The gates story so far
Along the way, we've learned about these unresolved problems:
- The Corps relies on divers to clean out the trenches in which the gates sit, rather than an automated cleaning system. Over the years, the use of those divers has been somewhat spotty, as I'll show in my next post.
- During construction in 2006, the trenches at Orleans Avenue and 17th Street were filled and covered with jet grout to a depth of at least three feet. All that had to be jackhammered out only after the Corps discovered the problem in advance of a tropical system. One of the needles at the site was lowered in the trench at the time they found this problem. Here's a picture of that needle after it was freed:
Two years later, they were still chiseling the stuff out. Inserts meant to keep silt from clogging the trenches, promised in 2006, were never installed. The upshot is the public does not know what condition the trenches are in currently, though smart money suggests "not good."
- The gates themselves are not built in accordance with the Corps' own design standards. Among the weaknesses are vertical framing, unreinforced guide columns, and point concentration of storm surge forces into a handful of locations on the main frames of the structures.
- The gates are lowered through the guide beams with no means to ensure they (the gates) don't rattle around as water flows around them. This increases the potential of damage to the gates and the guides, as well as creating the possibility of the gates binding up. The Corps' Operation and Maintenance Manual for the gates recognizes this as a possibility in the instructions for lowering the gates:
"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."
The Corps' engineering manual and their own consultant's report calls for a series of rollers on the guides to ensure against these problems. There are no rollers.
And it's not like the Corps doesn't know rollers are needed. They are building ten of them into each of the two vertical lift gates at the Seabrook closure just a couple of miles from the outfal canals, also along the Lake Pontchartrain lakefront. That design also includes two pours of concrete around the guide beams, horizontal reinforcement for the gate segments, and basically everything the current outfall canal gates do not. Frankly, the Seabrook design amply demonstrates how flimsy the outfall canal gates are.
- At 17th Street, the gates are sealed only on one side, and at London and Orleans Avenues, there is no sealing (a cosmetic seal on the lake side of the structures at those two sites is purely decorative). Thus, the gates are actually - as built - leaky.
Here's the location of the "seals" on the front of the London Avenue structure, away from the sealing surfaces between the needles and the guide beams (photo here):
A detail from the applicable London Avenue as-built drawing shows the seals from above:
This design includes a 4.5" gap between the needles and the structure:
- At all three sites, the hydraulic power units and control boxes for the winches - which lower and raise the gates - are placed directly in the line of fire of storm-driven waves and overwash.
At 17th Street, the units are actually closer to the water than the gates themselves:
These winch control units are not enclosed in storm-hardened enclosures and remain completely exposed to waves and storm-thrown debris. This arrangement places the equipment and the operators - and by extension, the city - at risk.
- The SCADA control system at all three sites appears to be extremely vulnerable to weather induced failures, demonstrated by the five day outage in December, 2009. During this outage, water levels in both the 17th Street and London Avenue canals exceeded the criteria for closure of the gates on each canal, but neither set of gates was dropped. On the London Avenue canal, the water depth exceeded the Safe Water Elevation during this outage as well.
In the next part, we'll take a look at how some of these moving parts interact during a real event. In this case, it'll be the closure of the London Avenue gates on September 12-14, 2009