the guide columns shall be performed to determine what is causing the gate to hang up. Any defects found shall be repaired, and the procedure shall be repeated."
Overall - despite the lack of reinforced guide columns, rollers along those guide columns, and more sealing surfaces - for what it is, the design of the needles and guides at 17th Street is pretty good.
The story is much different at the other two sites.
Seals and gaps at London and Orleans Avenues
I won't be relying as much on drawings in this section, because unlike at 17th Street there are so few design features to point out on the Orleans and London gates. However, the two as-built drawings for the needles are here:
London Ave. Canal Interim Closure Structure: Needle Gate - Sections and Details, drawing 16 of 34
Orleans Ave. Canal Interim Closure Structure: Needle Gate - Sections and Details, drawing 16 of 26
Just to bottom-line it before getting into the details:
1) The seals are outside of the structure, performing no sealing function at all. There are no seals between the needles and the guides, as there are at 17th Street, likely allowing water through.
2) There are no wedges to force the needles against the guide columns and keep the needles fixed in place when lowered, as there are at 17th Street.
3) There are no angles on the fronts of the needles to prevent side-to-side movement, as there are at 17th Street.
4) The gaps around the needles within the guide columns are huge, meaning the gates are constantly banging around and likely allowing water through.
As the list above demonstrates, the change from 17th Street to the London/Orleans sites is stunning. Beginning with the seals...
The essence of a seal is that it is a compressible material that gets smushed to fill a gap between two adjacent hard surfaces. Think of the seal on a car door, or weatherstripping on a house's front door, or the lid and rubber ring on a mason jar. At London and Orleans Avenues, the seals fail to meet this basic definition, because they are not placed between the two adjacent surfaces of concern, the needles and the guide columns. Instead, they are decoratively attached to the lake side of the guides and are pushed by a single surface of the needles into the lake.
Starting with this Corps photo
of the London gates on September 14, 2009:
The Corps also took a detail shot
the same day that really clarifies the location of the seals:
Yes, they are located outside the needles, bolted to the lakeside outside surface of the guide columns. They've always been there, as this May 27, 2006 shot (via SCPR Flickr
) during the construction of the Orleans Avenue gates shows:
A detail from the applicable London Avenue as-built drawing
shows the location when looking from above:
Let's put this in real world terms. How well do you think the seals on your car doors would do if they were attached to the outside panels of the car? What if you tried to weatherstrip your house's front door by putting flexible edging on the siding? What if you put the lid upside down on your mason jar, with the rubber ring facing outward?
All of these are the equivalent of the "sealing" arrangement at London and Orleans Avenues; that is, there are no seals at London and Orleans
. How did this happen?
Going back to the bid drawings for the London Avenue gates
, from January, 2006, they show how the interface between the needles and the guide columns was originally designed:
The top diagram shows the original issued-for-bid design. Unlike at 17th Street, there are no wedges or other mechanisms to secure the needles horizontally, only the sloped surfaces at the bottom of the needle and in the trench (on a remarkable side note, the bid drawings do not show a detail of the needle in the trench).
So the needles, even if properly seated in the trench, would tilt back and forth in the guide columns like a tree getting buffetted by wind. As the bottom diagram shows, when waves come in from the lake and push the needles toward the city, the needles would move away from the "seals," opening a gap, which would close but reappear on the back side of the needle as it saw pressure from outflowing rainwater and turbulence from the Corps' pumps. And since there's no seals between the needles and the guides, and there's nothing like wedges keeping the gates from continually rattling in the guide beams, water would flow around the needles quite easily.
After award of the gates contracts in January, 2006, a modification was made to the London and Orleans designs, ostensibly to remedy this problem. The location of the seals was not changed, which would have been best. Instead, two MC6x12 channels (depth 2.5") and two 1/2" plates would be welded to the lake side of each needle, to provide a kinda-sorta sealing surface against the j-seals. Of course, this would never work:
One cannot seal against water when a seal is pressed between a single surface and water
. As I wrote above, water flow, waves, and storm surge are not like snow - they're not going to provide totally uniform pressure along the entire submerged length of the j-seals, pressing them against the new channels. This is especially true, not only because water doesn't behave that way, but also because there is still nothing in the design (like 17th Street's wedges) to keep the needles from rattling in the guide columns. Their only fixed point of contact is in the trench.
