Follow up on flappers vs. siphons
The Corps publishes all their engineering regulations, standards, and manuals online. These are what they use to design everything.
Theoretically, the one they shold be using for the construction of the floodgates and pumps is this one, EM 1110-2-3105, Engineering and Design - Mechanical and Electrical Design of Pumping Stations. Chapter 7, which is only 3 pages long, deals with how the discharges of pumps should be designed.
It talks about two possible configurations: over-the-levee, or through-the-protection. Since the common 9'-0" discharges of the pumps go through the protection (i.e. the gate structure), that's the one that applies.
Here's what the Corps says should happen with that type of discharge:
"In general, two means shall be provided to prevent backflow when the discharge is through the protection. Discharge lines through the protection should terminate with a flap gate to prevent back flow. In addition to the flap gate, provisions should be made for emergency shutoff valves, emergency gates, or individual stop log slots to place bulkheads in case of flap gate failure."
Neither of these were included in the designs for the pumping discharges at the gates. As a reminder, here is what the discharges look like:
Keep in mind that during storm surge conditions, the water level would be about two to four feet higher.
In any case, there is even a picture of a flap gate on page 6 of the chapter that has all the drawings in the manual. Here's the relevant detail from that picture:
Why can't the Corps follow their own manuals?