As detailed in my post "The Seals," at 17th Street there are provisions to keep the gates in place horizontally. Namely, mating wedges on the needles and the guide columns which force the gates against the back (city) face of the guides (diagram adapted from 17th Street as-built drawing here):
There is no similar system at the London or Orleans gates, meaning the gates can rattle around in the guides when lowered or raised, as well as when they are seated in the trenches.
But what keeps the gates in place vertically? The answer is dogging pins. Dogging pins are large cylinders slid into circular slots on the needles to restrain them from moving. There are two for every gate section (pictures from USACE Flickr stream here and here):
At all three sites, the dogging pins slide into slots near the top of each needle when the needle is placed in the trench (picture here):
This serves to "lock" the needle in place, as well as letting the operators know that the needle is firmly seated in the trench. The pins are very similar to the lock on a double-hung window; that lock pushes the sashes into the framework of the window, ensuring a tight fit.
There is a minor difference between the 17th Street and London/Orleans designs in how the gates are stored when they are not lowered. At 17th Street, the needles hang off a second set of dogging pins placed at the top of the main framework (from this drawing)...
Here's a picture of the upper pins and their frameworks at 17th Street. This picture, from the 2007 SAME presentation I cited in my earlier post on the trench, shows the pins before the framework for the winches was added. This would place this photo in the mid-2006 timeframe:
At London Ave. and Orleans Ave., the dogging pins at the platform level do double duty, supporting the needles in a second set of circular slots (from London Ave. bid drawings):
As mentioned above, the pins signify to the Corps personnel that the gates are firmly seated in the trench. Remember that the Corps is supposed to be having their diving contractor clearing those trenches regularly, as well as immediately before any closure of the gates during a storm. So if the pins don't go in the slots on the needles, that means the trench is not clean, which is bad.
Here's what the 2009 Operating Manual for the gates says about the pins:
"To close a gate, it will need to be lifted slightly to allow the dogging pins to be removed. A come-along may be needed to remove the dogging pin. Once the dogging pins are removed the winch can lower the gate until the lifting cables go slack. Once the cables are slack the dogging pins should be inserted into the holes near the top of the gate that are meant for locking the gate in the closed position. If the dogging pins can be inserted when the gate is down, this will confirm that the notch in the sill is free from obstructions. The eye bolt may need to be removed from the dogging pin and a sledge hammer used to move the pin into place. This process for closing the gates is identical to that described in subparagraph 184.108.40.206. If gates do not fully close and conditions are no longer suitable for divers, pre-positioned sandbags within reach of the crane shall be utilized to seal gaps."
One can see those pins getting pulled before lowering the gates during the May 27, 2009 media availability in this Times-Picayune video, starting at 0:43:
If one looks at the dogging pins closely, and then considers the lack of horizontal restraint on the gates at London and Orleans, one might wonder, "If the gates are rattling around during a storm, what keeps them from knocking the dogging pins out?"
The answer is a second set of pins. The Corps had the contractors add cotter pins which slide through the eyebolts on the ends of the much bigger dogging pins. These cotter pins are bent pieces of threaded rod which screw into welded nuts. Here's a few pictures (from here, here, and here):
A similar set of second pins is also installed on the dogging pins at 17th Street.
Effectively, the threaded rods are the last line of defense for the gates. That is, the threaded rods restrain the dogging pins, and the dogging pins restrain the gates, and the gates restrain the storm surge. Of course, the gates are pretty heavy, but so were the wall sections that collapsed along the canals. Water can do amazing, scary things.