Additionally, at London Avenue there was another change made to the design of the needles and guides that negated the addition of the MC6 channels and the 1/2" plates. Perhaps to accomodate the new channels and plates on the needles, or perhaps for some other unrelated reason, the guides were increased in size. They went from W18x119's (18.97 inches deep) to W21x132's (21.83 inches deep). The needles did not change in size; they were probably already being fabricated at the time.
So, despite an extra 3 inches of thickness at the "sealing surfaces" from the new channels and plates, the even bigger guide columns resulted in the storm-surge-applied gap between the needles and the j-seals staying about the same as before: 0.8" rather than 0.9". Even more troubling, the larger guides at London Avenue leave a giant 4.5" gap between needle and guide flange, up from just 1.6" in the bid drawings. And worst of all, there is still no seal on the city side of the needles (or any side for that matter), nor is there a mechanism for ensuring the gates do not rattle, opening up these gaps. So this problem was never addressed.
[Note: the drawings for Orleans Avenue followed a slightly different path. In the Orleans bid drawings
, there were no MC6 channels or plates to provide a "sealing surface," but the guides were already W21x132's. In the as-built Orleans drawings, the MC6's and plates have been added, making the final Orleans versions exactly the same as the London versions.]
The 4.5" gap between the needles and the front flange of the guides can be seen in this Corps photo taken during the May 27, 2009 hurricane exercise:
The permanent damage to the "seals" from pushing them outward continuously is also clear in this photo from the same event:
If one can see gaps between the sealing surfaces and the seals on a sunny day, how many and how big are the gaps along the seals during a design-or-bigger storm event, with the needles rattling around front to back and side to side (they move side to side because there's no angles to stop them, like at 17th Street)? The water would be at least ten feet higher than what is shown in the photo above, with at least two to three foot swells on top of that. Water would be lapping at the top of the gates, if not washing over them. The gaps between the needles and the "seals" would probably be at least an inch, possibly more depending on how much those seals have gotten permanently flexed outward over the past four years.
And it's not like there isn't any guidance out there on how to do this. The Corps' Vertical Lift Gate Engineering Manual
says the following in its section on seals:
"Seals can be mounted on the skin plate side or the flange plate side, oriented so that the water pressure is acting on the stem of the J seal increasing the contact pressure of the seal."
That is exactly how the 17th Street seals are oriented and is exactly the opposite
of how the London/Orleans seals are installed. There is no way to increase the pressure on the seal when there is only one solid surface pressing on it.
In the December, 2006 Alternatives Consideration Report
mentioned above, B and V said much the same thing about the seals:
"Seals – J seals are used for water seals and are satisfactory for this purpose but not ideal. The J seals are pressure activated and the outboard installation location on the London Avenue and Orleans canal interim closure structure gates will not be as effective."
In engineer-ese, "will not be as effective" means "they're crap."
That 2006 report also pointed out the problem with the gap between the needles and their supports, and seemingly made reference to the modifications detailed above:
"The fit of the gates within the guides exceeds what would be considered acceptable and has already resulted in a field modification to make more functional."
Again, to translate: "exceeds what would be considered acceptable" = "it's too big."
The Corps knew about all these problems. They decided
not to fix them.
The inadequacy of the seal design at the London and Orleans sites, combined with the sizeable gaps between the needles and the guides at those sites, makes me very wary that the gates there actually fulfill their primary duty of stopping the water. At the very least, the design is ripe for creating conditions where needles get bound up while being lowered or raised. I believe that is what happened during the May 27, 2009 pre-hurricane-season exercise/show for the cameras. From the Corps press release
describing that event:
"At around 10 a.m. today, gates at the London Avenue Canal began to close to block the expected surge from entering the canals when a cable broke, keeping one of the 11 gates in the closed position.
'The cable breaking was not part of the exercise, but the team was still prepared and reacted extremely well. They had a spare cable onsite and were able to replace the cable within an hour,' said Emergency Management Chief Mike Stack. 'For a real event, we would have cranes on hand as a backup to raise or lower gates should the need arise.'"
In sum, it appears the London Avenue and the Orleans Avenue gates structures (and to a lesser extent the 17th Street structure) are far more permeable
than the Corps would have the public believe. While that is troubling enough, it's more perplexing that these problems have never been corrected in the four years the structures have been in service. The Corps anticipates using these gates at least four more years. I bet they will probably never get fixed, but it makes one wonder how much other work throughout the rest of the system is just for show like the seals at London and Orleans?